Article pubs.acs.org/est
Evaluating the Performance of Diffusive Gradients in Thin Films for Predicting Ni Sediment Toxicity David M. Costello,*,† G. Allen Burton,† Chad R. Hammerschmidt,‡ and W. Keith Taulbee§ †
School of Natural Resources & Environment, University of Michigan, 440 Church St., Ann Arbor, Michigan 48109, United States Department of Earth & Environmental Sciences, Wright State University, 3640 Colonel Glenn Hwy., Dayton, Ohio 45435, United States § Great Lakes Environmental Center, 1295 King Ave., Columbus, Ohio 43212, United States ‡
S Supporting Information *
ABSTRACT: Diffusive gradients in thin films (DGTs) rapidly measure labile fractions of metal and are promoted as an assessment tool for bioavailability. Using macroinvertebrate community composition as a response, this study compared the predictive ability of DGT-measured Ni with acid volatile sulfide (AVS) and organic carbon (OC) corrected Ni [(SEMNi−AVS)/f OC] and total Ni concentrations. In two experiments, sediments were amended with Ni and placed within either a streamside mesocosm or deployed in situ. DGTmeasured Ni concentrations (CDGT) increased with increasing total Ni, were greater at depth, and decreased over time. Relationships between Ni CDGT and sediment geochemistry indicated a shift in Ni partitioning from AVS-bound to Fe- and Mn-associated Ni. In both experiments, DGT-measured Ni poorly predicted the invertebrate response to metal, whereas models that included total Ni or (SEMNi−AVS)/f OC effectively predicted the invertebrate response for the streamside mesocosm and in situ experiments, respectively. CDGT overestimated the available Ni fraction, possibly due to sampling either nonbioavailable solid-phase Ni or Ni irrespective of cations competing at the biotic ligand. We suggest that CDGT cannot replace (SEMNi−AVS)/f OC for predicting invertebrate response to sediment Ni, and greater understanding of metal species lability to DGTs is needed before assuming equivalence between bioavailable and DGT-labile metals in sediments.
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INTRODUCTION Heavy metals in sediment present one of the most compelling challenges to those who study and regulate pollutants. Due to their high reactivity with redox and pH-sensitive compounds, metals in the environment exist in numerous chemical forms, only a fraction of which negatively affect biota.1−4 In sediments, where binding ligands are replete, regulation based on total metal concentrations can overestimate the risk.3 The concept of bioavailability was developed to describe the fraction of metal that may negatively influence biota, and numerous approaches have been developed to measure this fraction.1,3 Currently, the equilibrium partitioning (EqP) approach3 is a widely used procedure for estimating bioavailable metals with the goal of predicting whether or not sediments may be toxic. However, diffusive gradients in thin films (DGT) have been increasingly advocated as an alternative method for rapid assessment of metal bioavailability in sediments.5,6 The EqP approach involves acidification of wet sediment followed by measurement of acid volatile sulfide (AVS) and simultaneously extracted metal (SEM) to estimate bioavailable metal (e.g., SEM−AVS).3 In theory, the SEM fraction is a measure of dissolved and redox-sensitive, solid-phase metal species, and any SEM in excess of AVS is considered potentially bioavailable. Although metal sulfides can account for much of the nontoxic metal, it is recognized that SEM in excess of AVS © 2012 American Chemical Society
may be bound to organic carbon (OC) or iron and manganese oxides, further reducing the fraction of bioavailable metal.3,4,7 The influence of organic carbon on reducing metal bioavailability has been incorporated into EqP models through the use of an OC-corrected calculation [i.e., (SEM−AVS)/ f OC],3,8 yet no similar correction has been proposed for Fe and Mn oxides. Thus, the bioavailable metal measured by EqP methods is a composite measure of porewater metal and nonsulfidic, redox-sensitive, solid-phase metal. The EqP method has been useful for predicting thresholds, below which sediments are nontoxic to macroinvertebrates.3 However, above nontoxic thresholds, there is uncertainty in the concentration at which toxic effects are expected, most likely due to binding by ligands unaccounted for in EqP models (e.g., Fe oxides).3 In sediments, the DGT technique can be used to calculate porewater metal flux and concentrations in situ.5,9 The standard DGT device is a porous hydrogel covered by a filter membrane and backed by a Chelex resin, which is deployed in situ to sample metals over a short time period (usually
