K.V. Heal*, D.A. Hepburn** and R.J. Lunn*** *University of Edinburgh, School of Geosciences, Crew Building, West Mains Road, Edinburgh EH9 3JN, UK (E-mail:
[email protected]) **School of the Built Environment, Heriot Watt University, Riccarton, Edinburgh EH14 4AS, UK (E-mail:
[email protected]) ***Department of Civil Engineering, University of Strathclyde, John Anderson Building, 107 Rottenrow, Glasgow G4 0NG, UK (E-mail:
[email protected]) Abstract Since removal and disposal of sustainable urban drainage system (SUDS) sediment can incur high maintenance costs, assessments of sediment volumes, quality and frequency of removal are required. Sediment depth and quality were surveyed annually from 1999 –2003 in three ponds and one wetland in Dunfermline, Scotland, UK. Highest sediment accumulation occurred in Halbeath Pond, in the most developed watershed and with no surface water management train. From comparison of measured potentially toxic metal concentrations (Cd, Cr, Cu, Fe, Ni, Pb, Zn) with standards, the average sediment quality should not impair aquatic ecosystems. 72 –84% of the metal flux into the SUDS was estimated to be associated with coarse sediment (. 500 mm diameter) suggesting that management of coarse sediment is particularly important at this site. The timing of sediment removal for these SUDS is expected to be determined by loss of storage volume, rather than by accumulation of contaminants. If sediment removal occurs when 25% of the SUDS storage volume has infilled, it would be required after 17 years in Halbeath Pond, but only after 98 years in Linburn Pond (which has upstream detention basins). From the quality measurements, sediment disposal should be acceptable on adjacent land within the boundaries of the SUDS studied. Keywords Pond; potentially toxic metals; sediment depth; sediment quality; sustainable urban drainage system; wetland
Water Science & Technology Vol 53 No 10 pp 219–227 Q IWA Publishing 2006
Sediment management in sustainable urban drainage system ponds
Introduction
Uncertainty concerning maintenance costs is one of the main barriers to the implementation of sustainable urban drainage systems (SUDSs) (McKissock et al., 2003) which aim to minimise the impact of urban development on receiving watercourses. Sediment accumulates in SUDS ponds and wetlands over time due to the operation of chemical, physical and biological processes. Since removal and disposal of SUDS sediment can incur high maintenance costs, assessments of the frequency of sediment removal, sediment volumes and sediment quality are required. Results are presented here from annual surveys of sediment depth and sediment quality in four SUDS in the Duloch Park development, Dunfermline, Scotland, UK. The aims of the investigation were to: (a) characterise sediment accumulation and quality in SUDS ponds; (b) investigate sediment particle size and metal concentration relationships and; (c) provide recommendations for the design and maintenance of SUDS ponds, with regards to sediment issues. Methods
The four SUDS studied (three ponds and one wetland) are located in the Duloch Park area of the Dunfermline East Expansion Area (DEX), a new 7.5 km2 residential, retail, leisure and light industrial development on the eastern edge of the town of Dunfermline, central Scotland, UK (568 40 N, 38 240 W). Over a period of 20 years from doi: 10.2166/wst.2006.315
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the mid-1990s, approximately 5 km2 of greenfield, predominantly agricultural land, is being converted to a 200,000 m2 industrial park, 5,500 homes, an 18 ha leisure park, three schools and a district shopping centre. The development area comprises the headwaters of four streams which drain through the town of Dunfermline and were already responsible for downstream flooding and water quality deterioration due to the conventional urban drainage infrastructure in the town. As a result, a comprehensive drainage plan was developed for the site involving local (e.g. swales, permeable paving, detention basins) and regional SUDS (ponds and wetlands) to control stormwater flows and quality. At the time of its development, DEX was the largest site in the UK at which SUDS had been included in the initial site design. A detailed discussion of the SUDS techniques employed at Duloch Park is given in Roesner et al. (2001). The main characteristics of the SUDS studied are summarised in Table 1. The ponds and the wetland were designed following the guidance in CIRIA (2000) to hold four and three times the design treatment volume (Vt), providing residence times of 21 and 14 days, respectively, during wet conditions. Halbeath Pond, Linburn Pond and Pond 7 have a similar design comprising two basins, separated