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Partitioning of trace elements in contaminated estuarine sediments: The role of environmental settings
Ecotoxicology and Environmental Safety 110 (2014) 246–253
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Ecotoxicology and Environmental Safety journal homepage: www.elsevier.com/locate/ecoenv
Partitioning of trace elements in contaminated estuarine sediments: The role of environmental settings Mohmmad M. Shaike n, Bibhash Nath, Gavin F. Birch School of Geosciences, University of Sydney, Sydney, NSW 2006, Australia
art ic l e i nf o
a b s t r a c t
Article history: Received 15 July 2014 Received in revised form 3 September 2014 Accepted 5 September 2014
Estuarine sedimentary environments safeguard aquatic ecosystem health by attenuating and transforming catchment-derived contaminants. Currently these environments are under severe stress from trace element contamination due to urbanization. Sediments of Sydney estuary (Australia) are highly elevated in a range of metals due to a long period of intense urbanization and industrialization, which has had a considerable influence on coastal ecosystem health and functioning. A three-stage sequential procedure following Bureau Communautaire de Référence (Community Bureau of Reference—BCR) technique was applied to sediments collected from Sydney estuary to determine their quality, elemental partitioning and ecosystem risk in three human-impacted environmental settings (i.e., mangrove-dominated, stormwater-dominated and industrial-dominated sites) and a control site in this coastal ecosystem. In all three environmental settings, Pb and Zn concentrations exceeded Australian Interim Sediment Quality Guidelines-High (ISQG-High) values and were mostly associated with the reducible and acid soluble fractions, respectively. Copper and Cr also exceeded ISQG-High values (especially in the industrial-dominated site), however the majority of these metals were associated with the oxidizable fraction. Arsenic and Ni concentrations were mostly below ISQG-High values (except one of the stormwater-dominated sites) and were associated with the residual fraction. These results suggest that the most easily mobilized metal was Zn followed by Pb and these metals together presented a risk to estuarine ecosystems in the three selected environmental settings. However, these metals are not always the most abundant in tissue of mangroves, oysters or prawns suggesting other mechanisms are important in a complex uptake process. & 2014 Elsevier Inc. All rights reserved.
Keywords: Sydney estuary Sediments Trace elements Sequential extraction Environmental settings
1. Introduction Sydney estuary has been subjected to considerable anthropogenic stress for a prolonged period due to extensive urbanization and industrialization (Birch et al., 2013a). Sediments in the estuary are markedly elevated in metals (Birch and Taylor, 1999), nutrients (Birch et al., 1999), polycyclic aromatic hydrocarbons (McCready et al., 2000, 2003) and organochlorine compounds (Birch and Taylor, 2000, 2002a) and the waterway is classified as “severely modified” by the National Audit of estuaries (NLWRA) (OzCoast, 2000; NLWRA, 2001). Metal bioavailability (McCready et al., 2003, 2004), sediment quality (Birch and Taylor, 2002a–c) and toxicity (Birch et al., 2008) investigations have been completed. Results indicated that although metals were highly bioavailable using weak extraction techniques, response to a range of toxicity tests was highly varied and ecological risk posed by dissolved metals was low (Simpson et al., 2002). Metal uptake by oysters, mussels
n
Corresponding author. E-mail address: [email protected] (M.M. Shaike).
http://dx.doi.org/10.1016/j.ecoenv.2014.09.007 0147-6513/& 2014 Elsevier Inc. All rights reserved.
and prawns was also highly variable and was not always related to sediment metal concentrations (Birch and Apostolatos, 2013; Birch et al., 2013b, 2014; Lewtas et al., 2014). A deeper understanding of the risks posed by trace elements may be gleaned by determining the major phases with which sedimentary contaminants are associated (Luoma, 1983). Many selective and/or sequential extraction procedures have been used to provide information on chemical phases in estuarine/marine sediments. Zimmerman and Weindorf (2010) provided a detailed review of the few commonly followed sequential extraction procedures for various types of sediments and formation conditions. A sequential extraction procedure (SEP) was developed to differentiate organic- and pyrite-bound trace elements in acidic wetland sediments (Sohlenius and Öborn, 2004; Claff et al., 2010). Galán et al. (1999) developed a procedure to remove poorly crystalline Fe oxy-hydroxides in soils heavily contaminated by acid mine drainage. However, after considering sediment composition, metal contents, contamination levels and environmental settings, we decided to follow the procedure detailed by Zhou et al. (2011). The procedure is best known as the Bureau Communautaire de Référence (Community Bureau of Reference—
M.M. Shaike et al. / Ecotoxicology and Environmental Safety 110 (2014) 246–253
BCR) technique which is the method accepted by the European Community (Ure et al., 1993; Rauret et al., 1999). The objectives of the current study were to determine the major phases with which trace element contaminants are associated in sediments of different environmental settings and to assess these data to determine potential adverse biological effects. Data from the present study will give an indication of the stability of anthropogenic trace elements in the major environmental settings of Sydney estuary and to other estuaries with similar anthropogenic stress.
2. Materials and methods 2.1. Selection of study sites The current investigation focuses on three human-impacted environmental settings of Sydney estuary, i.e., a mangrove-dominated, a stormwater-dominated and an industrial-dominated site. Sampling sites were selected in each of these environments on the basis of metal contamination status (especially Cu, Pb and Zn concentrations) and similar sedimentological (grain size) characteristics (Birch et al., 2013a). Sydney estuary is historically dominated by extensive, moderate to dense strands of intertidal mangrove forests along the coastal fringes of major embayments (Burchett et al., 1984; McLoughlin, 2000). A human-impacted site in Homebush Bay (SC 1) and an impacted site in Hen and Chicken Bay (SC 4) were selected as these sites are surrounded by dense mangrove strands and are both mantled by fine-grained sediment (Fig. 1). Stormwater-dominated sites were selected in Glades Bay (SC 2) on the north side of the estuary and near the mouth of Hawthorne Canal in Iron Cove (SC 5), the site highly enriched in Cu, Pb and Zn (Birch et al., 2013a). Approximately 86 percent of the catchment area of the estuary is urbanized and industrialized and many industrial-dominated areas have been identified previously
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in this estuary (Birch and Taylor, 2000). A site adjacent to the former ‘Austral Bronze’ factory in Hen and Chicken Bay (SC 3) was selected as a Cu-enriched location, while another site opposite to the ‘old Balmain Power Station’ (SC 6) was chosen as a Pb and Znenriched site (Birch et al., 2013a). In addition, a site dominated by fine sediments and low metal concentrations was selected as a ‘control’ site (SC 7) in the Middle Harbour (Birch et al., 2013a). 2.2. Field sampling One sediment core (up to 50 cm long) was taken at each site (total seven cores) using a plastic push-corer (Birch et al., 2011). Intact sediments cores were sealed and stored in airtight, metalfree polyethylene bags and taken to the laboratory on ice and stored at 4 1C prior to pre-treatment to reduce the effects of microbiological activity. 2.3. Sample preparation Cores were split in half, logged and a representative sample taken from 0–4, 14–18, 26–30 and 34–38 cm sediment depths. Samples were air-dried at 25 1C for 48 h and ground to fine powder with a mortar and a pestle for metal extractions. A portion of the sample was wet-sieved using 62.5 mm nylon mesh for the determination of sediment texture, i.e., percentages of mud (o62.5 mm), sand (62.5–2000 mm) and gravel ( 42000 mm) (Birch, 2003). 2.4. Sediment digestion Trace element extractions (‘near-total’) were carried out on airdried and ground sediment samples. Samples weighing 0.4 g were digested with a 4 ml mixture of binary acid solution (1:1 HNO3:HCl) and 10 ml ultra-purity water (Birch et al., 2011) followed by heating at 120 1C in a block digestion system for
Fig. 1. Map of the study area showing sampling locations in three contaminated environmental settings and a control site in Sydney estuary (Australia). SC 1 and 4 represent mangrove-dominated sites, SC 2 and 5 represent stormwater-dominated sites and SC 3 and 6 represent industrial-dominated sites. The control site is in Middle Harbour represented by SC 7.
