Development of a methodology to determine the appropriate buffer ...

Development of a methodology to determine the appropriate buffer zone width and type for developments associated with wetlands, watercourses and estuaries Deliverable 1: Literature Review

D.M. Macfarlane J. Dickens F. Von Hase

INR Report No: 400/09

Deliverable 1: Literature Review

Prepared for: Department of Water Affairs and Forestry Directorate: Water Abstraction and Instream Use

Submitted to: Water Research Commission May 2009

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CONTENTS 1.

INTRODUCTION ...................................................................................................... vii 1.1.

Background to the study ........................................................................................... 1

2.

DEFINING AQUATIC BUFFER ZONES ........................................................................... 2

3.

FUNCTIONS AND VALUES OF AQUATIC BUFFERS ....................................................... 3 3.1. 3.1.1. 3.1.2.

Sediment removal ..................................................................................................... 3 Characteristics affecting the ability of the buffer to remove sediment ................. 4 Proposed buffer widths for sediment removal ...................................................... 6

3.2. Nutrient removal ....................................................................................................... 7 3.2.1. Characteristics affecting the ability of the buffer to remove excess nutrients ...... 9 3.2.1.1. Factors affecting Phosphorous assimilation ......................................................... 11 3.2.1.2. Factors affecting Nitrogen assimilation ............................................................... 11 3.2.2. Proposed buffer widths for nutrient removal ...................................................... 17 3.3. 3.3.1. 3.3.2.

Removal of toxics (bacteria, metals, pesticide) ...................................................... 19 Buffer characteristics affecting the ability of buffer to remove toxics ................ 20 Proposed buffer widths for toxic removal............................................................ 23

3.4. 3.4.1.

Influencing microclimate and water temperature.................................................. 24 Buffer characteristics that affect microclimate and temperature of associated water resources .................................................................................. 25 Proposed buffer widths for water temperature and microclimate control ......... 26

3.4.2. 3.5. 3.5.1. 3.5.2.

3.6. 3.6.1.

Maintaining adjacent habitat critical for life needs of semi-aquatic species ......... 27 Buffer characteristics that affect the ability of the buffer to maintain the adjacent habitat critical for the life needs of semi-aquatic species ..................... 29 Proposed buffer widths for maintaining habitat required for semi-aquatic organisms ............................................................................................................. 31

3.6.2.

Maintaining adjacent habitat critical for aquatic species ....................................... 35 Buffer characteristics that affect the ability of the buffer to maintain the services critical for the life needs of aquatic species ........................................... 35 Proposed buffer widths for maintaining habitat required for aquatic species .... 36

3.7. 3.7.1. 3.7.2.

Maintenance of general wildlife habitat ................................................................. 36 Buffer characteristics that affect general wildlife habitat .................................... 37 Proposed buffer widths for maintaining general wildlife habitat ........................ 38

3.8. 3.8.1.

Screening adjacent disturbances ............................................................................ 39 Buffer characteristics that affect the ability of the buffer to screen against adjacent disturbances .......................................................................................... 40 Proposed buffer widths for screening against adjacent disturbances ................. 41

3.8.2. 3.9. i

Maintaining habitat connectivity ............................................................................ 42

Deliverable 1: Literature Review 3.9.1. 3.9.2. 3.10. 3.10.1. 3.10.2.

3.11. 3.11.1. 3.11.2. 3.12. 3.12.1. 3.12.2.

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Buffer characteristics that affect the ability of the buffer to maintain habitat connectivity .......................................................................................................... 43 Proposed buffer widths for maintaining habitat connectivity ............................. 44 Channel Stability and Flood Attenuation ................................................................ 45 Buffer characteristics affecting the ability of the buffer to provide channel stability and attenuate floods .............................................................................. 46 Proposed buffer widths for providing channel stability and attenuating floods .................................................................................................................... 47 Providing aesthetic appeal ...................................................................................... 47 Buffer characteristics affecting the ability of the buffer to provide aesthetic appeal ................................................................................................................... 48 Proposed buffer widths for providing aesthetic appeal ....................................... 48 Groundwater recharge............................................................................................ 48 Buffer characteristics affecting the ability of the buffer to promote groundwater recharge.......................................................................................... 48 Proposed buffer widths for promoting groundwater recharge ........................... 48

3.13.

Limitations of buffer zones ..................................................................................... 49

3.14.

Summary ................................................................................................................. 52

BUFFER WIDTH DELINEATION METHODOLOGIES ..................................................... 56 4.1. 4.1.1. 4.1.2. 4.1.3. 4.1.4. 4.2.

Approaches to buffer zone determination ............................................................. 56 Fixed width methodologies .................................................................................. 56 Combining the fixed-width approach with site-specific variables ....................... 57 Fixed width buffer with an added ‘setback’ or restricted management zone ..... 58 Site specific buffer width methodologies ............................................................. 58

Examples of buffer zone delineation methodologies applied in different countries ................................................................................................................. 59 4.2.1. Australia ................................................................................................................ 60 4.2.1.1. State of Victoria (Barling and Moore, 1994) ........................................... 60 4.2.1.2. City of Bellingham, Western Australia (Northwest Ecological Services LLC, 2006) .................................................................................. 60 4.2.1.3 Water quality protection note: Vegetation buffers to sensitive water resources (Department of Environment, 2005)......................................61 4.2.1.4. Swan River Plain, Western Australia (WA Planning Commission, 2005)........................................................................................................ 64 4.2.2. North America ...................................................................................................... 70 4.2.2.1. Quantitative review of riparian buffer width guidelines from Canada and the United States (Lee et al, 2004) ................................................... 70 4.2.2.2. Island County (Washington) matrix approach to buffer delineation (Environmental Law Institute, 2008) ....................................................... 72 4.2.2.3. Criteria based delineation for Chesapeake Bay in the states Maryland, Virginia and Pennsylvania (Palone and Todd, 1997) ............. 73 4.2.2.4. City of Everett, Washington State (Pentec Environmental, 2001)........... 76

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Deliverable 1: Literature Review 4.2.2.5.

Guidelines for wetland protection using buffers in Western Washington (Granger et al., 2005).......................................................... 78

4.2.3.

England .................................................................................................................... 84

4.2.4. 4.2.4.1. 4.2.4.2.

South Africa ............................................................................................................. 84 Guideline for buffers to red listed species in Gauteng Province (Pfab, 2008)...... 85 GDACE Requirements for Biodiversity Assessments (Directorate of Nature Conservation, 2008).............................................................................................. 85 Guidelines for estuarine, stream and wetland buffers in the Eastern Cape Province– ECBCP Handbook (Berliner et al., 2007) .............................................. 85 Western Cape, Cape Town City (Roads and Stormwater Dep., 2008) .................. 86 Developing guidelines to determine appropriate buffers for the protection of freshwater wetlands from various land use impacts in KwaZulu-Natal (Graham and de Winnaar, 2009) ......................................................................... 86

4.4.4.3. 4.4.4.4. 4.4.4.5.

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CONTEXTUALIZING BUFFER WIDTH METHODOLOGIES WITHIN AN APPROPRIATE SUPPORTING FRAMEWORK............................................................................ 91 5.1.

Criteria and procedures for varying from a standard buffer width ........................ 91 5.1.1.1. Buffer Averaging...................................................................................... 91 5.1.2. Scenarios for reducing buffer widths ................................................................... 92 5.1.2.1. Reasonable Use ....................................................................................... 92 5.1.2.2. Reduced intensity of impacts from proposed land uses .......................... 92 5.1.2.3. Where roads or infrastructure already exists within the buffer .............. 94 5.1.3. Scenarios for increasing buffer widths ................................................................. 94 5.1.3.1. Buffers with attributes not specifically included in the buffer methodology ........................................................................................... 94 5.1.3.2. Buffers used by species sensitive to disturbance ..................................... 94

5.2.

Allowable uses within the buffer ............................................................................ 95

5.3. 5.3.1. 5.3.2. 5.3.3. 5.3.4. 5.3.5. 5.3.6.

Best management practices to enhance and ensure effective buffer function ..... 96 Water quality protection ...................................................................................... 96 Microclimate and water temperature control ..................................................... 98 Provision of habitat for wildlife ............................................................................ 98 Maintaining channel stability and flood control .................................................. 99 Improving aesthetic appeal .................................................................................. 99 Promoting groundwater recharge ........................................................................ 99

5.4.

Provisions for the delineation and demarcation of buffers and their maintenance over time ........................................................................................... 99

5.5.

Ownership and enforcement considerations. ...................................................... 100

6. CONCLUSION………………………………………………………………………………………………………..101

7.

LITERATURE REVIEWED ......................................................................................... 102

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LIST OF FIGURES Figure 1.

Percent effectiveness of riparian buffers at removing sediment and pollutants (Pentec Environmental 2001, adapted from Desbonnet et al. 1994). ............................................ 4

Figure 2.

Factors that influence the oxygen content of soils (adapted from Blanché, 2002). ........ 14

Figure 3.

Factors that influence the organic carbon concentration and therefore denitrification rate of the soil, with high organic C favouring denitrification (adapted from Blanché, 2002)................................................................................................................................. 15

Figure 4.

Summary of recommended buffer widths for the provision of different buffer functions and values. Boxes represent average upper and lower recommended widths while lines represent upper and lower ranges (note that upper range for habitat for semi-aquatic species is 2200m). ............................................................................................................ 55

Figure 5.

Proposed process for determination of wetland buffer requirements ........................... 65

Figure 6.

Generalized wetland buffering concept (Welker Environmental Consultancy in WA Planning Commission, 2005) ............................................................................................ 67

Figure 7.

Influence of different criteria on the water resource value when determining buffer width................................................................................................................................. 74

Figure 8.

Model for wetland buffer width determination according to land use in Kwazulu-Natal (Source: Graham and de Winnaar, 2009) ......................................................................... 87

Figure 9.

Depiction of a three-zone buffer approach developed for the Chesapeake Bay Watershed (From Welsch 1991)....................................................................................... 95

LIST OF TABLES Table 1.

Proposed buffer widths for sediment removal according to various authors. .................. 6

Table 2.

Proposed buffer widths for nutrient removal according to various authors. Cells highlighted in green apply to N removal only and those highlighted in brown apply for P removal only. .................................................................................................................... 17

Table 3.

Proposed buffer widths for toxics removal according to various authors. Cells highlighted in green apply to pathogen removal while those highlighted in brown apply for removal of pesticides. ................................................................................................. 23

Table 4.

Proposed buffer widths for microclimate and water temperature control according to various authors. ................................................................................................................ 26

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Table 5.

Proposed buffer widths for maintaining the habitat required by wetland dependant species according to various authors. Rows highlighted in green refer to South African examples........................................................................................................................... 32

Table 6.

Proposed buffer widths for maintaining the habitat required for instream species according to various authors. ........................................................................................... 36

Table 7.

Proposed buffer widths for maintaining the habitat required for effective general conservation according to various authors. ..................................................................... 38

Table 8.

Proposed buffer widths for screening wildlife against disturbances according to various authors. ............................................................................................................................ 41

Table 9.

Proposed buffer widths for maintaining habitat connectivity according to various authors. ............................................................................................................................ 44

Table 10.

Proposed buffer widths for maintaining streambank stability and increasing flood attenuation according to various authors. ....................................................................... 47

Table 11.

A summary of the Best Management Procedures (BMP) available and their effectiveness relating functions required (e.g. water quality improvement) and land urban land use. The results are based on a subjective assessment where ‘5’ indicated the highest positive and ‘-5’ the lowest negative aspect of a particular BMP (Braune et al., 1999). . 51

Table 12.

Generic representation of the importance of different attributes in affecting buffer functions and values......................................................................................................... 53

Table 13.

Summary of sediment and pollutant removal effectiveness and wildlife habitat value based on buffer width (Desbonnet et al 1994) ................................................................ 54

Table 14.

Default buffer dimensions to protect Public Drinking Water Source Areas (PDWSAs) ... 63

Table 15.

Wetland management category objectives ..................................................................... 64

Table 16.

Relative importance of wetland attributes to achievement of management aims objectives (Extract only) ................................................................................................... 66

Table 17.

Guidelines to Land uses and their associated threats (adapted from Welker Environmental Consultancy, 2002) .................................................................................. 67

Table 18.

Separation and management for the wetland categories (adapted from WA Planning Commission, 2005) ........................................................................................................... 69

Table 19.

Mean (S.E.) buffer widths summarized for jurisdictions from Canada and the United States combined (Lee et al, 2004). ................................................................................... 71

Table 20.

Mean number of modifying factors and the percentages of jurisdictions using different modifying factors assessed across all jurisdictions .......................................................... 71

Table 21.

Habitat buffers (Environmental Law Institute, 2008)....................................................... 73

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Table 22.

Water Quality Buffers (Environmental Law Institute, 2008) ............................................ 73

Table 23.

Water Quality adjustments due to slope (Environmental Law Institute, 2008) .............. 73

Table 24.

Buffer widths for specific buffer functions (as cited in Palone and Todd, 1997) ............. 75

Table 25.

Site criteria as suggested by Palone and Todd (1997) to be incorporated into buffer width................................................................................................................................. 75

Table 26.

Stream category definitions for the City of Everett ......................................................... 76

Table 27.

Wetland category definitions for the City of Everett ....................................................... 77

Table 28.

Minimum buffer widths for the Municipality along the separate categories .................. 77

Table 29.

Description of different wetland categories used to inform buffer width determination. .......................................................................................................................................... 79

Table 30.

Width of buffers in western Washington if impacts from land use and wetland functions are NOT incorporated....................................................................................................... 79

Table 31.

Land use impact categories that can result in high, moderate, and low levels of impacts to adjacent wetlands. ....................................................................................................... 80

Table 32.

Suggested buffer widths with land use impacts taken into consideration. ..................... 81

Table 33.

Width of buffers suggested in order to protect Category IV wetlands in western Washington (for wetlands scoring less than 30 points for all functions). ........................ 81

Table 34.

Width of buffers suggested in order to protect Category III wetlands in western Washington (for wetlands scoring 30 – 50 points for all functions). ............................... 82

Table 35.

Width of buffers suggested in order to protect Category II wetlands in western Washington (for wetlands scoring 51-69 points for all functions or having the “Special Characteristics” identified in the rating system). ............................................................. 82

Table 36.

Recommended buffer width for Category I wetlands (for wetlands scoring 70 points or more for all functions or having the “Special Characteristics” identified in the rating system). ............................................................................................................................ 83

Table 37.

Recommended buffers for different rivers (adapted from Berliner et al., 2007) ............ 85

Table 38.

Minimum buffer widths for different wetland types in the presence of various land uses. Secondary land uses are listed in order of disturbance within each primary category increases (adapted from Graham and de Winnaar, 2009) ............................................... 88

Table 39.

EIS Categories and suggested buffer width increases (adapted from Graham and de Winnaar, 2009) ................................................................................................................. 89

Table 40.

Catchment position and suggested associated buffer increases (Source: Graham and de Winnaar, 2009) ................................................................................................................. 89

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Table 41.

Slope and soil porosity considerations for buffer width (adapted from Graham and de Winnaar, 2009) ................................................................................................................. 89

Table 42.

Habitat integrity effects on buffer width modification (Source: Graham and de Winnaar, 2009)................................................................................................................................. 90

Table 43.

Examples of measures to minimize impacts to wetlands from proposed change in land use that have high impacts (Granger et al., 2005) ........................................................... 93

LIST OF ANNEXURES Annexure 1. Summary of the different buffer zone functions and attributes influencing their ability to perform these functions and values...............................................................................................112 Annexure 2. List of selected methods for buffer delineation reviewed as part of this study………119

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1. INTRODUCTION 1.1. Background to the study South Africa's water ecosystems are under increasing pressure, with impacts such as regulation of flow by impoundments; pollution; over-extraction of water; and the breakdown of natural biogeographical barriers all affecting the ecological condition of these resources. Wetland systems have been particularly heavily impacted with an estimated 50% of South Africa’s wetlands having been destroyed or converted (Department of Environmental Affairs and Tourism, 2005). This increasing pressure on water resources has resulted in significant impacts on aquatic biodiversity. Fourteen freshwater and seven estuarine fishes in Southern Africa, most of them endemic, are threatened with extinction due to anthropogenic environmental effects, such as deteriorating water quality and loss of habitat (Skelton, 2006). South Africa, in particular the Southwestern Cape, was declared to be one of the top centres for threatened amphibian species, according to the IUCN red list (Baillie et al., 2004), due to habitat destruction and because such species are often sensitive to changes in water quality. Many invertebrate populations are also declining due to poor water quality and habitat destruction (of wetlands and riparian areas), with Dragonflies (Odonata) being a prime example (Kinvig and Samways, 2000). Samways and Taylor (2004) found that 7.4% of South African dragonflies, all of them endemic, are Red-listed by the IUCN, mostly as a result of the destruction of the riparian zone. Wetland dependant bird species, such as cranes are also under threat. Populations of wattled cranes (Bugeranus carunculatus), were previously estimated at 13 000–15 000 birds in Africa but now number less than 8 000 individuals (Beilfus et al., 2005). For Wattled Cranes in South Africa, the picture is particularly severe with only 230 individuals remaining (Unknown, 2005). The reason for these declines has been attributed to the degradation and disturbance of their wetland breeding grounds and foraging habitat (Beilfus et al., 2005). Indeed, only 2 % of the required habitat for Wattled Cranes reportedly remains, due to the destruction (for agriculture or dams) and disturbance (by fires, trampling or human activities) of breeding wetlands and the loss of surrounding grasslands, which they used for foraging, to agriculture or forestry (Unknown, 2005). Declining water quality and the loss of aquatic habitats not only have implications for aquatic ecosystems and associated biota, but water quality and quantity are central to the continued sustainability of the human populations and society that are reliant on these resources (Blanché, 2002). A range of legislation has therefore been promulgated to protect aquatic resources in South Africa. This includes the National Environmental Management Act (NEMA), the Conservation of Agricultural Resources Act (CARA), the National Water Act (NWA), the Environment Conservation Act (ECA), and the National Forests Act (NFA) amongst others. Despite the value of aquatic systems to humans and the environment however, it is evident that present policies are failing to adequately protect South Africa’s water resources, as there is growing water scarcity and rapid declines in aquatic biodiversity (Johnson et al., 2001). This highlights the need to identify additional approaches to protect water resources to ensure the sustainability of the nation’s water resources in the interests of all water users. The first step towards the protection of water resources is their identification, classification and delineation. To date, good progress has been made to define and delineate wetland and riparian areas, culminating in the production of “A practical field procedure for the identification and delineation of Wetland and Riparian areas”, (Department of Water affairs and Forestry, 2005). A 1


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recent project undertaken for the South African National Biodiversity Institute (SANBI) has also contributed significantly to developing a national system for wetland classification (SANBI, 2009). Although definitions of water resources vary, these tools both provide the information necessary to identify, delineate and classify estuaries, and inland wetlands including river systems. Once identified, the reserve, resource class and resource quality objectives are some of the key legislative tools developed to reverse or prevent impacts to aquatic systems. Implementation of these tools is still in the early stages however and often does not specifically address site-level impacts to aquatic resources. There is therefore a clear need to develop and implement other sitespecific mechanisms to protect the impacts of adjoining land uses on water resources. The establishment of buffer zones to protect aquatic resources is a common approach used to protect aquatic resources from the effects of adjacent development and / or land use change. In this regard, Blanché (2002) argues that the implementation of buffers would lead to a significant improvement to water quality in South Africa. To date however, no national guidelines or legislation for buffer zone establishment exists, except in the case of forestry operations where a minimum of a 20m buffer has been generally accepted and applied (Department of Water affairs and Forestry, 2005). The Department of Water affairs and Forestry has therefore requested the Department of Environmental Affairs and Tourism and Department of Agriculture, in terms of their shared wetland mandate, to contribute to the development of a buffer zone guideline by contributing funds to the Water Research Commission to undertake the required study. The Institute of Natural Resources was appointed in 2009 to lead this study with a range of other supporting organizations. This report sets out the findings of the first deliverable of the project, which involved a review of buffer zone functions, values and limitations and a review of buffer width determination methodologies applied both locally and abroad.

