Metal speciation and bioavailability in contaminated estuary sediments, Alameda Naval Air Station, California

Measurements of simultaneously extracted metals (SEM), acid volatile sulfide (AVS), and invertebrate toxicity were combined with X-ray absorption spectroscopy (XAS) to evaluate metal speciation and ecological hazard of contaminated sediments from the Seaplane Lagoon, Naval Air Station Alameda (CA). This site is characterized by moderate to low toxicity in surface sediments and by metal concentrations in sediments and porewaters that increase with depth. Standard l-h Sigma SEM/AVS measurements for surface sediments were compared with time-series (0.25-24 h) measurements of metal and sulfide release from sediments at 30 cm. Results show that AVS is rapidly and completely evolved after 1 h, but metal extraction continues with time and is not complete after 24 h. Sediment-water interface tests of invertebrate toxicity using sand dollar embryos (D. excentricus) and adult amphipods (E. estuarius) exposed to intact cores showed no to low toxicity in surface sediments. In sediments from 30- and 60-cm depth, high toxicity in several replicates was attributed to factors other than metal concentrations, such as high dissolved ammonia or low dissolved oxygen concentrations. Metal speciation and bonding determined from XAS show that cadmium (100%), zinc (approximate to 80%), and manganese (approximate to 50-70%) are associated with monosulfide phases in the sediments, The remaining fraction of zinc and manganese and all of the chromium and lead are ligated by oxygen atoms, indicating association with oxide, carbonate, or silicate minerals. Iron is present in the sediments in two fractions, as Fe(ll) in the sulfide phase pyrite and as oxygen-ligated octahedral iron, probably associated with clay minerals. Bulk chemical measurements of porewaters and sediments, and speciation information from XAS, suggest that AVS could be accounted for by volatilization of porewater sulfide. Our results indicate that metals are removed from porewaters by formation of monosulfide phases only for cadmium and partially for zinc and manganese but not for lead or chromium, even though these are reduced, anoxic sediments typical of a restricted marine estuary environment. Comparison of geochemical, spectroscopic, and biological data provides new insight for the interpretation of Sigma SEM/AVS measurements and points out the need for synergistic biological/geochemical tests for determining potential ecological hazard.