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Table 1 Laboratory conditions for Bureau Communautaire de Référence (Community Bureau of Reference—BCR) sequential extraction procedure. Steps Chemical associations
Laboratory procedures
1
1 g of fresh samples. Added 20 ml of 0.11 M CH3COOH, shake for 16 h at room temperature followed by centrifugation at 3500 rpm for 10 min to collect supernatant for later analysis. The residue was transferred to step 2 procedure. Added 20 ml of 0.5 M NH2OH.HCl (pH adjusted to 1.5 with HNO3), shake for 16 h at room temperature followed by centrifugation at 3500 rpm for 10 min to collect supernatant for later analysis. The residue was transferred to step 3 procedure. Added 5 ml of 30% H2O2, left to react for 2 h at room temperature, followed by heating in a water bath at 85 1C for 1 h, added another 5 ml of 30% H2O2 and heated in a water bath at 85 1C for 1 h. Finally added 25 ml of 1 M CH3COONH4 (pH adjusted to 2 with HNO3), shake for 16 h at room temperature followed by centrifugation at 3500 rpm for 10 min to collect supernatant for later analysis. The residue was transferred to step 4 procedure. Hot binary acid digestion (1:1 HNO3:HCl), added 4 ml, heated in a block digestion system at 120 1C for 2.5 h or until near dryness. Adjusted to volume to 30 ml and left overnight to settle and finally stored for later analysis.
Water soluble, exchangeable and carbonate bound fractions Oxide bound fractions (mainly Fe- and Mn-oxides) Organic matter and sulphide bound fractions
2 3
4
Residual fractions
Note: all reagents used were of analytical grade; step 4 was an addition to the BCR procedure (after Ure et al., 1993 and Rauret et al., 1999) to obtain residual fractions.
Table 2 Trace element concentrations in bulk sediment fractions of three contaminated environmental settings in Sydney estuary (Australia) and at the control sites. Parameters (n¼8)
As
Cd
Co
Cr
Cu
Ni
Pb
Zn
Mangrove settings range mean sd
23–57 40 15
0.78–14 3.5 4.2
8.5–14 12 1.7
53–250 163 62
66–600 261 228
16–51 26 11
290–2871 1248 1129
409–1272 814 329
Stormwater settings range mean sd
19–215 62 72
bdl-2.1 1.8 0.23
bdl-23 12 5.0
71–732 253 212
98–287 200 56
15–169 47 51
204–3791 981 1365
393–961 745 189
Industrial settings range mean sd
25–44 32 5.9
0.82–4.9 2.6 1.8
8.4–11 9.6 0.88
73–214 123 54
339–765 481 165
17–33 25 5.6
130–1644 755 601
343–1875 1213 600
Control sites range mean sd
5.1–21 13 6.4
bdl-0.80 0.88 na
bdl-6.3 6.4 na
5.9–20 15 6.2
12–24 17 5.8
2.7–9.1 6.9 2.9
39–107 67 32
36–153 91 48
ISQG-Lowa ISQG-Higha
20 70
1.5 10
na na
80 370
65 270
21 52
50 220
200 410
Note: All data are in mg/g (dry weight); bdl—below detection limit; na—not available; sd—standard deviation. a
Australian Interim Sediment Quality Guidelines Values (ANZECC and ARMCANZ, 2000).
approximately 2.5 h, or until a near-dry condition (modified USEPA method 200.8 Rev 4.4 methods; USEPA, 1994). Digested samples were cooled to room temperature and made up to 30 ml with ultra-high purity water (18.2 mΩ cm). Solutions were left overnight to settle and solute was transferred to 10 ml polypropylene tubes and stored at 4 1C until analysis. 2.5. Sequential extraction procedure A three-step sequential extraction procedure, widely known as the Bureau Communautaire de Référence (Community Bureau of Reference—BCR) technique, used in this work (Ure et al., 1993; Rauret et al., 1999) is commonly employed for partitioning of trace elements in estuarine sediments. The chemical phases targeted were: water soluble, exchangeable and carbonate bound phases (step 1); oxide bound phases (especially Fe- and Mn-oxides) (step 2); and organic and sulfide-bound phases (step 3). At the end of step 3, the residue was digested with a 4 ml mixture of binary acid solution (1:1 HNO3:HCl) (see Section 2.4 for detailed procedure) to obtain residual phases which was designated as step 4 (Table 1). 2.6. Data quality The digested samples (both ‘near-total’ and sequential phases) were analyzed for trace elements (As, Cd, Co, Cr, Cu, Ni, Pb and Zn)
using a Varian Vista AX CCD simultaneous ICP–OES. Two blanks and a reference material (AGAL-10) and triplicates of an unknown sample per batch of 20 samples were used to measure background laboratory contamination, accuracy and precision of analytical methods (Nath et al., 2013). Analytical accuracy, reported as recovery, was 95 percent to 105 percent and precision was o5 percent relative standard deviation for all elements. No procedural contamination was detected. The recovery during the sequential extraction procedure was calculated from ‘near-total’ metal concentrations obtained through hot acid digestion using the following equation: percent recovery¼ [(Σ of metals in step 1 þstep 2 þstep 3 þstep 4)/‘neartotal’ metals in hot acid digestion] 100. The recoveries were between 95 and 110 percent, which is within the acceptable range for estuarine sediments (Li et al. 2007; Canuto et al., 2013).
