The purpose of this review paper is to present the technical basis for establishing sediment quality criteria using equilibrium partitioning (EqP). Equilibrium partitioning is chosen because it addresses the two principal technical issues that must be resolved: the varying bioavailability of chemicals in sediments and the choice of the appropriate biological effects concentration. The data that are used to examine the question of varying bioavailability across sediments are from toxicity and bioaccumulation experiments utilizing the same chemical and test organism but different sediments. It has been found that if the different sediments in each experiment are compared, there is essentially no relationship between sediment chemical concentrations on a dry weight basis and biological effects. However, if the chemical concentrations in the pore water of the sediment are used (for chemicals that are not highly hydrophobic) or if the sediment chemical concentrations on an organic carbon basis are used, then the biological effects occur at similar concentrations (within a factor of two) for the different sediments. In addition, the effects concentrations are the same as, or they can be predicted from, the effects concentration determined in water‐ only exposures. The EqP methodology rationalizes these results by assuming that the partitioning of the chemical between sediment organic carbon and pore water is at equilibrium. In each of these phases, the fugacity or activity of the chemical is the same at equilibrium. As a consequence, it is assumed that the organism receives an equivalent exposure from a water‐only exposure or from any equilibrated phase, either from pore water via respiration, from sediment carbon via ingestion; or from a mixture of the routes. Thus, the pathway of exposure is not significant. The biological effect is produced by the chemical activity of the single phase or the equilibrated system. Sediment quality criteria for nonionic organic chemicals are based on the chemical concentration in sediment organic carbon. For highly hydrophobic chemicals this is necessary because the pore water concentration is, for those chemicals, no longer a good estimate of the chemical activity. The pore water concentration is the sum of the free chemical concentration, which is bioavailable and represents the chemical activity, and the concentration of chemical complexed to dissolved organic carbon, which, as the data presented below illustrate, is not bioavailable. Using the chemical concentration in sediment organic carbon eliminates this ambiguity. Sediment quality criteria also require that a chemical concentration be chosen that is sufficiently protective of benthic organisms. The final chronic value (FCV) from the U.S. Environmental Protection Agency (EPA) water quality criteria is proposed. An analysis of the data compiled in the water quality criteria documents demonstrates that benthic species, defined as either epibenthic or infaunal species, have a similar sensitivity to water column species. T...

The purpose of this review paper is to present the technical basis for establishing sediment quality criteria using equilibrium partitioning (EqP). Equilibrium partitioning is chosen because it addresses the two principal technical issues that must be resolved: the varying bioavailability of chemicals in sediments and the choice of the appropriate biological effects concentration.The data that are used to examine the question of varying bioavailability across sediments are from toxicity and bioaccumulation experiments utilizing the same chemical and test organism but different sediments. It has been found that if the different sediments in each experiment are compared, there is essentially no relationship between sediment chemical concentrations on a dry weight basis and biological effects. However, if the chemical concentrations in the pore water of the sediment are used (for chemicals that are not highly hydrophobic) or if the sediment chemical concentrations on an organic carbon basis are used, then the biological effects occur at similar concentrations (within a factor of two) for the different sediments. In addition, the effects concen-

