Groundwater vulnerability assessment: two case ...

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Quarterly Journal of Engineering Geology, 28, 179-194.

0481-2085/95 $07.00 © 1995 The Geological Society

Groundwater vulnerability assessment: two case studies using GIS methodology K. M. Hiscock, A. A. Lovett, J. S. Brainard & J. P. Parfitt School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK

Abstract In the first case study presented here, a geographic information system (GIS) is used to create a groundwater vulnerability map of the Midlands and northwest of England by overlaying regional information on the solid geology, Quaternary drift cover and soil cover. The map reveals that areas of extreme and high groundwater vulnerability occur in the vicinities of Birmingham, Liverpool and Manchester. In the second study, a GIS is used to create a groundwater vulnerability map for southeast England and to combine this information with results from a routing model for the transport of hazardous aqueous waste within the region. The routing model utilizes an accident-minimizing scenario and expresses the potential pollution threat to groundwater as the number of tanker-kilometres directed over each groundwater vulnerability class. It is concluded that a GIS methodology is very suitable for groundwater vulnerability mapping, providing an ability to integrate multiple layers of information and to derive additional information, for example on pollution risks. A GIS also allows flexibility in the revision of maps should existing information become obsolete, or revision of the groundwater vulnerability classification scheme be necessary. Keywords: Geographic information systems, groundwater contamination, hazardous waste, hydrogeological maps, risk analysis

Introduction In England and Wales groundwater abstraction accounts for 35% of the total public freshwater supply, and for 20% of all abstractions (Department of the Environment 1992). To safeguard the quality of water intended for human consumption and to protect groundwater against pollution by certain dangerous substances and diffuse pollution by nitrate, the European Communities have enacted controlling legislation that is applicable in the UK (Council of European Communities 1980a, 1980b and 1991, respectively). It is the National Rivers Authority (NRA) who, under the Water Resources Act 1991 (Section 84), have the responsibility to protect the quality of groundwater. In promoting a national policy for groundwater protection, the N R A is seeking to influence the decisions of others whose actions can affect the quality of groundwater, for example in response to consultation under planning

legislation (NRA 1992). Inherent in this policy is the concept of groundwater vulnerability which recognizes that the risk of pollution from a given hazardous activity is greater in certain hydrogeological situations than in others. The concept of groundwater vulnerability and groundwater pollution risk adopted by the N R A and in this paper is in keeping with that presented by Foster & Hirata (1988) who define aquifer vulnerability as a function of: (a) the accessibility of the saturated zone, in a hydraulic sense, to the penetration of pollutants; and (b) the attenuation capacity of the strata overlying the saturated zone as a result of the physical retention of, and chemical reaction with, contaminants. These two components of aquifer vulnerability interact with the following two components of subsurface contaminant loading to determine the groundwater pollution risk: (a) the mode of contaminant disposition in the subsurface, in particular the magnitude of any associated loading; and (b) the contaminant class in terms of its mobility and persistence. An example of a methodology to evaluate groundwater vulnerability is that of the United States Environmental Protection Agency (Aller et al. 1987). This methodology is designed to permit systematic evaluation of the groundwater pollution potential in any hydrogeological setting anywhere in the United States. The system has two major components: (i) the designation of mappable units, termed hydrogeological settings; and (ii) the superposition of a relative rating system having the acronym DRASTIC. Inherent in each hydrogeological setting are the physical characteristics that affect groundwater pollution potential. The most important mappable factors considered to control the groundwater pollution potential are: depth to water (D); net recharge (R); aquifer media (A); soil media (S); topography (slope) (T); impact of the vadose zone (I); and hydraulic conductivity of the aquifer (C). In the UK, and in the implementation of national

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Average A and B and C for D (D=3) Maximise B and C for D (D=4)

