Predicting the toxicity of metal-contaminated field sediments using interstitial concentration of metals and acid-volatile sulfide normalizations

We investigated the utility of interstitial water concentrations of metals and simultaneously extracted metal/acid-volatile sulfide (SEM/AVS) ratios to explain the biological availability of sediment-associated divalent metals to benthic organisms exposed in the laboratory to sediments from five saltwater and four freshwater locations in the United States, Canada, and China. The amphipod Ampelisca abdita or the polychaete Neanthes arenaceodentata were exposed to 70 sediments from the five saltwater locations, and the amphipod Hyalella azteca or the oligochaete Lumbriculus variegatus were exposed to 55 sediments from four freshwater locations in 10-d lethality tests. Sediment toxicity was nor related to dry weight metals concentrations. Almost complete absence of toxicity in spiked sediments and field sediments where metals were the only known source of contamination and where interstitial water toxic units (IWTUs) were <0.5 indicates that toxicity associated with sediments having SEM/AVS ratios <1.0 from two saltwater locations in industrial harbors was not metals-related as these sediments contained <0.5 IWTU. Metals-associated toxicity was absent in 100% of sediments from the remaining three saltwater field locations, where metals were the only known source of contamination and SEM/AVS ratios were less than or equal to 1.0. Two-thirds of 45 sediments from seven saltwater and freshwater field locations having both IWTUs >0.5 and SEM/AVS ratios >1.0 were toxic. Toxicity was observed less often when SEM/AVS ratios >1.0 (39%) or IWTUs >0.5 (55%) were used alone. The difference between the molar concentrations of SEM and AVS (SEM AVS) can provide important insight into the extent of additional available binding capacity, The magnitude by which AVS binding has been exceeded, and, when organism response is considered, the potential magnitude of importance of other metal binding phases. For these reasons, SEM - AVS should be used instead of SEM/AVS ratios as a measure of metals availability. Over all published experiments with both metal-spiked and field sediments, SEM - AVS and IWTUs accurately (99.2%) identified absence of sediment toxicity and with less accuracy (79.1%) identified the presence of toxicity.