Effective metal concentrations in porewater and seawater labile
Transcription
Effective metal concentrations in porewater and seawater labile
Marine Pollution Bulletin 44 (2002) 956–976 www.elsevier.com/locate/marpolbul Baseline Edited by Bruce J. Richardson The objective of BASELINE is to publish short communications on different aspects of pollution of the marine environment. Only those papers which clearly identify the quality of the data will be considered for publication. Contributors to Baseline should refer to ‘Baseline—The New Format and Content’ (Mar. Pollut. Bull. 42, 703–704). Effective metal concentrations in porewater and seawater labile metal concentrations associated with copper mine tailings disposal into the coastal waters of the Atacama region of northern Chile M.R. Lee a a,* , J.A. Correa a, H. Zhang Departamento de Ecologıa and Center for Advanced Studies in Ecology and Biodiversity, Facultad de Ciencias Biol ogicas, Pontificia Universidad Catolica de Chile, Casilla 114-D, Santiago, Chile b Department of Environmental Science, IENS, Lancaster University, Lancaster LA1 4YQ, UK Tailings dumping on the coast of the Atacama region takes place in the area around the small city of Cha~ naral. The coast of this area is open to the Pacific Ocean swell and consists of rocky shores interspersed with sandy beaches of coarse sediments. Freshwater in this area is a scarce resource and significant freshwater input to the coastal environment occurs in rare flash floods only. As a result of the desert nature of the area population density is low, there is no agricultural activity and the only industrial activity is mining. Therefore, the only significant source of pollution in this costal environment is that derived from the mining activities, providing a unique opportunity to study the effects of tailings disposal and elevated metal concentration in the absence of the confounding effects of other groups of pollutants. Mine tailings from the copper mines at Potrerillos (now closed) and El Salvador have been dumped into coastal waters since 1939. Initially the tailings were deposited on to the beach at Playa Cha~ naral (Fig. 1) and as a result the shore line is now a kilometre further out to sea than it had been originally (Castilla, 1983). In the * b Corresponding author. Fax: +56-35-431670. E-mail address: [email protected] (M.R. Lee). 1970s the tailings were diverted to a new dumping point ten kilometres to the north of Cha~ naral, adjacent to Playa Palito (Fig. 1). Again a large tailings beach formed in the littoral zone between Caleta La Lancha and Caleta Agua Hedionda, with significant amounts of tailings mixed in with the natural sediments on the beaches around Caleta Palito. In 1990, after local court action, a tailings settlement dam was constructed inland at Pampa Austral (60 km east of Cha~ naral) and since then only ‘clear water’ tailings have been delivered to the coast with a legal maximum concentration of 2000 lg Cu l1 (Castilla, 1996). The data included in this report were collected as part of a larger study on the effects of copper on the coastal environment and specifically as part of a sediment quality triad. The objective of the latter was to understand the effects of the tailings disposal on the littoral meiofaunal communities of the area. The sampling sites were divided a priori into three groups: the northern sites, Puerto Pan de Azucar (Pue), Frente Isla Pan de Azucar (Fre), Playa Blanca (Bla); the central ‘impacted’ sites, Caleta La Lancha (Lan), Caleta Agua Hedionda (Hed), Palito 1000 m Norte (Mil), Playa Palito (Pal), Palito 2000 m Sur (Dos), Playa Cha~ naral (Cha); and the reference sites, Las Piscinas (Pis), Torres del Inca (Tor), Playa Zenteno (Zen). See Fig. 1 for the locations of the Baseline / Marine Pollution Bulletin 44 (2002) 956–976 957 Fig. 1. A map of the Cha~ naral area showing the locations of the sampling sites and the tailings dumping point. sampling sites, all of which will here after, in figures and tables, be referred to by their three letter codes. The effective concentrations of metals in the sediment porewater, August 1999, and the concentrations of labile metals in the overlying seawater, March 2000, were determined using the diffusive gradients in thin-film (DGT) technique (Davidson and Zhang, 1994; Zhang and Davidson, 1995; Zhang et al., 2001). The technique of DGT has been pioneered and developed at Lancaster to measure metals and to study their chemical speciation in natural waters (Davidson and Zhang, 1994; Zhang and Davidson, 1995, 2000) and their effective concentrations in soils and sediments (Zhang et al., 2001). It accumulates solutes quantitatively on a binding agent after their passage through a well-defined diffusion layer. A polyacrylamide hydrogel, of known thickness (Dg), is commonly used as the dif- fusive layer, while for trace metals Chelex-100, incorporated into a second gel layer, serves as the binding agent. The two gels are enclosed in a small plastic