FIG. 1. Diagram to illustrate two types of'map algebra' for organizing map analyses. In (a) traditional algebraic operations are used to combine maps, and in (b) boolean (logical) algebraic operations are used to combine maps. Algebraic operations used within a GIS facilitate the cartographic modelling of numerous data planes or maps (after yon Braun, 1988). policy, the methodology of the N R A relies partly on producing a series of 53 groundwater vulnerability maps, covering the whole of England and Wales, to provide a framework for decision making. This approach defines vulnerability as a function of: (a) the nature of the overlying soil; (b) the presence and nature of any overlying superficial or glacial deposits; (c) the nature of the geological strata forming the aquifer; and (d) the thickness of the saturated zone or thickness of confining beds. The vulnerability maps are being produced manually from the overlay of information on hydrogeology and soils at a scale of 1 : 100 000, and will be digitized for use within a geographic information system (GIS) when the full map series is produced (Robins, pers. comm.). Examples of currently available vulnerability maps, with descriptions of how they were compiled, are included in publications by Palmer (1987), the N R A (1992) and Robins et al. (1994). The presentation of groundwater vulnerability in the form of maps is ideally suited to management by a GIS in which multiple layers of information can be integrated in different combinations. This paper demonstrates the application of a GIS methodology for deriving regional groundwater vulnerability maps, and extends the application of GIS technology to evaluate the potential pollution threat to groundwater from a specific hazar-

dous activity. Two case studies are presented: the first is an evaluation of groundwater vulnerability in the Midlands and northwest of England; the second is an assessment of the potential pollution threat to groundwater arising from the transport of hazardous aqueous waste in southeast England.

Geographic information systems A GIS is a computer system for the capture, storage, manipulation, analysis and display of spatially referenced information. The GIS software is designed to manipulate spatial data with output in the form of maps and tabular reports, or as data files generated for interfacing with numerical models. An important feature of a GIS is the ability to generate new information by the integration of existing diverse datasets sharing a compatible spatial referencing system (Goodchild 1993). The inputs to a GIS may include: remotely sensed data from satellites or aircraft; existing digitized databases of, for example, road networks or terrain elevation; and information from maps, tables or reports. The common characteristic is that each type of data input describes the attributes of recognizable point,

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FIG. 2. Maps of (a) solid geology, (b) Quaternary drift cover and (c) soil permeability for the Midlands and northwest of England derived from digitizing existing regional maps. In each case, a vector file is created using a manual line digitizer and then converted to a raster file with a 1 km z grid cell size. The soil permeability map is limited to available information for Midland and Western England, with the area not digitized coinciding with unimportant, in terms of groundwater vulnerability, Carboniferous strata and older rocks.

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linear or areal geographical features. Details of the features are usually stored in either vector (points, lines and polygons) or in raster (grid cells) formats. Although a GIS is designed to handle large amounts of data, some pre-processing is usually necessary to convert the data to a form that can be integrated into the GIS. One of the most common techniques is classification, in which data are interpreted within a sampling framework to provide information on the general characteristics of the landscape such as vegetation, land use, soil or geology. The selection of data sources should be influenced by their accuracy and resolution, together with the nature of the problem to be investigated. The importance of establishing data quality objectives (for example: what decisions are to be made with these data?; what level of uncertainty in the results can be tolerated?) in advance of analysis is an important consideration. It is also necessary to be aware of the analytical techniques required to solve the problem and whether the data are suitable for such operations. Within a GIS, data manipulation can take the form of a set of mathematical operations which, like traditional algebra, may be combined to perform a variety of calculations (Berry 1993). As shown in Fig. 1, this map algebra forms a structure for spatial statistics and cartographic modelling permitting concurrent analysis of numerous data layers or maps.

Application of G IS in groundwater vulnerability assessment Examples of the application of GIS in groundwater vulnerability assessment can be broadly grouped into two categories. The first category includes those studies which combine geoscientific information, either raw data or results from groundwater modelling, with information characterizing the hazardous activity in order to assess either the environmental impact or human exposure risk resulting from groundwater contamination. Published studies in this first category include: von Braun (1988, 1993); Evans & Myers (1990); Halliday & Wolfe (1991); Fiirst (1992); Harper et al. (1992); Padgett (1992); and Pipes et al. (1994). The work presented in this paper is in this first category and demonstrates the application of GIS in assessing groundwater vulnerability (case study 1) and evaluating the potential pollution threat to groundwater from a specific hazardous activity (case study 2). In the second category, geoscientific information is handled within a GIS and then interfaced with a groundwater model to assist input and display of large amounts of groundwater data at both the pre- and postprocessor stages of modelling (EI-Kadi et al. 1994). In studies by Baker et al. (1993) and Rifai et al. (1993) a

