TECHNICAL BASIS AND PROPOSAL FOR DERIVING SEDIMENT QUALITY CRITERIA FOR METALS

In developing sediment quality criteria (SQC) for metals, it is essent ial that bioavailability be a prime consideration. Different studies h ave shown that while dry weight metal concentrations in sediments are not predictive of bioavailability, metal concentrations in interstitia l (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 ca tionic metals of environmental concern (cadmium, copper, nickel, lead, zinc) forming insoluble sulfide complexes with minimal biological ava ilability. Short-term (10-d) laboratory studies with a variety of mari ne and freshwater benthic organisms have demonstrated that when AVS co ncentrations in spiked or field-collected sediments exceed those of me tals simultaneously extracted with the AVS, interstitial water metal c oncentrations remain below those predicted to cause effects, and toxic ity 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 a ddition, field colonization experiments, varying in length from severa l months to more than 1 year, with cadmium- or zinc-spiked freshwater and marine sediments, have demonstrated a lack of biological effects w hen there is sufficient AVS to limit interstitial water metal concentr ations. These studies on metal bioavailability and toxicity in sedimen ts serve as the basis for proposed SQC for the metals cadmium, copper, nickel, lead, and zinc. Specifically, four approaches for deriving cr iteria are described: (a) comparison of molar AVS concentrations to th e summed molar concentration of the five metals simultaneously extract ed with the AVS; (b) measurement of interstitial water metal concentra tions and calculation of summed interstitial water criteria toxic unit s (IWCTU) for the five metals, based upon final chronic values from wa ter quality criteria documents; (c) calculation of summed IWCTU based upon sediment AVS concentrations and metal-specific partitioning of th e metals to organic carbon; and (d) calculation of summed IWCTU based upon partitioning of the metals to a minimum binding phase sorbent (ch romatographic 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 w ill not be toxic, but not necessarily when metal toxicity will be mani fested. Currently, approaches (a) and (b) are the most useful in terms of predicting metal bioavailability and deriving SQC. Further researc h is required, however, to fully implement approaches (c) and (d). Add itional research also is required to thoroughly understand processes c ontrolling bioaccumulation of metals from sediments by benthic organis ms, as well as accumulation of metals by pelagic species that ingest m etal-contaminated benthos.