device that is immersed in solution. The diffusion coefficient (D) of metal ions within the diffusive gel can be independently measured. Fick’s first law of diffusion can then be used to determine the concentration (Cb ) of metal in the bulk solution from the mass (M) of metal accumulated in the resin, as shown in Eq. (1). Cb ¼ MDg=ðDtAÞ ð1Þ The DGT device provides a direct measurement of the effective concentration CE in sediment (Zhang et al., 2001). When the DGT device is deployed in sediment it locally lowers the concentration of metal ions and induces the re-supply from sediment to porewater in the same manner as biological organisms. It measures the 958 Baseline / Marine Pollution Bulletin 44 (2002) 956–976 concentration from the porewater plus the effective additional concentration supplied from the solid phase. This effective concentration, CE , is what would be experienced by the organisms living in the sediments under the worst possible case, i.e. when the membrane transport is rapid. The sampler consists of a plastic base and cap, which are used to hold the gel ‘sandwich’ in place. The base layer of the ‘sandwich’ is a gel impregnated with Chelex beads, which bind the labile metals effectively. The second layer is the diffusion gel, with a large enough pore size to allow metal ions and complexes to diffuse through. The third layer is a 0.45 lm cellulose-nitrate filter that protects the gel surface from mechanical damage. The gels and plastic holders were supplied by DGT Research Ltd. UK (www.dgtresearch.com). The DGT samplers, including three blanks, were assembled less than a week before they were due to be used. The plastic bases and caps were acid washed (10% HNO3 for 48 h) and then rinsed in ultrapurTM water prior to assembly. The samplers were assembled in a laminar flow chamber to prevent contamination and stored at 4 °C, with a small amount of ultrapurTM water to prevent desiccation, in plastic bags prior to use. Samplers were transported to and from the study site in coolers. Due to the dynamic nature of the littoral environment on this part of the Chilean coast it was not possible to leave the samplers in situ. Therefore, for the porewater samples three replicate cores were collected from each site. The cores were collected using PVC tubing 45 mm in diameter, 150 mm in length, which had been acid washed prior to use (10% HNO3 for 48 h). The core tubes were inserted into the sediment to a depth of approximately 50 mm, removed with the sample and a cap placed on the top of the tube. The sample was then inverted and a DGT unit placed face down in the sediment. The cores were then placed in ZiplocTM bags and then into a cooler. Seawater samples (three replicate samples for each site) were collected from the surf zone on each beach. Water was collected from the surf using a plastic bucket. One litre of seawater was placed into a ZiplocTM bag along with a DGT unit. The bag was then placed in a second ZiplocTM bag for added security/ durability and the bags placed in a cooler. The exposure time of the DGT sampler to the water was at least 24 h for each sample. However, exposure times varied from site to site due to logistical constraints. At the end of the exposure, the DGT units were removed from the samples, rinsed in ultrapurTM water, placed in clean plastic bags and stored in a cooler for the return journey to the laboratory. In the laboratory the Chelex gels were removed and placed into 1.5 ml micro-centrifuge tubes with 0.8 ml of 10% HNO3 (Merck suprapur) and then sent by courier to the University of Lancaster for analysis. Samples were analysed by inductively coupled plasma-mass Table 1 The results of ANOVA tests on the effective metals concentrations data for porewater and labile metal concentrations for seawater, collected from 12 sites in the Cha~ naral area Sample Metal F p Porewater Cu Al As Cr Mn Ni Zn 4.806 1.880 8.958 9.526 0.963 52.385 2.542 0.001 0.095 0.0001 0.0001 0.503 0.0001 0.027 Seawater Cu Al As Cr Mn Ni Zn 45.106 6.145 2.463 9.737 4.159 1.577 16.606 0.0001 0.0001 0.031 0.0001 0.002 0.169 0.0001 spectrometry (ICP-MS, Varian Ultramass) using a direct injection nebulizer (CETAC). During the analyses a quality control solution was used for every 10 samples to monitor the performance of the machine. The blanks, which included all steps in the sampling process–– prepartion of the DGT device, handling and analysis, had concentrations of less than 2 ng l1 for all the metals studied. The effective concentrations in the porewater and the labile concentrations in the seawater were quantified for the following metals: Cu, Al, As, Cr, Mn, Ni, and Zn. The subscripts pw and sw refer to porewater and seawater respectively. The data for all the metals, both porewater and seawater, were normally distributed. However, Cupw , Cusw , Mnpw , Nisw , and Znsw all needed to be log transformed to achieve normal distribution. Analysis of variance (ANOVA) tests (Table 1) for each of the metals, both in porewater and