DISTRIBUTION OF HEAVY METALS IN NEAR-SHORE SEDIMENTS ...

DISTRIBUTION OF HEAVY METALS IN NEAR-SHORE SEDIMENTS OF THE SWAN RIVER ESTUARY, WESTERN AUSTRALIA ANDREW W. RATE∗ , ALISTAIR E. ROBERTSON and AUDREY T. BORG Soil Science and Plant Nutrition Group, University of Western Australia, Nedlands, Australia (∗ author for correspondence, e-mail: [email protected])

(Received 4 January 1999; accepted 15 November 1999)

Abstract. Estuarine systems adjacent to urban areas are at risk of contamination by contaminants from anthropogenic sources, such as heavy metals. We anticipated that the sediments of the Swan River estuary, which runs through metropolitan Perth in Western Australia, would show metal contamination related to industrialization and inputs of stormwater. Total Cu, Pb and Cd concentrations, and Cu, Pb, Cr and Zn in operationally-defined fractions, were determined in separate sampling exercises in near-shore sediments of the upper Swan River estuary. Total metal concentrations in sediments were not high (maximum values of 297 mg kg−1 for Cu, 184 mg kg−1 for Pb and 0.9 mg kg−1 for Cd) when compared with Australian environmental assessment guidelines for soils. On the basis of linear regressions between sediment metal concentrations and physicochemical properties of the sediments (pH, organic carbon, particle size distribution), no single parameter could explain the variation in metal concentrations for all metals. Sediment organic carbon content was positively correlated with Cu concentration; Cu concentrations also increased significantly with increasing clay content and decreasing sand content. Pb concentrations showed a significant increase with increasing sediment pH, and were approximately three-fold higher in sediments adjacent to stormwater drain outfalls than in sediments remote from drains; no such effect was observed for Cu or Cd. No effect of distance downstream was observed. Sequential extraction of sediments showed that most of the metals were in relatively immobile forms, for example bound to Fe oxides, or only extractable by aqua regia. The enhanced concentrations of Pb near stormwater outfalls suggest that vehicle-derived Pb may be an important contributor of Pb to the estuary. Keywords: cadmium, copper, estuarine sediments, heavy metals, lead, sequential extraction

1. Introduction The absolute concentration of a metal ion in a sediment will depend on the original metal content of the parent material, any juvenile inputs of metal (e.g. from volcanism or pollution), and the ability of the sediment to retain metals by adsorption or precipitation mechanisms. Metal ions in soils and sediments are partitioned between the different phases present, and will predominantly be associated with solid phases including: organic matter; oxyhydroxides of iron, aluminium and manganese; phyllosilicate minerals; carbonates; and sulfides (Bolt and van Riemsdijk, 1987; Gambrell, 1994). In addition, metal ions are retained on these solid phases by different mechanisms (ion exchange, outer- and inner-sphere surface complexation (adsorption), precipitation or co-precipitation). The combination of retaining solid Water, Air, and Soil Pollution 124: 155–168, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.