GIS is interfaced with a groundwater flow model to delineate well-head protection areas. Harris et al. (1993) demonstrate the coupling of a GIS with a threedimensional, finite element model, and Turner (1989, 1992) discusses advanced use of three-dimensional GIS for modelling groundwater contamination. Interactive ground and surface water resources modelling for environmental management, where systems are designed to integrate GIS, large databases, simulation models, expert systems and tools for graphical display, are discussed by Loucks & Fedra (1987) and Fedra & Diersch (1989).

Case study 1: Groundwater vulnerability in the Midlands and Northwest or England Introduction In this case study a GIS is used to create a groundwater vulnerability map of the industrialized Midlands and northwest of England. In order to input and manipulate a variety of spatial map data, the raster-based IDRISI software package (Graduate School of Geography, Clark University, Worcester, MA, USA) was used. In this (and the following) case study, it should be recognized that the estimated groundwater vulnerability for the region is the intrinsic vulnerability. It might be expected, for example, that the vulnerability of groundwater to agricultural chemical contamination in rural areas would be much greater, compared with non-rural areas, even though the intrinsic vulnerability may actually be less than in non-rural areas. The Permo-Triassic sandstone forms a major aquifer, extensively used for supplying water to Birmingham, Liverpool and Manchester. The sandstone consists of reddish-brown, evenly bedded, poorly cemented, locally pebbly, fine- to coarse-grained sandstones with thin beds and lenses of dark red mudstone. These sandstones form a multi-aquifer sequence with high intergranular and fissure permeabilities. The aquifer is mainly unconfined in the northwest region and is considered, in the context of this case study, to be extremely vulnerable to surfacederived pollution. Elsewhere, over much of the Midlands, the aquifer is overlain and confined by the low permeability Mercia Mudstone Group. The Mercia Mudstone Group comprises red silty mudstones with thin beds of fine-grained sandstone and siltstone which produce minor supplies, mainly from springs. Given these small supplies, the Mercia Mudstone Group is considered here as a minor aquifer, parts of which, in the absence of coveting deposits, are classed as having a high groundwater vulnerability. Below the Permo-Triassic sandstone are Carboniferous and other Palaeozoic deposits forming cyclic

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GROUNDWATER VULNERABILITY ASSESSMENT sequences of variable thickness and extensively faulted coals, limestones, shales, sandstones and mudstones of little significance for public water supply. Taken together, these deposits are considered to form a nonaquifer and are classed as having a low groundwater vulnerability.

Data inputs and manipulation In mapping the groundwater vulnerability of this region, three physical variables were considered: the nature of solid geology; the presence and nature of Quaternary drift cover; and the presence and nature of soil cover. Data from small-scale maps of these parameters were used since larger scale maps would provide unnecessary detail for such a large region. In the absence of groundtruth data, these maps were taken to be accurate, although as might be expected with small scale maps, inaccuracies must arise through the generalization involved in their construction. The British National Grid was used as a common geographic co-ordinate system. Geological information is contained in four, 1:250 000 solid geology (old series) maps published by the Institute of Geological Sciences (1978). Three geological classes were differentiated for digitizing: the Permo-Triassic sandstone; the overlying Mercia Mudstone Group; and the underlying Carboniferous strata and older rocks. Minor geological units within the Carboniferous strata were not differentiated. The resulting digitized regional geology map is shown in Fig. 2a. Major fault lines (there are a number of en echelon faults in the region with a general north-south trend) and the coastline were digitized separately as vector files in order to overlay these details on the final vulnerability map. Information on Quaternary drift cover is contained in the first edition 1:625 000 Quaternary (south sheet) map published by the Institute of Geological Sciences (1977). Three Quaternary drift cover classes were differentiated for digitizing: absence of drift cover; sands and gravels (including both glacial and fluvial deposits); and glacial till. Glacial till includes boulder clay and morainic drift comprising poorly sorted material that is predominantly fine-grained. The glacial till is generally less than 10m thick but locally exceeds 70m in drift-filled glacial channels. For the purpose of vulnerability mapping, the glacial till is considered to have a low vertical hydraulic conductivity capable of protecting underlying deposits and solid strata. The resulting digitized drift cover map is shown in Fig. 2b. Information on soil cover is contained in the 1:250 000 Soils of Midland and Western England (Sheet 3 with Legend) map published by the Soil Survey of England and Wales (1983) and the accompanying regional bulletin (Ragg et al. 1984). From the soil sub-groups detailed on the map and in the legend, and from a