2. DEFINING AQUATIC BUFFER ZONES Definitions of buffer zones are variable, but in the case of aquatic buffer zones typically have a common purpose, to act as a barrier between activities such as human developments and sensitive aquatic environments thereby protecting them from adverse negative impacts. Aquatic buffer zones 1are typically defined from the edge of the identified aquatic resource, extending outward, ending at the interface with another land use. Buffers would therefore typically be applied from the delineated edge of a wetland, river or estuary. Due to their positioning adjacent to water bodies, buffers will typically incorporate riparian habitat, which (as defined by the National Water Act) includes the physical structure and associated vegetation of the areas associated with a watercourse which are commonly characterised by alluvial soils (deposited by the current river system), and which are inundated or flooded to an extent and with a frequency sufficient to support vegetation of species with a composition and physical structure distinct from those of adjacent land areas. The riparian zone is not the only vegetation type that lies in the buffer zone however and it may also incorporate stream banks, and terrestrial habitats depending on the width of the buffer zone applied. Activities within buffer zones are typically controlled or restricted in order to reduce the impact of adjacent land uses on aquatic resources. 1

‘ Buffer zones are strips of vegetated land (composed in many cases of riparian habitat and upland plant communities) which separate development or adjacent land uses from aquatic resources (rivers, wetlands & estuaries)’

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3. FUNCTIONS AND VALUES OF AQUATIC BUFFERS In developing a methodology and guideline for buffer zone determination, it is first important to understand the role that buffer zones can play in protecting aquatic resources and associated biota and in mitigating impacts from anthropogenic impacts. This section of the review therefore serves to provide a summary of a wide range of buffer functions and values including: • Sediment removal; • Nutrient removal; • Toxic removal; • Control of microclimate and water temperature; • Provision of habitat for wildlife; • Screening of adjacent disturbances; • Habitat connectivity; • Channel stability and flood attenuation; • Groundwater recharge; and • Aesthetic appeal. This review is approached in a structured manner by first providing evidence for each function and how it is performed and then highlighting specific characteristics or variables that affect a buffers ability to perform each function. This has been done in order to help inform the prioritization and selection of variables for inclusion in the buffer zone determination methodology. A review of buffer zone widths proposed to maintain each function is also provided together with a brief review of some of the limitations of applying buffer zones as a tool for protecting aquatic resources.

3.1. Sediment removal It has long been known that buffer zones are effective sediment traps, removing sediment from further upstream in the watershed and from runoff on adjoining lands thus reducing the sediment load of surface waters (Lowrance et al., 1988). Indeed, many would argue that this is one of the most important functions of buffer zones since it facilitates the removal of many other pollutants that are attached to the sediment, including nutrients, such as nitrates and phosphates, as well as toxics like faecal bacteria, microbes, pesticides and metals (Blanché, 2002). Sediment removal begins with the a reduction in the flow rate, which decreases the sediment carrying capacity of the water causing the excess sediment to drop out of suspension (Sheldon et al., 2003). This reduction in flow rate is caused mainly through the presence of vegetation, which increases surface roughness, increasing the resistance to flow (Blanché, 2002). A reduction in slope, typical of valley bottom settings also contributes to reducing the velocity of influent waters. Vegetation also mechanically filters the runoff, causing it to be deposited in the buffer zone, whilst the infiltration and deposition into the soil profile of sediment carried by runoff is a more minor process (Sheldon et al., 2003). The buffer zone not only causes sediment to deposit within it but also further away from it, this is termed the ‘backwater area’, as the reduction in flow velocity progresses backward from the riparian zone, causing deposition to occur (Blanché, 2002). Research shows that the relationship between the length covered by the runoff (buffer width) and sediment removal is not linear, with most sediment being deposited in outer portions of the buffer (Sheldon et al., 2003). In a study undertaken by Barling and Moore (1994) for example on forested buffers, the majority (91%) of sediment deposition took place within the first 0.25 to 0.6m of the

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outer edge of the buffer. For any further reduction in sediment load, of the remaining finer sediments (mostly silt and clay) , larger and larger buffer widths are required, for instance, 82% of clay is removed in the first 2.5m of a buffer strip but it takes 122m to remove 100% (Barling and Moore, 1994). This relationship is well illustrated in Figure 1 that illustrates the relative effectiveness of differing buffer widths in removing sediments and pollutants. It should be noted however high levels of outer edge deposition is mainly of the coarser sediments, as they require the least reduction in flow for deposition to occur (Gilliam, 1994).

Figure 1. Percent effectiveness of riparian buffers at removing sediment and pollutants (Pentec Environmental 2001, adapted from Desbonnet et al., 1994). Deposition of sediment in the outer edge of the buffer zone may lead to the formation of sandy fans that bury buffer vegetation. Barling and Moore (1994) for example, found that over time as the upper part of the buffer becomes buried in alluvial fans the subsequent sediment that enters the buffer may cause the formation of a wedge shaped sediment deposit to advance down the buffer. Burying of the vegetation by sediment has led to widespread concern about the long-term viability of buffers as traps for sediment as any subsequent sediment entering the buffer zone could potentially flow over it (Blanché, 2002; Barling and Moore, 1994). Potential loss of sediment from buffers during large flood events has also been highlighted as a concern in instances where sediment accumulation occurs within the buffer zones. Despite these concerns, there is evidence to show that the storage capacity of a buffer zone may be constantly renewed, by vegetation germination and regrowth on the deposited sediment, suggesting that buffers may act as viable long term sediment filters.

3.1.1.

Characteristics affecting the ability of the buffer to remove sediment

Flow rate is one of the main factors influencing sediment attenuation, as its reduction causes the deposition of excess sediments. Flow rate is a function of width, slope and soil permeability of the buffer (Sheldon et al., 2003), which are thus all contributing factors to the sediment attenuation capacity of the buffer zone.

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Slope is arguably most important factor affecting flow rate, and for the buffer to be effective, should be low enough to prevent the formation of concentrated flows (rills and channels). Sheldon et al., (2003), reported that the maximum slope should be between 5-10 degrees to prevent concentrated flows while Blanché (2002) suggested it should be no greater than 15 degrees. The manner in which surface flows enter the buffer can also affect the ability of the buffer to trap sediments. In this regard, uniform (laminar) flow that is usually slow and shallow, deposits more sediment relative to flow that enters at points, as it is usually rapidly flowing and deep (Barling and Moore, 1994). Topography of upland areas may therefore also be important, with steep areas generally giving rise to concentrated flows whereas uniform flows are more typical of flatter areas (Barling and Moore, 1994). Where concentrated flows occur, particularly those with a large volume and high velocity (e.g. below storm water drains), effective removal of excess sediments is unlikely to occur, with sediments likely to be carried straight through the buffer zone into the water resource (Gilliam, 1994). Such concentrated flows may actually increase the sediment load of the runoff by scouring (Sheldon et al., 2003), which could cause the buffer zone to become a net source of sediment, instead of the desired sink (Basnyat et al., 1999). The next important factor in sediment removal is surface roughness of the buffer zone itself. This is increased by the presence of dense vegetation, large amounts of woody debris as well as rocks and other obstructions that can help slow water movement (Sheldon et al., 2003). Surface roughness is particularly important in steep areas prone to the formation of concentrated flows or at discharge points where surface roughness can reduces the flow rate and cause sediments to settle more easily (Chapman and Kreutzwiser, 1999). Vegetation characteristics have been highlighted as an important contributor in reducing sediment loads. In this regard, Barling and Moore (1994) found that taller and thicker vegetation was more effective at reducing flow rate and removing sediment than lower, less dense vegetation. Sparse vegetation on the other hand offers low resistance and is therefore not very effective at sediment removal. Indeed, if vegetation is short enough to become submerged by the runoff, then the amount of sediment deposition that takes place is very low as there is little flow reduction and filtering (Barling and Moore, 1994). A further vegetation factor increasing sedimentation accumulation rate, is the vegetation spacing, with uniformly spaced stems being more effective than clumps of stems in dense and sparse vegetation types (Blanché, 2002). Due to the range of vegetation factors affecting sediment deposition, there has been much debate on whether forested or grassed buffer zones are more effective at sediment removal. Studies in the USA (Gilliam, 1994), showed that a forested riparian zone was very effective, removing >90% of the sediments entering it, but that a grassy riparian zones was just as effective, removing 90% of the sediments. Forest and grass each have their own benefits however which should also be considered, with robust forest riparian areas proving effective sediment traps for sediments from upstream flooding, reducing the sediment load of surface waters (Lowrance et al.,1988). Grass on the other hand, may provide a more useful cover in areas with concentrated flows, by reducing erosion, in the buffer zone as well as of the stream bank (Barling and Moore, 1994). The vegetations effectiveness varies widely across watersheds however (Basnyat et al., 1999), due to the many structural differences of the vegetation between them. It is also worth noting that natural vegetation is generally regarded as better at maintaining the natural sediment loading of an area, than unnatural vegetation. Kent (1994) for example found that more often than not, natural vegetation was more effective at sediment removal than buffers that were disturbed or consisted of unnatural vegetation. Richardson et al. (2007) also found that in the

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South African context alien vegetation in the riparian zone was a serious cause of perturbation. He argues that this is because sediment removal is closely related to the vegetation structure, especially at ground level, with alien vegetation usually having highly different structure from natural vegetation, altering the ecosystem functioning (Richardson et al., 2007). In some instances however, the structure of invasive plants, may well result in an increase in sediment removal. This may result in less than desirable consequences on sediment dynamics however. For example in South Africa the invasive shrub Sesbania punicea is very successful at removing sediment from runoff. In fact it is so successful at trapping sediment that it creates large flat sediment deposits along the banks of a river. This is the exact habitat it prefers and it grows prolifically, destroying wildlife habitat and severely altering river geomorphology, reducing the channel width, which would increase the effects of flooding (Richardson et al., 2007). Also in South Africa (the Eastern and Western Cape), in the context of coastal wetlands and estuarine systems, the presence of alien Rooikrans (Acacia Cyclops) around these aquatic environments has altered the sediment transportation systems. This adversely affects the geomorphology of estuaries (such as the frequency with which it opens and closes), which in turn threatens many estuarine organisms as well as depleting fish stocks. Many of the beaches on the neighbouring coast are also being depleted of sediment, which in turn threatens coastal developments (Richardson and Van Wilgen, 2004). Another mechanism by which sediment is removed is its infiltration into the soil profile, where it becomes trapped. Infiltration and the characteristics that influence it (as discussed under nutrient removal) therefore also play a role in sediment removal. This is particularly true for finer clay particles, as the more the water infiltrates the more fine sediment is trapped in the soil profile (Blanché, 2002). Buffers with coarse-grained, well drained and organic rich soils are thus more effective at removing sediment by infiltration than those in areas with fine grained, poorly drained and organic poor soils (Kent, 1994). This is because the hydraulic conductivity of coarse grained soil is high (Reichenberger et al., 2007), allowing large volumes of runoff to infiltrate. It should be noted however that although the infiltration capacity of the soil is determined mainly by its texture and associated conductivity it also increases with increasing soil structure and the presence of macro pores at the surface. Thus, clay soils with abundant macro pores, such as shrinking cracks and earthworm channels, can exhibit high infiltration capacities (Reichenberger et al., 2007). Although flow, slope, vegetation density, surface roughness, topography and soil characteristics all influence sedimentation, the ability of a buffer zone to perform this function is largely influenced by the size of the suspended particles. Heavier sediment particles (sand) settle faster than lighter ones (silt and clay), thus the nature of the sediment in the watershed and that entering the buffer determine its effectiveness in removing the sediments; the larger the particle size, the easier its removal (Chapman and Kreutzwiser, 1999). The difference can be quite pronounced with Barling and Moore (1994) showing that sand was deposited over 3m of the buffer’s edge, silt over 15m and clay over 122m in a forested buffer. The nature of sediment inputs therefore has a major bearing on the ability of the buffer to trap sediments.

3.1.2.

Proposed buffer widths for sediment removal

Table 1 below provides a breakdown of proposed buffer zone widths for sediment removal based on a range of studies between 1973 and 2005. These range from minimum widths of 1m to widths of more than 100m. Higher buffer widths have typically been advocated for steeper slopes or to ensure efficient removal of fine sediments. Table 1.

6

Proposed buffer widths for sediment removal according to various authors.


2009

2000

Min. Buffer (m) 1

Max. Buffer (m) 10

Desbonnet et al.

1994

2

25

Norman

1996

3

15.2

Recommended widths. Cited in Blanché (2002) 60% removed at 2 m; 80% removed at 25 m Needed for silt removal

Norman

1996

3

122

Needed for clay removal

Ghaffarzadeh et al.

1992

5

15

Desbonnet et al.

1994

5

15

Castelle et al.

1994

10

60

Palone and Todd

1997

17

50

Young et al.,

1980

24.4

24.4

Lynch, Corbett and Mussallem

1985

30

30

Wong and McCuen

1982

30.5

61

CWP & USEPA

2005

50

50

Broderson

1973

61

61

Horner and Mar

1982

61

61

CWP & USEPA

2005

100

Or more

85% removed in 9.1 m buffers. Cited by Castelle and Johnson (2000) Grassy buffers with 5% slope or less, as cited by McMillan (2000) Recommended widths. Cited in Blanché (2002) Recommended widths. Cited in Blanché (2002) 92% removal via vegetated buffers. As cited by Castelle et al. (1994) 75% - 80% of sediments removed from wetland. Cited by Castelle and Johnson (2000) 90% removal at 30 m, 95% removal at 61 m. As cited by Castelle et al. (1994) Slopes 15%. Cited by Biohabitats Inc. (2007)

Author

Date

Dosskey

Comments

3.2. Nutrient removal The main polluting nutrients of surface waters are nitrogen (N) and phosphorus (P), which enter a water body either dissolved in surface and subsurface flows or attached to sediment particles in surface runoff (Sheldon et al., 2003). Increases in levels of these nutrients may result in excessive outbreaks of algae and diatoms that can have adverse effect on both freshwater and estuarine environments. Buffer zones, however, have been shown to vastly reduce the amount of nutrients, entering a water body (Gilliam, 1994; Fischer and Fischenich, 2000; Castelle et al., 1994; Dosskey,

7


2009

2001; Essential Environmental Services, 2005; Winning, 1997; Department of Environmental Affairs and Tourism, 2005; Biohabitats Inc., 2007; and Blanché, 2002) Barling and Moore (1994) showed that up to 97% of N and 78% of P in runoff is sediment bound. This implies that the most important mode of nutrient removal is their co-deposition with sediments (Gilliam, 1994). Thus the effectiveness of a buffer zone at nutrient attenuation greatly depends on its sediment attenuation ability. However the buffer zone is also able to control dissolved nutrient loads by other pathways; namely plant uptake (immobilization in plant material) and chemical and microbial immobilisation pathways, including denitrification (removal of N into the atmosphere (Blanché, 2002)). N and P are important macronutrients to plants; N is used in the formation of proteins, nucleic acids and organic chemicals, such as chlorophyll and hormones (Blanché, 2002), whilst P is used to synthesize nucleic acids and ATP. Both are not usable in their natural form, though, and can only be taken up by plants as nitrates and phosphates, which are taken up by plant roots as the runoff percolates through the root zone of the vegetation in the buffer zone (Kent, 1994). Thus plant uptake does not remove N and P but temporarily stores them in new growth, seeds and storage organs, reducing the amount of nitrates available to be released to the water resource (Blanché, 2002). When plants die, the N and P are released back into the soil where most are recycled through microbial, physical and chemical attenuation mechanisms. Some of the nutrients will nevertheless enter the water body in a soluble form. This amount, though, is far lower than if there were no buffer zone (Blanché, 2002) because some of the nutrients are converted into non-biologically available forms by denitrification, for N and by other chemical reactions for P (Drizo et al., 1997 and Blanché, 2002). Denitrification is the key factor for N removal, with denitrifying bacteria in the soil converting nitrates into N gas, which is lost to the atmosphere and nitrites, which are assimilated by microorganisms into proteins, nucleic acids and other organic chemicals or are taken up by plants (Blanché, 2002). Other kinds of denitrifying bacteria reduce nitrate to other nitrogenous gases, such as nitric and nitrous oxides, which are also released into the atmosphere (Blanché, 2002). Some of the N taken up by micro-organisms will be converted into forms that are non-biologically available and therefore are not recycled into the soil when the organism dies (Blanché, 2002). Thus microorganisms convert biologically available nitrates into immobilized forms of N, which are released into the soil when they die or into the atmosphere as N gas (Blanché, 2002). P on the other hand, is immobilised more slowly, with the adsorption of P to substrate (sediment) providing the main means of orthophosphate (the biologically available form) immobilisation (Drizo et al., 1997). The other chemical means of immobilising P is by chemical precipitation of P with metals (Fe, AL, Mn), rendering it non-bioavailable (Drizo et al., 1997). Micro-organisms also convert some P into non-biologically available forms, effectively removing it from the buffer (Drizo et al., 1997). Although there is good evidence that buffers can remove nutrients, there is some concern over their ability to retain these nutrients. Indeed, Blanché (2002) showed that in areas that experience repeated rainfall events, buffer vegetation often becomes smothered in sediment. Once this has occurred, any additional nutrients entering in the surface runoff will be carried right through the buffer and deposited in the water body. Large flood events may also wash accumulated nutrients into the river, negating the effect of retention in the buffer and impacting on the downstream ecology (Barling and Moore, 1994). Barling and Moore(1994) also showed that the nutrient concentration of runoff leaving the buffer strip can be higher than that entering it if the sediment bound nutrient is released into the water adding to that which is already dissolved. Vegetation also

8


2009

has a limited capacity for nutrient uptake, restrained by the amount of plant growth, with nutrients being recycled back into the buffer zone when the plant dies, adding to the nutrient influx available to enter the water body (Dosskey, 2001). Thus plants merely recycle nutrients, with only small amounts being removed by herbivory (Blanché, 2002), unless they are managed in such a way that the plants are harvested and removed from the buffer zone (Dosskey, 2001). This is consistent with research undertaken on constructed wetlands by Spieles and Mitsch (1999), which reveals that the assimilation capacity of such systems is limited and cannot be maintained indefinitely without upkeep and maintenance. The capacity is linked to the rate at which the nutrients become immobilised, namely by denitrification or conversion to non-biologically available forms in the case of N and adsorption to substrate, chemical precipitation and conversion to non-biologically available forms in the case of P. These in turn are influenced by the soil and hydrogeologic characteristics as described in the following section.