Abstract-In developing sediment quality criteria (SQC) for metals, it is essential that bioavailability be a prime consideration. Different studies have shown that while dry weight metal concentrations in sediments are not predictive of bioavailability, metal concentrations in interstitial (pore) water are correlated with observed biological effects. A key partitioning phase controlling cationic metal activity and toxicity in the sediment-interstitial water system is acid-volatile sulfide (AVS). Acid-volatile sulfide binds, on a mole-to-mole basis, a number of cationic metals of environmental concern (cadmium, copper, nickel, lead, zinc) forming insoluble sulfide complexes with minimal biological availability. Short-term (10-d) laboratory studies with a variety of marine and freshwater benthic organisms have demonstrated that when AVS concentrations in spiked or field-collected sediments exceed those of metals simultaneously extracted with the AVS, interstitial water metal concentrations remain below those predicted to cause effects, and toxicity does not occur. Similar observations have been made in life-cycle laboratory toxicity tests with amphipods and chironomids in marine and freshwater sediments spiked with cadmium and zinc, respectively. In addition, field colonization experiments, varying in length from several months to more than 1 year, with cadmium-or zinc-spiked freshwater and marine sediments, have demonstrated a lack of biological effects when there is sufficient AVS to limit interstitial water metal concentrations. These studies on metal bioavailability and toxicity in sediments serve as the basis for proposed SQC for the metals cadmium, copper, nickel, lead, and zinc. Specifically, four approaches for deriving criteria are described: (a) comparison of molar AVS concentrations to the summed molar concentration of the five metals simultaneously extracted with the AVS; (b) measurement of interstitial water metal concentrations and calculation of summed interstitial water criteria toxic units (IWCTU) for the five metals, based upon final chronic values from water quality criteria documents; (c) calculation of summed IWCTU based upon sediment AVS concentrations and metal-specific partitioning of the metals to organic carbon; and (d) calculation of summed IWCTU based upon partitioning of the metals to a minimum binding phase sorbent (chromatographic sand). For a number of reasons, SQC derived using these approaches generally should be considered ''no effect'' values, i.e., with these techniques it is possible to predict when sediment metals will not be toxic, but not necessarily when metal toxicity will be manifested. Currently, approaches (a) and (b) are the most useful in terms of predicting metal bioavailability and deriving SQC. Further research is required, however, to fully implement approaches (c) and (d). Additional research also is required to thoroughly understand processes controlling bioaccumulation of metals from sediments by benthic organisms, as well as accumulation of metals by pelagic s...

An extension of the simultaneously extracted metals/acid-volatile sulfide (SEM/AVS) procedure is presented that predicts the acute and chronic sediment metals effects concentrations. A biotic ligand model (BLM) and a pore water-sediment partitioning model are used to predict the sediment concentration that is in equilibrium with the biotic ligand effects concentration. This initial application considers only partitioning to sediment particulate organic carbon. This procedure bypasses the need to compute the details of the pore-water chemistry. Remarkably, the median lethal concentration on a sediment organic carbon (OC)-normalized basis, SEM*(x,OC), is essentially unchanged over a wide range of concentrations of pore-water hardness, salinity, dissolved organic carbon, and any other complexing or competing ligands. Only the pore-water pH is important. Both acute and chronic exposures in fresh- and saltwater sediments are compared to predictions for cadmium (Cd), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) based on the Daphnia magna BLM. The SEM*(x,OC) concentrations are similar for all the metals except cadmium. For pH = 8, the approximate values (micromol/gOC) are Cd-SEM*(xOC) approximately equal to 100, Cu-SEM*(x,OC) approximately equal to 900, Ni-SEMoc approximately equal to 1,100, Zn-SEM*(x,OC) approximately equal to 1,400, and Pb-SEM*(x,OC) approximately equal to 2,700. This similarity is the explanation for an empirically observed dose-response relationship between SEM and acute and chronic effects concentrations that had been observed previously. This initial application clearly demonstrates that BLMs can be used to predict toxic sediment concentrations without modeling the pore-water chemistry.

Numerous studies have shown that dry weight concentrations of metals in sediments cannot be used to predict toxicity across sediments. However, several studies using sediments from both freshwater and saltwater have shown that interstitial water concentration or normalizations involving acid‐volatile sulfide (AVS) can be used to predict toxicity in sediments contaminated with cadmium, copper, nickel, lead, or zinc across a wide range of sediment types. Six separate experiments were conducted in which two or three sediments of varying AVS concentration were spiked with a series of concentrations of cadmium, copper, lead, nickel, or zinc or a mixture of four of these metals. The amphipod Ampelisca abdita was then exposed to the sediments in 10‐d toxicity tests. Amphipod mortality was sediment dependent when plotted against dry weight metals concentration but was not sediment dependent when plotted against simultaneously extracted metal (SEM)/AVS or interstitial water toxic units (IWTUs). Sediments with SEM/AVS ratios <1.0 were seldom (2.3%) toxic (i.e., caused <24% mortality), while sediments with SEM/AVS ratios >1.0 were frequently (80%) toxic. Similarly, sediments with <0.5 IWTU were seldom toxic (3.0%), while sediments with >0.5 IWTU were toxic 94.4% of the time. These results, coupled with results from related studies, demonstrate that an understanding of the fundamental chemical reactions which control the availability of cadmium, copper, lead, nickel, and zinc in sediments can be used to explain observed biological responses. We believe that using SEM/AVS ratios and IWTUs allows for more accurate predictions of acute mortality, with better causal linkage to metal concentration, than is possible with sediment evaluation tools which rely on dry weight metal concentrations.