The nature and distribution of copper, lead, and zinc in soils of a highly urbanised sub-catchment (Iron Cove) of Port Jackson, Sydney.
Heavy metals are not biodegradable and are generally poorly mobile (Mermut et al. 1996); hence, metals tend to accumulate in soils with time and frequently constitute a substantial store of heavy metals. Factors controlling mobilisation of these potential pollutants within the soil profile include weathering and leaching, which break down soil minerals; adsorption of ions onto clay minerals and humus; surface enrichment of elements by plant material; and mobilisation or fixation by soil microorganisms (Brooks 1972). However, uptake and bioaccumulation of metals by plants are dependent on soil acidity, texture, and organic matter content, and on other factors influencing bioavailability (Chlopecka et al. 1996).
Possible point- and diffuse-sources of heavy metals to urban soils are numerous. Atmospheric deposition may be an important diffuse source of heavy metals for soils and may effect extensive areas and at great distances from the point of emission. Land use is frequently an important point source controlling metals distribution in urban catchments. Industrial areas and zones adjacent to major roads are commonly associated with elevated heavy metals in soils (Lagerwerff and Specht 1970; Albasel and Cottenie 1985; Tiller 1989; Thornton 1991; Munch 1993; Chon et al. 1998; Zupancic 1999). Dwellings, especially older houses protected with Pb-based paints, have been shown to influence metal concentration in soil (Davies et al. 1987; Culbard et al. 1988).
Iron Cove catchment (~1500 ha), located in inner western Sydney, is heavily modified by industrial and residential land-uses and infrastructure. Main roads, i.e. Hume Highway, Parramatta Road, and Old Canterbury Road, as well as a railway goods line and a passenger railway line transect the catchment (Fig. 1). Two canals (Hawthorne and Iron Cove Canals) drain the catchment and discharge into Iron Cove, an embayment of Port Jackson. Iron Cove catchment has a varied land-use history. In the 19th Century, the catchment and adjoining areas were developed for agriculture for the rapidly growing colony. Later, and well into the 20th Century, the catchment supported heavy industry, e.g. coal and chemical works and brass foundries (Soiling and Reynolds 1997), as well as major port operations in nearby Rozelle Bay. Timber yards, slaughterhouses, tanneries, breweries, distilleries, and refineries were also present in and near Iron Cove catchment (Markus and McBratney 1996). Iron Cove catchment is presently highly urbanised (>90%), with a moderate commercial and industrial base (Birch et al. 1999), with only about 2% of the catchment being parkland (Chapman and Murphy 1989).
The aim of the current research was to determine the nature, concentration, and source of Cu, Pb, and Zn in topsoil in Iron Cove catchment and to compare concentrations with ANZECC and NH&MRC (1992) environmental investigation limit standards and NEPM (1999) schedule B (1) draft guidelines. A chemical assessment of metal bioavailability in the soil was undertaken to determine the environmental implications of possible metal contamination
A1:10 000 scale map of Iron Cove catchment was divided into squares 200 by 200 m and from each of the resulting 374 squares, a topsoil sample was collected. Samples were recovered randomly from the dominant land-use type in each square (Fig. 2), e.g. road verges, gardens, parks, schools, and sport fields by inserting 4 stainless steel rings (60 mm in diam. and 25 mm deep) into the soil and combining the material into a single sample to reduce small-scale spatial variation. The 2-ram size fraction was removed by sieving through a nylon mesh on site. Physical attributes (texture, colour, wetness, organic content) of the sample, as well as a description of the site (road verge, house, factory), terrain type (grass, garden), and the type and distance from possible contaminant sources (road, factory), were documented on site.
[FIGURE 2 OMITTED]
Approximately 1.2 g of dried sample was digested in an aqua regia solution (2 mL HN[O.sub.3], 2 mL HCl, 10 mL water) on a hotplate at 120[degrees]C for 2 h. Samples were analysed for a suite of heavy metals (As, Ag, Al, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Zn) (only Cu, Pb, and Zn are reported in the current work because these metals showed consistently high concentrations throughout the catchment) using a Perkin Elmer inductively coupled mass spectrometer (ICP-MS). Precision, determined by repeat analyses of an internal laboratory standard (ILS 3), was <10% relative standard deviation for all metals analysed and recoveries were 95-105% of an International Reference Material (AGAL-10).
Grain size normalisation
Three normalisation procedures were tested during the current investigation and the results of this assessment are discussed in detail elsewhere (Birch and Snowdon 2004). All samples were subjected to post-extraction normalisation (PEN), a new procedure which uses the residue after digestion as the normalising fraction (Birch 2003) and elemental (Al) normalisation (Grant and Middleton 1990), whereas 30 of the samples were randomly selected for size normalisation (Brook and Moore 1988; Loring 1991).