3.2.1.

Characteristics affecting the ability of the buffer to remove excess nutrients

When considering the factors affecting nutrient assimilation, it is necessary to differentiate between N and P separately since the processes driving their assimilation varies considerably. There are, however, some basic factors which affect both N and P removal that are reported on first before focusing more specifically on the technical aspects of P and N assimilation. One of the common factors affecting uptake of N and P is the time they spend in the buffer zone (Blanché, 2002). This is mainly linked to infiltration since the infiltration rate of the soil must be such that it enables water to be stored in the soil for a long enough period for effective plant uptake and chemical immobilisation processes to occur (Blanché, 2002). Effective infiltration is achieved by buffer characteristics that cause a reduction in the flow rate, similar to those needed for sediment removal; this includes slope, type and amount of flow, infiltration rate, buffer width, soil characteristics and the type and condition of the vegetation (Kent, 1994). Slow shallow lateral subsurface and uniform surface flow were found by Blanché (2002) to be the most effective as they increase the time spent by the runoff in the buffer zone, allowing more effective infiltration. Barling and Moore (1994) found that uniform flows were 61-71 and 70-95% more effective at removing N and P, respectively, than concentrated flows, which allowed only a small percentage of the fast flowing water will percolate into the root zone and be taken up by the plants (Kent,1994). This means buffer zones and the upland areas above them with lower slopes (Blanché, 2002) and smoother topography (Kent ,1994), which are less likely to cause concentrated flows, will have better nutrient attenuation abilities (Barling and Moore, 1994). Nutrient attenuation from groundwater is linked to the contact time with the plant rooting zone. Since groundwater levels are also related to slope, with the water table usually being parallel and closer to the surface on gentle slopes, uptake from groundwater is usually greater on shallow slopes. The positive effects of a gentle slope are limited to a certain slope threshold however. Indeed, Blanché (2002) reported that the positive effects of gentle slope are only true for slopes between 510 degrees and no greater than 15 degrees. Any greater slope will decrease the retention time and promote the formation of concentrated flows (rills and channels), reducing nutrient attenuation. While flow from upland areas is typically considered, it is also worth noting that riparian vegetation can also improve the quality of water flowing in the river and create appropriate flow conditions for the removal of nutrients. Indeed, slow shallow types of flow, typical during inundation, enable the deposition of sediments, removing the sediment attached nutrients, as well as promoting effective infiltration, allowing the removal of dissolved nutrients (Blanché , 2002). Density of vegetation plays a significant role in this regard, with denser vegetation being more effective at slowing flow and

9


2009

allowing it to infiltrate. This is aided by root and stem structures as well as the amount of leaf litter. Infiltration is also aided by the deep root systems of some riparian plants which may provide infiltration channels deep into the soil (Blanché, 2002). Once infiltration has occurred, other plant characteristics affect the amount of uptake that can occur from the subsurface flows. The density and type (forest vs. Grassland) of vegetation, structure, density and condition of the plants are the main influencers of the ability of the buffer zone to uptake nutrients (Basnyat et al., 1999). The great differences in these characteristics between forest, herbaceous and grassland buffers have thus led many to speculate which vegetation type is more effective at removing nutrients. Interestingly however, in a compilation of research from various authors around the world, by Blanché (2002), on whether forests or grasslands are better at removing N the results were inconclusive. They do however vary according to the delivery pathway of the nutrients, either as surface or ground flow (Blanché, 2002). For surface flow grassland and herbaceous buffers proved slightly more effective at nutrient removal than forested ones because they are more effective at reducing the flow rate. They also have a more uniform density at ground level, with the numerous stems, thatch and roots of grasses being more effective at attenuating sediments, to which nitrates are attached than the few large trunks, fallen branches and litter provided by a forest. Grasses are also more effective at nutrient removal in high rainfall areas, as the resistance of forests to multiple runoff events is low, as their leaf litter, which provides most of the resistance to flow, is easily washed away by multiple or heavy rain events. In situations where vegetation becomes covered in sediment, grasses are also able to regrow far quicker than trees, resuming sediment and nutrient removal after a shorter period (Blanché, 2002). In terms of subsurface flows, though, forests and scrublands seem to be slightly more effective at attenuating nutrients, although results are once again highly variable (Blanché, 2002). This is mainly due to their more varied root length and depth intercepting not only shallow but also deeper groundwater flow, enabling more efficient plant uptake as well as the fact that forests are generally more active, due to their large extent underground, increasing the capacity of the buffer vegetation for nutrient uptake (Blanché, 2002). Between buffers with similar vegetation types, though, species composition may also play an important role, with Basnyat et al., (1999) reporting that native and non-native vegetation with similar structure and density had vastly different nutrient uptake levels species. This is supported by Richardson and Van Wilgen (2004), who showed that in the Western Cape Port Jackson willows (Acacia saligna) changed the nutrient dynamics and cycling of the soil relative to the natural fynbos vegetation. The productivity of different species or vegetation types is also a major factor in determining nutrient uptake and plants with high productivity, especially annuals, are regarded as most efficient at removing nutrients, by uptake. Thus the more annuals there are in the vegetation the better its nutrient removal ability will be (Chapman and Kreutzwiser, 1999). However, as these plants are annual, they will release these nutrients as they decompose, but because they take up nutrients mostly in spring and summer, when downstream ecological systems are most biologically active, they do help retain nutrients when the river is most active (Chapman and Kreutzwiser, 1999). Therefore the plant uptake ability is affected by season, with less nutrients being taken up during the winter, when plants are dormant, allowing more nitrates to escape into the water body (Gilliam, 1994). In many such cases trees would therefore be more effective as they remain active deep underground during winter, taking up nutrients. This was supported by Haycock and Pinay (1993), who showed that poplars were more effective than grasses at removing nutrients during the winter (summer rates were almost equal) in England, as they remain more active and intercept more runoff due to their roots penetrating deep into the soil.

10


2009

The soil characteristics of the buffer zone also indirectly affect nutrient removal by influencing vegetation type and growth and therefore uptake. Plant uptake, is influenced by the moisture content and organic matter concentrations of the soil, with higher soil moisture increasing root uptake potential and 110mm.1111anspiration and carbon from organic matter providing the energy needed for uptake to occur (Blanché, 2002). Therefore moist organic rich soils are likely to be the most efficient in removing nutrients, if infiltration occurs, as they promote vigorous plant growth, which increases nutrient uptake (Blanché, 2002).

3.2.1.1.

Factors affecting Phosphorous assimilation

Given that the primary mechanism of phosphorous removal is co-deposition with sediments (Barling and Moore, 1994), buffer characteristics affecting P co-deposition are the same as those for sediment removal (Gilliam, 1994). These include characteristics contributing to slow, shallow uniform flow, gentle slope, increased surface roughness and high infiltration rates (Kent, 1994). The remaining P is dissolved in the runoff and contributes the greatest amount to the P loading of water bodies as the removal of dissolved P is relatively ineffective (Gilliam, 1994). The plant uptake and chemical processes that remove dissolved P, though, require time to be effective as previously discussed. Therefore factors increasing the time spent by dissolved P in the buffer zone, as discussed above, will increase its removal; this is mainly determined by the infiltration rate and capacity in the buffer zone, increased by greater width, slow shallow uniform flow, gentle slope, high soil conductivity and vegetation density (Blanché, 2002). Once dissolved P has infiltrated into the soil chemical precipitation of P with metals is a major component of its removal, thus the soil chemistry will undoubtedly affect its ability to remove P, with shale providing optimal chemical properties for P precipitation (Drizo et al., 1997). In fact shale has the capacity to precipitate 0.8g P/kg shale, due to its high iron and manganese content. P adsorption to soil particles, particularly the negative clay ones, is also very important (Drizo et al., 1997), thus soil with moderate clay content will be effective at removing P by this means. If clay content is too high, though, the infiltration capacity is decreased rendering the removal of dissolved P ineffective (Blanché, 2002). P is also removed by plant uptake thus species composition and the factors increasing plant productivity will increase plant uptake and increase P removal (Chapman and Kreutzwiser, 1999). These include organic rich and moist soils (Blanché, 2002) and possibly annual plants, which have a high capacity to absorb P on a seasonal basis but must be removed from the buffer if the nutrients they assimilate are not to be released back into it when they die (Chapman and Kreutzwiser, 1999). It is thus apparent that the huge variation of factors, even between buffers in close proximity to one another affects the mechanisms of co-deposition, plant uptake and chemical immobilisation of P (Gilliam, 1994). In order to accurately assess the ability of a buffer to uptake P, Basnyat et al. (1999) therefore suggest that generalizations should therefore be avoided and each buffer should be investigated separately.

3.2.1.2. Factors affecting Nitrogen assimilation The effectiveness of nitrogen assimilation can also vary greatly, even between similarly sized buffers. Peterjohn and Correl (1984) recorded a level >94% but Cooper (1990) 20%, by volume, organic matter in low clay soils and >30% organic matter in clay soils (Blanché, 2002).

13


Yes

Water Inundation

No

Shallow

Water Table Depth

Deep

Occurring

Aerobic Respiration

Not occurring

Lengthy

Water Residence Time

Brief

Occurring

Decomposition in Soil

Not Occurring

Shallow Aquiclude

Hydrogeology

Deep Aquiclude

High

Soil Moisture Content

Low

Anaerobic Soil

Figure 2.

2009

Aerobic Soil

Factors that influence the oxygen content of soils (adapted from Blanché, 2002).

Factors that may lead to elevated levels of soil carbon include large amounts of senescent vegetation, levels of plant productivity, levels of nutrient leaching and soil moisture content as illustrated in Figure 3. Even if the extent of organic soil in a buffer zone is small however, its importance must not be underestimated. This is demonstrated in a study by Cooper (1990) who found that although only 12% of the soils in the riparian zone around a New Zealand stream were organic these organic soils carried out between 56-100% of the denitrification within the riparian zone over a 12 year period.

14


High

Organic Matter Content

Low

Present

Deciduous Species

Absent

High

Plant Productivity

Low

Low

Nutrient Leaching

High

High

Soil Moisture Content

Low

High Organic Carbon

Figure 3.

2009

Low Organic Carbon

Factors that influence the organic carbon concentration and therefore denitrification rate of the soil, with high organic C favouring denitrification (adapted from Blanché, 2002).

Apart from aerobic conditions and organic carbon levels, low pH may also affect dentirification processes. Low pH is also linked to C availability but not from dead organic matter, rather from plant exudates in the soil, which are correlated to levels of primary production (Blanché, 2002). The low pH controls the rate of denitrification by providing the H+ ions utilized during the process. However the levels of H+ ions are inversely controlled by the level of denitrification in a negative feedback loop, with prolonged denitrification depleting the H+ ion concentration. Thus continued productivity is required to maintain low pH levels in the soil which in turn enables denitrification to proceed (Blanché, 2002). Highly productive therefore contribute to nitrogen uptake in a range of different ways. Although anaerobic conditions, high organic C and low pH are the chemical properties of the soils that favour denitrification, there are many physical soil features and hydrogeological settings that further influence the denitrification process. These features not only affect denitrification but may also influence plant uptake of N and P and for completeness, are discussed below. Soil texture; the proportions of clay, silt, sand and organic matter, affects the chemical properties and soil moisture content as well as root development, therefore influencing the vegetation type and rates of denitrification. Coarse textured soils allow faster movement of water, reducing the time spent by nutrients in the buffer and therefore reducing the efficiency of nutrient uptake and denitrification. The better aeration of these soils is also unfavourable to denitrification as it increases

15


2009

the oxidation/reduction potential. Clay soils are also unfavourable for nutrient attenuation, due to their low permeability, reducing the amount of infiltration that occurs. However, when mixed clay soils are present water is retained for longer and organic content is high, resulting in optimum levels of denitrification (Blanché, 2002). Of further benefit is the increased denitrification during drier times, caused by the good water retention properties of these soils that maintain anaerobic conditions for longer periods (Blanché, 2002). The size of sediment particles in the buffer zone also influences its nutrient removal ability, as the greater surface area created by smaller particles retains far more nutrients than coarser grained sediment. Soils with high clay content, but not high enough to prevent infiltration, are therefore much better at filtering nutrients, which can then be removed by plant uptake or denitrification. For this reason areas such as floodplains that usually contain a higher degree of fine sediments are typically regarded as very important in nutrient removal (Gilliam, 1994). The efficiency of nutrient removal from water that enters the buffer zone as subsurface flow is dependent on a number of hydrogeological settings. First is the depth of the water table, as it is important to note that the biologically active layer of the soil, in which most the nutrient removing processes occur, is only 1-2m deep (Blanché, 2002). Thus if the water table in the buffer zone is lower than this most the nutrients in the groundwater will not be removed and enter the water body as groundwater recharge (Kent, 1994). If a shallower water table is present, though, there will be a more effective removal of nutrients, as they experience greater upwelling; which is the upward flow of water to the root zone generated by evepotranspiration. As this groundwater nears the surface in the buffer zone the nutrients are taken up by plants or denitrified, reducing the amount that enters the water body (Kent, 1994). The rate at which upwelling occurs is determined by the water potential gradient between the root zone and the groundwater, which is determined by the soil matrix, depth of the water table and hydraulic conductivity of the soil, with greater conductivity increasing the rate of upwelling and therefore nutrient removal (Blanché, 2002). Where present, the thickness of a surficial aquifer (an aquifer located within close proximity to the land surface) also determines whether the biologically active zone will have much of an effect on it, with deeper aquifer’s allowing possibly nutrient laden groundwater to effectively bypass the buffer and enter the stream (Blanché, 2002). The presence of a shallow impermeable layer or aquiclude may also significantly improve levels of nutrient removal by preventing nutrient percolation into deeper groundwater and the dilution of nitrate laden shallow groundwater by deeper groundwater. This can vastly increase nutrient removal as the shallow groundwater passes in close proximity to the roots, becoming easily available for uptake (Blanché, 2002). Increased soil moisture also promotes denitrification rates as it depletes oxygen in the soil, reducing the oxidation/reduction potential and dissolving nitrates, enabling their utilization by denitrifying bacteria. It was shown that when the WFP (water filled pore space) increased from 80-100% the rate of denitrification increased 5 fold, showing that this is a crucial factor in the rate of denitrification, especially in the range of 60-100% WFP (Blanché, 2002). Also a fluctuating shallow water table or soil moisture levels constantly shift the aerobic/anaerobic boundary optimizing the conditions for denitrification (Blanché, 2002). Furthermore high soil moisture leads to leaching, which supplies deeper soils with organic C, increasing the depth at which denitrification can occur, with a corollary reduction in surficial denitrification only significant in non-organic soils (Blanché, 2002). Soil drainage; the duration of periods of saturation, ranges from very poorly to very well drained soils, with the time it remains saturated decreasing from poor to well drained soils. Soil drainage is related to soil conductivity, with well drained soils having good conductivity, meaning water can pass quickly through it relative to soil with poor drainage, which takes longer. Both drainage schemes are

16


2009

useful for different nutrient removal mechanisms, with well drained soils being well suited for the nutrient removal of sediments attached nutrients, as the sediment becomes caught in the soil profile. Poorly drained soils, on the other hand, create favourable conditions for denitrification, by promoting the formation of anaerobic conditions. Thus soils with moderate drainage would enable co-deposition of nutrients and denitrification (Blanché, 2002).

3.2.2.

Proposed buffer widths for nutrient removal

Table 2 below provides a breakdown of proposed buffer zone widths for nutrient removal based on a range of studies undertaken on this complex field. These range from minimum widths of 4.6m to widths of more than 200m. Higher buffer widths have typically been advocated for higher proportional removal of nutrients.

Table 2.

Proposed buffer widths for nutrient removal according to various authors. Cells highlighted in green apply to N removal only and those highlighted in brown apply for P removal only.

1993

Min. Buffer (m) 4.6

Max. Buffer (m) 9.1

Lowrance et al.

1995

7

7

Sufficient nitrate removed by plant uptake and microbial denitrification in forested buffer zones

Desbonnet et al.

1994

9

60

Thomson et al.

1978

12

36

Small buffers (6 m) removed 60% of nitrogen; much larger buffers (60 m) are needed for 80% removal Removal efficiencies ranged from 44 to 70%. Cited in McMillan (2000)

Desbonnet et al.

1994

12

85

The averaged (from the reviewed literature) removal efficiencies were not linear, and wider buffers were necessary for an increase in effectiveness

Shisler, Jordan and Wargo

1987

19

19

80% and 89% of phosphorus and nitrogen removed respectively by forested buffers, as cited in Castelle and Johnson (2000)

Edwards et al.

1983

30

30

Peterjohn and Correll

1984

50

50

Author

Date

Dillaha

17

Comments 70% and 84% (respectively) suspended solids and associated nutrients removed with vegetated filter strips. As cited by Todd (2000)

Mcmillan (2000) cites 50% phosphorus removal at 30 m “Dramatic reduction in nutrient loads from crops” in forested buffer strips, as cited in Belt and O’Laughlin (1994)


Author

Date

Vanderholm and Dickey

1978

Dillaha

1993

Min. Buffer (m) 260

Max. Buffer (m) 260

4.6

9.1

2009

Comments 80% of nutrients, solids, and BOD removed from feedlot runoff on shallow (less than 0.5%) buffer slopes, as cited by Castelle et al. (1998) 54% removal of nitrogen at 4.6 m and 73% at 9.1 m through vegetated filter strips. Cited in Todd (2000) Recommended widths. Cited in Blanché (2002)

Castelle et al.

1994

5

90

Patty et al.

1997

6

20

Daniels and Gilliam Palone and Todd. Dosskey

1996

6

20

1997

10

40

47% removal of nitrogen at 6 and 99% at 20 m. Cited in McMillan (2000) 47% to 99% of nitrogen removed. As cited by McMillan (2000) Recommended widths. Cited in Blanché (2002)

2000

18

30

Recommended widths. Cited in Blanché (2002)

Palone and Todd CWP & USEPA

1997

55

260

Recommended widths.

2005

100

100

Removal from shallow groundwater. Cited in Biohabitats Inc. (2007)

Dillaha

1993 4.6

9.1

61% and 79% removal of phosphorus via vegetated filter strips at 4.6 and 9.1 m respectively, as cited in Todd (2000)

Young et al.