by a shallow sill, with piped inflows (mainly in concrete inlet structures) and an outlet. In contrast to Halbeath Pond and Pond 7, Linburn Pond has a surface water management train, including six detention basins, in the watershed upstream of the pond. For safety reasons, the slopes at the edge of the ponds are gently sloping and were planted with common reed (Phragmites australis) in 1998. The Wetland is less highly engineered and was developed by flooding an existing marshy area. Sediment depth and quality were surveyed annually from 1999 to 2003 in the four SUDS as part of an ongoing investigation of the performance, biological quality and maintenance costs of SUDS at Duloch Park. All measurements were conducted along transects across each SUDS: two for each of the ponds (one along the primary axis from the main inlet to outlet of each pond, and one perpendicular to this) and three for the Wetland (due to its larger area). Along each transect, water depth was measured and sediment cores were collected in a plastic liner (diameter 48 mm) within a stainless steel aquatic sediment sampler (Wildcow hand corer, length 0.51 m), equipped with a Lexan nosepiece and a rubber flutter valve to provide suction. The corer was attached to a steel extension rod and driven into the sediment by hand from a small boat attached to a rope (with distance markers) which was stretched across the pond surface. In this way sediment samples were collected from the same points each year to enable comparison of results between years. Wet sediment depth was measured in the cores in the field (27 –55 cores in each SUDS). Selected sediment cores (7 –14 in each SUDS) were transferred to sample bags and retained for laboratory analysis. Grab samples of sediment from inlet and outlet structures were also analysed. All wet sediment samples were weighed immediately on return to the laboratory in order to calculate wet bulk sediment density from the wet volume and mass. The sediment was subsequently prepared for analysis by air-drying and grinding by hand in a pestle and mortar. Particle size was determined by dry sieving 50 g sediment through 600 and 63 mm stainless steel sieves (BS410/1986, Endecotts) to separate the coarse sand, medium-fine sand, and silt and clay fractions, respectively. pH was measured with an electrode calibrated with pH buffers 7 and 9 in a suspension of 25 g air-dried sediment in 50 ml deionised water that had been stirred, left for 30 minutes and then re-stirred. The remaining sediment was oven-dried at 105 8C for 6 hours and then milled. Sediment organic matter content was determined by loss on ignition of 20 g oven-dried, milled sediment heated in a muffle furnace at 430 8C for 6 hours. Total potentially toxic metal concentrations (Cd, Cr, Cu, Fe, Ni, Pb, Zn) were measured by flame atomic absorption spectrometry (Solaar M Series, Unicam) in digests of the ashed samples prepared using
Table 1 Characteristics of SUDS studied at Duloch Park, Dunfermline, Scotland Characteristic
Halbeath Pond
Linburn Pond
Pond 7
Wetland
SUDS type Watershed land use Watershed area (ha)a % watershed developeda Construction date Inlets and outlet configuration
Pond Leisure development, highway 13.5 72 1997 1 inlet. 1 outlet (perforated standpipe).
Water storage volume (m3)a Pond surface area (m2)c Mean water depth (m)d
4600 6489 1.49
Pond Residential, highway, undeveloped grassland 67.5 27 1998 4 inlets. 1 outlet (weir plate with 4 equal-sized 908 V-notches). 15495 9848 2.24
Pond Residential, highway 67 35 1998 3 inlets (1 added in 2003). 1 outlet. 5120 4992 1.15
Wetland/pond District park, residential, highway 197b 22 1998 2 inlets (1 open channel, 1 piped). 1 outlet. 14325 18633 0.94
a
From Spitzer and Jefferies (2003) Wetland is in subcatchment c Calculated in ArcView v.3.2 (ESRI) GIS from scanned design drawings d From measurements conducted annually along transects across SUDS, 1999–2003 b
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hydrochloric and nitric acids (Allen et al., 1974). Total nitrogen and phosphorus concentrations were determined by automated colorimetry (Autoanalyser II, Bran & Luebbe) in Kjeldahl digests (prepared using the procedure of Taylor (2000)) of milled, oven-dried samples. All analyses were performed in duplicate, with certified reference materials and blanks for laboratory quality control. In 1999 and 2000, six separate sediment samples from each SUDS were collected and freeze-dried prior to analysis for hydrocarbons by infrared spectroscopy at the Scottish Environment Protection Agency’s Riccarton laboratory. Maps of interpolated sediment depth and sediment quality were produced from the core measurements at the transect points in each SUDS using the spatial analyst tool (spline function) within the ArcView v.3.2 (ESRI) GIS (Geographical Information System) package. To investigate relationships between sediment particle size and metal concentration (Hepburn, 2004), 100 g of 23 selected sediment samples from Halbeath and Linburn Ponds in 1999 and 2002 were dry sieved through 63, 125, 250 and 500 mm stainless steel sieves (BS410/1986, Endecotts). Approximately 8 g of each sieved fraction was retained, milled and then analysed for total potentially toxic metals using the procedure described above. Results and discussion