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consideration of the soil physical characteristics (soil texture, structure, water regime and the presence of distinctive layers such as peat or gravel) of the associated soil series, three general soil permeability classes were differentiated for digitizing as shown in Table 1. The classes are broad and designed to represent the relative risk of infiltration through the soils. They also take no account of losses by run-off. A fourth, unclassified category was created for soils in dense urban and industrial areas. The resulting digitized soil permeability map is shown in Fig. 2c. TASLE 1. Ordering of typical soil sub-groups into soil permeability classes for use in groundwater vulnerability mapping in the Midlands and northwest of England Soil sub-group Brown calcareous earths Brown earths Brown sands Brown alluvial soils Argillic brown earths Humic and brown rankers Rendzinas Pelosols Podzols and stagnopodzols Gley soils Peat soils Stagnogley soils Stagnohumic gley soils Urban and industrial areas

Soil permeability class High

Moderate

Low Unclassified

The scheme used to differentiate soil permeability classes is similar to that devised by the Soil Survey and Land Research Centre for the NRA (1992). Soils with a high permeability class (or high leaching potential according to the N R A 1992) have little ability to attenuate diffuse source pollutants and readily transmit liquid discharges. Those of moderate permeability (intermediate leaching potential) have a moderate ability to attenuate diffuse source pollutants and can possibly transmit a wide range of pollutants. Soils of low permeability (low leaching potential) are generally clayey soils with the ability to attenuate diffuse source pollutants and impede vertical water movement. Inaccuracies associated with digitizing the base maps can be expressed as a root mean square (RMS) error calculated for the positional accuracy of digitized control points. This statistic indicates the translocational or 'drift' error for each set of digitized lines with respect to their position in reality. For the geology, drift cover and soil vector files the RMS errors were 1,381 m, 23 m and 123 m, respectively. The larger error in the geology file results from the assemblage of data from four base maps. However, the errors are acceptable given the small scale of the maps digitized and the objectives of the study.

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TABLE 2. Relative groundwater vulnerability classification scheme for the Midlands and northwest of England. Vulnerability is designated extreme, high, moderate or low depending on the two-stage process, shown as steps (a) and (b) in the table, to combine information on solid geology, Quaternary drift cover and soil permeability (a) Initial groundwater vulnerability map classification Quaternary drift cover class Drift cover absent Sands and gravels Glacial till

Solid geology class

Permo-Triassic sandstone

Mercia Mudstone Group

Carboniferous strata and older rocks

Extreme Extreme High

High Moderate Low

Low Low Low

(19)Final groundwater vulnerability map classification Combined solid geology/ Quaternary drift cover vulnerability classification

High

Moderate

Low

Unclassified soil class (urban and industrial areas)

Extreme High Moderate Low

Extreme High Moderate Low

Extreme High Moderate Low

High Moderate Moderate Low

Extreme High Moderate Low

Soil permeability class

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Derivation of the vulnerability map To produce the groundwater vulnerability map for the Midlands and northwest of England, the geology, drift cover and soil maps were combined to determine the areal extent of combinations of classes present. The operations performed are summarized in the flow diagram in Fig. 3, and the initial and final groundwater vulnerability map classifications defined during the overlay operations are given in Tables 2a and 2b, respectively. In essence, the extreme and high relative vulnerability classes associated, respectively, with the Permo-Triassic sandstone (major aquifer) and Mercia Mudstone Group (minor aquifer in parts) are lessened, in terms of vulnerability, by the presence of sands and gravels or glacial till. A moderate vulnerability category is defined for sands and gravels overlying the Mercia Mudstone Group, and a low vulnerability category is defined for the Carboniferous strata (non-aquifer) regardless of overlying Quaternary drift cover. These four vulnerability classes are then combined with the three soil permeability categories to produce the final relative groundwater vulnerability classification. The effect of the unclassified soil class in dense urban and industrial areas is to not alter the initial vulnerability classification based on the combination of solid geology and Quaternary drift cover present. The derived classes given in Table 2b are therefore based on informed judgement, subjectively weighing the importance of the hydrogeological unit considered against the mitigating effects of the nature and presence