1980

21

27

Removal of 67% and 88% of phosphorus at 21 and 27 m respectively. Cited in McMillan (2000)

CWP & USEPA

2005

50

50

Shallow slope. Cited in Biohabitats Inc. (2007)

CWP & USEPA

2005

100

100

Steep slope. Cited in Biohabitats Inc. (2007)

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2009

3.3. Removal of toxics (bacteria, metals, pesticide) Toxic pollutants, such as bacteria, pesticides, metals and other chemicals may all affect the quality of water resources and thus their suitability for aquatic biota and for human use. These toxics may enter a water body in three different ways. They may be attached to the sediment, be dissolved in the runoff (Reichenberger et al., 2007, Chapman, 1999 and Kent, 1994) and, in the case of pesticides, may be carried on the wind as spray-drift (Reichenberger et al., 2007). Toxics attached to sediment and dissolved pesticides and metals enter the water body via similar pathways that may be either diffuse or point source. The nature of the pollutant typically varies according to land use, with metals originating mainly from urban runoff (Biohabitats Inc., 2007) and pesticides from cultivated fields as well as urban areas (Reichenberger et al., 2007). Diffuse sources include direct surface runoff from drained agricultural lands, base flow seepage, surface and subsurface runoff and soil erosion from treated fields as well as spray-drift during application, and deposition after volatilization (Reichenberger et al., 2007) in agricultural areas or road runoff in urban areas (Biohabitats Inc., 2007). Point source pathways include farmyards and accidental spills in agricultural areas (Reichenberger et al., 2007) and sewage plants, sewer overflows, accidental spills, applications to urban surfaces (such as roads, parking lots etc.) and stormwater drains in urban areas (Biohabitats Inc., 2007). The entry of micro-organisms (bacteria and viruses) is of particular concern in aquatic resources subject to high levels of human use and in domestic areas as they are a public health threat and can render fish and shellfish resources unexploitable (Kent, 1994). They typically enter the water body in diffuse source runoff that has come into contact with domestic waste products or faecal matter (animal and human). The runoff then detaches and transports the micro-organisms suspended in solution to the water body (Tate et al., 2004). Micro-organisms may also be delivered to the water body from point sources, such as sewage outflows (Kent, 1994). The contamination of water bodies by faecal matter is indicated by the presence of E.coli and Salmonella, which are enteric faecal microbes, indicating the potential contamination by more threatening pathogens (Tate et al., 2004). For example, Bilharzia (Schistosoma haematobium) may enter a water body through faecal contamination, thus by reducing faecal contamination, one can reduce the number of eggs entering the water body and thus lower the infection rate (Cowan, 1995). According to the type (sediment attached, dissolved, suspended or spray-drift) and mode of entry of toxics, they can be removed in different ways by the buffer zone. The same processes that cause sedimentation, including flow rate reduction, filtration and infiltration, will lead to the removal of sediment attached toxics, mainly positively charged metals, chemicals and certain pesticides, by the buffer zone (Blanché, 2002). These trapped toxics may then be buried, chemically broken down or temporary assimilated into plant tissue (Chapman, 1999). Processes that are responsible for removing dissolved toxics are much more problematic however as they may require large residence times, as is typically the case with dissolved nutrients (Kent, 1994 and Reichenberger et al., 2007). For this to be the case runoff must therefore first infiltrate into the soil, where it can be retained, providing the time needed to remove the dissolved toxics. This is often not the case however and is the reason why most toxics that enter a water body after passing through the buffer zone are dissolved (Kent, 1994). Another process that removes many of the positively charged metals and pesticides from the solution is their sorption to the negatively charged soil particles in the buffer zone, preventing them entering the water body (Reichenberger et al., 2007 and Biohabitats Inc., 2007). Once absorbed to

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the soil the toxic metal, chemical or pesticide may then be buried, chemically broken down or temporarily assimilated into plant tissue, just like the sediment attached toxics (Chapman, 1999). The function of naturally occurring micro-organisms in the soil of the buffer zone is also very important as they actively transform metals, specifically into less harmful or immobile substances (Biohabitats Inc., 2007). Many of the pesticides and other organic chemicals are also removed as they are actively metabolized by soil micro-organisms as an energy source, thus reducing the amounts entering the stream (Blanché, 2002). Removal of micro-organisms is slightly different from that of other toxics and is thus dealt with separately. Some micro-organisms may indeed be attached to the sediment and deposited with sediment, just as with sediment attached nutrients and other toxics (Kent, 1994). Many, however, are suspended freely in the runoff and to be removed they must settle out from the solution, through a reduction in flow rate, just as with sediments (Kent, 1994). They may then infiltrate into the soil and/or adsorb to soil or organic material (Tate et al., 2004). The primary mechanism for the removal of micro-organisms in runoff, though, is infiltration (Tate et al., 2004). This is usually coupled with their adsorption to soil particles, hindering their passage to the water body, resulting in their eventual death. For parasitic oocysts, such as Cryptosporidium parvum (a diarrheal disease mainly spread by recreational water activities) however, they may not die, but must be retained in the buffer during their ‘infective stage’ so as to not contaminate the water body (Tate et al., 2004). In the case of pesticides, although edge-of-field runoff may be a far more significant than spray drift for nonpoint-pollution of surface waters from agricultural fields (Schulz, 2001), buffers do also help reduce contamination of surface waters by reducing spray-drift as the vegetation in the buffer may acts as a windbreak, preventing the pesticide droplets from crossing the buffer zone (Reichenberger et al., 2007). In this regard, it is worth noting that Padovani and Capri (2005) estimated that only 0.4 – 5.8% of pesticide applied to a crop is lost as spray-drift. This may amount to substantial quantities in areas with high levels of pesticide use however and under such circumstances buffers may play an important role in reducing the levels of pesticides entering water resources. This is demonstrated in a study on the effectiveness of wooded buffers in New Zealand by Reichenberger et al. (2007) who found that buffers are very important for blocking spray-drift and therefore reducing the levels of pesticides entering the water resource.

3.3.1.

Buffer characteristics affecting the ability of buffer to remove toxics

Due to the range of potential toxics from adjacent land uses, it is not possible to identify a standard set of characteristics that affect the ability of the buffer zone to assimilate such toxics. Factors affecting the assimilation of different types of toxics are therefore described in order to provide insights into the characteristics of buffer zones that will be effective in different circumstances. In terms of the sediment attached and dissolved toxics, the same buffer characteristics that aid in the removal of sediment and dissolved nutrients will generally increase the efficiency of toxics removal (Blanché, 2002). By implication then, slow, shallow uniform flow, denser vegetation and increased surface roughness as well as coarser soil texture will improve removal of sediment attached toxics. This is confirmed by Reichenberger et al. (2007), who argues that the buffer zone will only effectively remove pesticides if the runoff enters the buffer zone as slow uniform flow, as concentrated flows (rills, gullies, ditches, tile drains) render the buffer useless at toxics removal. This was also demonstrated in the Lourens River, South Africa as buffers that had a higher number of erosion rills (concentrated flows) and steeper slopes were less effective at removing pesticides, as the concentrated flows due to the steep slope carrying the pesticide right through the buffer zone into the river (Dabrowski et al., 2002). They don’t allow the settling of sediments and sediment

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attached pesticides, which is enabled in areas of lower slope that create more uniform flows that are more effective at allowing the deposition of sediment attached pesticides (Dabrowski et al., 2002). Many of the dissolved toxins, especially metals and pesticides, are also removed in much the same manner as soluble nutrients and are thus affected in similar ways, favouring conditions that increase their retention time in the buffer (Chapman, 1999 and Reichenberger et al., 2007). This is increased by slow uniform flow, dense vegetation, a high infiltration rate and high porosity, increasing the retention of runoff into the buffer soil enabling it to better remove the dissolved toxics (Chapman, 1999). This is backed up by Dabrowski et al. (2002) who found that low slope and uniform flow enable infiltration to occur, necessary to enable the breakdown of the pesticides to take place. Infiltration is also influenced by soil texture, which should be intermediate to allow optimal infiltration and storage of runoff; if it is too coarse water will pass through it too quickly and if it is too fine, it will be impermeable to the runoff (Blanché, 2002). Once infiltrated plant uptake is responsible for removing metals and chemicals and thus buffers that have high productivity, such as those with moist and organic rich soils, will be the most effective at removing toxics by this means (Blanché, 2002). The chemical character of the soil within the buffer is also an important characteristic, as this may have unique attributes that help in removing specific metals, chemicals or pesticides by means of chemical reactions that immobilize these toxics (Chapman, 1999). Increased clay content of the soil is also useful in removing toxics as the negatively charged clay particles bind to positively charged pollutants, especially metals and organic pesticides (Blanché, 2002). Increased organic carbon content of the soil is also likely to facilitate adsorption and breakdown of pesticides, due to the presence of organoclays with higher C.E.C. (cation exchange capacity) making such soils more able to adsorb and breakdown pesticides (Dabrowski et al., 2002). Removal of pathogenic micro-organisms typically requires similar buffer attributes, such as gentle slope, slow uniform flow, dense vegetation and good soil permeability for their removal. The velocity of the contaminated water entering the buffer (and associated residence time), is however regarded as a particularly important attribute in affecting the ability of buffers to remove pathogens. Increased velocity increases the detachment and flushing transport or micro-organisms from substrates in the upland and buffer areas, increasing the amount delivered to the water body (Tate et al., 2004). An investigation on the rate of Cryptosporidium parvum oocyst delivery to a water body, across a buffer by Tate et al. (2004) concurred that the rate of delivery was related to the velocity of the surface runoff, with increasing velocity causing the micro-organisms to dislodged more easily from the substrate, resulting in greater concentrations entering the water body. The main factors influencing the velocity of the runoff from adjacent lands are slope and the rate of rainfall, with an increase in either causing the runoff velocity to increase, thereby increasing microorganism delivery to a water body (Tate et al., 2004). Besides influencing their transport, runoff also influences micro-organism mortality, which is largely due to desiccation (Biohabitats Inc., 2007) and therefore the link between runoff velocity and residence time is also important in determining micro-organism removal (Kent, 1994). In this regard, Kent (1994) found that even a short residence time can vastly reduce the number of pathogens. He showed that even though domestic sewage in a particular study originally contained more pathogens than stormwater runoff, the stormwater contributed more pathogens to the water body. This is because it delivered water at a higher velocity, giving little time for the desiccation or death of the pathogens to occur, whilst the sewage was delivered at a far slower velocity, resulting in the desiccation and death of a larger portion of the pathogens. He then demonstrated that just 7m of buffer was needed to reduce both these amounts to an acceptable level.

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As desiccation is a large contributor to pathogen mortality, soil moisture levels may also affect buffer zone effectiveness, drier soils promoting water absorption and desiccation (Biohabitats Inc., 2007). For example, hookworm disease (Strongyloidiasis) and threadworm can survive in the film of moisture surrounding soil particles, thus buffer zones with drier soils will carry less of these parasites, reducing infection rates (Cowan, 1995). However, the pathogen removal ability of the buffer is mainly dependent on the physiochemical interactions that occur between the soil and the pathogen. The different chemical characteristics of different soil types will promote the adsorption of different types of pathogens, some pathogens, such as Cryptosporidium parvum can actively desorb themselves from particles (Tate et al., 2004). Size and shape also play a role, with small narrow types, such as E.coli and Salmonella being far more difficult to remove as they can escape entrapment far more easily than larger cylindrical types, including parasitic oocysts (Tate et al., 2004). Even once caught, though, the survivability of the micro-organisms influences the buffers effectiveness at removing them, as some micro-organisms may survive up to 27 weeks in the soil, enabling them to possibly be dislodged once again and delivered to the water body (Kent, 1994). Thus the degree to which the micro-organisms can survive determines the distance they can cover, which can vary from 2-837m (Kent, 1994). Therefore to factor out the attributes specific to the type of micro-organism slow flow, greater infiltration and filtration should be the primary buffer zone characteristics considered when aiming to remove microbes in general (Tate et al., 2004). Along with the characteristics of the buffer zone that affect its ability to remove sediment attachedand dissolved-pesticides, there are also other factors affecting pesticide attenuation. One such factor is that pesticide removal is inversely related to soil water content, with wetter soils being less effective at removing them than drier ones. The concentration of pesticides in the inflow is also important, with nominal concentrations creating the best conditions for retention due to greater soil absorption (Reichenberger et al., 2007). Specific pesticide characteristics are also important in that this affects the mechanism of removal, which can be either by co-deposition with sediment, or by immobilization from solution, which depends on the absorbing properties of the pesticide. This is given by the Koc (Freundlich sorption coefficient normalized to soil organic carbon content), with pesticides that have a Koc greater than ca. 1000 L kg-1 (Reichenberger et al., 2007) being highly absorptive, resulting in most of the pesticide being lost as co-deposition with sediment (Reichenberger et al., 2007). An example of this is in the Lourens River Catchment, South Africa, where the pesticide azinphos-methyl (AZP) is mostly found in the water, even though chloropyrifoc (CPF) and endosulfan (END) were also used, as the later two tended to attach to the sediment, due to their Koc greater than ca. 1000 L kg-1 (Dabrowski et al., 2002). This sediment was then deposited, allowing the effective removal of CPF and END but AZP entered the water body as it has a Koc less than ca. 1000 L kg-1 and remained in solution, entering the river with ease. Besides their Koc values, the half-life of the pesticide is the other of its physiological properties influencing whether it can be removed by the buffer (Dabrowski et al., 2002), as it only need be retained long enough to break down (Dabrowski et al., 2005). It was further noted by Adriaanse (1997) that the half life of pesticide is less whilst in solution than if attached to sediment (Adriaanse, 1997) so buffers that can retain runoff for long periods, due to a high infiltration rate and/or low slope (sometimes creating backwater pools) would be more suitable for the natural breakdown of the pesticide than impervious, steep buffers. Characteristics of buffer zones required to reduce spray drift is also worth noting. This is influenced primarily by vegetation height, with taller trees blocking more spray-drift than shorter ones (Reichenberger et al., 2007). This is because taller vegetation lowers the angle of incidence of the exposed channel to on-coming drift (Dabrowski et al., 2005). At a finer scale, plant structure may also influence the buffer’s ability to intercept spray-drift, with factors such as leaf stage and leaf area affecting interception levels. In terms of leaf stage Reichenberger et al. (2007) found that a an

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average tree blocked 50% of the spray, a bare tree 25% and a tree in full leaf stage, 90%. The effect of leaf area on interception rates was also demonstrated in a study by Dabrowski et al. (2005) on the Lourens River, South Africa, where the broader leafed Juncus capensis blocked more spray drift than the thinner leafed Fuirena hirsute and various Pycreus species (Dabrowski et al., 2005). The importance of plant structure for intercepting spray-drift is further illustrated by the fact that it is used in the Kenaga nomogram model that is used by the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA) in the USA to estimate pesticide residue on plants. Wind speed and turbulence of the area may also affect the ability of the buffer to stop spray drift, with higher wind speeds and greater turbulence resulting in more pesticide being transported through and over the buffer, making it less effective at blocking spray-drift (Reichenberger et al., 2007). This is supported by Dabrowski et al. (2005), who showed that wind speed and direction are hugely influential on the amount of drift-spray that can reach the water body across the buffer. It has therefore been recommended that buffers should be wider in windier areas (Reichenberger et al., 2007).

3.3.2.

Proposed buffer widths for toxic removal

As with other buffer functions, recommended buffer widths for removal of toxics varies significantly, based on site characteristics and specific toxins being considered. A range of buffer widths proposed for pathogen control and pesticide removal are illustrated in Table 3 below and range from 2m for a moderate load reduction of pesticides to a conservative figure of 50m. Table 3.

Proposed buffer widths for toxics removal according to various authors. Cells highlighted in green apply to pathogen removal while those highlighted in brown apply for removal of pesticides.

1977

Min. Buffer (m) 3.8

Max. Buffer (m) 3.8

Doyle, Stanton and Wolf

1977

4

4

Faecal coliform bacteria levels reduced by grass buffers, as cited in Castelle and Johnson (2000)

Grismer

1981

30

30

Grass filter strips removed 60% of faecal coliform bacteria. Cited in McMillan (2000)

Young et al.

1980

35

35

Grass buffers reduced microorganisms to acceptable amounts, as cited in McMillan (2000)

CWP & USEPA

2005

50

-

Author

Date

Doyle, Stanton and Wolf

23

Comments Faecal coliform bacteria levels reduced by forested buffers. Cited in Castelle and Johnson (2000)


2009

2007

Min. Buffer (m) 2

Max. Buffer (m) 18

Reichenberger et al.

2007

2

18

Load reduction efficiencies were on average 62% (2m) and 99% (18m) for sediment attached pesticide. Compiled from different authors.


2007

2

18

Load reduction efficiencies were on average 65% (2m) and 85% (18m) for dissolved pesticide. Compiled from different authors.

Winkler

2001

5

-

50% reduction using German regulatory model EXPOSIT 1.1. cited in Reichenberger et al. (2007)

Winkler

2001

10

-

90% reduction using German regulatory model EXPOSIT 1.1. cited in Reichenberger et al. (2007)

Winkler

2001

20

-

97.5% reduction using German regulatory model EXPOSIT 1.1. cited in Reichenberger et al. (2007)

CWP & USEPA

2005

50

-

Author

Date


Comments Load reduction efficiencies were on average 65% (2m) and 95% (18m) for all pesticide types. Compiled from 277 variables from different authors.

3.4. Influencing microclimate and water temperature Another potential benefit of maintaining riparian areas and associated buffer zones, is the positive effect that such areas can have on influencing local microclimate and temperate of water resources. There are two main microclimates present in a buffered aquatic system that are affected by the buffer zone; that on the boundary of the water body, which in turn affects water temperature, as well as that within the vegetation of the riparian or buffer zone (Ghermandi et al., 2009 and Pentec Environmental, 2001). Vegetation occurring along the stream bank may affect the microclimate of the stream area nearest the stream bank and reduce water temperatures. The potential influence of shading on water temperatures is well illustrated by a study carried out by Ghermandi et al. (2009) who showed that light interception from shading can reduce water temperature by 4-8 degrees Celsius. This can have serious consequences for many species as water temperature plays a key role in the lifecycles of many species (Barling and Moore, 1994). One particular study in New Zealand showed that in midsummer only the shaded areas of certain streams had temperatures low enough to support fish compared to un-shaded areas (Ghermandi et al., 2009). The occurrence of streamside vegetation also has a significant effect on aquatic plant growth, as light incidence is the main variable

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controlling productivity in shaded streams (Blanché, 2002). Thus removing vegetation is likely to increase stream primary productivity, increase the risk of eutrophication and change the species structure and community composition in the water body (Barling and Moore, 1994). The potential affect on community composition is demonstrated in a study by Ghermandi et al. (2009) who found that shading reduced phytoplankton and algal productivity by up to a 44%. Maintaining streamside vegetation may therefore be an effective means to control eutrophication in small to medium sized streams that are at risk of eutrophication. The lower temperatures caused by shading, also has important consequences for other water quality variables besides temperature, such as the dissolved oxygen concentration (DO), which increases with lower temperatures. This increases the capacity of the stream to contain life and assimilate organic wastes, further increasing water quality (Ghermandi et al., 2009). Buffer zones can be of further benefit to maintaining water temperatures by receiving runoff from impervious surfaces, which may be much warmer than normal, and allowing it to cool down and infiltrate before it enters the stream (Biohabitats Inc., 2007). They may even be used to mitigate for thermal pollution, effectively bringing down the temperature, improving the water quality and the habitability of the water body for instream organisms (Ghermandi et al., 2009). Another means, by which the buffer maintains cooler water temperatures, is by increasing the amounts of groundwater discharge to a stream, which usually have lower temperatures (Biohabitats Inc., 2007). The amount of shading in the riparian zone also affects the animals and plants that naturally occur as illustrated by Samways and Taylor (2004) who found that the vegetation and dragonfly communities along South African streams were very specific in selecting the amount of shading in their habitat. These characteristics can be altered by a range of factors including vegetation clearing, agricultural practices or infestation by alien species. Indeed, in the areas of South Africa with low lying riparian vegetation, such as the fynbos, and grassland, Samways and Taylor (2004) showed shading was increased by the presence of alien tree species, such as Acacia mearnsii and Acacia longifolia. This in turn affected the air and soil temperatures as well as soil moisture content, thus greatly influencing the species which could inhabit this area (Samways and Taylor, 2004).