Sediment depths varied within each SUDS, with sediment initially accumulating near inlets and in the primary basins, before accumulating throughout the SUDS (see Figure 1 for Linburn Pond). Mean measured sediment depths for each SUDS were combined with surface area and sediment density data to estimate sedimentation rate, change in sediment volume and dry mass, and % of volume infilled for each SUDS (Table 2). The figures were calculated on a cumulative basis to smooth out interannual variations in sediment depth measurements. Table 2 shows that the greatest increases in sediment volume and mass and the highest mean annual sedimentation rate occur in Halbeath Pond and the Wetland. The increase in sediment dry mass is greater in Halbeath Pond than the Wetland because the Wetland sediment has a higher organic content and is less dense. Some of the values for change in sediment volume and mass for Linburn Pond and Pond 7 are negative due to interannual variation in measured sediment density and depth. The final column in Table 2 provides information relevant for estimating when sediment removal from the ponds will be required, depending on the acceptable threshold for loss of storage volume. For example, if sediment removal is required when 25% of the SUDS storage volume has infilled, excavation would be necessary in Halbeath Pond every 17 years, whereas for Linburn Pond it would not be necessary for 98 years (although the distribution of sediment between the two basins in each pond would also need to be taken into consideration). These estimates concur with the frequency of sediment excavation of 25
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Figure 1 Maps of interpolated sediment depths (m) in Linburn Pond, Duloch Park, Dunfermline, in (a) 1999 and (b) 2003
Table 2 Estimates of sedimentation rate, changes in sediment volume and dry mass, and % of volume infilled for four SUDS at Duloch Park, Dunfermline, 1999 –2003 SUDS
Increase in sediment volume
Increase in sediment dry mass
SUDS volume infilled
(cm a21)
(m3)
(t)
(%)
1.0 0.4 20.2 0.8
335 197 2 44 717
454 290.5 12.8 131
7 1 21 5
Halbeath Linburn Pond 7 Wetland
years suggested by Yousef et al. (1994a) in order to minimise the potential of metal transport from SUDS basin sediments to groundwater. The estimated sedimentation rates in the Duloch Park SUDS lie within the range reported from other studies (Table 3). Concentrations of most potentially toxic metals (Cr, Cu, Ni, Zn) increased significantly (1-way ANOVA, p , 0.05) in the sediments in all SUDS from 1999/2000 to 2001/2002 (see Figure 2), though concentrations of most metals returned to 1999/2000 values in 2003. There are a number of possible explanations for the observed increases in metal concentrations in sediments. The most likely explanation is that they are due to increasing traffic as site development has progressed. Chromium and nickel are often elevated in highway drainage due to corrosion of metal plating and wear of bearings and other moving parts in engines (Makepeace et al., 1995). Other studies have also found evidence of metal accumulation in SUDS. For example, Lind and Karro (1995) measured higher loadings of copper, lead and zinc in soil beneath an infiltration device in Go¨teborg, Sweden, which had received runoff for eight years from a highway with a mean daily traffic flow of 11,400 vehicles, compared to soil from a reference site at the same distance from the road which had not been used for infiltration. An alternative explanation may be an increase in metal inputs to the SUDS surfaces from atmospheric deposition (wet and dry) from 1999 to 2002. Analytical error can be ruled out as a cause since certified reference materials (of known metal concentration) were analysed at the same time as the sediment samples as a quality control procedure. Furthermore, repeat analyses of some of the samples showed similar results. Nevertheless, comparison with standards for aquatic sediments and contaminated soils (Table 4) showed that the average SUDS sediment quality at Duloch Park should not generally pose a threat to the aquatic environment and would not be classified as a special waste if excavated.