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FIG. 3. Flow chart of GIS (IDRISI) analytical operations used to combine information on geology, Quaternary drift cover and soil permeability to compile a groundwater vulnerability map for the Midlands and northwest of England.

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GROUNDWATER VULNERABILITY ASSESSMENT

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FIG. 4. Relative groundwater vulnerability map for the Midlands and northwest of England. Outcrop areas of the major PermoTriassic sandstone aquifer have the greatest groundwater vulnerability; mainly extreme or high depending on the protection provided by Quaternary drift cover and soil cover (Table 2). Large areas of Carboniferous and older strata, which have little importance for public water supplies, are classed as low vulnerability.

of overlying drift and soil cover. The range of combinations include, for example, the important (in terms of water resources) Permo-Triassic sandstone aquifer which, when covered by glacial till and low permeability soil, is classed as having a moderate vulnerability to groundwater pollution. On the other hand, the unimportant Carboniferous and older strata, where covered by sands and gravel and high permeability soil, are classed as having low vulnerability. The resulting relative groundwater vulnerability map,

shown in Fig. 4, is at best only as accurate as the least accurately digitized base map. Inaccuracies arise from digitizing, during vector-to-raster conversion of the datasets to a 1 km 2 grid cell resolution, and error propagation in the overlay operations. Comparing Fig. 4 with Fig. 2a, it is noticeable that areas of sandstone outcrop have the greatest vulnerability to groundwater pollution; mainly extreme or high, and occasionally moderate. Conversely, large areas of Carboniferous strata and older rocks have low

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groundwater vulnerability. Also, when comparing Fig. 4 with Fig. 2c, which indicates urban and industrial areas as an unclassified soil class, it is evident that areas of extreme vulnerability coincide with major population centres, namely the corridor between Manchester and Liverpool, the areas northeast and southwest of Birmingham and the industrial area surrounding Wolverhampton. Interestingly, it is apparent in Fig. 4 that once the major fault lines are laid on top of the final vulnerability map, a few fault lines connect areas classified as low vulnerability to those classified as high or extreme vulnerability. Depending on their nature, these faults could provide pollution pathways and so should be considered in the assessment of the true vulnerability at specific locations. Further considerations in the evaluation of vulnerability could include: (i) the actual thickness and nature of Quaternary drift cover, particularly the heterogeneity of the glacial till; and (ii) the quality of groundwater in the Permo-Triassic sandstone aquifer which, in some areas, for example in the vicinity of the Mersey estuary, is already contaminated with saline water, thus decreasing the importance of the aquifer locally for water supply purposes.

Case study 2: Modelling the pollution threat to groundwater from the transport of hazardous aqueous waste in southeast England Introduction In this case study a GIS is used to evaluate the potential pollution threat to groundwater arising from an accident during the transport of hazardous aqueous chemical waste from its production point to disposal site. The vector-based ARC/INFO software package (Environmental Systems Research Institute, Inc., Redlands, CA, USA) was used to analyse two scenarios: in the first, waste streams are routed by the shortest travel time; and in the second, the outcome of a strategy that minimizes cargo-threatening road accidents is evaluated. Comparing the results of these scenarios enables an assessment to be made of the benefit, in terms of reducing pollution risk, of adopting the second, accident-minimizing (or accident-avoidance) scenario. In assessing the two scenarios, the potential threat to groundwater resources is expressed as the amount of tanker traffic (in terms of the number of tanker-kilometres) passing over each groundwater vulnerability class. For reasons of data availability and the need to limit the destinations of waste transportation streams, the study area is centred on Greater London and neighbouring counties in southeast England.