3.4.1.

Buffer characteristics that affect microclimate and temperature of associated water resources

The amount of vegetation overhanging the stream is the primary characteristic affecting the microclimate of the water body. This is affected by channel width, which should be 5m or less for the shading to have any real impact. For any greater width, the area that is shaded by the vegetation is insignificant relative to the total area of the water body to substantially reduce water temperature (Ghermandi et al., 2009). The presence of streamside vegetation is therefore most important in small streams; due to the large amount of shading they receive relative to their small size (Barling and Moore, 1994). The vegetation characteristics also affect the degree of shading on the perimeter of the water body, with the greater canopy height of taller forest vegetation providing greater shading area than short grassland vegetation (Pentec Environmental, 2001). The width of the vegetation buffer as well as the density of the foliage in the buffer also influence the amount of shading, with wider buffers composed of denser vegetation being more effective. Channel orientation relative to the sun also affects the effectiveness of shading with channels that are perpendicular to the axis of the suns movement being most effectively shaded by the vegetation, whilst those parallel to the sun will receive less (Pentec Environmental, 2001, Ghermandi et al., 2009).

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Apart from the effect of vegetation structure on water temperature, changes in natural structure of the riparian zone and associated buffer will also affect the suitability of the buffer zone for different species and so affect the use of the area by biota.

3.4.2.

Proposed buffer widths for water temperature and microclimate control

Clearly, maintenance of natural habitat is necessary to maintain microclimates necessary to support native biota, and requires greater management intervention than simply setting aside a buffer zone of a particular width. Recommendations of various studies are worth noting however and suggest that buffers of up to 40m may be necessary to maintain streamside temperatures and vegetation characteristics in forested systems. Buffer widths required to maintain water temperature and maintain natural microclimate are likely to be significantly smaller in vegetation types of lower stature (e.g. grasslands) that have a naturally smaller influence on water temperature. Proposed buffer widths for microclimate and water temperature control according to various authors.

Table 4.

Author

Date

Palone and Todd

1997

Min. Buffer (m) 5

USDA NRSC

2003

9.4

Dosskey

2000

12

18


Palone and Todd

1997

15

60

Recommended widths for maintaining water temperature.

Broderson

1973

15.2

15.2

Provides adequate shade for a small stream to regulate the temperature. Cited by Castelle 1994 (02-02)

Castelle et al.

1994

20

35


Lynch et al.

1985

30

30

Maintains stream temp within 1oC of former temp. Adjacent to logging activities. Cited by Castelle 1994 (0202)

Harper and MacDonald

2001

40

40

Influence of large aquatic systems on adjacent upland forest composition and structural complexity

26

Max. Buffer (m) 20

Comments Recommended widths. Cited in Blanché (2002) No specific motivation provided


3.5.

2009

Maintaining adjacent habitat critical for life needs of semiaquatic species

The terrestrial ecology of many semi-aquatic species is often being underappreciated or overlooked by managers and conservation planners mainly because these species make only brief visits to terrestrial habitats when nesting, and hibernacula are rarely observed. However, while these animals are seasonally found in streams and wetlands, much of their life history occurs in terrestrial habitats (Semlitsch and Bodie, 2003). Indeed, a review of international literature reveals that many semi-aquatic organisms, especially amphibians but also freshwater turtles, snakes, birds, mammals and insects, depend not only on wetlands for survival but also on adjacent terrestrial habitats, which they use to complete their life cycle (Semlitsch, 1997). This is no different in South Africa, with Graham and de Winnaar (2009)) identifying 46 protected bird, 31 protected mammal, 5 protected reptile, 8 protected frog and 17 protected dragonfly species that depended on upland habitat adjacent to water bodies for their survival in KwaZulu-Natal alone. Many of South Africa’s threatened wetland bird species are also dependant to some degree on adjacent grasslands. Indeed, Allan et al. (1997) reported that there were 90 bird species (25 threatened and 5 endemic) associated with both wetland and grassland habitats whose continued survival depends on the protection of wetlands as well as the surrounding grassland areas. Similar scenarios exist in the Eastern highlands of Zimbabwe and fynbos biome, with many threatened or endemic bird species using both wetland and adjacent grassland or fynbos habitats, respectively (Allan et al., 1997). There are a further 34 reptile taxa, 105 amphibian species as well as numerous mammal and invertebrate species associated with wetlands that also reportedly utilize habitats adjacent wetland areas (Cowan, 1995; Graham and de Winnaar, 2009; and Samways and Taylor, 2004). There are two key aspects that makes habitat adjacent to wetlands important for the survival of semi-aquatic species, firstly it may be required for the successful recruitment of juveniles and secondly, it may be required to maintain optimal adult survival rates (Semlitsch, 1997). The importance of buffers for breeding, was reported by Kent (1994) who noted that most waterfowl use the cover provided by habitat adjacent to wetlands for nesting (Kent, 1994). The importance of the buffer zone in allowing successful recruitment is also true for many reptile species, which bury their eggs on the land adjacent to the aquatic environment (Cowan, 1995). Examples of South African reptiles include the Nile crocodile crocodylus niloticus, Cape Terrapin Pelomedusa subrufa and African Rock Python Python sebae natalensis (Cowan, 1995). Mammals, such as two species of otters (both threatened), a mongoose and many species of rodents, also use the riparian zones to create dens or burrows used to breed in (Cowan, 1995). This led Cowan (1995) to the conclusion that the loss of riparian habitat was one of the greatest threats to these species, especially otters, and that the creation of a buffer zones would greatly benefit this threatened species (Cowan, 1995). Semlitsch, (1997) argues that the terrestrial area is not only important for the recruitment of juveniles but also for adult survival as was noted for many amphibians, reptiles, birds and insects that use the upland habitat for cover, foraging and migration and hibernation. A local example of a species reliant on upland habitat for the survival is the iconic and critically endangered Wattled Crane. Wattled Cranes require wetlands as breeding habitat, however, the grassland areas surrounding suitable breeding wetlands are as important to successful breeding as the wetland itself as they are widely used for foraging (McCann et al., 2000). McCann et al.(2000) report that only 2% of wetlands suitable for breeding of this species are surrounded by such areas and thus note the importance of buffer zones that incorporate suitable foraging habitat for the survival of this species.. This is not only the case with the Wattled Crane and the same issue is relevant to a range of other critically endangered species, including the Whitewinged Flufftail (Saothrura ayresi) and Blue Swallow (Hirundo atrocaerulea) (Graham and de Winnaar, 2009). In coastal wetlands and estuaries 27


2009

adjacent upland habitat are equally important, providing a refuge to shorebirds and waders that use the water body as their feeding grounds, predating on fish and invertebrates, during high tide (Kent, 1994). Most of South Africa’s frog species use freshwater wetlands to breed, as they lay their eggs in the water which later supports the aquatic larvae (Graham and de Winnaar, 2009). However 69 of the 105 South African frogs are predominantly terrestrial, foraging and sheltering in the in upland areas for the majority of the year (Cowan, 1995). These areas are also used for the maturation of juveniles, which forage and find shelter in these areas. The maintenance of the upland areas adjacent to wetlands is therefore essential to enable the successful maturation of juvenile and survival of adult frogs, necessary to increase the viability of these populations (Cowan, 1995). Declining frog populations is indeed a global problem (Graham and de Winnaar, 2009) and one of the factors contributing to this decline is the loss of required upland habitats adjacent to wetlands (Hero and Morrison, 2004). As an example, 20 of the 40 threatened frog species in Australia, including 91.7% of the lowland species are threatened primarily because of the loss of their required upland habitats (Hero and Morrison, 2004). The importance of buffer zones to amphibian conservation is particularly important in the South African context with 21 species already listed as threatened in the IUCN Redlist of which 20 are endemic (Bailie et al, 2004) Of the South Africa reptiles at least 34 taxa are associated with wetlands, although the ecology of most is unknown thus making it more difficult to discern the effects that buffer zones would have on them as the degree to which they depend on wetlands and adjacent habitat varies (Cowan, 1995). The best studied species that require wetlands are the African rock python, spotted skaapsteker, aurora snake, rhombic night adder and slender legless skink which all require water in which to submerge themselves as well as for cover from which to ambush prey (Cowan, 1995). Even though most wetland reptiles can tolerate a wide variety of habitats it is clearly evident that they require wetlands for their survival although they mostly inhabit the land adjacent to the wetland (Cowan, 1995). The importance of creating a buffer zone for the conservation of wetland associated reptiles is therefore also very important. This is further illustrated by Cowan (1995) who found that wetlands, which lacked adjacent vegetation, had a far less diverse reptile assemblage. For example an investigation on wetlands bordered directly by sugarcane fields found that only 7 reptile taxa remained, whilst the urbanisation of land adjacent to water bodies in Durban, KwaZulu-Natal has caused the possible extinction of 4 out of 8 wetland species in the area (Cowan, 1995). Cowan, (1995) points out an interesting trend in semi-aquatic reptiles in South Africa that he relates to their adaptation to aridity. Most species found in the South-western region of the country are adapted to drier conditions compared to those found in the more tropical Eastern and Northern parts. As a result, the number of wetland dependent reptile taxa increases from south to north and west to east, with KwaZulu-Natal and the Transvaal supporting the most wetland dependent species, with 28 and 21, respectively. The Free State, Cape and Namibia, on the other hand, only supported 11, 12 and 14 species, respectively (Cowan, 1995). Despite a lower number of species, it is worth noting that the majority of the 12 endemic wetland reptiles occur in the Cape however (Cowan, 1995). Cowan (1995) also highlights that both wetland reptile and amphibian species in South Africa, occurring in areas with consistent low minimum temperatures 4.3 ha in extent • Small wetlands: 0.3 – 2.3 ha in extent • Large wetlands: > 2.3 ha The authors also noted that most approaches had categories reserved for areas of exceptional value such as historic or natural sites where much wider buffers were typically applied. Surrogates for waterbody size were also used in some jurisdictions such as stream order rather than a measure of channel width.

4.2.2.2.

Island County (Washington) matrix approach to buffer delineation (Environmental Law Institute, 2008)

Based on this County’s draft ordinance (November 2007), buffers are prescribed for a few types of especially sensitive wetlands (i.e. coastal lagoons, bogs and estuaries), with wider buffers prescribed for more intensive land uses adjacent these systems. For other wetlands, this approach establishes matrices for buffer determination which are based on habitat condition, land use intensity and wetland sensitivity (influenced by slope and the presence/absence of surface water outlets). Wetlands lacking outlets and those bordered by steep slopes are regarded as more sensitive (especially to the accumulation of sediment/contaminants) and therefore receive wider buffers. In this methodology, two buffers are calculated separately, one for wildlife habitat and the second for water quality. The habitat buffer width is adjusted according to habitat functionality, with wider buffers being applied for wetlands with higher habitat functionality ratings (Table 21). The water quality calculation includes varying land use intensities and sensitivity of different wetlands by incorporating wetland types (A-E), and whether or not there is a surface water outlet from the wetland, which was regarded as impacting on the sensitivity of the water resource to water quality impacts (Table 22). Water quality buffers are then modified by a buffer multiplier based on slope adjacent to the wetland (Table 23). Once the two different buffer widths have been calculated, the larger buffer is then applied.

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Table 21. Habitat buffers (Environmental Law Institute, 2008)

Land use Intensity

Habitat Buffers Habitat Functions Score 42-48 ft 39-41 ft 32-38 ft 125 ft 100 ft 75 ft

Low

50 or higher 150 ft

Moderate

225 ft

175 ft

150 ft

110 ft

High

300 ft

200 ft

175 ft

150 ft

Less than 32 Use Water Quality and Slopes Tables

Table 22. Water Quality Buffers (Environmental Law Institute, 2008) Water Quality Buffers Wetland Category Wetland Outlet A B C Yes 40 ft 35 ft 30 ft No 75 ft 50 ft 40 ft Yes 90 ft 65 ft 55 ft No 105 ft 90 ft 75 ft Yes 125 ft 110 ft 90 ft No 175 ft 150 ft 125 ft

Land Use Intensity Low Moderate High

D 25 ft 35 ft 45 ft 60 ft 65 ft 90 ft

E 20 ft 25 ft 30 ft 40 ft 40 ft 50 ft

Table 23. Water Quality adjustments due to slope (Environmental Law Institute, 2008)

Slope Gradient 5-14% 15-40% >40%

4.2.2.3.

Slope Adjustment Additional Buffer Multiplier 1.3 1.4 1.5

Criteria based delineation for Chesapeake Bay in the states Maryland, Virginia and Pennsylvania (Palone and Todd, 1997)

This approach is a site-specific one which utilizes a few scientific criteria to set a framework for assessing the buffer width needed to protect any aquatic resources in forest riparian buffers in the region of Chesapeake Bay. The four criteria considered are: 1. Existing or potential value of the resource to be protected 2. Specific water quality and/or habitat functions described 3. Site, watershed and buffer characteristics 4. Intensity of adjacent land use Assessment of these criteria is suggested to be undertaken based on the availability of scientific information, the work of ‘field observers’ and subjective analysis. No specific guidelines have been provided in this document and it is suggested that this framework be used to inform the development of detailed buffer zone methodologies in the region. Once all criteria have been

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evaluated and a resultant buffer width has been arrived at, scope is given to modify this width according to socio-economic needs such as management goals and constraints. In this way, scientific criteria guide width decisions, but are modified by socioeconomic variables where the risk and benefits of the decisions can be identified and discussed. Criterion 1: Existing or potential value of the resource to be protected Inclusion of this criterion recognizes that a wetland of higher value requires a larger buffer than one of lower value. In general terms, narrower buffers are adequate when the stream, wetland, shore zone, or lake is of relatively low functional value. Although the determination of “value” can involve subjective judgment, scientific information can be applied to assist in this assessment. For example, states routinely rate the value of fish habitat based on potential natural condition or the target species being managed. The Chesapeake Bay Program has identified priorities for stream blockage removal based on value to migratory fish. Streams in watersheds providing municipal water supply or recreational use would likewise be considered of high functional value. Aquatic systems with a high disturbance regime or ones that are dominated by non-native species may be considered of lower functional value. In this methodology, watershed value, water quality and habitat conditions are used to assess the potential value of the resource and therefore inform the necessary buffer extent. This concept is illustrated in Figure 7 below:

Low

Watershed value

High

Low

Water quality value

High

Low

Habitat values

High

Min. Buffer width Figure 7. width

Max. Buffer width

Influence of different criteria on the water resource value when determining buffer

Criterion 2: Specific water quality and/or habitat functions described This step requires the assessors to define the specific functions the buffer is designed to fulfil. The required buffer functions are chosen according to the desires of the protector, and the applicable buffer width is taken from a simple table that gives the minimum ranges according to the desired function (Table 24). Should any multiple functions yield overlapping results, the maximum width will be chosen in order to fulfil all desired functions. The following table is a guideline to these widths:

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Table 24. Buffer widths for specific buffer functions (as cited in Palone and Todd, 1997) Buffer function Wildlife habitat Flood mitigation Nitrogen removal Water temp/ moderation

Buffer Width (feet) 40-270 60-225 55-260 15-60

Bank stabilization and aquatic food web

15-40

Criterion 3: Site, watershed and buffer characteristics Numerous site specific criteria for buffer width influence its functions and they are given in Table 25. To establish the maximum or minimum width, the assessors should include criteria where environmental sustainability is not endangered and economic cost is considered, such as to inhibit the unnecessary loss of agricultural land and ensure ecological functioning. Table 25. Site criteria as suggested by Palone and Todd (1997) to be incorporated into buffer width Criteria Watershed condition Slope Stream order Soil depth and erodibility Hydrology Flood plains Wetlands Stream banks Vegetation type Stormwater system Criterion 4: Intensity of adjacent land use The impact of neighbouring land uses on a wetland will affect the required width of the buffer zone. The density, intensity and magnitude of the land use are factors influencing the width, with higher intensities etc. Necessitating larger buffers. This is depicted in Figure 8 on the following page.

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Low

Land use intensity

High

Low

Land use density

High

Low

Land use magnitude

High

Low

Potential nitrate yield

High

Min. Buffer width

Figure 7.

2009

Max. Buffer width

Land use influence on riparian buffer width (Palone and Todd, 1997)

The term ‘Land use magnitude’ in the figure above indicates the magnitude of the impact on the wetland and ‘land use intensity’ refers to the intensity with which the land is being used (e.g. density of residential houses).

4.2.2.4.