seawater, indicated that all metals showed significant (p < 0:05) between-site variation except Alpw , Mnpw , and Nisw . The effective concentrations of metals in the porewater were consistently higher than the labile concentrations measured for the seawater. Seawater concentrations of copper, aluminium, chromium, manganese, and zinc can be visually associated with the expected distribution of tailings impact. However, for concentrations in the porewater, this only applies to copper. Only in the case of copper did the seawater and porewater concentrations vary in the same way between sites (Fig. 2). The maximum and minimum concentrations of labile metals in both the porewater and the seawater for each of the metals are presented in Table 2. The concentrations of copper in both the porewater (effective) and seawater (labile) were higher in the central sites than they were at the northern and reference sites (Fig. 2a). Copper concentrations were also higher at the northern sites than at the reference sites. Effective Baseline / Marine Pollution Bulletin 44 (2002) 956–976 959 Fig. 2. The effective porewater metal concentrations and seawater labile metal concentrations (lg l1 ) recorded at each of the sites (bars represent standard errors). Table 2 The maximum and minimum effective concentrations in porewater and labile concentrations in seawater for each of the metals (lg l1 ) collected from 12 sites in the Cha~ naral area Porewater Copper Aluminium Arsenic Chromium Manganese Nickel Zinc Seawater Max Min Max Min 1449.59 (Hed) 820.92 (Bla) 6.14 (Hed) 15.08 (Fre) 12.38 (Pal) 43.71 (Pis) 1263.18 (Pis) 6.43 (Pis) 179.78 (Zen) 1.82 (Zen) 3.75 (Zen) 3.17 (Zen) 18.01 (Cha) 493.52 (Zen) 41.42 (Hed) 189.48 (Mil) 1.22 (Hed) 2.94 (Cha) 4.24 (Hed) 3.11 (Fre) 631.13 (Cha) 0.16 (Pis) 34.87 (Pue) 0.13 (Lan) 1.05 (Pue) 1.22 (Zen) 1.33 (Tor) 225.18 (Tor) porewater concentrations of aluminium did not vary significantly between sites (Fig. 2b). In seawater, however, the concentrations of labile aluminium tended be higher at the sites closer to Caleta Palito. Seawater concentrations of labile arsenic fluctuated from site to site with no apparent pattern emerging (Fig. 2c). On the other hand, effective arsenic concentrations in the porewater fluctuated little, but with a peak at Caleta Agua Hedionda. The speciation of arsenic is currently under investigation using the DGT technique. Preliminary results indicated that the DGT device used in this work (using Chelex-100 as binding agent) only measure 960 Baseline / Marine Pollution Bulletin 44 (2002) 956–976 Fig. 3. A plot of effective porewater metal concentration versus seawater labile copper concentration with regression analysis. The data was logðx þ 1Þ transformed prior to plotting. As3þ . Effective porewater chromium concentrations did not vary much between sites (Fig. 2d) although moving northwards, concentrations tended to increase. Concentrations of chromium in the seawater were highest at the central impacted sites, with a steady decline in concentration moving northwards from Playa Cha~ naral. The chromium measured by DGT (with Chelex-100 resin) is Cr3þ only (Ernstberger et al., 2000). The effective porewater concentrations of manganese did not vary significantly between sites (Fig. 2e) whereas labile concentrations in the seawater were lowest at the reference sites. The nickel concentrations in both the porewater (effective) and the seawater (labile) did not appear to vary much between sites (Fig. 2f), except for a peak value recorded in the porewater at Las Piscinas. Effective porewater zinc concentrations did not show much variation between sites (Fig. 2g). However, the reference sites clearly had lower labile concentrations in the seawater, with the peak concentration occurring at Playa Cha~ naral with a steady decline in concentrations moving northwards. Of the Pearsons correlations between the porewater (effective) and seawater (labile) concentrations of a given metal, only copper had a significant correlation (0.596, p < 0:001). Regression analysis of porewater (effective) versus seawater (labile) copper concentrations (Fig. 3) indicated that there was a significant positive relationship between the two (F ¼ 55:283, p < 0:0001). Regression analysis yielded the following equation: y ¼ 0:577x 0:033, R2 ¼ 0:847 (where x is the effective porewater concentration and y the labile seawater concentration) indicating that the concentration of labile copper in the seawater increased with the effective concentration of copper in the porewater. Spearman’s rank correlation tests were carried out with the metal concentrations in both the porewater (effective) and the seawater (labile), and a ranking of the sites based on the amount of tailings present. Rankings by the amount of tailings present (Table 3) were based upon the visual observation of the sites outlined above, and were therefore subjective. The sites were ordered so that the sites with least amount of tailings receiving the lowest rank. Caleta