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Sedimentation rate
Table 3 Sedimentation rates determined in other SUDS/urban ponds Reference
Fa¨rm (2002) Marsalek et al. (1997) Rowney et al. (1986) Striegl (1987)
Yousef et al. (1994b)
Method
3 sediment cores 19 sediment cores 14 sediment cores Probing and sediment cores
Sedimentation
Pond
Main watershed
rate (cm a21)
characteristics
land use
3.2
Detention pond constructed 1998 2.0 (mean) On-stream stormwater pond constructed 1982 0.46 (mean) Urban lake constructed 1961 0.2 (mean) Impounded 1889. Drained and sediments removed 1970. 10–66 sediment 0.46 (mean of 9 ponds) 9 detention ponds cores constructed 1961 –1986 In situ 2.26 (mean of 9 ponds) measurements U.S. EPA model 1.23 (mean of 9 ponds)
Highway Commercial Residential Residential
Urbanised and highway
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(a)
K.V. Heal et al
450 400 350 300 250 200 150 100 50 0
(b) Cr
70
b
50
b
b
40 b
b
c
a,b
b,c
c
a
aa
a
a,b
a
a
a,b a,b
a
a
a,c
b a,b a,b
a a,c c a,c b
a
10
aa
aa
a
a,b
a,b
30 20
a
0 Halbeath Linburn
Pond 7 Wetland
(c)
Halbeath Linburn
Pond 7 Wetland
(d) Ni
250
b b
200 b
b b
150
c
100 a a
b,c
c
c
50
b
60
b
1999 2000 2001 2002 2003
Cu
a,c
a a
a
a,c aa
a a
0 Halbeath Linburn
Pond 7
450 400 350 300 250 200 150 100 50 0
b
b a,b a,b a,b a
a,b aa a
Halbeath Linburn
Wetland
1999 2000 2001 2002 2003
Zn
b,c b,c a,b b,c a,b a
Pond 7
b,c
a,b a,b
a
Wetland
Figure 2 Total metal concentrations (mg kg21 dry weight) in SUDS sediments at Duloch Park, Dunfermline, 1999 – 2003. All bars show the mean value ^ one standard deviation. Bars with different letters are significantly different from other bars for the same pond
Concentrations of chromium, copper and zinc in sediments from the Duloch Park SUDS studied were of similar magnitude to those reported for other SUDS/urban lakes (Table 5). Cadmium and lead concentrations were lower than at some other sites, probably due to the phasing out of leaded petrol in the 1980 s, but iron and nickel Table 4 Sediment total potentially toxic metal, nitrogen, phosphorus and hydrocarbon concentrations in the Duloch Park SUDS for all samples 1999 –2003 compared with standards for aquatic sediments and contaminated land. Units are mg kg21 dry weight, except for Fe (%). Values are mean ^ 1 standard deviation. Values in bold exceed aquatic sediment and/or contaminated land standards Determinand
Cd Cr Cu Fe Ni Pb Zn N P Hydrocarbonsc a
Linburn
Pond 7
Wetland
Standards
Standards for
(n 5 49)
(n 5 77)
(n 5 62)
(n 5 123– 126)
for aquatic
contaminated
sedimentsa
landb
10 110 110 4 75 250 820 4800 2000 1500
30 200 – – 75 450 – – – –
0.21 ^ 0.54 0.22 ^ 0.42 0.32 ^ 0.39 0.39 ^ 0.94 70.7 ^ 65.8 78.2 ^ 87.0 118 ^ 110 76.7 ^ 102 18.8 ^ 9.22 20.9 ^ 15.3 16.3 ^ 6.42 17.4 ^ 7.44 4.41 ^ 1.10 4.74 ^ 1.68 3.87 ^ 0.873 7.16 ^ 3.04 63.3 ^ 48.4 68.4 ^ 39.8 83.9 ^ 61.4 63.6 ^ 57.5 26.3 ^ 31.5 25.4 ^ 19.6 18.2 ^ 9.46 22.6 ^ 17.2 78.4 ^ 72.9 110 ^ 89.4 77.0 ^ 24.8 93.1 ^ 43.1 977 ^ 575 1850 ^ 998 1300 ^ 705 3550 ^ 2340 720 ^ 915 696 ^ 568 560 ^ 208 664 ^ 662 89.2 ^ 100 523 ^ 590 515 ^ 943 171 ^ 197
Severe effect level, Ontario Ministry of Environment (1993) Soil guideline values for residential land use without plant uptake, DEFRA and The Environment Agency (2002a, b, c, d) c n ¼ 6 b
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Halbeath
Table 5 Total potentially toxic metal concentrations reported in sediment from other SUDS/urban ponds. Units are mg kg21 dry weight, except for Fe (%). Values are mean ^ 1 standard deviation Reference
Number samples
Cd
Cr
Cu
Fe
Ni
Pb
Zn
Fa¨rm (2002)a Mallin et al. (2002)b Marsalek et al. (1997)c Meseure and Fish (1989)d
3 4 5 4—44
0.431 ^ 0.368 0.150 ^ 0.153 1.2 ^ 0.3 4
25.7 ^ 9.07 1.72 ^ 1.31 110 ^ 25 –
– 0.0304 ^ 0.0320 2.98 ^ 0.21 –
38.7 ^ 12.4 0.723 ^ 0.764 32 ^ 5 –
34.0 ^ 9.85 3.35 ^ 3.71 125 ^ 50 16
189 ^ 73.7 25.7 ^ 45.2 319 ^ 124 –
Rowney et al. (1986)e Striegl (1987) Yousef et al. (1990)
33—34 7—15 28
2.55 ^ 1.03 4 5^6
– – 19 ^ 17
51.3 ^ 24.1 3.58 ^ 6.00 63 ^ 26 17 ^ 4 47 ^ 16 18 ^ 5 17 ^ 4 25.7 ^ 9.97 250 10 ^ 14
– 1.94 –
– – 10 ^ 10
58.3 ^ 32.9 1590 92 ^ 193
114 ^ 38.9 210 37 ^ 101
a