3. Results 3.1. Trace element concentrations in environmental settings Mean As, Co, Cr and Ni concentrations in core sediments in the environmental settings follow the trend: stormwater-dominated site4mangrove-dominated site4industrial-dominated site, Cu and Zn follow the reverse trend (Table 2). Arsenic concentrations gradually decreased up core and showed maxima at intermediate depths
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Fig. 2. Trace element concentrations in core sediments (at 0–4, 14–18, 26–30 and 34–38 cm depths) of three contaminated environmental settings (mangrove-SC 1 and 4, stormwater-SC 2 and 5, and industry-SC 3 and 6) of Sydney estuary (Australia) and at the control site (SC 7). Australian Interim Sediment Quality Guidelines (ISQG) values are shown (ANZECC and ARMCANZ, 2000). Green dotted lines are for ISQG-Low and red dotted lines are for ISQG-High values of respective elements. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. Trace element partitioning (step 1—acid soluble, step 2—reducible, step 3—oxidizable and step 4—residual fractions) in core sediments (at 0–4, 14–18, 26–30 and 34–38 cm depths) of three contaminated environmental settings (mangrove-SC 1 and 4, stormwater-SC 2 and 5, and industry-SC 3 and 6) of Sydney estuary (Australia) and at control sites (SC 7). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
(Fig. 2). Cobalt and Cr also decreased up core and showed greater concentrations at intermediate and deeper depths. Nickel concentrations increased up core in a mangrove-dominate site (SC 1) while remainder of the sites had gradual decreasing trends. Both Cu and Zn concentrations increased up core in both mangrove-dominated sites and industrial-dominated sites (Fig. 2). Lead concentrations follow: mangrove-dominated site4stormwater-dominated site4industrialdominated site trend. Lead concentrations mostly decreased up core in all the environmental settings except one industrial-dominated site (SC 3). Cadmium concentrations were highest in mangrove-dominated sites followed by industrial- and stormwater-dominated sites. Cadmium concentrations gradually decreased up core with an exception of a mangrove-dominated site which contained 14 mg/g of Cd at intermediate depths (Fig. 2). Copper concentrations in sediments exceeded ISQG-High values in both industrial-dominated sites and three samples from the mangrove-dominated site (SC 4), however stormwater-
dominated sites remained below ISQG-High values, but were above ISQG-Low values. Lead and Zn concentrations exceeded ISQG-High values in all three environmental settings. Concentrations of As, Cd, Cr and Ni were mostly below ISQG-High and above ISQG-Low values with the exception of the stormwater-dominated sites where few samples had values above ISQG-High for As, Cr and Ni (Fig. 2). Additionally, a sample from a mangrove-dominated site had Cd concentrations above ISQG-high values. No ISQG guidelines are available for Co, however, distributions in the different environmental settings were similar. The concentrations of all metals in control sites (i.e., in Middle Harbour) were below ISQG-Low values, except Pb concentration in two samples (Fig. 2). 3.2. Partitioning of trace elements in environmental settings The concentrations of trace elements in different stages of sequential extraction are shown in Fig. 3. The concentrations of As and Ni
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were highest in the residual fraction and lowest, although mostly not measureable were in the acid soluble fraction (i.e., bound to water soluble, exchangeable and carbonate phases) for As and in the reducible fraction (i.e., bound to Fe-/Mn-oxides) for Ni in three environmental settings. The concentrations of partitioned As and Ni in all environmental settings gradually decreased up core, except Ni in mangrove site SC 1 which increased up core. In the control site, the highest concentrations of As and Ni were in the oxidizable fraction (i.e., bound to organic matter and sulphides) and lowest in the reducible fraction and were mostly not measureable for As and in the acid soluble fraction for Ni. The highest concentrations of Cr and Cu in three environmental settings were in the oxidizable fraction and lowest concentration in the acid soluble fraction for Cr and in the residual fraction for Cu. The concentrations of partitioned Cu were maximum at intermediate depths in SC 3 for all phases. The concentrations of reducible Cu were increased up core in SC 4 while in SC 1 the partitioned Cu concentrations were increased up core for acid soluble, oxidizable and residual fractions. Chromium showed maximum concentrations at intermediate depths in SC 1, 3 and 4 for reducible fractions, oxidizable and residual fractions. At the control site, the highest concentration of Cr was in the residual fraction and Cu in the oxidizable fraction, while the lowest concentration was in the acid soluble fraction though mostly not measureable for these metals. The highest concentration of Pb in three environmental settings and in the control site was in the reducible fraction, while the lowest concentration of Pb in three environmental settings was in the residual fraction, however control site samples had the lowest concentrations in the acid soluble fraction. The concentrations of partitioned reducible Pb in SC 3 and 4 increased up core while decreased up core in rest of the environmental settings. The highest concentration of Zn was in the acid soluble fraction and the lowest concentrations were in the residual fraction in three environmental settings, whereas control site samples showed the highest concentrations in the reducible fraction and the lowest concentration in the oxidizable fraction. The concentrations of partitioned Zn increased up core in mangrove-dominated sites, especially acid soluble fractions in SC 1 and oxidizable fractions in SC 4 while in other settings the concentrations were at their highest levels at intermediate depths or at the bottom of the cores. There were no reliable data for Cd and Co due to low ‘near-total’ concentrations in these sediments.