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phase and retention mechanism determines the bioavailability and thus potential toxicity of metal ions (Tessier and Campbell, 1987). The relative importance of any solid phase for retaining a metal will depend on the identity and concentration of the metal, the abundance of the solid phase, and the controlling parameters of pH and redox potential (Eh ). The pH of a sediment and its interstitial water affects metal retention in several ways; development of pH-dependent charge at weakly acidic surface functional groups on organic and inorganic colloids determines the extent of ion exchange reactions. Similarly, protons and hydroxide ions also compete with adsorbing cations and anions, respectively (Sposito, 1986; Bolt and van Riemsdijk, 1987). In addition, sediment Eh influences metal ion speciation by controlling the solubility of some adsorptive phases (that is, FeI I I - and MnI V oxyhydroxides) (Stone and Morgan, 1987), and by controlling the concentration of anions such as sulfide which form very insoluble compounds with many trace metal ions (Gambrell, 1994). Estuarine sediments in urban areas are often found to be contaminated with heavy metals (Förstner and Wittman, 1979). These metals are derived from multiple sources including: motor vehicle traffic (Culbard et al., 1983; Francek, 1997); waste dumps or sewage overflows (Birch et al., 1996); and industries, especially mining or smelting (Farrell and Calder, 1988; Fergusson, 1990; Alloway, 1995). A major transport mechanism for heavy metals is stormwater from urban runoff (Wei and Morrison, 1993; Deely and Fergusson, 1994; Birch et al., 1996; Line et al., 1996). In many instances, metals transported by stormwater have been shown to be retained in estuaries or other wetland systems (for example, Wei and Morrison, 1993; Deely and Fergusson, 1994; Reinelt and Horner, 1995). The Swan River estuary in Western Australia is situated in a major urban area, with numerous stormwater discharges directly into the estuary. In a previous study, Eyre and McConchie (1993) found elevated concentrations of metals in the Swan estuary, near sources such as a recreational marina and a single stormwater drain. More recently, Gerritse et al. (1998b) used radiochemical dating techniques to show that Zn, Cd, Pb and Cu concentrations in the estuary had risen significantly since European settlement. This study was performed to determine: (i) whether established capacity-controlling parameters (sediment pH, organic matter content or particle size distribution) (Salomons and Förstner, 1983) could explain the variability in cadmium, copper and lead concentrations in near-shore sediments of the Swan River estuary; (ii) whether metal concentrations were higher near stormwater outfalls than in parts of the estuary remote from stormwater drains (and thus whether stormwater might be contributing Cd, Cu or Pb to the estuary), and; (iii) whether metal concentrations increased down the estuary, suggesting a cumulative effect of urbanization on sediment quality. A sequential extraction procedure was used to provide additional information on metal speciation in sediments.

HEAVY METALS IN THE SWAN ESTUARY, WESTERN AUSTRALIA

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Figure 1. Swan River estuary location map and approximate positions of sampling sites (# remote from stormwater drain outfalls; adjacent to and downstream from stormwater drain outfalls; 1 sediments subjected to sequential extraction procedure).

2. Materials and Methods 2.1. S TUDY

AREA

The estuary of the Swan and Canning Rivers (Figure 1) is a large (53 km2 ) microtidal estuary which passes through metropolitan Perth in Western Australia (32◦ S, 116◦ E). The river systems have a combined catchment of approximately 139 000 km2 ; the climate is Mediterranean with winter-dominant rainfall of approximately 800 mm yr−1 at the coast decreasing to 250–300 mm yr−1 at the eastern (inland) limits of the catchment. 2.2. S EDIMENT

SAMPLING

Near-shore sediment samples were collected at 25 positions in June 1996, from locations adjacent to and immediately downstream from stormwater outfalls, and locations remote (>1 km) from stormwater outfalls. Sediments were also collected at 8 locations in June 1997, using a plastic corer. Water column depth was approximately 60 cm, and the depth of sediment sampled was 30 cm. Sediments collected

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in 1996 were oven-dried at 105 ◦ C for ≥24 hr, and used for aqua regia ‘total’ metal determinations only; those collected in 1997 were air-dried at ≤40 ◦ C, and were used for determining the mode-of-occurrence of metals using a sequential extraction procedure (see below). The ≤1 mm size fractions from both sets of samples were retained for analysis following gentle hand-grinding to break up aggregates. 2.3. S EDIMENT

CHARACTERISATION

Sediment pH was determined in 1:5 sediment: water suspensions using a calibrated combined glass/reference electrode (Thomas, 1996). Organic carbon concentrations in the sediment samples were determined by wet oxidation with K2 Cr2 O7 and H2 SO4 (Allison, 1965). The proportions of sand, silt and clay-sized particles were determined by a sedimentation method (Day, 1965) following destruction of organic matter with hydrogen peroxide. 2.4. ‘T OTAL’

METAL ANALYSES

Metals were determined in sediment samples using an adaptation of an aqua regia digestion method (McGrath and Cunliffe, 1985). Duplicates of approximately 1 g of dry sediment were predigested overnight in 15 mL of aqua regia; sediment-acid mixtures were then digested at 140 ◦ C until near-dryness (ca. 5 hr). Digests were filtered (Whatman No. 42) into vials and diluted to known volume with deionised water. Cadmium, copper and lead were determined in diluted aqua regia digests by atomic absorption spectrophotometry using a Perkin Elmer 403 instrument with automatic background correction. 2.5. S EQUENTIAL

