Sediment toxicity data and excess simultaneously extracted metals from field-collected samples: Comparison to United States Environmental Protection Agency benchmarks

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US Environmental Protection Agency (USEPA) Procedures for the Derivation of Equilibrium Partitioning Sediment Benchmarks (ESBs) for the Protection of Benthic Organisms: Metal Mixtures are based on the principle that metals toxicity to benthic organisms is determined by bioavailable metals concentrations in porewater. One ESB is based on the difference between simultaneously extracted metal (SEM) and acid volatile sulfide (AVS) concentrations in sediment (excess SEM). The excess SEM ESBs include a lower uncertainty bound, below which most samples (95%) are expected to be “nontoxic” (defined as a bioassay mortality rate ≤24%), and an upper uncertainty bound, above which most samples (95%) are expected to be “toxic” (defined as a mortality rate >24%). Samples that fall between the upper and lower bounds are classified as “uncertain.” Excess SEM ESBs can, in principle, be improved by normalizing for organic carbon (OC). OC is a binding phase that reduces metals bioavailability. OC normalization should improve the accuracy of bioavailable metal concentration estimates, thus tightening uncertainty bounds. We evaluated field-collected sediments from 13 studies with excess SEM, OC, and bioassay data (n = 740). Use of the OC-normalized excess SEM benchmarks did not improve prediction accuracy. The ESB model predicts OC-normalized excess SEM exceeding the upper benchmark even when toxicity is not observed, because error in the OC normalization model increases at low OC concentrations. To minimize the likelihood of incorrectly identifying nontoxic samples as toxic, we recommend that OC normalization of excess SEM should not be considered for sediments with an OC concentration <1% and is questionable for sediments with an OC concentration of 1%–4%. Additional focused studies are needed to confirm or refine the minimum sediment OC concentrations that are applicable for reducing uncertainty in toxicity predictions due to excess SEM. Integr Environ Assess Manag 2022;18:174–186. © 2021 SETAC KEY POINTS One of the US Environmental Protection Agency's equilibrium partitioning sediment benchmark approaches for metals is based on the molar difference between acid volatile sulfide (AVS) and simultaneously extracted metal (SEM), which is referred to as excess SEM. Organic carbon (OC) normalization of excess SEM is intended to reduce uncertainty in toxicity predictions by considering a binding phase, in addition to AVS, that reduces metal bioavailability. Based on a synthesis of bioassay and chemistry data from field-collected samples, we argue that the theory behind OC normalization breaks down at low OC concentrations and that OC normalization should not be applied in sediments with OC <1% and is questionable in sediment with OC between 1% and 4%.