Reduction in variance in normalised data improved comparability of data and produced spatially more consistent distributions than total sediment data and assisted in the identification of source. Results presented in the current work are normalised using the PEN method and are statistically similar to size-normalised data (Birch 2003; Birch and Snowdon 2004).
Thirty soil samples were selected to determine the bioavailability of Cu, Pb, and Zn in soil using EDTA and HCI digests (Batley 1987; Ying et al. 1992). Approximately 1 g of sample was placed into individual centrifuge tubes and 25 mL of 1 M HCI added. This procedure was repeated using 25 mL of 0.05 ',t EDTA. Samples were shaken for 24 h and placed in a centrifuge for 10 min (4000 r.p.m.). The HCl and EDTA extracts were expressed as a percentage of total sediment concentration using aqua regia digest.
As is common with contaminant data for soils, concentrations in the current investigation arc asymptotic and skewed to the right, and therefore Spearman product moment correlation coefficient analysis was carried out to determine the relationship between metal concentrations.
Heavy metal distributions were spatially presented using MapInfo for total soil samples and normalised data. Multiple indicator Kriging (MIKI) was used for depicting the spatial distribution of Pb concentrations in the soil (McBramey 1999). MIKI, through interpolation of the point data, estimates the concentration of metals at unsampled locations and can be used to predict the probability that metal concentrations are above a guideline value.
Results and discussion
Concentrations of heavy metals in topsoil in Iron Cove catchment varied considerably. Of the 12 metals analysed, 8 (As, Cd, Cr, Cu, Mn, Ni, Pb, Zn) are included in Australian guidelines for soils and these frequently occurred at concentrations above ANZECC and NH&MRC (1992) environmental investigation limits (EIL) in Iron Cove catchment. However, mean concentrations of total soil metals analysed were below EIL, except for Cu, Pb, and Zn. Zinc also has an average metal concentration above the Assessment of Site Contamination Guidelines (NEPM 1999) (Table 1).
Background concentrations of heavy metals
Mean concentrations of total soil Cu, Pb, and Zn were 62, 410, and 343 mg/kg, respectively; however, because of the skewed distribution of the results the 50 percentile better defined the data, i.e. 44, 203, and 224 mg/kg, respectively. The 10, 25, 50, 75, and 90 percentiles are given in Table 2 for improved definition of the distribution.
To identify areas of environmental concern, it is important to establish to what the extent these metals are enriched over pre-anthropogenic (background) levels. Variable composition and texture of the topsoil in Iron Cove catchment make it difficult to determine background concentrations on total soil chemistry, and normalised data were used to reduce the confounding due to variable grain size for background determinations (Birch 2003). Other attempts at determining background metal concentrations in Port Jackson catchment soils have also been problematic. Soils from an undeveloped parkland reserve (Middle Harbour) were found to be contaminated, presumably by atmospheric Pb (Graham 1999), and soils from a core taken in another 'pristine' reserve (Lane Cove) were altered by pedogenic processes (Hodge 2002). Background values for Port Jackson catchment shales, which may be the parent material for Iron Cove soils, are estimated at 39, 23, and 120 mg/kg for Cu, Pb, and Zn, respectively (e.g. Lester 1987). However, these rocks have undergone considerable diagenetic change and cannot be used for determining background concentrations in modern soils. In the current study, background concentrations were determined using frequency distributions of the metals, assuming that the population with lowest metal concentrations approximates the composition of the pre-anthropogenic soil, as has been undertaken in other works (Fergusson 1984; Lottermoser 1997). Mean concentrations of these background populations were estimated as 30, 75, and 75 for Cu, Pb, and Zn, respectively. Using the pre-anthropogenic concentrations for Cu, Pb, and Zn in soils estimated in the current study provided mean metal enrichments of 2, 5.5, and 4.6 and maximum enrichments of 36, 112, and 99 times above background for Cu, Pb, and Zn, respectively
Distribution and source of heavy metals
Concentrations of Cu, Pb, and Zn had similar spatial distributions in soils in Iron Cove catchment (Figs 3, 4, 5), and many metals correlated to each other strongly, e.g. Cu and Pb ([r.sup.2] = 0.70), Cu and Zn ([r.sup.2] = 0.73), and Pb and Zn ([r.sup.2] = 0.85), suggesting similar contaminant sources. Consistently high metal concentrations in topsoil throughout the catchment suggest an airborne source, possibly from smelters, coal-fired power stations, and fertiliser manufactures (e.g. Alloway 1990). From 1917 to 1927, scrap-metal works, as well as dye users and manufacturers, were located in the catchment and adjoining areas (Links 1998). Defence stores, Balmain Power Station, and the Monsanto chemical factory were situated in the north-east of the catchment. Balmain Power Station may have contributed metals via fly ash as has occurred elsewhere (Baker and Senft 1995; Kiekens 1995), and emissions from other industries, e.g. the Defence stores, current and past metal manufacturers, and engineering works, may have been sources of metals to the catchment via atmospheric transport (e. g. Barzi et al. 1996). Fertilisers and pesticides produced north of the catchment in the 1890s may have been a source of metals to the atmosphere (e.g. Alloway 1990).
