THE BEHAVIOR OF NATURAL AND ANTHROPOGENIC OSMIUM IN LONG-ISLAND SOUND, AN URBAN ESTUARY IN THE EASTERN US

The Os concentration and Os-187/Os-186 distributions in surface sedime nts of Long Island Sound (eastern U.S.) provide a way of determining t he sources and estuarine transport of Os. The contribution of anthropo genic Os from sewer outfalls from the New York City region supplies a tracer with a characteristic Os-187/Os-186 Of about 1. The Os concentr ation of the bulk surface sediment increases steeply moving toward New York City in the westernmost Sound and generally follows the concentr ation of organic carbon, The Os-187/Os-186 ratio of bulk surface sedim ent increases from west to east in the westernmost part of the Sound a nd is effectively constant in the central Sound. We interpret these re sults as indicating that the surface bulk sediments of the Sound conta in a low Os-187/Os-186 component, perhaps as a reduced coating associa ted with organic remains from sewer outfalls. The acid hydrogen peroxi de leach fraction has an average Os-187/Os-186 of 9.5 in the central S ound, significantly higher than both the bulk sediment value and the p robable sea water value of about 8. The leach fraction in thr westernm ost part of the traverse is less radiogenic than the central Sound and follows the Os isotope trend of the bulk sediment. Liquid effluent fr om a New York City sewer outfall contains 30 pg l(-1) of dissolved Os with a Os-187/Os-186 of about 2.5, consistent with its being an end-me mber of the west-east sediment pattern recorded in the leach fractions of the westernmost cores. The leachable Os from the central Sound pre dominantly reflects Os in ferromanganese oxyhydroxide coatings from co ntinentally derived sediments with Os-187/Os-186 ratios more radiogeni c than seawater. The distribution patterns of anthropogenic and natura l Os, with their characteristic isotopic signatures in the Sound, and the insights gained from the behavior of other particle-reactive speci es, indicates that very little Os in solution may pass through the est uarine gauntlet.