EXTRACTION OF METALS

Cr, Cu, Pb and Zn were fractionated in sediment samples collected in 1997 using an adaptation of the method of Shuman (1985) (refer to Table I). Following the extraction with Mg(NO3 )2 , sediment was rinsed with deionised water and air dried prior to sub-sampling for the remainder of the sequential extraction procedure. Metals were determined in extracts by ICP-MS (Perkin-Elmer Elan 6000) following dilution with deionised water to a total dissolved solids concentration of less than 0.5%. This extraction scheme does not directly address carbonate-bound metals, which were expected to be co-extracted by the Mn oxide-bound extractant (0.1 mol L−1 NH2 OH·HCl), which was the first acidic (pH 2) extractant in the sequence. 2.6. N UMERICAL

METHODS

Correlation analyses, single and multiple regression analyses, and one-way analyses of variance were performed using Microsoft Excel software.

Step

Metal fraction

Extracting solution

Mass of sediment (g)

Volume of solution (mL)

Conditions

1 2

Exchangeable Organically bound

10 1

40 20

Shake 2 hr Shake 16 hr

3

Mn oxide bound

1

20

Shake 30 min

4

Bound to amorphous Fe oxides Bound to crystalline Fe oxides

Mg(NO3 )2 , 1 mol L−1 0.1 mol L−1 Na2 P4 O7 + 0.1 mol L−1 NaOH 0.1 mol L−1 NH2 OH·HCl, pH 2 0.2 mol L−1 (NH4 )2 C2 O4 + 0.2 mol L−1 H2 C2 O4 as step 4 + 0.1 mol L−1 ascorbic acid

1

50

Shake 4 hr in dark

1

50

30 min, 98 ◦ C

5


TABLE I Sequential extraction scheme for fractionation of Cr, Cu, Pb and Zn in sediments (after Shuman, 1985)

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TABLE II Summary of metal concentrations and other chemical properties in sediment samples

Mean (n = 25) Standard deviation Minimum Maximum Median 10th percentile 90th percentile

Sediment metal concentration (mg kg−1 )

pH

%

Cd

Cu

Pb

pH

OCa

clay

silt

sand

0.32 0.23 0.00 0.94 0.226 0.09 0.60

30.4 53.5 2.6 296.9 19.2 5.0 44.3

47.1 44.1 4.9 184.1 28.8 10.7 110.8

6.60 0.96 4.63 7.87 6.96 5.35 7.58

2.11 1.53 0.72 5.84 1.49 0.78 4.09

10.0 7.7 1.5 31.0 7.6 3.4 20.4

7.8 11.3 0.0 47.5 3.9 0.7 19.6

82.2 14.9 38.8 96.3 89.6 61.8 95.1

a Organic carbon.

3. Results Sediment metal concentrations and relevant chemical properties are summarized in Table II. The predominantly sandy nature of the sediments is evident from Table II. There was a statistically sufficient range of values to relate sediment properties (pH, and organic carbon, clay, silt and sand contents) with metal concentrations by means of regression analysis. Examination of histograms of the distribution of metal concentrations and sediment properties showed that Cd, Cu and Pb concentrations, and silt content, had approximately symmetrical distributions when log-transformed. Regressions involving these variables were therefore performed using log10 values rather than with untransformed data. Table III shows that metal concentrations in the sediment samples were significantly correlated with one another. Cadmium concentrations showed no significant correlations with other sediment properties. Copper concentrations were positively correlated with organic carbon content and silt content, and negatively correlated with sand content. Lead concentrations were positively correlated with sediment pH. No metal concentration showed a dependence on distance downstream. Multiple regressions did not improve the ability to predict metal concentrations from sediment properties, relative to the single-parameter regressions from which correlation coefficients were derived. Of all the sediment properties examined, only Pb concentration and pH were affected by the proximity to a stormwater drain outfall (Table IV). Both Pb concentration and pH were significantly higher adjacent to drain outfalls than in sediments distant from drains. Metal fractionation results were somewhat variable, with a general trend for all metals to be found predominantly in the residual fraction, with mean values of

log10 [Cd] log10 [Cu] log10 [Pb] pH OCa (%) %clay log10 (%silt) % sand kmb a b c d e

log10 [Cd]

log10 [Cu]

log10 [Pb]

pH

OCa

% clay

log10 (% silt)