[FIGURE 3-5 OMITTED]
Consistently high concentrations of Cu, Ph, and Zn were located in the north-eastern part of the catchment where urbanisation is oldest and where major, high-volume roads and railway lines converge. A large proportion (33% of samples) of soils in the catchment, especially to the east of the goods railway line and north of Parramatta Road and the Hume Highway, had Cu, Pb, and Zn concentrations >100, >800, and >480 mg/kg, respectively, in total soil. Soil adjacent to roads is commonly found to be elevated in heavy metals (Lagerwerff and Specht 1970; David and Williams 1975; Albasel and Cottenie 1985; Tiller et al. 1987; Munch 1993; Zupancic 1999). Copper is incorporated into automotive fuels, engines, lubricants, brake linings, and tyres (Harrison 1979; Mielke et al. 2000). Zinc is used in lubricating oils and the tyres of motor vehicles and train wheels (Onyari et al. 1991). The coarser particles (>2 mm) of Pb released from vehicle exhausts are deposited within 50 100 m of a road, but most Pb particles are smaller and are dispersed further from the source (Tiller 1989). Studies of major highways elsewhere show a correlation between distance from a road and heavy metal soil concentrations (Mielke et al. 1984; Mielke 1994; Hydo-Taek et al. 1995), and metal concentrations have been shown to increase with traffic volumes on Sydney streets (Scollen 1998; Birch and Scollen 2003). Soil samples taken <10 m from a road in the current study had higher mean Pb and Zn concentrations than those taken from a greater distance, and significant correlations were found between Cu and the distance from the road. Major roads (e.g. Parramatta Road) as well as other roads with high traffic volumes, a goods railway line, and the main suburban railway line pass through this area. Trains and motor vehicles stopping and starting cause parts to wear, releasing Cu, Pb, and Zn to the environment (Harrison 1979). Metal contamination may also be associated with the use of herbicides and pesticides in the railway line corridors and in Pb-based paints required to protect railway support equipment locally (Links 1998) and elsewhere (Tiller 1992; Marsh 1996). Railway yards and engineering facilities are also known sources of heavy metals (Thornton 1991; Barzi et al. 1996).
The mean Pb concentration was higher for samples taken near houses built pre-1960 (481 mg/kg) than near houses built later (335 mg/kg) in the present study, and the highest Pb concentration was from a site in the front yard of a pre-1900 house. The nearby road had a low traffic volume, and the sample was taken < 1 m from a painted wall. The roof and the paint (which may have been Pb-based due to the age of the dwelling) are possible sources of Pb. The flashing and washers used on older corrugated iron roofs may also be a source of Pb, as has been determined in an adjoining catchment (Ford and Dale 1996).
Soil quality and risk assessment
In Iron Cove catchment, 34, 33, and 56% of the topsoil samples had Cu, Pb, and Zn concentrations above the ANZECC and NH&MRC (1992) guidelines, respectively (Table 1). This is similar to heavy metal distributions in an adjoining suburb (Glebe), where 50% of topsoil samples have Cu, Pb, and Zn above ANZECC and NH&MRC guidelines (Markus and McBratney 1996).
Kriging was carried out for I00 thresholds, producing 100 estimated Pb distributions (quantiles). The 0.1, 0.5, and 0.9 quantiles had a 10%, 50%, and 90% probability of exceeding guideline thresholds. The 0.5 quantile represents the median (50 percentile) probable Pb distribution in the Iron Cove catchment and on this basis a large portion of the eastern part of the catchment had >50% probability of exceeding the ElL for Pb. The probability of Pb concentrations being greater than the ANZECC and NH&MRC (1992) guideline (300 mg/kg) was also highest (50-60%) in the north-east part of the catchment (Fig. 6).