City of Everett, Washington State (Pentec Environmental, 2001)

The City of Everett’s Municipal Code regulates the buffer widths around wetlands and other water resources within the boundaries of the Municipality. The method developed to inform buffer width requirements is based on a fixed width methodology which is informed by a classification of aquatic resources with greater buffer widths required for more important or sensitive resources. This width may under special circumstances be extended or reduced based on further site-specific assessments. The first step required when undertaking an assessment under this code is to classify the resources into one of four categories based on a set of standard definitions. Table 26 lists these and all streams within the region are classified into one of these Categories under the Sensitive Areas Ordinance (SAO), the same applies to wetlands (Table 27). Table 26. Stream category definitions for the City of Everett Category I

Streams inventoried as shorelines of the state under the City’s Shoreline Master Program, or those used by salmonids

Category II Category III

Perennial streams that are smaller than Category I streams Streams that are naturally intermittent or ephemeral and are not used by salmonids in any portion of the stream

Category IV

Naturally occurring, intermittent swales

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Table 27. Wetland category definitions for the City of Everett Category I

High quality native wetland communities, documented or qualifying Category I or II Natural Heritage wetland sites; or Regionally rare wetlands of irreplaceable ecological functions; or Wetlands greater than or equal to 5 acres with 3 or more wetland classes and open water

Category II

Wetlands greater or equal to 1 acre in size, have 40 to 60% open water in dispersed patches, and have 1 or 2 wetland classes; or Wetlands ≥ 1 acre in size, and have a forested wetland class; or Riparian wetlands of any size

Category III Category IV

Wetlands that do not qualify as Category I or II wetlands Wetlands that are hydrologically isolated, have an area ≤1 acre; and contain one vegetation class (80% or greater dominance by invasive species)

Once water resources have been classified, a standard minimum buffer width is applied based on the category assigned (Table 28). These minimum widths are assigned so as to ensure an acceptable level of protection to the most important functions of the resource (e.g. 30 m for 70 % effectiveness in removing sediments and pollutants for wetland and stream Categories 1 – See Table 13). Table 28. Minimum buffer widths for the Municipality along the separate categories Wetlands Category I Category II Category III Category IV Streams Category I Category II Category III Category IV

Minimum buffer width (m) 30 25 15 7.5 Minimum buffer width (m) 30 15 7.5 3.3

The City has provisions through which it may increase the buffer width if, after site-specific analyses, it has been established that: • Sensitive species or habitats are present or the neighbouring land is subject to severe erosion; • The standard buffer width allocation includes minimal or degraded vegetation; • The wetland includes land of 25% slope or steeper; • Sensitive fish and wildlife species/habitats could be adversely impacted. For streams and wetlands, the buffer width may be decreased by buffer averaging as long as the wetland functions are not compromised. Riparian buffers may be decreased by buffer enhancement to provide increased buffer functions in formerly altered riparian areas. In all cases, however the buffer may not be reduced by more than 50% of the standard width or 6 m, whichever is greater.

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4.2.2.5.

2009

Guidelines for wetland protection using buffers in Western Washington (Granger et al., 2005)

This recently published guideline document is not mandatory but has been compiled to act as a guide to the various jurisdictions in Washington State in order for them to decide on buffer width policies required to in line with requirements set out in the Growth Management Act (GMA of 1990) of the Washington State Government. The approach highlights the importance of considering acceptable risk in informing buffer decisions. The authors recognize that scientific information on buffers clearly states that buffers are important, that they perform many functions that are critical to maintaining wetland functions, and that a wide range of buffer widths provides a variety of benefits depending on a number of factors. Determining the required buffer width is however largely an exercise in assessing the science and deciding on an acceptable level of risk. To illustrate this point, a regulation that sets a 100m buffer around every wetland would significantly reduce the risk of those wetlands from human activities in the immediate vicinity of the wetland and would be characterized as a relatively ‘low risk’. On the other hand, setting a regulation that requires a 20m buffer for all wetlands could be regarded as a ‘high risk’ approach since a 20m buffer will not protect many wetland functions. In this approach, risk is addressed by tailoring the degree of protection based on a number of factors identified as important in the scientific literature. This includes the type of wetland, functions it performs and the type and intensity of adjacent land use. The widths recommended in this approach represent those from the middle of the range of buffers suggested in the literature, thereby representing a moderate risk approach to buffer determination. Three alternative approaches for protecting wetland functions using buffers have been proposed in this guideline document. This starts with a simple buffer allocation informed by wetland category to more complex approaches aimed at accommodating key considerations highlighted in the literature. The three alternative approaches include: • Buffer Alternative 1: Width based only on wetland category. • Buffer Alternative 2. Width based on wetland category and the intensity of impacts from proposed land uses. • Buffer Alternative 3. Width based on wetland category, intensity of impacts, and wetland functions/characteristics. Each of these approaches is briefly outlined to provide an indication of the criteria included in each of the different approaches. Alternative I: Width based only on wetland category. This is a fixed buffer width approach informed simply by categorization of wetland systems into one of four categories. It is important to note however that while this is the simplest approach, it is also the most conservative, with widths recommended in this approach being the widest of those recommended in Alternatives 2 and 3 as discussed below. Buffers proposed are also sufficient to protect the wetland from proposed land uses that have the greatest potential impact on wetland systems. A brief description of each of these categories used in this approach is included in Table 29 below:

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Table 29. Description of different wetland categories used to inform buffer width determination. Category I

Description These wetlands represent a unique or rare wetland type, are more sensitive to disturbance than most wetlands, are relatively undisturbed and contain some ecological attributes that are impossible to replace within a human lifetime, or provide a very high level of functions

II

Wetlands which are difficult, though not impossible, to replace, and provide high levels of some functions. These wetlands occur more commonly than Category I wetlands, but they still need a relatively high level of protection.

III

Generally, wetlands in this category may have been disturbed in some way and are often less diverse or more isolated from other natural resources in the landscape than Category II wetlands. These have the lowest levels of functions and are heavily disturbed.

IV

Wetland systems included • Relatively undisturbed estuarine wetlands larger than 1 acre • Wetlands that are identified by scientists of the Washington Natural Heritage Program/DNR as high quality wetlands • Bogs larger than ½ acre • Mature and old-growth forested wetlands larger than 1 acre • Wetlands in coastal lagoons; or • Wetlands that perform many functions well. • Estuarine wetlands smaller than 1 acre, or disturbed estuarine wetlands larger than 1 acre; • A wetland identified by the Washington State Department of Natural Resources as containing “sensitive” plant species; • A bog between ¼ and ½ acre in size; • an interdunal wetland larger than 1 acre; or • Wetlands with a moderately high level of functions. • Wetlands with a moderate level of functions; or • Interdunal wetlands between 0.1 and 1 acre in size.

• These are wetlands that should be replaceable, and sometimes may be improved. However, experience has shown that replacement cannot be guaranteed in any specific case. These wetlands may however provide some important functions, and should be protected to some degree.

Once wetlands have been classified, a standard wetland buffer is applied based on the importance of the wetland system being considered. Standard buffer widths proposed are outlined in Table 30. Table 30. Width of buffers in western Washington if impacts from land use and wetland functions are NOT incorporated Category IV III II I

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Buffer width (m) 16 47 94 94


2009

Alternative 2: Width Based on Wetland Category and Modified by the Intensity of the Impacts from Proposed Land Use This alternative increases the flexibility by recognizing that not all changes in land uses are likely to have the same impacts on water resources. For example, the construction of a new house on 1 hectare of land close to a wetland expectedly has less impact than the construction of 20 houses on the same land. In order to guide this assessment, proposed land uses have been categorised into those with potentially high, moderate, and low impacts to wetlands respectively (Table 31). Buffer widths are then proposed based on an integration of wetland categories and impacts associated with different land uses (Table 32). Table 31. Land use impact categories that can result in high, moderate, and low levels of impacts to adjacent wetlands. Impact category

High

Moderate

Low

80

Types of Land Use • • • • • • •

Commercial Urban Industrial Institutional Retail sales Residential (more than 1 unit/acre) Conversion to high-intensity agriculture(diaries, nurseries, greenhouses, growing and harvesting crops requiring annual tilling and raising and maintaining animals)

• • • • • • • •

High intensity recreation (golf courses, ball fields) Hobby Farms Residential (1 unit/acre or less) Moderate-intensity open space (parks with biking, jogging) Conversion to moderate-intensity agriculture (orchards, hay fields) Paved trails Building of logging roads Utility corridor or right-of-way by several utilities and including access/maintenance road

• Forestry (cutting of tress only) • Low-intensity open space (hiking, bird watching, preservation of natural resources) • Unpaved trails • Utility corridor without a maintenance road and little or no vegetation management


2009

Table 32. Suggested buffer widths with land use impacts taken into consideration.

Wetland Category IV III II I

Buffer width in m Land Use with Land Use with low Impact Moderate Impact 8 13 23 34 47 70 47 70

Land Use with High Impact 16 47 94 94

Alternative 3: Width based on wetland category, intensity of impacts, and wetland functions/characteristics. This alternative refines proposed buffer widths based on an assessment of wetland functioning. This requires an assessor to evaluate the wetland according to water quality, hydrologic and habitat functions as stated in “The Wetland Rating system for Western Washington” as published by the Department of Ecology in 2004 (Hruby, 2004). This methodology requires the assessor to rate a variety of criteria based on the type of wetland present, which is then used to allocate points to each of the wetland functions. When rating habitat functionality for example, users are required to assess the potential of the wetland to provide habitat for many species by rating a range of wetland characteristics such as vegetation structure, hydroperiods, plant species richness, interspersion of habitats and special habitat features. A separate assessment is then undertaken to assess the opportunity of the wetland to provide habitat based on a range of characteristics such as buffer characteristics, connectivity, adjacency to other priority habitats and location relevant to other wetlands in the landscape. These ratings are then added to provide a habitat score for the wetland. Resultant scores are then used to allocate the wetland into one of four categories reflecting the importance of each resource: • Category IV = Score 70 Recommended buffer widths are then modified for each wetland category based on the provision of certain functions and impacts from associated land uses. Tables 33 – 35 provide details of the proposed buffer widths for each wetland category, with additional recommended measures for protection identified for category I and II wetlands. Table 33. Width of buffers suggested in order to protect Category IV wetlands in western Washington (for wetlands scoring less than 30 points for all functions). Wetland Characteristic Score for all 3 basic functions is less than 30 points

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Buffer Widths by Impact of Proposed Land Use Low – 7.6 m Moderate – 12.2 m High – 15.3 m


2009

Table 34. Width of buffers suggested in order to protect Category III wetlands in western Washington (for wetlands scoring 30 – 50 points for all functions). Wetland Characteristics

Buffer Widths by Impacts of Proposed Land Use

Moderate level of function for habitat (score for habitat 20-28 points) Not meeting above characteristics

Low – 23.4 m Moderate – 33.5 m High – 45.7 m Low – 12.1 m Moderate – 18.3 m High – 24.4 m

Table 35. Width of buffers suggested in order to protect Category II wetlands in western Washington (for wetlands scoring 51-69 points for all functions or having the “Special Characteristics” identified in the rating system). Wetland Characteristics

Buffer Widths by impacts of Proposed Land Use

Other Measures recommended for protection

High level of function for habitats (score for habitat 29-36 points)

Low – 30.5 m Moderate – 45.7 m High – 70.0 m

Maintain connections to other habitat areas

Moderate level of function for habitat (score for habitat 20-28 points)


No recommendations at this time

High level of function for water quality improvement and low for habitat (score for water quality 2432 points; habitat less than 20 points) Vernal pool


No additional surface discharges of untreated runoff

Low – 30.5 m Moderate – 45.7 m High – 70.0 m OR Develop a regional plan to protect the most important vernal pool complexes – buffers of vernal pools outside protection zones can then be reduced to: Low – 12.1 m Moderate – 18.3 m High – 24.4 m Buffer width to be based on score for habitat functions or water quality functions

No intensive grazing or tilling in the wetland

Riparian forest

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Riparian forest wetlands need to be protected at a watershed or sub-basin scale (protection of the water regime in the watershed) Other protection based on

Deliverable 1: Literature Review Wetland Characteristics

Buffer Widths by impacts of Proposed Land Use

2009

Other Measures recommended for protection needs to protect habitat and/or water quality functions

Not meeting above criteria



Table 36. Recommended buffer width for Category I wetlands (for wetlands scoring 70 points or more for all functions or having the “Special Characteristics” identified in the rating system).

Wetland Characteristics

Buffer Widths by Impact of Proposed Land Use


Natural Heritage Wetlands

Low – 39 m Moderate – 59.3 m High – 78.1 m

Bogs

Low – 39 m Moderate – 59.3 m High – 78.1 m

Forested

Buffer width to be based on score for habitat functions or water quality functions

Alkali


High Level of function for habitat (score for habitat 29 – 36 points)


Moderate level of function for habitat (score for habitat 20 – 28 points)


No additional surface discharges to wetland or its tributaries No septic systems within 300 ft Restore degraded parts of buffer No additional surface discharges to wetland or its tributaries Restore degraded parts of buffer If forested wetland scores high for habitat, need to maintain connectivity to other natural areas Restore degraded parts of buffer No additional surface discharges to wetland or its tributaries Restore degraded parts of buffer Maintain connections to other habitat areas Restore degraded parts of buffer No recommendations at this time

High level of function for water improvement (24 – 32 points) and low for habitat (less than 20


83

No additional surface discharges of untreated runoff

Deliverable 1: Literature Review Wetland Characteristics

2009

Buffer Widths by Impact of Proposed Land Use




points)

Not meeting any of the above characteristics

It is worth noting that special conditions for possible changes in recommended buffer width are also included in the method. A reduction in width may be considered for example, where measures to minimize impacts of adjacent land uses are implemented. A minimum buffer width is still required however to maintain a minimum level of buffer functionality. Conditions for increasing buffer width are also included to cater for some of the buffer characteristics not specifically catered for in the methodology. Buffers for example are increased by 50% if the slope is greater than 30% where buffers have been specifically established to maintain water quality functions. Allowance is also made to increase the buffer to cater for species sensitive to disturbance.

4.2.3.

England

The specific use of buffering mitigation methods in England and Wales has been investigated by a small number of projects, including the Woburn Erosion Reference Experiment (e.g. Quinton & Catt, 2004) and the Defra BUFFERS project (PE0205). While some research has been undertaken, discussions with scientists from the Environmental Agency (Dr.Everard, M. , pers comm.) and Department of Environment, Food and Rural Affairs (Defra) (Mr. McGonigle, D., pers comm.) revealed that no generic buffer zone guidelines or buffer determination methods are currently applied. At this stage, establishment of buffer zones is therefore regarded as an option rather than a requirement and is typically applied in agri-environmental schemes. A tool has however been developed to assist resource managers in evaluating the effectiveness of established buffer zones in meeting water quality and habitat objectives (Ducros and Joyce, 2003). While not a formal approach to inform buffer zone determination, the buffer zone inventory and analysis form (BZIEF) developed highlight a range of criteria that may be useful in informing the identification of variables for inclusion in a buffer zone procedure.

4.2.4.

South Africa

At present in South Africa, buffer zones are legally set at 20-40m, however, this does not take into account the different characteristics present in different catchments (Blanché, 2002). Discussions with a range of stakeholders from different provinces around the country revealed that standards are highly variable, with no nationwide standards for the establishment of aquatic buffer zones except in the case of forestry, where a fixed width buffer of 20 m is applied between the edge of a wetland and the plantation, or a water abstraction site (Kotze, 2004). In Mpumulanga, the Mpumalanga Tourism and Parks Agency typically recommends buffers of 30m in built up areas while buffers of 20m are typically proposed in areas of open veld (Cowden, G., pers comm.). Buffers typically recommended by the Eastern Cape Department of Economic Development& Environmental Affairs in the Eastern Cape range from 32 – 50m for rivers and wetlands while buffers for estuaries are informed by the sensitivity of the receiving environment

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(Gabula, S., pers comm.). It is worth noting however that the Province has a piece of legislation which prohibits development without a permit within 1000m of the high water mark of the sea and tidal rivers (Environmental Conservation Decree No 9 of 1992). Where a permit is Issued, buffer zones have to be determined however there is no formal guideline currently available to inform these decisions (Gabula, S., pers comm.). In Kwazulu-Natal, no formal guidelines have been approved although the Department of Agriculture and Environmental Affairs has developed interim guidelines pertaining to the runoff from hardened surfaces such as roads and storm water outflows into wetlands, for which a buffer of 15 m and 20 m respectively is applied (DAEA, 2002). Further details on available guidelines in the country are briefly presented below, with a strong focus given to a detailed methodology currently being developed by Graham and de Winnaar (2009) for application by Ezemvelo KZN-Wildlife in KwaZulu-Natal.

4.2.4.1.

Guideline for buffers to red listed species in Gauteng Province (Pfab, 2008)

Although not specific to aquatic species, this guideline suggests that buffer zones of 200 m and 600 m be established from the edge of any population of red-listed plant species in urban and rural areas respectively, in order to sustain their populations.

4.2.4.2.

GDACE Requirements for Biodiversity Assessments (Directorate of Nature Conservation, 2008).

In the Province of Gauteng, the Department of Agriculture, Conservation and Environment (GDACE) requires buffers of 32 m and 100 m to be established for rivers/streams in urban and non-urban settings respectively. For wetlands, buffers of 30 m and 50 m have been proposed for urban and non-urban areas respectively. The guidelines do however make provision for extension of these buffers in the case of endangered species being present.

4.2.4.3.

Guidelines for estuarine, stream and wetland buffers in the Eastern Cape Province– ECBCP Handbook (Berliner et al., 2007)

Due to the lack of national guidelines for buffer establishment, a range of fixed-width buffers have been proposed for application in the Eastern Cape. These range from 32 to 100m for streams (Table 37) while a standard 50m buffer was proposed for all wetland systems. For estuarine systems, the guidelines indicate that no new development should take place below the 5 m contour from the high water mark in order to protect the most important estuary drivers and processes. Table 37. Recommended buffers for different rivers (adapted from Berliner et al., 2007) River Criterion used

Buffer Width (m)

Rationale

Mountain streams and upper foothills of all 1:500 000 rivers

50

These longitudinal zones generally have more confined riparian zones than lower foothills and lowland rivers are generally less threatened by agricultural practices

Lower foothills and lowland rivers of all 1:500 000 rivers

100

These longitudinal zones generally have more confined riparian zones than mountain streams and upper foothills and are generally threatened by agricultural practices. These larger buffers are particularly important to lower the amount of crop-spray reaching the river

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River Criterion used

Buffer Width (m)

Rationale

All remaining 1:50 000 streams

32

Generally smaller upland streams corresponding to mountain streams and upper foothills, smaller than those designated in the 1:500 000 rivers layer. They are assigned the riparian buffer required under South African legislation.

4.2.4.4.

Western Cape, Cape Town City (Roads and Stormwater Department., 2008)

The draft floodplain management policy for the city recognizes the potential benefits associated with establishing buffer zones along aquatic systems. Although no specific methodology is given, the policy does propose that buffer widths be based on classification of the watercourse or wetland in terms of recognized national classification system followed by an assessment of the ecological importance and sensitivity of the system. For watercourses, buffer width is also adjusted on the basis of the width of the active channel. Buffers are measured from the top of the bank or the delineated edge of the wetland. Within the metropolitan area, several of the significant watercourses and wetlands have been identified and buffers have been determined which vary between 30 – 75 m for wetlands and 10 – 40 m for watercourses. Furthermore, for concrete channels, minimum buffers of 10 m are required. Where no buffers have been determined, the developer is required to procure the services of a suitably qualified freshwater ecologist to recommend buffer widths in accordance with generally accepted practice at own cost prior to any detailed planning being undertaken. It is also worth noting that the draft policy recognizes the importance of increasing standard buffer widths to allow for a range of site-specific characteristics. These include the presence of sensitive habitats, fauna or flora which may require wider buffers for adequate protection; the intensity of adjacent land use and the nature of anticipated impacts; and the chemical and/or botanical characteristics of the buffer area which may alter the efficacy of the buffer to mitigate against identified impacts.

4.2.4.5.