Nitric acid digestion Digestion with H2O2 and nitric acid c Nitric, hydrochloric, perchloric and hydrofluoric acid extraction d Extraction for 24 hours at room temperature with H2O2 and nitric acid e Nitric, perchloric and hydrofluoric acid digestion b
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concentrations were higher. The higher iron concentrations in Duloch Park SUDS sediments is attributable to former coal mining activity and ferruginous staining has been observed in ditches near the Wetland. Concentrations of potentially toxic metals in SUDS sediments measured by Mallin et al. (2002) are considerably lower than those reported from other studies, apart from cadmium. Sediment quality is highly variable in many SUDS ponds as reflected in the large values for standard deviation. In the Duloch Park SUDS the highest concentrations of potentially toxic metals were measured in sediments near the inlet and this pattern has also been reported by Meseure and Fish (1989). From analyses of metal concentrations in different sediment particle size fractions it was estimated, unexpectedly, that 72–84% of the metal flux into the SUDS ponds is associated with coarse sediment (. 500 mm diameter). One plausible explanation for this observation is that fine sediments are washed rapidly from watershed surfaces compared to coarse sediments, so there is more time for accumulation of metals to occur on the coarser sediments. Conclusions
Accumulated sediment will require removal in the future, but the timing of removal will vary between SUDS and is expected to occur in the order: Halbeath (earliest) – Linburn – Pond 7 – Wetland (latest). Although metal concentrations generally increased in sediments from 1999/2000 to 2001/2002, they only exceeded severe effect levels for aquatic organisms in a few samples. Consequently the timing of sediment removal is expected to be determined by reduction in the retention time for storm runoff, due to sediment infilling the SUDS storage volume, rather than by accumulation of contaminants to unacceptable levels in the sediment. It is recommended that sediment removal is not conducted until required in order to minimise disturbance in the ponds. The disposal route for excavated sediment will be dependent on the sediment quality at the time of removal but the measurements to date suggest that disposal may be acceptable on adjacent land within the boundaries of the SUDS as recommended by the National SUDS Working Group (2003). The majority of the metal flux into the SUDS is associated with coarse sediment meaning that control of construction runoff is particularly important at Duloch Park. The twobasin design of Halbeath and Linburn Ponds and Pond 7 is regarded as a positive feature for sediment management. Sediment accumulates preferentially in the primary basin so that removal should only be required from here, thereby minimising the disturbance to the entire pond. Inclusion of a larger shallow forebay at the main inlets would also provide an area for preferential accumulation of coarse sediment which could be removed with minimal disturbance to the pond ecosystem. A surface water management train upstream of the ponds, as for Linburn Pond, is recommended to minimise the frequency of sediment removal. Detention basins are particularly valuable features in a surface water management train since accumulated sediment is highly visible and can be removed relatively easily (as long as access for sediment removal is included in the detention basin design). Acknowledgements
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The authors are grateful to Taylor Woodrow and Scottish Water for funding, to the Urban Water Technology Centre, University of Abertay Dundee, for support and advice and to the Scottish Environment Protection Agency for hydrocarbons analyses. Hannah Bird, Clare Dunsmore, Marina Xenophontos, Maggie Keegan, Nick Forrest, Marc Giamblanco and James Wright assisted with sediment sampling, laboratory analysis and data analysis. The technical support of Andrew Gray, John Morman, Alan Pike, Alex Jackson and James Smith is acknowledged.
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