4. Discussion 4.1. Sediment quality and toxicity in environmental settings The presence of high concentration of trace elements in sediments, especially Cu, Pb and Zn above ISQG-high values, has been previously reported throughout major embayments of Sydney estuary, irrespective of the environmental settings (Birch and Taylor, 1999). Birch et al. (2013a) reported that the contemporary catchment surface processes are mainly responsible for the present-day elevated metal concentrations in Sydney estuary. The embayments studied are the most contaminated locations in Sydney estuary due to poor industrial practice in the past and their legacy remains, especially in the industrial-dominated sites (Birch and Taylor, 1999). The source of Cu, Pb and Zn in the studied embayments of the Sydney estuary was likely to be derived from industrial activities and that has been highlighted through a significant increase in up-core accumulations in the industrialdominated site (i.e., in core SC 3). The presence of high-metal concentrations in mangrove-dominated sites result from significant accumulation of fine-grained sediments (o 62.5 mm, mean: 88 percent) and the metals were mainly sourced from a highly developed catchment in proximity to this site. Fine sediments generally have a strong capacity to sequester land-derived metal
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contaminants and these environments were regarded as highly beneficial in their ability to accumulate metals and safeguard coastal ecosystems (Harbison, 1986; Saenger et al., 1991). The present-day increase of Cu and Zn in both mangrove-dominated sites was likely to be derived directly from either anthropogenic sources or by re-suspension and remobilization of contaminated sediments. Currently, the major source of metals to this estuary has changed from industrial to stormwater (Birch et al., 2013a) which has significantly contributed to elevated metal concentrations in stormwater-dominated sites, especially Zn in SC 2 which increased up core. High concentrations of As, Co, Cr and Ni in stormwater-dominated environments are likely to be derived from residential and road land uses (Birch et al., 2013a). The presence of significant Pb levels in the mangrove-dominated site at the deeper levels resulted from prior industrial activities in this bay (Birch et al., 2013a). Sedimentary metals (‘near-total’ concentrations) are assessed to indicate potential adverse biological effects on flora in Sydney estuary (ANZECC and ARMCANZ, 2000). Sediment metal concentrations at the control site located in the least developed and impacted region of Sydney estuary in Middle Harbour were mostly below ISQG-Low values and are unlikely to cause adverse biological effects to flora. Lead (Pb) and Zn concentrations exceeded ISQG-High values in the three environmental settings and pose a high risk of adverse biological effects (Birch et al., 2013a; Nath et al., 2014). Copper concentrations in industrial-dominated settings and a mangrove-dominated site were above ISQG-High values suggesting a high risk of adverse biological effects in this environment. The increasing up-core concentrations of Cu, Pb and Zn above ISQG-High values in Hen and Chicken Bay (i.e., core sites SC 3 and 4) suggests considerable ecological risk in this embayment. Arsenic, Cr and Ni concentrations were mainly below ISQGHigh values, suggesting minor ecological risk. 4.2. Trace element bioavailability in environmental settings The mobility and bioavailability of metals are partly determined by chemical speciation (Yuan et al., 2004; Zhou et al., 2010). Generally, the anthropogenic fraction of metals in the sediments are present in most labile forms (Cuong and Obbard, 2006; Canuto et al., 2013). Similarly, our results showed a large proportion of only Zn to be associated with the acid soluble fraction, whereas Cu and Pb were mostly associated with the oxidizable and reducible fractions, respectively in all environmental settings. The likely origin of these metals was industrial activities in the past and contemporary stormwater loadings. Arsenic and Cr were mostly associated with the oxidizable and residual fractions, while Ni was evenly distributed between the four fractions. Approximately 60 percent of Zn in these sediments are associated with the water soluble, exchangeable and carbonate-bound fractions and is the most mobile and bioavailable metal in all three environmental settings. The data show that no particular environmental setting controls trace element partitioning, rather that environment controls trace element contents. Li et al. (2007) reported 40 percent of Zn in water soluble, exchangeable and carbonate bound fractions in sediments collected from three different environments, i.e., mudflat, mangrove swamp and sewage outlet wetland in Pearl River Estuary, China. Passos et al. (2010) reported 430 percent of Zn in water soluble, exchangeable and carbonate bound fractions in sediments of Poxim River estuary, Sergipe State, northeast Brazil. They suggested that the sources of these metals were likely due to the presence of anthropogenic material. Copper was mainly (approximately 50–60 percent) bound to organic and sulphide phases and their partitioning increased up core in mangrove- and industrial-dominated settings of Hen and Chicken Bay. However, liberation of Cu in this estuary will require a change in the redox
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potential, e.g., re-suspension and oxidation of the sulfidic sediments into overlying oxic waters (Morgan et al., 2012), possibly during storms or dredging activities. Additionally, the presence of significant amount of Cu in oxidizable fractions of three settings is mainly due to its affinity for organic matter and sulfidic phases to form metal sulfides. Many monosulfides, except Cu, are dissolved in the first fraction owing to slight acidity and presence of minor oxygen and much of the environmentally relevant sulfidizedmetal fraction is extracted before the ‘organic/sulfidic’ fraction, whereas the later sulfidic fraction is possibly pyritised metal (Passos et al., 2010). The majority of Pb (about 60–70 percent) was associated with Fe and Mn oxides suggesting formation of stable phases of Pb in sediments (Ramos et al., 1994) and would therefore require the environment to become more reducing to be mobilized, possibly by deep burial and increase in the reducing environment. Most As and Ni in the sediments of the three environmental settings were associated with residual fractions followed by oxidizable fractions, which suggested these metals tend to accumulate in the refractory metal-organic compounds and are moderately stable in these environments. 4.3. Environmental consequences Higher bioavailability of metals in estuarine sediments is of considerable environmental significance due to potential bioaccumulation and toxicity to plants and biota (Luoma, 1983; John and Leventhal, 1996). Significant levels of acid soluble Cu in industrial(SC 3) and mangrove-(SC 4) dominated sites of Hen and Chicken Bay increase ecological risks at this embayment as does a high proportion of acid soluble and reducible Pb at intermediate depths in Homebush Bay mangrove-dominated site (SC 1) and two core sites (SC 5 and 6) in Iron Cove. Arsenic, Cr and Ni are unlikely to be an ecological risk given the low concentrations in the acid soluble fraction. However, a slight change in environmental conditions, such as Eh, pH or salinity, could potentially increase bioavailability of these metals (Passos et al., 2010). Zinc poses a very high ecological risk due to the high (54 to 65 percent) proportion of acid soluble fractions in surficial sediments in the three environmental settings. Previous studies in Sydney estuary have shown that bioaccumulation of Cu is moderately high in mangrove pneumatophores (Nath et al., 2014) and fine nutritive roots (Chaudhuri et al., 2014) and is moderately elevated in tissue of oysters (Birch et al., 2013b) and prawns (Lewtas et al., 2014), whereas Zn is moderately enriched in all four of these media. Lead has moderate to low bioaccumulation in mangrove tissue and is in low concentrations in oyster and prawn tissue. These bioaccumulation patterns of metals in different environmental media suggest a complex interplay of metal bioavailability and a complex uptake mechanism.
5. Conclusion Lead and Zn concentrations in core sediment in all three environmental settings investigated exceeded ISQG-High values and most of these metals were associated with the reducible and acid soluble fractions, which suggest increased mobility and bioavailability and increased environmental risk. Copper and Cr in these estuarine sediments exceeded ISQG-High values and were mostly associated with the oxidizable fraction, followed by reducible fraction, indicating reduced mobility and bioavailability and a lower ecological risk. The mean concentrations of As and Ni were mostly below ISQG-High values and the majority of these metals are associated with the residual phase suggesting minor environmental risk. The sources of trace elements in the Sydney estuary were derived from industrial activities with concomitant increase
through present-day stormwater loadings and re-suspension and remobilization of contaminated sediments.
