Chromium exists in sediments in two oxidation states: Cr(III), which is relatively insoluble and nontoxic, and Cr(VI), which is much more soluble and toxic. Chromium(VI) is thermodynamically unstable in anoxic sediments, and acid-volatile sulfide (AVS) is formed only in anoxic sediments; therefore sediments with measurable AVS concentrations should not contain toxic Cr(VI). If this hypothesis holds true, measuring AVS could form the basis for a theoretically based guideline for Cr in sediments. Ten-day water-only and spiked sediment toxicity tests with the amphipod Ampelisca abdita were performed with Cr(VI) and Cr(III), along with sediments collected from a site contaminated with high concentrations of Cr. In sediments where AVS exceeded analytical detection limits, Cr concentrations in interstitial water were very low (<100 microg/L) and no significant toxicity to A. abdita was observed. In sediments in which AVS was not significantly greater than zero, Cr concentrations in interstitial waters increased significantly, with greater than 90% of the Cr present as Cr(VI), and mortality of A. abdita was elevated. These results demonstrate that measurements of AVS and interstitial water chromium can be useful in predicting the absence of acute effects from Cr contamination in sediments.

Abstract-Previous experiments conducted with freshwater sediments spiked with silver have shown that, when expressed on a dry weight basis, the toxicity of silver is sediment-specific and dependent on the form of silver added (e.g., AgNO 3 , Ag 2 S). This study was conducted to assess the usefulness of silver interstitial water toxic units (IWTU) and acid volatile sulfide (AVS) concentrations in predicting the biological effects of silver species across sediments, regardless of the species of silver present. Two saltwater sediments were spiked with a series of concentrations of silver. The amphipod, Ampelisca abdita, was then exposed to the sediments in ten-day toxicity tests. Amphipod mortality was sediment-specific when expressed on a dry weight basis, but not when based on IWTU or simultaneously extracted metal (SEM)-AVS. Sediments with an excess of AVS relative to SEM had IWTU Ͻ0.5, and were generally not toxic. Sediments with an excess of SEM relative to AVS had silver IWTU Ͼ0.5, but no measurable AVS, and were generally toxic. Sediments with measurable AVS were not toxic. Reanalysis of the previously published data from the freshwater sediments spiked with silver showed mortality to be correlated with nominal SEM-AVS and with silver IWTU. Taken together, these results support the use of AVS and silver IWTUs in predicting the toxicity of silver in sediments.

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