Developing guidelines to determine appropriate buffers for the protection of freshwater wetlands from various land use impacts in KwaZulu-Natal (Graham and de Winnaar, 2009)

This method uses a step-wise approach to define an appropriate buffer width based on ecological and biophysical features of a site. Figure 9 shows the conceptual buffer delineation model they have developed. This starts with calculation of minimum buffer widths based on wetland type and land use / development. This width may then be modified by taking ecological criteria into consideration such as whether or not important species are present (Buffer A) or for wetlands located within catchments with low ecological importance and sensitivity ratings (Buffer B). Separate buffer calculations are also made on the basis of biophysical attributes of the site (Buffer C). Further details of the steps followed are included to provide further details and clarity on the methodology developed.

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Land use/ Development

2009

Buffer modifiers (Ecological or biophysical)

Ecological Wetland Type • Linear • Point

Important species NOT present

Important species present

Buffer A

EIS ≤ 2

Buffer B

Biophysical Habitat Integrity + EIS ≥ 3

Slope (%) and soil porosity + Stream Order or HGM type

Figure 8.

Buffer C

Model for wetland buffer width determination according to land use in KwazuluNatal (Source: Graham and de Winnaar, 2009)

Step 1: Minimum buffer widths (to protect core wetland habitat and aquatic functioning) are calculated based on a simple classification of wetland types and land use categories. Wetland types are broadly grouped as riverine, lacustrine and palustrine systems and with minimum buffer widths ranging from 15 – 175m applied (Table 38)

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Table 38. Minimum buffer widths for different wetland types in the presence of various land uses. Secondary land uses are listed in order of disturbance within each primary category increases (adapted from Graham and de Winnaar, 2009) Buffer widths (m) for wetland types

Land use

Disturbance

Low

Riverine

Lucastrine/ Palustrine

National Park

15

20

Nature Reserve

15

20

Conservancy

32

32

Eco-estates

32

32

Low density

32

40

Medium density

32

40

High density

45

50

Cultivation (Subsistence)

32

45

Cultivation (Commercial)

32

45

Planted pastures

32

45

Cultivation (Irrigation)

45

50

Forestry

50

75

Feedlots

70

75

Industrial development

Low intensity

45

50

High intensity

70

75

Services/ Infrastructure

Pipelines

32

40

Communication/Power lines

32

40

Roads

32

40

Sewerage farms

45

50

Septic tanks

70

75

Landfills

100

110

Mines

150

175

Primary category

Secondary category

Protected/ Conservation areas

Residential developments

Agriculture

High

Mining/Landfills

Step 2: Buffer widths are modified based on ecological considerations. This includes considering whether or not endangered or threatened species are present in the wetland and the ecological importance and sensitivity of the wetland concerned. If threatened or endangered species are known to occur, minimum buffer requirements are proposed in order to sustain minimum habitat requirements. Proposed buffer widths range from 30m to 1260m based largely on recommendations made in other studies. The result of this assessment results in a recommended “Buffer A”.

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2009

An ecological Importance and Sensitivity (EIS) parameter is also included, and acts as an alternative for wetlands where no records of important species are present. This is based on desktop ratings for each quaternary catchment that highlights those catchments considered ecologically important and sensitive to perturbations (available for the entire country). EIS values range from low to very high and are used as the basis for increasing the minimum buffer widths proposed in Step 1 (Table 39). The result of this assessment results in a recommended “Buffer B”. Table 39. EIS Categories and suggested buffer width increases (adapted from Graham and de Winnaar, 2009) EIS Category

Buffer increase in %

Very high (1) High (2) Moderate (3) Low (4)

75 50 25 0

Step 3: The next buffer is calculated by assessing a range of landscape characteristics surrounding that can influences ecological and hydrological processes and therefore the level of impact that a land use activity has on the wetland. These include wetland hydrogeomorphic type, slope and soil porosity and integrity of surrounding habitat. The range of modifiers proposed for each of the biophysical variables considered is represented in Tables 40 to 42, below. The result of this assessment results in a recommended “Buffer C”.

Table 40. Catchment position and suggested associated buffer increases (Source: Graham and de Winnaar, 2009) Hydrogeomorphic types Floodplain Valley bottom with channel

Buffer increase in % 115

Valley bottom without channel

Hillslope with channel

50

Hillslope without channel

Depression

0

Table 41. Slope and soil porosity considerations for buffer width (adapted from Graham and de Winnaar, 2009) Flat (less than 1%)

Inclined (1-5%)

Steep (more than 5%)

High

Medium

Low

0% increase

15% increase

30% increase

Slope class

Soil porosity class Buffer change

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Table 42. Habitat integrity effects on buffer width modification (Source: Graham and de Winnaar, 2009) Habitat integrity class

Condition factor

Buffer change

Highly modified High Poor Heavy Heavy 80% increase

Moderate

Pristine

30% increase

Low alien plant density Good basal cover Light erosion No agriculture 0% increase

Step 4: Once a range of different buffer widths have been calculated (Buffers A – C), the maximum width is determined and used as a basis for recommending a required buffer width for the wetland concerned. This method therefore follows a cautious approach by selecting the buffer required to provide the highest level of protection based on the assessment undertaken

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5. CONTEXTUALIZING BUFFER WIDTH METHODOLOGIES WITHIN AN APPROPRIATE SUPPORTING FRAMEWORK Any guidelines or regulations designed to ensure that buffer functions are maintained and protected, needs to consider broader issues than simply providing an indication of buffer width. A full range of guidelines for the protection of wetland buffers should ideally be addressed (Granger et al, 2005). These include: • Criteria and procedures for varying from a standard; • Allowable uses within the buffer; • Best management practices to enhance and ensure effective buffer function; • Provisions for the delineation and demarcation of buffers and their maintenance over time, and • Ownership and enforcement considerations. A brief review of these aspects is provided in this chapter of the review in order to further inform thinking regarding buffer zones in the South African context.

5.1. Criteria and procedures for varying from a standard buffer width An important aspect that needs to be considered when developing a buffer zone determination methodology is the degree of flexibility built into the method and the types of concessions that would be considered. Buffer averaging is a common approach applied to introduce a flexible approach to maintaining buffer functions and is discussed below. In addition, a range of scenarios are discussed where either decreasing or increasing a proposed buffer widths should be contemplated.

5.1.1.1.

Buffer Averaging

Buffer averaging is a method used either to balance buffer width with specific needs for development, or for tailoring a buffer to maximize protection of natural features within or surrounding the wetland (Granger et al., 2005). For example, a standard buffer width of 100 m around a wetland could be varied through buffer averaging while still maintaining an average 100 m buffer width to ensure that the same total buffer area as initially envisaged is maintained. This is reportedly done to allow developments closer proximity to the wetland than usual so as to fit a given ‘footprint’ onto a site. Such an approach could also be used to protect natural features (e.g. a population of trees) that would otherwise have fallen outside of the generic buffer delineation. Buffer averaging can also provide connections to neighbouring habitats or deal with cases where old developments have reduced a buffer area to a width less than required. Such an approach, referred to locally as “give and take” is currently applied in the South African forestry industry, particularly where plantations were previously established in wetland and riparian areas (Kotze, 2004). The system allows for some encroachment of plantations within the recommended 20 m buffer. To ensure that the concept does not in practice become one of taking only, the following approach has been proposed (Kotze, 2004).

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All wetlands (and their associated 20 m buffers) in forestry areas must be delineated via a mapping scale of 1: 10 000 (or smaller); inevitably some small wetlands may be omitted. Any buffer area that is ‘taken’ has to be balanced through increasing (by an area equivalent to or greater than that ‘taken’) another buffer on the same estate. Any reduced buffer area has to be documented and substantiated in an auditable format to be available to the Responsible Authorities or Certification Bodies.

• •

It has however been recommended that buffer reduction should only take place under well justifiable conditions. Such conditions might encompass cases such as wetlands that will remain non functional even in the event of a 20 m buffer allocation. Indeed, it is possible that the achieved results of buffer creation could be improved if an equivalent area of plantation was removed from a better functioning or larger wetland system. In allowing such an approach, Kotze (2004) cautions that such an approach should never be applied indiscriminately across an estate, and a minimum of 80 percent of the buffers in the estate should adhere to the minimum buffer requirement. If buffer averaging is contemplated, a minimum buffer width (designated width or percentage of the standard buffer width) is typically required (Granger et al., 2005). Documentation to provide evidence that the process will not impair the buffer functioning is also typically required. Ideally, buffer widths should be narrowed in areas where the least disturbance will be caused and widened areas where it will result in most benefits.

5.1.2.

Scenarios for reducing buffer widths

5.1.2.1.

Reasonable Use

A further situation in which generic buffer widths may need to be reduced on a case-by-case basis is when the protection of the buffer results in a property owner being denied reasonable use of their land (Granger et al., 2005). For example, if a landowner has a parcel that was sized for a single family house and a wetland covers 80% of the property, establishing a buffer around the wetland would render the plot undevelopable. In such a case, the owner would have a strong case that the buffer would deny them all reasonable use of their property. However, a reduced buffer might enable the construction of a single house on the plot. Granger et al. (2005) therefore argues that regulations should allow for buffer width reduction in case reasonable use would be denied. If such an allowance is made, it is however suggested that the applicant be required to demonstrate the lack of feasible alternatives to reducing the buffer (e.g. revising the development design) and that wetland functions will not be compromised upon. Any reduction should also potentially be coupled with stricter mitigation measures to limit impacts on the aquatic resource. This may entail the erection of permanent fencing and manipulation of buffer characteristics (e.g. re-vegetation) as the reduction of buffers increases the risk of degradation and encroachment over time.

5.1.2.2.

Reduced intensity of impacts from proposed land uses

In many instances it may be possible to reduce the potential risk of activities on a water resource through a range of on-site mitigation measures. Since buffers are designed, in many cases, to reduce risk of impacts on adjacent aquatic resources, a reduction in risk from the adjoining land use should ideally translate in a relaxation of buffer requirements. So, for example, if a buffer width of 50m is proposed for cultivated agriculture based on a modified fixed-width approach and the applicant agrees to apply a no-till approach that reduces risk, there may be good reason to reduce

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buffer width accordingly. Some additional examples of measures that may be implemented to minimize impacts from land uses with potentially high impacts are outlined in Table 43 below. Table 43. Examples of measures to minimize impacts to wetlands from proposed change in land use that have high impacts (Granger et al., 2005) Examples of disturbance Lights

Noise Toxic runoff*

Stormwater runoff

Activities and Uses that Cause Disturbances • Parking lots • Warehouses • Manufacturing • Residential • Manufacturing • Residential • Parking lots • Roads • Manufacturing • Residential areas • Application of agricultural pesticides • Landscaping • • • • • • • • •

Parking lots Roads Manufacturing Residential areas Commercial Landscaping Impermeable surfaces Lawns Tilling

Examples of Measures to Minimize Impacts • Direct lights away from wetland

• Locate activity that generates noise away from wetland • Route all new, untreated runoff away from wetland while ensuring wetland is not dewatered • Establish covenants limiting use of pesticides within 150 ft of wetland • Apply integrated pest management • Retrofit stormwater detention and treatment for roads and existing adjacent development • Prevent channelized flow from lawns that directly enters the buffer

• Infiltrate or treat, detain, and disperse into buffer new runoff from impervious surfaces and new lawns Pets and human • Residential areas • Use privacy fencing; plant dense disturbance vegetation to delineate buffer edge and to discourage disturbance using vegetation appropriate for the ecoregion; place wetland and its buffer in a separate tract Dust • Tilled fields • Use best management practices to control dust * These examples are not necessarily adequate for minimizing toxic runoff if threatened or endangered species are present at the site. Change in water regime

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Where roads or infrastructure already exists within the buffer

In instances where roads or other infrastructure are already established it may be appropriate to permit limited activities within such a buffer that would not normally be permitted as long as such activities do not allow further deterioration of the buffer (Granger et al., 2005). For example, a high impact land use (e.g. building a new road) may be proposed adjacent a wetland for which a buffer of 30m is required to maintain buffer functions. If the activity involves expanding an existing road that is already 10m from the wetland, including an additional vegetated buffer on the upland side of the road would not help to buffer the effect of road upgrading on the wetland concerned. Where alternative developments are proposed (e.g. construction of a new office block), and impacts to the wetland and associated functions are likely to increase, it may still be necessary to maintain an additional buffer area between the road and the new development.

5.1.3.

Scenarios for increasing buffer widths

5.1.3.1.

Buffers with attributes not specifically included in the buffer methodology

The literature review clearly highlights a range of buffer attributes that affect the functionality of buffer zones (Table 12). The most obvious attributes include slope, vegetation characteristics, surface water flow characteristics and soil variables. Where the buffer methodology does not specifically address these issues, the risk of incorrectly allocating an appropriate buffer width is increased unless conservative buffer widths have been proposed. Indeed, in many methodologies, buffer widths are proposed based on an assumption that the buffer is vegetated with indigenous natural vegetation (e.g. fixed-width approaches). In reality however, the buffer may be unvegetated, sparsely vegetated or vegetated with alien species that reduce the functional value of the buffer zone. In such a scenario it may be necessary to either require rehabilitation of the buffer zone or to increase the width of the buffer zone to ensure that adequate functions are provided (Granger et al., 2005). A similar scenario could exist where the buffer occupies land of extremely steep slope, poorly suited to perform certain functions (e.g. sediment control). In such a scenario it may be necessary to increase the buffer to incorporate areas of lower slope to ensure that some assimilation of sediment or nutrients does take place.

5.1.3.2.

Buffers used by species sensitive to disturbance

A number of methodologies make specific allowance for greater buffer widths where rare, threatened or endangered species or other important species that are particularly sensitive to disturbance make use of the wetland or associated buffer zone. This is critical to ensure that species-specific habitat requirements of priority species are not compromised by proposed developments. Some buffer requirements for aquatic and semi-aquatic species have already been proposed for South Africa (Table 5) but little information is readily available for a number of priority species. Compiling more detailed guidance on the buffer width needs of priority species would certainly help to ensure better protection of these species.

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5.2. Allowable uses within the buffer Once a buffer has been established, options for use should also be considered. In many instances, buffers are regarded as no-use areas however there may be instances where certain activities are compatible with maintaining the functions and values of the buffer for which it was established. Allowing certain activities within the buffer may also increase the perceived value of buffer areas, particularly in areas with high demands on available space. An approach to maximize buffer functionality whilst recognizing the need for water access and views has been proposed by Welsch (1991) and is presented in a document produced by Fischer and Fischenich (2000). He proposes a three-zone riparian buffer that provides a framework through which water quality, habitat and other objectives can be accomplished. This is pictorially presented in Figure 9, with each of the three proposed zones briefly described below.

Figure 9.

Depiction of a three-zone buffer approach developed for the Chesapeake Bay Watershed (From Welsch 1991).

Zone 1: This zone begins at the stream or lake edge and is the area that provides streambank stabilization and habitat for both aquatic and terrestrial organisms. Primary function of this zone includes provision of shade and input to the lake or river of detritus and large woody debris from mature forest vegetation. Vegetation in this zone also helps reduce flood effects, stabilizes the bank, and removes some sediments and nutrients. Vegetation should be composed of native trees and shrubs of a density that permits understory growth. This zone should be a ‘no touch’ zone, however, limited shoreline access may be provided. Access paths should be constructed to minimize erosion, soil compaction and disturbance to habitat. The width of this zone varies between 5 and 8m (Fischer and Fischenich 2000). Zone 2: This zone extends inland from Zone 1 for a minimum of 3 meters up to several hundred meters, depending on the objective, lake type, soil type, slope or topography, and land use. The objective of this zone is to provide a managed forest area with vegetation composition and character

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similar to natural forests in the area. Limited and well-constructed paths that do not significantly increase overland flow to the lake may be permitted in some situations (Fischer and Fischenich 2000). The primary function of Zone 2 is to remove sediment, nutrients and other pollutants from surface and groundwater. This zone, in combination with Zone 1, also provides most of the enhanced habitat benefits, and allows for recreation and aesthetic benefits. Some activities may be permitted in this zone such as periodic harvesting of trees, sawtimber or pulp, growing nuts, berries, and fruits for commercial purposes, or leasing lands out for hunting (Washington County Soil and Water Conservation District, 1999). Zone 3: This zone typically contains grass or herbaceous filter strips and provides the greatest water quality benefits by slowing runoff, infiltrating water, and filtering sediment and its associated chemicals. The minimum recommended width of Zone 3 is 4.5 meters when used in conjunction with Zones 1 and 2 or 11m when used alone. The primary concern in this zone is initial protection of the water resource from overland flow of non-point source pollutants such as herbicides and pesticides applied to lawns, agricultural fields and timber stands. Properly designed grassy and herbaceous buffer strips may also provide quality habitat for several upland wildlife species (Fischer and Fischenich 2000). The importance of allowing use within buffer zones is likely to be particularly important in heavily transformed landscapes where buffer zones have already been transformed and re-vegetation is a costly option for reducing impacts. Basnyat et al. (1999) for example, showed that orchards that were mature and given minimal amounts of fertilizer acted as sinks for nutrients and sediment whereas intensively managed orchards were sources of pollutants. Thus if land use within a “buffer’ was to continue, it would make most sense to obtain produce from natural vegetation (such as hay) or to plant mature orchards, so as to improve water quality. In such a situation, the buffer zone could alternately be called the ‘streamside management zone’, which no longer implies that it must be unutilized land (Basnyat et al., 1999).

5.3. Best management practices to enhance and ensure effective buffer function In Chapter 3, we outlined the attributes of buffer zones that affect the suitability of buffers in performing a range of functions. This provides a useful starting point in providing recommendations on management practices that should be applied to buffer zone management. For example, for maintaining water quality functions, surface roughness and plant productivity are important factors influencing the levels of attenuation functions. Managing buffer zones in a way that protects or improves such attributes would therefore help to ensure that water quality functions are maximized to reduce potential impacts on aquatic resources. In this section, management practices that can be implemented to help maximize the full range of buffer functions are briefly discussed based on findings from the literature review.

5.3.1.