Acknowledgments This research was partly supported by the University of Sydney postdoctoral fellowship scheme (Grant no. G135714). We thank Tom Savage for technical assistance.
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Partitioning of trace elements in contaminated estuarine sediments: The role of environmental settings Mohmmad M. Shaike n, Bibhash Nath, Gavin F. Birch School of Geosciences, University of Sydney, Sydney, NSW 2006, Australia
art ic l e i nf o
a b s t r a c t
Article history: Received 15 July 2014 Received in revised form 3 September 2014 Accepted 5 September 2014
Estuarine sedimentary environments safeguard aquatic ecosystem health by attenuating and transforming catchment-derived contaminants. Currently these environments are under severe stress from trace element contamination due to urbanization. Sediments of Sydney estuary (Australia) are highly elevated in a range of metals due to a long period of intense urbanization and industrialization, which has had a considerable influence on coastal ecosystem health and functioning. A three-stage sequential procedure following Bureau Communautaire de Référence (Community Bureau of Reference—BCR) technique was applied to sediments collected from Sydney estuary to determine their quality, elemental partitioning and ecosystem risk in three human-impacted environmental settings (i.e., mangrove-dominated, stormwater-dominated and industrial-dominated sites) and a control site in this coastal ecosystem. In all three environmental settings, Pb and Zn concentrations exceeded Australian Interim Sediment Quality Guidelines-High (ISQG-High) values and were mostly associated with the reducible and acid soluble fractions, respectively. Copper and Cr also exceeded ISQG-High values (especially in the industrial-dominated site), however the majority of these metals were associated with the oxidizable fraction. Arsenic and Ni concentrations were mostly below ISQG-High values (except one of the stormwater-dominated sites) and were associated with the residual fraction. These results suggest that the most easily mobilized metal was Zn followed by Pb and these metals together presented a risk to estuarine ecosystems in the three selected environmental settings. However, these metals are not always the most abundant in tissue of mangroves, oysters or prawns suggesting other mechanisms are important in a complex uptake process. & 2014 Elsevier Inc. All rights reserved.
Keywords: Sydney estuary Sediments Trace elements Sequential extraction Environmental settings
1. Introduction Sydney estuary has been subjected to considerable anthropogenic stress for a prolonged period due to extensive urbanization and industrialization (Birch et al., 2013a). Sediments in the estuary are markedly elevated in metals (Birch and Taylor, 1999), nutrients (Birch et al., 1999), polycyclic aromatic hydrocarbons (McCready et al., 2000, 2003) and organochlorine compounds (Birch and Taylor, 2000, 2002a) and the waterway is classified as “severely modified” by the National Audit of estuaries (NLWRA) (OzCoast, 2000; NLWRA, 2001). Metal bioavailability (McCready et al., 2003, 2004), sediment quality (Birch and Taylor, 2002a–c) and toxicity (Birch et al., 2008) investigations have been completed. Results indicated that although metals were highly bioavailable using weak extraction techniques, response to a range of toxicity tests was highly varied and ecological risk posed by dissolved metals was low (Simpson et al., 2002). Metal uptake by oysters, mussels
n
Corresponding author. E-mail address: [email protected] (M.M. Shaike).
http://dx.doi.org/10.1016/j.ecoenv.2014.09.007 0147-6513/& 2014 Elsevier Inc. All rights reserved.
and prawns was also highly variable and was not always related to sediment metal concentrations (Birch and Apostolatos, 2013; Birch et al., 2013b, 2014; Lewtas et al., 2014). A deeper understanding of the risks posed by trace elements may be gleaned by determining the major phases with which sedimentary contaminants are associated (Luoma, 1983). Many selective and/or sequential extraction procedures have been used to provide information on chemical phases in estuarine/marine sediments. Zimmerman and Weindorf (2010) provided a detailed review of the few commonly followed sequential extraction procedures for various types of sediments and formation conditions. A sequential extraction procedure (SEP) was developed to differentiate organic- and pyrite-bound trace elements in acidic wetland sediments (Sohlenius and Öborn, 2004; Claff et al., 2010). Galán et al. (1999) developed a procedure to remove poorly crystalline Fe oxy-hydroxides in soils heavily contaminated by acid mine drainage. However, after considering sediment composition, metal contents, contamination levels and environmental settings, we decided to follow the procedure detailed by Zhou et al. (2011). The procedure is best known as the Bureau Communautaire de Référence (Community Bureau of Reference—
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BCR) technique which is the method accepted by the European Community (Ure et al., 1993; Rauret et al., 1999). The objectives of the current study were to determine the major phases with which trace element contaminants are associated in sediments of different environmental settings and to assess these data to determine potential adverse biological effects. Data from the present study will give an indication of the stability of anthropogenic trace elements in the major environmental settings of Sydney estuary and to other estuaries with similar anthropogenic stress.
2. Materials and methods 2.1. Selection of study sites The current investigation focuses on three human-impacted environmental settings of Sydney estuary, i.e., a mangrove-dominated, a stormwater-dominated and an industrial-dominated site. Sampling sites were selected in each of these environments on the basis of metal contamination status (especially Cu, Pb and Zn concentrations) and similar sedimentological (grain size) characteristics (Birch et al., 2013a). Sydney estuary is historically dominated by extensive, moderate to dense strands of intertidal mangrove forests along the coastal fringes of major embayments (Burchett et al., 1984; McLoughlin, 2000). A human-impacted site in Homebush Bay (SC 1) and an impacted site in Hen and Chicken Bay (SC 4) were selected as these sites are surrounded by dense mangrove strands and are both mantled by fine-grained sediment (Fig. 1). Stormwater-dominated sites were selected in Glades Bay (SC 2) on the north side of the estuary and near the mouth of Hawthorne Canal in Iron Cove (SC 5), the site highly enriched in Cu, Pb and Zn (Birch et al., 2013a). Approximately 86 percent of the catchment area of the estuary is urbanized and industrialized and many industrial-dominated areas have been identified previously
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in this estuary (Birch and Taylor, 2000). A site adjacent to the former ‘Austral Bronze’ factory in Hen and Chicken Bay (SC 3) was selected as a Cu-enriched location, while another site opposite to the ‘old Balmain Power Station’ (SC 6) was chosen as a Pb and Znenriched site (Birch et al., 2013a). In addition, a site dominated by fine sediments and low metal concentrations was selected as a ‘control’ site (SC 7) in the Middle Harbour (Birch et al., 2013a). 2.2. Field sampling One sediment core (up to 50 cm long) was taken at each site (total seven cores) using a plastic push-corer (Birch et al., 2011). Intact sediments cores were sealed and stored in airtight, metalfree polyethylene bags and taken to the laboratory on ice and stored at 4 1C prior to pre-treatment to reduce the effects of microbiological activity. 2.3. Sample preparation Cores were split in half, logged and a representative sample taken from 0–4, 14–18, 26–30 and 34–38 cm sediment depths. Samples were air-dried at 25 1C for 48 h and ground to fine powder with a mortar and a pestle for metal extractions. A portion of the sample was wet-sieved using 62.5 mm nylon mesh for the determination of sediment texture, i.e., percentages of mud (o62.5 mm), sand (62.5–2000 mm) and gravel ( 42000 mm) (Birch, 2003). 2.4. Sediment digestion Trace element extractions (‘near-total’) were carried out on airdried and ground sediment samples. Samples weighing 0.4 g were digested with a 4 ml mixture of binary acid solution (1:1 HNO3:HCl) and 10 ml ultra-purity water (Birch et al., 2011) followed by heating at 120 1C in a block digestion system for
Fig. 1. Map of the study area showing sampling locations in three contaminated environmental settings and a control site in Sydney estuary (Australia). SC 1 and 4 represent mangrove-dominated sites, SC 2 and 5 represent stormwater-dominated sites and SC 3 and 6 represent industrial-dominated sites. The control site is in Middle Harbour represented by SC 7.