In geological terms, the study area encompasses the London Basin, a synclinal structure in Eocene and Cretaceous strata. The principal geological formations are the Eocene London Clay, the Lower London Tertiaries and the underlying Cretaceous Chalk, Upper Greensand and Gault Clay (Smith et al. 1976; Downing et al. 1979). The London Clay is a relatively impermeable, over-consolidated clay, but the Lower London Tertiaries are represented by a series of sands, loams and clays. In a few areas of the Basin, post-Eocene erosion has led to the removal of the London Clay, producing Lower London Tertiary inliers (Flavin & Joseph 1983). In the central and eastern parts of the Basin, the lower part of the Lower London Tertiaries is arenaceous and is referred to as the Basal Sands. The Basal Sands facies is commonly in hydraulic continuity with the underlying Chalk. The Chalk, which forms a major aquifer for public water supplies, is a soft, fine-grained limestone. The clays of the London Clay and the Lower London Tertiaries confine the Chalk aquifer under natural conditions. The Chalk is underlain by the Upper Greensand and the Gault Clay, an argillaceous deposit forming the base of the aquifer system. Other important hydrogeological units in the study area include the Cretaceous Lower Greensand and the Jurassic Corallian and the Great and Inferior Oolitic Limestones. The Lower Greensand has been extensively developed as a source of groundwater on the northern limb of the London Basin syncline due west of London (Mather et al. 1973). The principal outcrop of the Lower Greensand occurs in the area of Bedford where the Woburn Sands have been developed for water supply (Irving 1982). The outcrop of the Corallian Limestone aquifer is east of Oxford dipping southeastwards below low permeability Jurassic Kimmeridge Clay. Springs arising from the formation are the source of streams feeding the rivers of the middle Thames area (Alexander & Brightman 1985). The Great and Inferior Oolitic Limestones have been developed for water supply to the north and west of Oxford. Interbedded clays in the limestones can cause local perched water table conditions and springs (Morgan-Jones & Eggboro 1981). In geographical terms, a number of motorways converge on the study area connected by the M25 London orbital motorway. Road run-off is either gravity drainage to local surface drainage systems or oil interceptors to soakaways. Price et al. (1992) explain that interceptors, such as those located at the junction of the M1 and M25 motorways to the north of London, with soakaways in the Chalk aquifer, are designed to retain light, non-aqueous phase liquids such as oil and petrol; although the possibility remains that water soluble pollutants, or dense non-aqueous phase liquids could pass through the interceptors and enter a soakaway. Examples of the latter substances include pesticides, phenols and chlorinated organic liquids. Hence,

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GROUNDWATER VULNERABILITY ASSESSMENT road tanker accidents involving these substances are a serious potential threat to groundwater resources.

Data inputs and manipulation Four sources of data were used: waste consignment note records (London Waste Regulation Authority); road network and average vehicle speeds; vehicle accident probability data; and the vulnerability of groundwater supplies to aqueous surface pollution. Unlike the previous case study, for the purpose of mapping groundwater vulnerability, two physical variables were considered: the nature of the hydrogeological unit present; and the presence and nature of Quaternary drift cover. The choice of hydrogeological unit is logical in that it recognizes the hydraulic properties, and thus the water-bearing potential, of the area formations. Discounting soil cover as a source layer, the vulnerability map is simpler to implement and is adequate for the intended application in modelling the potential pollution threat to groundwater arising from the transport of hazardous aqueous waste. The consignment note records are the best available data on the transport of hazardous wastes arising in London and refer to single or multiple movements of a particular class of waste between two locations. For this study, data on aqueous waste streams between 1 April 1984 to 31 March 1985 were used. These include the Ordnance Survey grid reference for the production point, the hazard class of the waste involved and a code for the disposal location. Data for later years is either poorer in quality or non-existent. Parfitt (1990) discusses this situation in more detail. For 1984/85, 349 aqueous waste streams were recorded that involved one or more road tankers transporting waste. In some instances all waste producing companies on an industrial estate had an identical grid reference. Data on production points are therefore at a coarse spatial resolution. The road network is available in the UK digital database compiled by John Bartholomew & Son, Ltd from 1:253440 scale motorists' maps for 1990. Each class of road (motorway, A-road (single and dualcarriageway), B-road and minor road) is assigned a numeric code. To reflect the road network in 1985 accurately, verification work was undertaken using 1:50 000 and 1:250 000 Ordnance Survey maps, with all roads clearly absent on maps dated 1 January 1986 or later being eliminated from the 1990 network. Computer processing restrictions required a reduction in network size (by eliminating minor roads, except for those in the vicinity of producer and disposal sites) to approximately 7800 road segments. To calculate travel times along any section of road, average vehicle speeds were attributed to segments in the road network based on the class of road and using the data compiled in Table 3. Average road speeds were taken from official statistics provided by the