[FIGURE 6 OMITTED]
Total chemical concentrations provide little meaningful information on the availability of metals to biological resources (Marr et al. 1997). Bioavailability analyses conducted in the current study suggested that a large proportion of the heavy metals in Iron Cove catchment soils might be available to flora and fauna. Mean EDTA extractions indicated that 57, 70, and 46% of Cu, Pb, and Zn, respectively, in Iron Cove catchment soils might be bioavailable, whereas 64, 86, and 67% of Cu, Pb, and Zn was extracted by HCl (Table 3). McGrath and Cegarra (1992) found that Pb was the largest fraction of metal extracted by EDTA, similar to the current study. Lead is also the largest fraction extracted by HCl of the 3 metals analysed. This is in contrast to the findings of Chon et al. (1998), who found bioavailability to be in the order of Zn > Cu > Pb. The extractability of metals with HCl was generally higher than with EDTA, and this is consistent to the work of McCready et al. (2003) for sediments in Port Jackson and Ying et al. (1992) for sediment collected from central New South Wales estuaries. Concentrations of Pb and Zn extracted by EDTA and HCl had mean (bioavailable) values above the toxicity symptom limits (100 and 120 mg/kg, respectively) set by the NSW Department of Agriculture (Wong et al. 1996). A large proportion of Zn was taken up by lettuce grown in a pot trial using urban soil from Glebe, a suburb adjacent to the Iron Cove Catchment (Markus 1993). Although the current study did not analyse metal content of plants, isotopic dilution studies show that plants and EDTA remove a similar proportion of soil Zn (Tiller 1992).
Pre-anthropogenic soil concentrations of 30, 75, and 75 mg/kg were determined for Cu, Pb, and Zn, respectively, in the Iron Cove catchment, indicating mean enrichment over background of 2, 5.5, and 4.6 times for Cu, Pb, and Zn, respectively.
Soils across the entire Iron Cove catchment are enriched in Cu, Pb, and Zn, but are substantially elevated in the north-east part of area, probably due to the location of old houses, past industry, and the convergence of major roads and railway lines. Metal soil distribution patterns were more consistent when the confounding produced by variable grain size was reduced using normalisation techniques. Of the 374 samples analysed, 34%, 33%, and 56% had concentrations of Cu, Pb, and Zn above ANZECC and NH&MRC guidelines. The probability distributions produced by MIKI indicate that the north-east part of Iron Cove catchment has >50% probability of exceeding the ANZECC and NH&MRC guideline for Pb (300 mg/kg).
Selective extraction procedures using EDTA and HCl indicate large proportions (46 86%) of Cu, Pb, and Zn in the topsoil in the catchment may be bioavailable.
Table 1. Number of samples above ANZECC and NH&MRC (1992) Environmental Investigation Limit (EIL) concentrations All concentrations mg/kg Metal Mean concentration Samples Whole soil Normalised EIL >EIL % As 15 37 20 17 Cd 1 2 3 3 Cr 20 56 50 3 Cu 62 170 60 34 Mn 227 665 500 3 Ni 12 34 60 0.5 Pb 410 1069 300 33 Zn 343 927 200 56 Table 2. Percentiles for total soil concentrations Concentrations in mg/kg Percentiles: 10 25 50 75 90 Cu 15 26 44 74 132 Pb 43 88 203 436 921 Zn 76 120 224 400 716 Table 3. Bioavailability as a percentage of extractable for EDTA and HCI extracts TSI, toxicity symptom limit (Wong et al. 1996); percentages as portion of aqua regia extract (n = 30) Cu Pb Zn EDTA HCI EDTA HCI EDTA HCI Mean (%) 57 64 70 86 46 67 Minimum (%) 9 3 20 14 16 20 Maximum (%) 85 112 107 108 74 105 Mean (mg/kg) 31 36 366 424 142 214 TSI (mg/kg) 120 100 120
The authors thank Alex McBratney and Julie Markus for assistance with Multiple Indicator Kriging (MIKI) used for calculating the spatial distribution of the probability of Pb concentrations exceeding soil guidelines. We also thank an anonymous reviewer who provided valuable and constructive comments on the paper and Tom Savage for assistance with analyses.
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R. Snowdon (A) and G. E Birch (A,B)
(A) Environmental Geology Group, School of Geosciences, Sydney University, NSW 2006, Australia.
(B) Corresponding author; email: firstname.lastname@example.org
Manuscript received 6 February 2003, accepted 6 February 2004.
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|Author:||Snowdon, R.; Birch, G.F.|
|Publication:||Australian Journal of Soil Research|
|Date:||May 1, 2004|
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