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Table 1 Laboratory conditions for Bureau Communautaire de Référence (Community Bureau of Reference—BCR) sequential extraction procedure. Steps Chemical associations
Laboratory procedures
1
1 g of fresh samples. Added 20 ml of 0.11 M CH3COOH, shake for 16 h at room temperature followed by centrifugation at 3500 rpm for 10 min to collect supernatant for later analysis. The residue was transferred to step 2 procedure. Added 20 ml of 0.5 M NH2OH.HCl (pH adjusted to 1.5 with HNO3), shake for 16 h at room temperature followed by centrifugation at 3500 rpm for 10 min to collect supernatant for later analysis. The residue was transferred to step 3 procedure. Added 5 ml of 30% H2O2, left to react for 2 h at room temperature, followed by heating in a water bath at 85 1C for 1 h, added another 5 ml of 30% H2O2 and heated in a water bath at 85 1C for 1 h. Finally added 25 ml of 1 M CH3COONH4 (pH adjusted to 2 with HNO3), shake for 16 h at room temperature followed by centrifugation at 3500 rpm for 10 min to collect supernatant for later analysis. The residue was transferred to step 4 procedure. Hot binary acid digestion (1:1 HNO3:HCl), added 4 ml, heated in a block digestion system at 120 1C for 2.5 h or until near dryness. Adjusted to volume to 30 ml and left overnight to settle and finally stored for later analysis.
Water soluble, exchangeable and carbonate bound fractions Oxide bound fractions (mainly Fe- and Mn-oxides) Organic matter and sulphide bound fractions
2 3
4
Residual fractions
Note: all reagents used were of analytical grade; step 4 was an addition to the BCR procedure (after Ure et al., 1993 and Rauret et al., 1999) to obtain residual fractions.
Table 2 Trace element concentrations in bulk sediment fractions of three contaminated environmental settings in Sydney estuary (Australia) and at the control sites. Parameters (n¼8)
As
Cd
Co
Cr
Cu
Ni
Pb
Zn
Mangrove settings range mean sd
23–57 40 15
0.78–14 3.5 4.2
8.5–14 12 1.7
53–250 163 62
66–600 261 228
16–51 26 11
290–2871 1248 1129
409–1272 814 329
Stormwater settings range mean sd
19–215 62 72
bdl-2.1 1.8 0.23
bdl-23 12 5.0
71–732 253 212
98–287 200 56
15–169 47 51
204–3791 981 1365
393–961 745 189
Industrial settings range mean sd
25–44 32 5.9
0.82–4.9 2.6 1.8
8.4–11 9.6 0.88
73–214 123 54
339–765 481 165
17–33 25 5.6
130–1644 755 601
343–1875 1213 600
Control sites range mean sd
5.1–21 13 6.4
bdl-0.80 0.88 na
bdl-6.3 6.4 na
5.9–20 15 6.2
12–24 17 5.8
2.7–9.1 6.9 2.9
39–107 67 32
36–153 91 48
ISQG-Lowa ISQG-Higha
20 70
1.5 10
na na
80 370
65 270
21 52
50 220
200 410
Note: All data are in mg/g (dry weight); bdl—below detection limit; na—not available; sd—standard deviation. a
Australian Interim Sediment Quality Guidelines Values (ANZECC and ARMCANZ, 2000).
approximately 2.5 h, or until a near-dry condition (modified USEPA method 200.8 Rev 4.4 methods; USEPA, 1994). Digested samples were cooled to room temperature and made up to 30 ml with ultra-high purity water (18.2 mΩ cm). Solutions were left overnight to settle and solute was transferred to 10 ml polypropylene tubes and stored at 4 1C until analysis. 2.5. Sequential extraction procedure A three-step sequential extraction procedure, widely known as the Bureau Communautaire de Référence (Community Bureau of Reference—BCR) technique, used in this work (Ure et al., 1993; Rauret et al., 1999) is commonly employed for partitioning of trace elements in estuarine sediments. The chemical phases targeted were: water soluble, exchangeable and carbonate bound phases (step 1); oxide bound phases (especially Fe- and Mn-oxides) (step 2); and organic and sulfide-bound phases (step 3). At the end of step 3, the residue was digested with a 4 ml mixture of binary acid solution (1:1 HNO3:HCl) (see Section 2.4 for detailed procedure) to obtain residual phases which was designated as step 4 (Table 1). 2.6. Data quality The digested samples (both ‘near-total’ and sequential phases) were analyzed for trace elements (As, Cd, Co, Cr, Cu, Ni, Pb and Zn)
using a Varian Vista AX CCD simultaneous ICP–OES. Two blanks and a reference material (AGAL-10) and triplicates of an unknown sample per batch of 20 samples were used to measure background laboratory contamination, accuracy and precision of analytical methods (Nath et al., 2013). Analytical accuracy, reported as recovery, was 95 percent to 105 percent and precision was o5 percent relative standard deviation for all elements. No procedural contamination was detected. The recovery during the sequential extraction procedure was calculated from ‘near-total’ metal concentrations obtained through hot acid digestion using the following equation: percent recovery¼ [(Σ of metals in step 1 þstep 2 þstep 3 þstep 4)/‘neartotal’ metals in hot acid digestion] 100. The recoveries were between 95 and 110 percent, which is within the acceptable range for estuarine sediments (Li et al. 2007; Canuto et al., 2013).