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London Research Centre (1988) and the Department of Transport (1992). TABLE 3. Average off-peak vehicle speeds assigned to the digitized road network for south-east England using data given by the London Research Centre (1988) and the Department of Transport (1992) (for simplicity, the possible difference between average vehicle speeds in rural and urban areas outside of London is not considered) Class of road

Speed (km h-l)

Outside Greater London Motorway A-road, dual-carriageway A-road, single-carriageway B-road All other roads Within Greater London Motorway and A-road All other roads

113 97 89 64 48 60 34

The potential adverse effects to the environment of hazardous waste transport are not realized unless an accident occurs. Thus, minimizing the risk of an accident is clearly an important planning consideration. Frequencies of cargo-threatening accidents for heavy goods vehicles are contained in AUsop et al. (1986) based on estimates of the numbers of accidents in which four and five-axle vehicles overturned or suffered side damage. From Table 4 it is clear that, per 107 cargo lorrykilometres, motorways are the safest class of road. Unfortunately, the statistics make no distinction between single and dual-carriageway roads or differences in vehicle speeds, and are not specific to the study area. However, these national data were accepted in the absence of other information for later use in the accident-minimizing model scenario. TABLE 4. Annual frequency of cargo-threatening accidents for heavy goods vehicles (after Allsop et al., 1986) Class of road Motorway A-road B-road Minor road

Annual accident frequency per 107 cargo lorry-kilometres 1.15 3.65 11.40 8.31

Four hydrogeological units, digitized from the 1:625000 hydrogeology map of England and Wales (prepared by the Institute of Geological Sciences 1977), were recognized and grouped, according to the legend accompanying the hydrogeology map, into: major aquifers of regional importance; aquifers of local importance; minor aquifers of only limited potential,

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K . M . HISCOCK E T AL.

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GROUNDWATER VULNERABILITY ASSESSMENT

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Groundwater vulnerability classes:

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FIG. 5. Relative groundwater vulnerability map for Greater London and southeast England. Large areas of extreme and high vulnerability are associated, respectively, with outcrops of the important Cretaceous Chalk and Jurassic Oolitic Limestones, and outcrops of the locally important Lower Cretaceous Wealden Beds (Hastings Sands). Areas of moderate and low vulnerability include, respectively, the occasionally important Quaternary sands and gravels and Tertiary Beds, and the Eocene London Clay and Jurassic clays. Extensive areas of low vulnerability also occur where glacial till or clay-with-flints cover underlying hydrogeological units.

but possibly of local importance; and non-aquifers with negligible permeability, but possibly yielding domestic supplies. Additionally, four types of Quaternary drift cover, chosen on the basis of affording some protection to underlying aquifers, were differentiated for digitizing: peat; lacustrine clays, silts and sands; glacial till (boulder clay and morainic drift); and clay-with-flints. The source layer defining groundwater vulnerability was generated by overlaying the Quaternary drift cover with the hydrogeological units present in the region. The

groundwater vulnerability classification scheme is summarized in Table 5. Where overlying deposits are absent, the four hydrogeological units are classed as having extreme, high, moderate and low susceptibility to groundwater pollution, as appropriate. In areas where overlying deposits are present, the four hydrogeological units are interpreted as simply having low groundwater vulnerability. The resulting relative groundwater vulnerability map for Greater London and southeast England is shown in Fig. 5. The area of each groundwater