% sand

kmb

1

0.554d 1

0.508d 0.584d 1

0.181 0.112 0.429c 1

0.221 0.527d 0.092 –0.232 1

–0.067 0.281 –0.262 –0.278 0.423c 1

0.100 0.490d 0.272 0.023 0.695e 0.351 1

–0.108 –0.419c 0.044 0.147 –0.814e –0.669e –0.773e 1

–0.006 –0.030 0.265 0.522d –0.312 –0.535d –0.255 0.500d 1

Organic carbon. Distance downstream from sampling point 1. P ≤0.05. P ≤0.01. P ≤0.001.


TABLE III Matrix of correlation coefficients and significance of correlations between sediment metal concentrations and sediment chemical properties (n = 25)

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TABLE IV Effect of proximity to stormwater drains on metal concentrations and sediment properties

Adjacent to drain Not adjacent to drain Significance

Cd

Cu

Pb

0.320 0.244 ns

21.1 15.3 ns

54.0 17.1 p≤0.001

a Organic carbon. b Distance downstream from sampling point 1.

pH

% OCa

% clay

% silt

% sand

kmb

7.20 5.92 p≤0.001

1.8 2.4 ns

7.7 12.6 ns

7.6 28.1 ns

84.8 79.3 ns

15.6 11.0 ns

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Geometric mean concentration (mg kg−1 )

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TABLE V Proportions of metals (%) in sequential extraction fractions (mean (range) over 8 sites) Metal

Cu Pb Zn Cr

Exchangeable

0.7 (0.0–2.3) 0.0 (–) 1.7 (0.0–9.3) 0.8 (0.1–3.4)

Organic

1.7 (0.0–6.5) 0.4 (0.0–0.8) 0.7 (0.0–3.9) 0.8 (0.0–2.1)

Mn oxide

0.1 (0.0–0.6) 0.7 (0.2-1.8) 8.8 (1.5–43.6) 0.1 (0.0–1.2)

Amorphous Fe oxide

Crystalline Fe oxide

Residual

16.6 (2.6–34.3) 21.4 (7.7–49.6) 33.5 (4.7–81.0) 9.0 (0.8–22.9)

9.3 (1.8–19.7) 9.4 (3.4–21.1) 12.7 (0.0–33.3) 26.4 (5.9–70.2)

71.6 (48.7–92.2) 68.0 (38.2–87.7) 42.6 (2.3–81.4) 62.8 (6.7–91.9)

>60% of Cu, Pb and Cr reporting to this fraction (Table V). In contrast, approximately 43% of Zn was found in the residual fraction. The next most abundant fraction for Cu, Pb and Zn was the amorphous iron oxide fraction; crystalline iron oxides held more Cr than amorphous iron oxides. Iron oxide concentrations in sediments (amorphous + crystalline) were in the range 0.16–2.6%.

4. Discussion Cd concentrations were low, never exceeding the environmental investigation threshold for soils of 3 mg kg−1 (ANZECC/NHMRC, 1992). A similar situation was found for Pb; the relevant environmental investigation threshold for Pb is 300 mg kg−1 . In terms of current environmental guidelines, Cu was the only element which exceeded the investigation threshold value of 60 mg kg−1 , and this concentration was exceeded at only one site. In a study in a different part of the Swan estuary, Eyre and McConchie (Eyre and McConchie, 1993) found similar concentration ranges for the metals presented here, with the exception of very high metal concentrations near a marina (e.g. total Pb >1900 mg kg−1 ). Our measurements of metal concentrations also agree in magnitude with those measured recently in the Swan estuary by Gerritse et al. (1998b) and also with data reported from another similar estuarine system in Western Australia (Gerritse et al., 1998a). The high correlation found between sediment metal concentrations, however, might have been expected, given that input of heavy metals into the river sediments was likely to have occurred by similar mechanisms (urban runoff directly into the river, or via stormwater drains). The simultaneous accumulation of several heavy metals in estuarine sediments has been observed previously (Deely, 1993).

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Figure 2. Distribution of total Cu, Pb and Cd concentrations in near-shore Swan River estuarine sediments.


