The behavior of natural and anthropogenic osmium in Long Island Sound, an urban estuary in the eastern U.S.

The Os concentration and187Os/186Os distributions in surface sediments of Long Island Sound (eastern U.S.) provide a way of determining the sources and estuarine transport of Os. The contribution of anthropogenic Os from sewer outfalls from the New York City region supplies a tracer with a characteristic187Os/186Os of about 1. The Os concentration of the bulk surface sediment increases steeply moving toward New York City in the westernmost Sound and generally follows the concentration of organic carbon. The187Os/186Os ratio of bulk surface sediment increases from west to east in the westernmost part of the Sound and is effectively constant in the central Sound. We interpret these results as indicating that the surface bulk sediments of the Sound contain a low187Os/186Os component, perhaps as a reduced coating associated with organic remains from sewer outfalls. The acid hydrogen peroxide leach fraction has an average187Os/186Os of 9.5 in the central Sound, significantly higher than both the bulk sediment value and the probable sea water value of about 8. The leach fraction in the westernmost part of the traverse is less radiogenic than the central Sound and follows the Os wsotope trend of the bulk sediment. Liquid effluent from a New York City sewer outfall contains 30 pg l−1 of dissolved Os with a187Os/186Os of about 2.5, consistent with its being an end-member of the west-east sediment pattern recorded in the leach fractions of the westernmost cores. The leachable Os from the central Sound predominantly reflects Os in ferromanganese oxyhydroxide coatings from continentally derived sediments with187Os/186Os ratios more radiogenic than seawater. The distribution patterns of anthropogenic and natural Os, with their characteristic isotopic signatures in the Sound, and the insights gained from the behavior of other particle-reactive species, indicates that very little Os in solution may pass through the estuarine gauntlet.