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190

vulnerability class and the percentage each class represents of the total study area, as calculated by the GIS, are given in Table 6. TABLE 6. Areas of relative groundwater vulnerability classes in Greater London and south-east England Groundwater vulnerability class

Area (x 105 ha)

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8.150 2.464 5.325 16.330

(25.3%) (7.6%) (16.5%) (50.6%)

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In both scenarios, A R C / I N F O calculates the cumulative number of waste streams passing over each road segment. To derive the number of road tanker journeys per waste stream for each class of aqueous waste, the number of waste streams in each category are compared with official summary statistics contained in Parfitt (1990) on the tonnage assigned each year. For example, in 1984/85, there were 44 waste streams for inorganic alkalis and the total quantity consigned was 988 tonnes. This gives an average of 22.5 tonnes per waste stream, interpreted as a single 20-tonne tanker journey per waste stream. Informal enquiries indicate that an assumed average tanker capacity of 20 tonnes is not atypical. Thus, the number of tankers associated with each waste stream can be estimated.

GIS routing model results G IS routing model methodology Central to modelling waste transport is route selection. Roads identified as carrying waste streams were referenced as critical roads. For the purpose of routing within the GIS, the locations of the origin and destination sites for each waste stream were identified by means of Ordnance Survey grid references, and algorithms used to determine the optimal route between them via the road network. In this routing model it is assumed that the risk of groundwater pollution arises from either direct infiltration at an accident site or via drainage soakaways. A more accurate determination of the risk of groundwater pollution would require further information on the accident zone, for example, local factors such as slope of the ground surface and roadside drainage systems. Given that the actual routes taken by tanker drivers in 1984/1985 are unknown, the first model scenario, in which the shortest travel times are calculated, is taken to represent the base pattern of transportation routes. To find the shortest travel times, road impedance values were assigned by dividing the length of each road segment by its average speed to give the average time, in minutes, to traverse that segment of road. The second scenario adopted a strategy to reduce the risk of a tanker accident occurring by modelling routes that minimize the total accident 'impedance' value. A relative accident impedance value was assigned to each segment in the road network, defined as: [annual accident frequency per 107 cargo lorry-kilometres (the accident probability)] x [road segment length in kilometres]. Use of accident probability as a routing criteria, rather than minimal elapsed time on distance travelled, is used elsewhere in risk assessment studies (e.g. Barkan & Treichel 1992).

The outcomes of both scenarios suggest that wastes arising in Greater London relied heavily upon few transportation routes to disposal sites, with most of these on the outskirts of the conurbation itself. Closer inspection reveals heavily used corridors in east London and a definite west-east trend in the pattern of the waste streams, as well as significant predicted flows to Hampshire in the south of England and into those counties immediately north and west of London. For the scenario with routing by shortest travel time, the average estimated tanker journey length was 55 km. To illustrate the pattern of waste streams, the results obtained for the simulation incorporating the avoidance of cargo-threatening accidents are shown in Fig. 6. In comparison to the results obtained for routing by the shortest travel time, the pattern of critical roads shows wastes leaving the centre of London to circle the conurbation before continuing to the disposal sites via a few simpler, yet longer and more frequently used transportation routes. The average estimated tanker journey length is now 65km. Figure 6 indicates a pronounced flow of wastes eastwards from west to east London via the M25, the Dartford Tunnel and the A13 in the direction of large waste disposal facilities. Other notable flows are southwest to Southampton, west to Oxford and north to Northampton and Cambridge. Elsewhere, mainly in southern England, there is a nearvirtual absence of aqueous waste transport. From a comparison of Figures 5 & 6, of the critical roads radiating from Greater London, those that cross areas of extreme vulnerability include: the M3, in crossing a wide area of unconfined Chalk aquifer to the southwest of London; the M4 and M40 in crossing areas of Chalk outcrop to the west of the London Basin; the M1 in crossing areas of exposed Chalk, Lower Greensand and Jurassic Oolitic Limestones north of London; and the M11 in crossing part of the Cambridgeshire Chalk. Part of the southeastern section of

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GROUNDWATER VULNERABILITY ASSESSMENT

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