La Lancha (Lan), Caleta Agua Hedionda (Hed) and Playa Cha~ naral (Cha) are all sites composed of 100% tailings. The results of the Spearman’s rank test are presented in Table 4 and show that there was a high correlation between the tailings and Table 3 The study sites ranked by the amount of tailings present at each site, based on visual observation Site Ranking Pue Fre Bla Lan Hed Mil Pal Dos Cha Pis Tor Zen 6 5 4 11 11 8.5 7 8.5 11 2 2 2 Sites with the least amount of tailings were given the lowest ranks. Table 4 Spearman’s rank correlation of tailings and effective metal concentrations in porewater and labile metal concentrations in seawater from 12 sites in the Cha~ naral area Metal Seawater Porewater Cu Al As Cr Mn Ni Zn 0.913 0.405 0.146 0.430 0.799 0.732 0.831 0.913 0.135 0.444 0.245 0.025 0.345 0.242 Baseline / Marine Pollution Bulletin 44 (2002) 956–976 copper in both the porewater (0.913) and the seawater (0.913). In the porewater, only the effective copper concentration showed a high correlation with the tailings, with all other metals displaying low correlations (<0.500). In the seawater samples in addition to copper, manganese (0.799), nickel (0.732) and zinc (0.831) showed high correlations with the tailings. The other metals in the seawater had low correlations (<0.500) with the tailings. Copper is the principal component of past tailings discharges from the perspective of sediment toxicology. Only Cupw and Cusw are highly correlated with the distribution of the tailings and with each other. The porewater copper concentrations recorded here are considerably higher than those recorded by Alongi et al. (1991) for the Fly River Delta, Papua New Guinea. Alongi et al. (1991) is the only other study, to the authors knowledge, that reports the porewater copper concentrations in tailings discharges. Manganese, nickel and zinc were all highly correlated with the observed distribution of the tailings but only in the seawater. The metals recorded in the area under study can be divided into two groups: those present at natural background concentrations and those whose concentrations have been increased at some of the sites by anthropogenic sources. The metals present at natural background levels are characterised by concentrations that either fluctuate little between sites or appear to fluctuate in a pattern that can not be tied in with current knowledge of the anthropogenic sources, (i.e. at random). Within the porewater, only the effective concentrations of copper appear to be above background levels; therefore, all the other metals sampled are present, within the porewater, at natural background concentrations. For the seawater samples, the metals can be divided into four groups. The first group consists of arsenic and nickel which are present at background concentrations only. The second group of metals includes aluminium and manganese, which displayed elevated concentrations around the tailings discharge point at Caleta Palito and are probably associated with the ‘clear water’ tailings discharge. The third group includes chromium and zinc, where the highest concentrations occur at Playa Cha~ naral with a steady decline in concentration moving northward. The only obvious possible source of metals near Playa Cha~ naral is the port activity at Barquito located one kilometre to the south of Cha~ naral. The final, singlemetal group includes only copper, where the source of the metal in the seawater is clearly the tailings that have been deposited in the intertidal zone. In summary, of the metals reported in the present study, only copper appears to be ecotoxicologically important in the porewater. The tailings themselves, in addition to their physical impact, are also the primary sources of copper in the seawater adjacent to the sites. A comparison of the data presented here with data from 961 other sites is difficult as in the majority of cases the concentrations of metals within sediments are measured in the form of lg g1 DW of sediment. This measure of metals in not ecologically relevant from the point of view of ecotoxicology. It is well known that the bulk sediment concentrations of a metal are not predictive of the toxic effect of those sediments (Luoma, 1989; Di Toro et al., 1990; Chapman et al., 1998). When only bulk sediment concentrations are known, a host of other variables need to be measured and applied to complex models in order to estimate the bioavailable fraction of the metals. Therefore, it would seem more appropriate in the future to measure the biologically more appropriate fractions of the metals present using a methodology such as DGT. Acknowledgements The authors thank the International Copper Association for providing research funds for this study. CASEB, program 7, is also acknowledged. 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Environ. Sci. Technol. 35, 2602–2607. 0025-326X/02/$ - see front matter Ó 2002 Published by Elsevier Science Ltd. PII: S 0 0 2 5 - 3 2 6 X ( 0 2 ) 0 0 1 1 2 - 1