Equilibrium Partitioning Sediment Benchmarks (ESBs) for the Protection of Benthic Organisms: Procedures for the Determination of the Freely Dissolved Interstitial Water Concentrations of Nonionic Organics

This document describes procedures to determine the concentrations of nonionic organic chemicals in sediment interstitial waters. In previous ESB documents, the general equilibrium partitioning (EqP) approach was chosen for the derivation of sediment benchmarks because it accounts for the varying bioavailability of chemicals in different sediments and allows for the incorporation of the appropriate biological effects concentration. This provides for the derivation of benchmarks that are causally linked to the specific chemical, applicable across sediments, and appropriately protective of benthic organisms. In contrast to the previous ESB documents, the emphasis of this ESB document is to provide a summary of procedures for determining the freely dissolved concentrations of nonionic organic chemicals for deriving sediment interstitial water toxic units (IWTUs). In the last ten years, technologies have been developed allowing for the accurate estimation and measurement of the concentrations of nonionic organic chemicals in sediment interstitial waters. When the general EqP model (i.e., one-carbon model) was first proposed for deriving ESBs, methods for directly measuring interstitial water concentrations of nonionic organic chemicals were often overly technically difficult, cost prohibitive, or simply not available. The procedures described here are an alternative or complement to using the one-carbon general model for deriving ESBs. The one-carbon general model estimates the bioavailability of nonionic organic contaminants based on their measured sediment concentrations and sediment organic carbon content. The new technologies and resulting procedures described in this document include a two-carbon model incorporating black carbon along with natural organic carbon for making EqP-based predictions, direct measurements of interstitial water contaminants adjusted for dissolved organic carbon, and passive samplers to measure interstitial water concentrations directly or via the sediment. These procedures allow for the more accurate determination of the freely dissolved and potentially bioavailable concentrations of nonionic organic chemicals. These concentrations along with the final chronic values (FCVs), secondary chronic values (SCVs), or other relevant water-only toxicity values are used to derive IWTUs. Depending upon the toxicological endpoint, if the IWTUs are greater than one, benthic organisms may not be protected and adverse effects may result. This document is not intended as a methods manual but rather provides an overview of procedures for determining freely dissolved concentrations of nonionic organic chemicals. Throughout this document, the scientific literature cited provides greater methodological detail. ESB documents have been developed for two pesticides (endrin, dieldrin), polycyclic aromatic hydrocarbon (PAH) mixtures, metal mixtures, and a selection of 32 nonionic organic chemicals. The ESBs do not intrinsically consider the antagonistic, additive or synergistic effects of other sediment contaminants in combination with the individual nonionic organic chemicals discussed in this document or the potential for bioaccumulation and trophic transfer of these chemicals to aquatic life, wildlife or humans. However, for narcotic chemicals, additivity can be used to sum toxic effects.