3. Results 3.1. Trace element concentrations in environmental settings Mean As, Co, Cr and Ni concentrations in core sediments in the environmental settings follow the trend: stormwater-dominated site4mangrove-dominated site4industrial-dominated site, Cu and Zn follow the reverse trend (Table 2). Arsenic concentrations gradually decreased up core and showed maxima at intermediate depths
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Fig. 2. Trace element concentrations in core sediments (at 0–4, 14–18, 26–30 and 34–38 cm depths) of three contaminated environmental settings (mangrove-SC 1 and 4, stormwater-SC 2 and 5, and industry-SC 3 and 6) of Sydney estuary (Australia) and at the control site (SC 7). Australian Interim Sediment Quality Guidelines (ISQG) values are shown (ANZECC and ARMCANZ, 2000). Green dotted lines are for ISQG-Low and red dotted lines are for ISQG-High values of respective elements. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. Trace element partitioning (step 1—acid soluble, step 2—reducible, step 3—oxidizable and step 4—residual fractions) in core sediments (at 0–4, 14–18, 26–30 and 34–38 cm depths) of three contaminated environmental settings (mangrove-SC 1 and 4, stormwater-SC 2 and 5, and industry-SC 3 and 6) of Sydney estuary (Australia) and at control sites (SC 7). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
(Fig. 2). Cobalt and Cr also decreased up core and showed greater concentrations at intermediate and deeper depths. Nickel concentrations increased up core in a mangrove-dominate site (SC 1) while remainder of the sites had gradual decreasing trends. Both Cu and Zn concentrations increased up core in both mangrove-dominated sites and industrial-dominated sites (Fig. 2). Lead concentrations follow: mangrove-dominated site4stormwater-dominated site4industrialdominated site trend. Lead concentrations mostly decreased up core in all the environmental settings except one industrial-dominated site (SC 3). Cadmium concentrations were highest in mangrove-dominated sites followed by industrial- and stormwater-dominated sites. Cadmium concentrations gradually decreased up core with an exception of a mangrove-dominated site which contained 14 mg/g of Cd at intermediate depths (Fig. 2). Copper concentrations in sediments exceeded ISQG-High values in both industrial-dominated sites and three samples from the mangrove-dominated site (SC 4), however stormwater-
dominated sites remained below ISQG-High values, but were above ISQG-Low values. Lead and Zn concentrations exceeded ISQG-High values in all three environmental settings. Concentrations of As, Cd, Cr and Ni were mostly below ISQG-High and above ISQG-Low values with the exception of the stormwater-dominated sites where few samples had values above ISQG-High for As, Cr and Ni (Fig. 2). Additionally, a sample from a mangrove-dominated site had Cd concentrations above ISQG-high values. No ISQG guidelines are available for Co, however, distributions in the different environmental settings were similar. The concentrations of all metals in control sites (i.e., in Middle Harbour) were below ISQG-Low values, except Pb concentration in two samples (Fig. 2). 3.2. Partitioning of trace elements in environmental settings The concentrations of trace elements in different stages of sequential extraction are shown in Fig. 3. The concentrations of As and Ni
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were highest in the residual fraction and lowest, although mostly not measureable were in the acid soluble fraction (i.e., bound to water soluble, exchangeable and carbonate phases) for As and in the reducible fraction (i.e., bound to Fe-/Mn-oxides) for Ni in three environmental settings. The concentrations of partitioned As and Ni in all environmental settings gradually decreased up core, except Ni in mangrove site SC 1 which increased up core. In the control site, the highest concentrations of As and Ni were in the oxidizable fraction (i.e., bound to organic matter and sulphides) and lowest in the reducible fraction and were mostly not measureable for As and in the acid soluble fraction for Ni. The highest concentrations of Cr and Cu in three environmental settings were in the oxidizable fraction and lowest concentration in the acid soluble fraction for Cr and in the residual fraction for Cu. The concentrations of partitioned Cu were maximum at intermediate depths in SC 3 for all phases. The concentrations of reducible Cu were increased up core in SC 4 while in SC 1 the partitioned Cu concentrations were increased up core for acid soluble, oxidizable and residual fractions. Chromium showed maximum concentrations at intermediate depths in SC 1, 3 and 4 for reducible fractions, oxidizable and residual fractions. At the control site, the highest concentration of Cr was in the residual fraction and Cu in the oxidizable fraction, while the lowest concentration was in the acid soluble fraction though mostly not measureable for these metals. The highest concentration of Pb in three environmental settings and in the control site was in the reducible fraction, while the lowest concentration of Pb in three environmental settings was in the residual fraction, however control site samples had the lowest concentrations in the acid soluble fraction. The concentrations of partitioned reducible Pb in SC 3 and 4 increased up core while decreased up core in rest of the environmental settings. The highest concentration of Zn was in the acid soluble fraction and the lowest concentrations were in the residual fraction in three environmental settings, whereas control site samples showed the highest concentrations in the reducible fraction and the lowest concentration in the oxidizable fraction. The concentrations of partitioned Zn increased up core in mangrove-dominated sites, especially acid soluble fractions in SC 1 and oxidizable fractions in SC 4 while in other settings the concentrations were at their highest levels at intermediate depths or at the bottom of the cores. There were no reliable data for Cd and Co due to low ‘near-total’ concentrations in these sediments.