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The positive correlation of Cu concentrations in sediments with organic carbon content was expected on the basis of the known strong complexation between Cu and natural organic matter (Whitfield and Turner, 1987). The positive correlation of Cu concentrations with silt content and the negative correlation with sand content both reflect the role of finely-divided materials in retaining metals by surface reaction mechanisms (Bolt and van Riemsdijk, 1987), and the low trace element content of quartz sand. The positive correlation of Pb concentrations with sediment pH may reflect the tendency of cation adsorption to increase at high pH (Sposito, 1989; Gambrell, 1994). The finding that no metal concentration was related to distance downstream in this estuary may be related to the low level of metal contamination. This interpretation may be confounded, however, in this part of the Swan estuary because sediments become progressively sandier as distance downstream increases (Table III). Finer textured sediments upstream would have a tendency towards higher metal contents (Moore et al., 1989), and thus any cumulative effect of the urban area on metal concentrations in estuarine sediments may have been masked as a result. In addition, Birch (1996) found that sediment metal concentrations in estuaries in Sydney, Australia, decreased downstream from source due to dilution effects and the relative immobility of metals. It should also be noted that this study was performed on near-shore sediments rather than on deeper areas of the estuary where sediment accumulation might occur. Lead concentrations in sediments sampled adjacent to stormwater drain outfalls were significantly higher than those sampled remote from stormwater drains (Table IV), suggesting that Pb transported by stormwater was entering the estuary. Lead was likely to originate from traffic emissions and subsequent runoff from roads. Gerritse et al. (1998b) also attributed Pb concentrations in Swan estuary sediments to traffic runoff, based on the dates of Pb accumulation relative to other metals. Sediment pH, however, was also higher adjacent to stormwater drain outfalls, and it is possible that the higher Pb concentrations adjacent to drain outfalls were controlled by sediment pH rather than drain proximity. In this case, however, we would have expected to observe higher concentrations of Cd and Cu as well, and thus it seems most likely that the elevated Pb concentrations do in fact represent stormwater Pb inputs. The higher pH values observed near stormwater outfalls may reflect removal of finer sediment particles by stream flow, leaving a coarse residue which was observed to include a substantial proportion of carbonaceous material in the form of mollusc shell fragments. A site-by-site comparison of lead fractions as a function of pH (Figure 3) showed a trend for Pb extractable by 0.1 M NH2 OH·HCl (pH 2) to increase with increasing pH, from 0.4% at pH 6.7 to 1.8% at pH 8.3. Although the trend may represent an increase in Pb associated with carbonates with increasing sediment pH, the increase is small and insufficient to account for the differences in Pb concentration in Table IV. Eyre and McConchie (1993) found substantial proportions of Cu, Zn and Pb in forms other than residual; their three-stage sequential extraction procedure, how-

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Figure 3. Proportions of lead in sequential extraction fractions as a function of sediment pH.

ever, differed from that used in this study so results are not strictly comparable. Their extractant for ‘bioavailable’ metals was 30% HCl at 60 ◦ C, which would be likely to incorporate most of the fractions in this work apart from exchangeable (extracted separately) and residual. Partial extractions with dilute acid extract metals bound by Fe oxides (Chao, 1984), and the substantial amounts of metals bound by Fe oxides in this study may therefore corresponds well with the findings of Eyre and McConchie (1993). Most Cr in their study was in a residual form, as in this work. In contrast, and as in this work, ‘acid extractable’ Zn and Pb concentrations were substantially lower than comparable total concentrations in the Peel Inlet and Harvey Estuary, Western Australia (Gerritse et al., 1998a).

5. Conclusions The significant correlations between sediment metal concentrations in the Swan River estuary suggest a common source and transport mechanism for these contaminants. High Pb concentrations adjacent to stormwater drain outfalls were positively correlated with sediment pH, but were more likely to be a consequence of heavy metal input to river sediments in stormwater. Commonly accepted capacitycontrolling parameters (sediment pH, organic matter content or particle size distribution) could not explain all the variability in sediment metal concentrations, but the significant relationships that were observed correspond the known behaviour of heavy metals in sediments. Chemical fractionation of the sediment suggested that most of the metals were in non-bioavailable form. In addition, total metal concentrations were low, in particular for cadmium. For this reason, there is unlikely to be any significant impact on the estuarine ecosystem.


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