Water quality protection

Many attributes that affect the ability of buffers to assimilate nutrients are determined by factors that cannot be easily altered by management practices. These include attributes such as slope of the buffer, soil characteristics and groundwater levels (Table 12). There are a number of attributes 96


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that can be altered by management practices however, and these include the roughness of the soil surface, vegetation characteristics of the buffer and flow characteristics and size of particles of water flowing from adjoining land uses into the buffer zone. The first two attributes can be altered by management interventions within the buffer itself while the other attributes require management of flows prior to reaching the buffer zone. Maintaining shallow, lateral and uniform flows of surface flows is one of the primary attributes required to promote nutrient uptake and remove microbes and other toxics. Habitat management aimed at maintaining good basal cover is therefore likely to assist in nutrient removal within buffer zones. Activities that create preferential flow paths (e.g. animal tracks) or reduce the density of vegetation are likely to reduce the effectiveness of sediment rapping and nutrient assimilation however and should be avoided. On the contrary, introducing barriers to flow that increase surface roughness (e.g. introducing debris parallel to the slope) could help to improve attenuation capacity. Grasses are also reportedly more effective that trees and shrubs at removing nutrients from surface flows while woody species are better at removing pollutants from contaminated groundwater. Narrow (15- to 30-foot-wide) grass filter strips in particular, have been shown to be effective at removing coarse sediments and adsorbed pollutants as well as helping encourage sheetflow and infiltration of surface runoff, thus enhancing a buffer’s effectiveness at removing remaining pollutants (Granger et al., 2005). Altering the vegetation structure (such as specifically introducing narrow grass filter strips) may therefore help to improve nutrient uptake levels. It is worth noting however, that despite potential benefits associated with introducing non-native species, maintenance of natural vegetation is typically regarded as preferable (Richardson et al., 2007). This is due to the other consequences coupled with the introduction of non-native vegetation such has loss of wildlife habitat, alteration of river microclimate and organic inputs required to sustain naturally occurring aquatic species. Although buffer zones are effective at removing nutrients, that which is taken up by plants is potentially added back into the system, potentially entering the waterbody, when the plant dies (Dosskey, 2001). In instances where buffer zones are subject to high nutrient inputs, careful management may therefore be required in order to help achieve maximum beneficial impact on water quality (Cooper et al., 1995; Dorioz et al., 2006). Periodic harvesting of plant matter may therefore be required to remove nutrients from the buffer zone, as well as stimulating increased growth after such harvesting, which would result in even greater levels of nutrient uptake (Cooper et al., 1995; Dosskey, 2001). Although buffer functions can clearly be increased by management practices, it is important to note that buffers should be seen as secondary approaches to soil conservation / pollution reduction and used in conjunction with primary infield conservation practices to reduce impacts on aquatic systems (Barline and Moore, 1994; Braune and Wood, 1999). This may entail a range of interventions to increase storage and infiltration rates and to slow flows to facilitate sediment and nutrient removal. A range of management interventions aimed at reducing discharge rates (e.g. through attenuation dams) and dispersing flows (e.g. through level spreaders, water bars or stiffgrass hedges) should therefore be implemented in order to maximize the potential benefits of buffers in ameliorating poor water quality. Pre-treatment of contaminated water should also be implemented where practicable and other nutrient sources (e.g. fertilizer application) addressed to reduce the risk of contamination and reduce the burden on buffer strips.

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Microclimate and water temperature control

The ability of streamside buffers to regulate microclimate and control temperature of aquatic resources is almost entirely dependent on the vegetation characteristics of the buffer zone. Maintenance of natural vegetation characteristics through appropriate management measures is therefore the most appropriate approach to maintaining natural microclimate and temperature regimes. In instances where water levels have been artificially increased through thermal pollution, introduction of tall and dense vegetation such as trees and shrubs may be considered to increase shading of the affected water resource. This may however lead to loss of species richness, accumulation of leaves and organic matter and shading out of riparian grasses that could result in geomorphological changes, such as increased bank erosion, which could cause widening of the channel (Ghermandi et al., 2009).

5.3.3.

Provision of habitat for wildlife

Provision of habitat for wildlife includes a range of specific functions highlighted in chapter 3 and habitat provision in the buffer itself and in the broader landscape through connectivity that buffers can provide. Although buffer width has been highlighted as a key attribute in determining the usefulness of the buffer zone for wildlife, management of buffer is clearly a key requirement to ensure that these functions are maintained. Maintaining natural vegetation structure and composition is likely to most adequately cater for the needs of native wildlife species. There may however be some instances in which modification of existing buffer zones can be made to improve the suitability of this habitat for specific species (e.g. creation of artificial nesting sites). In general however, management measures aimed at maintaining natural disturbance regimes (e.g. grazing and fire) and reducing impacts from disturbance (e.g. alien vegetation) are likely to contribute meaningfully towards maintenance of habitat quality of the buffer zone. Reducing the intrusion of noise, light, people, and pets is another important consideration since this can significantly affect use by specific species. Granger et al. (2005) list a number of ways in which this can be accomplished. Buffers vegetated with dense trees and shrubs for example, are effective at reducing intrusion of noise and light. Projects can also be designed to reduce noise and light intrusion by locating noisy areas such as parking lots, playgrounds, and loading docks away from the edge of the buffer Lighting can also be designed and located so it points away from the wetland and its buffer. Fences or berms can be constructed to block noise and light. Fences can also be used to limit human and pet intrusion. Dense shrubs, particularly those with thorns, can be planted along the edge of a development to block noise and light and limit intrusion. Maintaining connectivity is another key consideration that can only be achieved through broader scale planning initiatives. Site design and layout of large developments can however play an important role in maintaining connectivity between aquatic systems and other terrestrial areas by establishing corridors to link such areas. While scientific literature indicates that corridors should be 100’s of meters wide to provide functions over an extended period (Bennett, 1998, 2003) it may still be beneficial to provide corridors of any size (Granger et al., 2005). Indeed, corridors as narrow as 30m may have some wildlife and habitat value (Desbonnet et al., 1994). Design of such corridors should however be undertaken with due consideration of particular species, particularly where rare, threatened or endangered species are known to utilize the area. Buffer averaging may also be a

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useful tool to ensure that important habitat attributes are retained without unduly constraining development opportunities.

5.3.4.

Maintaining channel stability and flood control

Although slope and soil attributes affect the ability of buffers to provide this function, maintenance of vegetation characteristics has been highlighted as a key attribute affecting the ability of buffer zones to provide this function (Table 12). Maintenance of natural vegetation is arguably more appropriate for maintaining stream bank stability and attenuating floods than unnatural vegetation, as it is typically better suited for the specific conditions and flooding regimes of the area (Fischer et al., 2000). Trees and shrubs are however more effective at attenuating floods than less robust, shorter vegetation communities and could be introduced to improve the level of flood attenuation and stream bank stability where natural flood levels have increased. Limiting disturbance levels, particularly of stream banks is also important in preventing bank erosion and may also help to maintain vegetation density required to attenuate floods.

5.3.5.

Improving aesthetic appeal

The aesthetic appeal of a buffer zone is more the side effect of its implementation, with the other functions (water quality improvement, wildlife habitat provision) being the priorities. Thus management should be based on what would be the most suitable management for the provision of these functions, not on aesthetics, which would in most cases happen naturally (Blanché, 2002). Aesthetics of buffer zones are determined largely by the vegetation attributes of the wetland buffer however and it may be necessary to implement specific management measures to improve aesthetics in areas that experience large levels of ecotourism or recreation to ensure that it continues to attract people. This may simply include pruning or mowing in more intensive managed areas, or implementing weed control activities in areas of less intense use.

5.3.6.

Promoting groundwater recharge

Levels of groundwater recharge from buffer zones are largely determined by factors that cannot be easily affected by management practices. These include characteristics of the soil in the buffer zone (e.g. conductivity), natural levels of saturation and slope of the buffer, affecting percolation rates (Table 12). Management of measures aimed at increase interception rates of vegetation and reducing velocities of water flowing into and through the buffer could however help to increase levels of groundwater recharge.

5.4. Provisions for the delineation and demarcation of buffers and their maintenance over time Another aspect that should be considered when developing guidelines for wetland buffer determination is how such areas should be demarcated to ensure that buffer widths do not “shrink” over time as a result of infringement from adjacent activities. A number of options exist to ensure that such areas are clearly demarcated (Granger et al., 2005). Temporary markers should perhaps

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be viewed as a minimum requirement, particularly when construction activities in adjacent land could result in disturbance of the wetland buffer. Such an approach is typically applied in the South African forestry industry where wooden pegs are routinely placed along the edge of the buffer to delineate a line in which planting may not take place. Permanent signs are another option, regularly used in Washington State, USA with the following wording, “Protected Wetland Area. Do Not Disturb. Contact [Local Jurisdiction] Regarding Uses, Restrictions, and Opportunities for Stewardship” (Granger et al., 2005). In some instances, the erection of fences may be necessary to protect the functions and values of particular wetland and buffer area. This may be necessary for example where high density domestic grazing takes place in the adjoining area or where such fencing is used to protect habitat for an important species.

5.5. Ownership and enforcement considerations. Ownership and enforcement of buffer management requirements are other considerations that can significantly affect the provision of benefits from buffer zones. Granger et al., (2005) provide a useful discussion on ownership options that is briefly presented below. There are basically two options for ownership of buffer zones: • The buffer area can be included in a separate tract or lot and held in common ownership by a homeowners association, agency, or non-profit organization OR • The buffer can be included in lots owned by adjacent landowners The second option is often pursued by a developer who wants to divide the buffer among individual lots in order to achieve a required minimum lot size. However, a study by Cooke in (Castelle et al. 1992) of buffer areas in two counties in western Washington, USA showed that buffers that were owned by many different lot owners were more likely to be degraded over time. Even with easement language on each lot owner’s deed specifying the buffer protection provisions, owners tend to clear buffer vegetation over time to expand lawns, build storage sheds, or serve other uses. If the buffer area is not held in some kind of common ownership, it is much more difficult to take enforcement action against those landowners who encroach upon its boundaries. Therefore, when feasible, wetlands and their buffer areas should be placed in a separate, non-buildable tract that is owned and maintained by an organization that is dedicated to protecting the buffer. The boundaries of the tract should be clearly marked to help prevent unintentional encroachments. Once buffer zones and ownership have been established, management of these areas is essential to ensure that buffer functions are maintained. Regular observation of buffer zones should therefore be undertaken to ensure that desired habitat elements are maintained and that development is not encroaching on established buffer areas. Such actions should ideally be prioritized for projects with large potential impacts on water resources or in instances where buffer zones provide essential habitat for rare, threatened and endangered species.

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6. CONCLUSION Scientific literature provides extensive evidence that buffers are an important tool for achieving protection of aquatic resources. They are able to provide a wide range of functions from improving water quality to providing stability to stream banks and providing habitat for a range of wildlife species. The ability of buffers to provide these benefits is dependent on a range of buffer attributes such as soil type, vegetation structure and slope and on off-site factors such as the nature of influent particles and rate of flow of influent waters. The optimal width for buffers is therefore variable, depending on site characteristics and the desired functions that need to be maintained. Recommended buffer zone widths therefore range considerably from as narrow as 10m for microclimate control to hundreds of meters to maintain habitat for important species. A range of different buffer zone methodologies have been developed in response to the need to provide adequate protection to water resources from adjacent activities. These range from simple fixed-width approaches to highly technical methodologies that require detailed assessments of individual water resources to determine an appropriate buffer width. Each approach has advantages and disadvantages that need to be understood and considered in designing a methodology appropriate for implementation in the South African context. Attention is also drawn in this review to the limitations of buffer zones in protecting aquatic resources and the range of management options that should be considered to adequately protect aquatic resources. If buffer zones are identified as the appropriate tool, appropriate management of these areas has been highlighted as a key requirement to ensure that buffer functions continue to be maintained.

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7. LITERATURE REVIEWED Adriaanse, P. I. (1997). “Exposure assessment of pesticides in field ditches; the TOXSWA model.” Pesticide Science 49: 210-212. Allan, D. G., J. A. Harrison, et al. (1997). “The impact of commercial afforestation on bird populations in Mpumalanga province, South Africa – insights from bird atlas data.” Biological Conservation 79: 173-185. Allen, T.F. and Starr, T.B. (1982) Hierarchy: Perspectives for Ecological Complexity. Chicago: Univ. Chicago Press Baillie, E. M., C. Hilton-Taylor, et al., Eds. (2004). 2004 IUCN Red List of Threatened Species. A Global Species Assessment. Cambridge, UK, IUCN Publications Services Unit. Barling, R. D. And I. D. Moore (1994). “Role of Buffer Strips in Management of Waterway Pollution: A Review.” Environmental Management 18(4): 543-558. Basnyat, P., L. D. Teeter, et al. (1999). “Relationships Between Landscape Characteristics and Nonpoint Source Pollution Inputs to Coastal Estuaries.” Environmental Management 23(4): 539-549. Beilfuss, R. D., T. Dodman, et al. (2005). “The status of cranes in Africa in 2005.” Ostrich: Journal of African Ornothology 78(2): 175-184. Beilfuss, R., Bento, C., Hancock, P., Kamweneshe, B., MCCann, K., Morrison, K. and Rodwell, L. (2002) Water, wetlands and Wattled Cranes: A regional monitoring and conservation program for southern Africa. Conference on Environmental Monitoring of Tropical and Subtropical Wetlands, Maun, Botswana, 4-6 December 2002 Belt, G.H. and J. O’Laughlin. 1994. Buffer strip design for protecting water quality and fish habitat. Western Journal of Applied Forestry 9(2): 41-45. Bennett, A.F. (1998, 2003). Linkages in the Landscape: The Role of Corridors and Connectivity in Wildlife Conservation. IUCN, Gland, Switzerland and Cambridge, UK. Xiv + 254 pp. Berliner, D., P. Desmet, et al. (2007). Eastern Cape Biodiversity Conservation Plan Handbook. King William’s Town, Department of Water Affairs and Forestry: 1-57. Biohabitats Inc. And City of Boulder Planning and Development Services (2007). Wetland and Stream Buffers: A Review of the Science and Regulatory Approaches to Protection. Boulder, CO. Blanché, C. (2002). The Use of Riparian Buffer Zones for the Attenuation of Nitrate in Agricultural Landscapes. Pietermaritzburg, University of natal. M.Sc.: 140. Bond, W. J., J. Midgley, et al. (1988). “When is an island not an island? Insular effects and their causes in fynbos shrublands.” Oecolgia Berl. 77: 515-521. Boyd, L. (2001). Wildlife use of Wetland Buffer Zones and their Protection under the Massachusetts Wetland Protection Act, Department of Natural Resources Conservation, University of Massachusetts: 1-30. 102


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Brady, P. And R. Buchsbaum (1989). Buffer Zones: The Environments Last Defence. Wenham, Massachusetts Audubon Society. Breen R., pers. comm. Program Officer. Wetland Section. Department of Environment, Water, Heritage, and the Arts Canberra, ACT, Australia. Broderson, J.M. (1973) Sizing buffer strips to maintain water quality. Master’s thesis. University of Washington. Seattle. Bruane, M. J. And A. Wood (1999). “Best management Practices Applied to Urban Runoff Quantity and Quality Control.” Water Science and Technology 39(12): 117-121. Budd, W., P. Cohen, et al. (1987). “Stream Corridor Management in the Pacific Northwest: I. Determination of Stream-corridor Widths.” Environmental Management 11(5): 587-597. Burke, V. J. And J. W. Gibbons (1995). “Terrestrial Buffer Zones and Wetland Conservation: A Case Study of Freshwater Turtles in Carolina Bay.” Conservation Biology 9(6): 1365-1369. Castelle, A. Y. and A. W. Johnson, 2000. Riparian Vegetation Effectiveness. National Council for Air and Stream Improvement (NCASI), Triangle Park, N.C., Technical Bulletin No. 799, 26 p., February 2000 Castelle, A. J., A. W. Johnson, et al. (1994). “Wetland and stream buffer size requirements – a review.” Journal of Environmental Quality 23(5): 878-882. Castelle, A. J., C. Conolly, et al. (1992). Wetland Buffers: Use and Effectiveness. Olympia, Washington Department of Ecology: 1-62. Castelle, A. J., S. Washington, et al. (1992). Wetland mitigation replacement ratios: defining equivalency. Olympia, Wash., Washington State Dept. Of Ecology. Cederholm, C.J. (1994) A suggested landscape approach for salmon and wildlife habitat protection in western Washington riparian ecosystems. Pages 78-90 in A.B. Carey and C.Elliott compilers. Washington Forest Landscape Management Project Progress Report. Washington State Department of Natural Resources, Olympia. Chapman, J.L. (1999) Ecology: principles and applications. 2nd edition. Cambridge University Press. ISBN: 0521588022 Chapman, S. And R. Kreutzwiser (1999). A Rapid Hydrologic Wetland Evaluation Technique for South Africa. Advances in Planning and Management of Watersheds and Wetlands. F. J.E. Harare, Weaver Press: 234. Chase, V., L. Deming, et al. (1995). Buffers for wetlands and Surface Waters: A Guidebook for New Hampshire Municipalities. Concord, Audubon Society of New Hampshire. Cooke, S.S. (1992). Wetland Buffers: Use and Effectiveness. Appendix A:Wetland buffers - A Field Evaluation of Buffer Effectiveness in Puget Sound. Washington Department of Ecology. Publication No. 92-10.

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Annexure 1. Summary of the different buffer zone functions and attributes influencing their ability to perform these functions and values. Buffer Function

Attribute

112

Attributes reducing functionality

Flow characteristics

• Slow, shallow and uniform flows

Topography

• Gentle slope (10%) • Rough topography

Soil characteristics

• • • • •

• Poor infiltration rates • Low permeability (high clay content) • Poorly drained • Organic poor

Vegetation characteristics

• Increased surface roughness • Taller, thicker vegetation • Dense vegetation at the ground surface • Uniformly spaced stems • Natural vegetation

• Low surface roughness • Sparse vegetation at ground surface • Vegetation submerged by runoff • Clumps of stems • Degraded vegetation

Size of influent particles

• Coarse sediment particles (sand)

• Finer sediment particles (silt/clay)

Flow characteristics

• Slow, shallow and uniform flows

• Fast flow • Large flow volumes • Concentrated flows (rills,

Sediment Removal

Nutrient Removal (general)

Attributes improving functionality

Highly permeability Good infiltration Well drained Organic rich Clay soils with macropores at surface

Attributes of varying influence

Vegetation type • Grassed buffer strips are more effective in reducing erosion in areas with concentrated flows • Forested buffers are effective at removing sediments from upstream sources.


Buffer Function

Attribute



2009

Attributes of varying influence

channels)

Note: The ratio of sediment attached and dissolved nutrients will determine the functionality of the buffer.

Topography

• Gentle slope (10%) • Rough topography


• • • • • • • • •

• Poor infiltration rates • Low permeability (high clay content) • Poorly drained • Organic poor • Dry soil

Vegetation characteristics

• • • • •

113

Highly permeability Good infiltration Well drained Organic rich Moist soil Greater conductivity High soil porosity Increased surface roughness Dense vegetation at the ground surface Uniformly spaced stems Natural vegetation High primary productivity High primary productivity Annual plant species

• Low surface roughness • Sparse vegetation at ground surface • Low primary productivity • Degraded vegetation Invasive plants

Vegetation type • Grassed buffer strips are more effective in areas with concentrated flows • Grass and herbaceous vegetation is most effective when surface flow is the major contributor of nutrients • Forested buffers are effective at removing sediments from upstream sources. • Forest is also more effective when subsurface flow is the main contributor as well as being more effective during winter months when grass species may be


Buffer Function

Attribute



Attributes of varying influence dormant

Size of influent particles

• Coarse sediment particles (sand)

• Finer sediment particles (silt/clay)

Groundwater

• • • • •

• Steep sloped buffer • Deep water table • Deep surficial aquifer.


• • • • •

Nutrient Removal (Nitrates only) Vegetation characteristics

Saturation frequency

114

Shallow sloped buffer Shallow water table (

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