4. Discussion 4.1. Sediment quality and toxicity in environmental settings The presence of high concentration of trace elements in sediments, especially Cu, Pb and Zn above ISQG-high values, has been previously reported throughout major embayments of Sydney estuary, irrespective of the environmental settings (Birch and Taylor, 1999). Birch et al. (2013a) reported that the contemporary catchment surface processes are mainly responsible for the present-day elevated metal concentrations in Sydney estuary. The embayments studied are the most contaminated locations in Sydney estuary due to poor industrial practice in the past and their legacy remains, especially in the industrial-dominated sites (Birch and Taylor, 1999). The source of Cu, Pb and Zn in the studied embayments of the Sydney estuary was likely to be derived from industrial activities and that has been highlighted through a significant increase in up-core accumulations in the industrialdominated site (i.e., in core SC 3). The presence of high-metal concentrations in mangrove-dominated sites result from significant accumulation of fine-grained sediments (o 62.5 mm, mean: 88 percent) and the metals were mainly sourced from a highly developed catchment in proximity to this site. Fine sediments generally have a strong capacity to sequester land-derived metal
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contaminants and these environments were regarded as highly beneficial in their ability to accumulate metals and safeguard coastal ecosystems (Harbison, 1986; Saenger et al., 1991). The present-day increase of Cu and Zn in both mangrove-dominated sites was likely to be derived directly from either anthropogenic sources or by re-suspension and remobilization of contaminated sediments. Currently, the major source of metals to this estuary has changed from industrial to stormwater (Birch et al., 2013a) which has significantly contributed to elevated metal concentrations in stormwater-dominated sites, especially Zn in SC 2 which increased up core. High concentrations of As, Co, Cr and Ni in stormwater-dominated environments are likely to be derived from residential and road land uses (Birch et al., 2013a). The presence of significant Pb levels in the mangrove-dominated site at the deeper levels resulted from prior industrial activities in this bay (Birch et al., 2013a). Sedimentary metals (‘near-total’ concentrations) are assessed to indicate potential adverse biological effects on flora in Sydney estuary (ANZECC and ARMCANZ, 2000). Sediment metal concentrations at the control site located in the least developed and impacted region of Sydney estuary in Middle Harbour were mostly below ISQG-Low values and are unlikely to cause adverse biological effects to flora. Lead (Pb) and Zn concentrations exceeded ISQG-High values in the three environmental settings and pose a high risk of adverse biological effects (Birch et al., 2013a; Nath et al., 2014). Copper concentrations in industrial-dominated settings and a mangrove-dominated site were above ISQG-High values suggesting a high risk of adverse biological effects in this environment. The increasing up-core concentrations of Cu, Pb and Zn above ISQG-High values in Hen and Chicken Bay (i.e., core sites SC 3 and 4) suggests considerable ecological risk in this embayment. Arsenic, Cr and Ni concentrations were mainly below ISQGHigh values, suggesting minor ecological risk. 4.2. Trace element bioavailability in environmental settings The mobility and bioavailability of metals are partly determined by chemical speciation (Yuan et al., 2004; Zhou et al., 2010). Generally, the anthropogenic fraction of metals in the sediments are present in most labile forms (Cuong and Obbard, 2006; Canuto et al., 2013). Similarly, our results showed a large proportion of only Zn to be associated with the acid soluble fraction, whereas Cu and Pb were mostly associated with the oxidizable and reducible fractions, respectively in all environmental settings. The likely origin of these metals was industrial activities in the past and contemporary stormwater loadings. Arsenic and Cr were mostly associated with the oxidizable and residual fractions, while Ni was evenly distributed between the four fractions. Approximately 60 percent of Zn in these sediments are associated with the water soluble, exchangeable and carbonate-bound fractions and is the most mobile and bioavailable metal in all three environmental settings. The data show that no particular environmental setting controls trace element partitioning, rather that environment controls trace element contents. Li et al. (2007) reported 40 percent of Zn in water soluble, exchangeable and carbonate bound fractions in sediments collected from three different environments, i.e., mudflat, mangrove swamp and sewage outlet wetland in Pearl River Estuary, China. Passos et al. (2010) reported 430 percent of Zn in water soluble, exchangeable and carbonate bound fractions in sediments of Poxim River estuary, Sergipe State, northeast Brazil. They suggested that the sources of these metals were likely due to the presence of anthropogenic material. Copper was mainly (approximately 50–60 percent) bound to organic and sulphide phases and their partitioning increased up core in mangrove- and industrial-dominated settings of Hen and Chicken Bay. However, liberation of Cu in this estuary will require a change in the redox
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potential, e.g., re-suspension and oxidation of the sulfidic sediments into overlying oxic waters (Morgan et al., 2012), possibly during storms or dredging activities. Additionally, the presence of significant amount of Cu in oxidizable fractions of three settings is mainly due to its affinity for organic matter and sulfidic phases to form metal sulfides. Many monosulfides, except Cu, are dissolved in the first fraction owing to slight acidity and presence of minor oxygen and much of the environmentally relevant sulfidizedmetal fraction is extracted before the ‘organic/sulfidic’ fraction, whereas the later sulfidic fraction is possibly pyritised metal (Passos et al., 2010). The majority of Pb (about 60–70 percent) was associated with Fe and Mn oxides suggesting formation of stable phases of Pb in sediments (Ramos et al., 1994) and would therefore require the environment to become more reducing to be mobilized, possibly by deep burial and increase in the reducing environment. Most As and Ni in the sediments of the three environmental settings were associated with residual fractions followed by oxidizable fractions, which suggested these metals tend to accumulate in the refractory metal-organic compounds and are moderately stable in these environments. 4.3. Environmental consequences Higher bioavailability of metals in estuarine sediments is of considerable environmental significance due to potential bioaccumulation and toxicity to plants and biota (Luoma, 1983; John and Leventhal, 1996). Significant levels of acid soluble Cu in industrial(SC 3) and mangrove-(SC 4) dominated sites of Hen and Chicken Bay increase ecological risks at this embayment as does a high proportion of acid soluble and reducible Pb at intermediate depths in Homebush Bay mangrove-dominated site (SC 1) and two core sites (SC 5 and 6) in Iron Cove. Arsenic, Cr and Ni are unlikely to be an ecological risk given the low concentrations in the acid soluble fraction. However, a slight change in environmental conditions, such as Eh, pH or salinity, could potentially increase bioavailability of these metals (Passos et al., 2010). Zinc poses a very high ecological risk due to the high (54 to 65 percent) proportion of acid soluble fractions in surficial sediments in the three environmental settings. Previous studies in Sydney estuary have shown that bioaccumulation of Cu is moderately high in mangrove pneumatophores (Nath et al., 2014) and fine nutritive roots (Chaudhuri et al., 2014) and is moderately elevated in tissue of oysters (Birch et al., 2013b) and prawns (Lewtas et al., 2014), whereas Zn is moderately enriched in all four of these media. Lead has moderate to low bioaccumulation in mangrove tissue and is in low concentrations in oyster and prawn tissue. These bioaccumulation patterns of metals in different environmental media suggest a complex interplay of metal bioavailability and a complex uptake mechanism.
5. Conclusion Lead and Zn concentrations in core sediment in all three environmental settings investigated exceeded ISQG-High values and most of these metals were associated with the reducible and acid soluble fractions, which suggest increased mobility and bioavailability and increased environmental risk. Copper and Cr in these estuarine sediments exceeded ISQG-High values and were mostly associated with the oxidizable fraction, followed by reducible fraction, indicating reduced mobility and bioavailability and a lower ecological risk. The mean concentrations of As and Ni were mostly below ISQG-High values and the majority of these metals are associated with the residual phase suggesting minor environmental risk. The sources of trace elements in the Sydney estuary were derived from industrial activities with concomitant increase
through present-day stormwater loadings and re-suspension and remobilization of contaminated sediments.
Acknowledgments This research was partly supported by the University of Sydney postdoctoral fellowship scheme (Grant no. G135714). We thank Tom Savage for technical assistance.
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