THE EFFECT OF SEDIMENT TRANSPORT ON THE GEOCHEMICAL DYNAMICS OF METAL POLLUTANTS IN HUDSON RIVER SEDIMENTS

Sediments in the Hudson Estuary are highly contaminated with organic and inorganic pollutants (e.g., PCB's, PAH's, Hg and Cd) as a result of past industrial inputs, both upriver and within the lower estuary system. These sediments, upon remobilization may become a significant non-point source for a number of metals. Here we present the results from a year long study on the impact of sediment transport and resuspension with respect to the geochemistry of Hg, Cu, Zn, Ag, Cd and Pb within the turbidity maximum zone (TMZ). During this study, we take sediment cores from two sites, 4 miles apart, at the southern and northern extents of the TMZ on 4 and 3 occasions, respectively.

Pore-water dissolved oxygen concentration at both sites suggest that approximately the top 1cm of the sediment is oxic year round, possibly reflecting frequent resuspension by tidal currents. Furthermore, pore-water profiles of redox sensitive elements indicate that the upper 8-10 cm is chemically active on an annual time-scale. This is attributed to along-channel sediment transport, deposition and subsequent development of anoxic conditions. An example of this temporal sequence is recorded at the southern site during the October 2000 and June 2001 sampling dates. Perturbation Fe and Mn porewater profiles indicate up to 8cm of sediment is freshly deposited. The pore-water data collected at this site indicates that redox conditions are re-established shortly thereafter. As redox conditions intensify, the sediments change from a source to a sink for some elements, depending on their reducing potential. This pattern is not observed in the northern site between October 2000 and June 2001, possibly reflecting a convergence front that remains to the south during this time. Based on crustal ratios the complete sediment core (core depth=20 cm) is enriched, relative to terrestrial material, in Cu, Zn, Ag, Cd and Pb approximately, 3, 3, 100, 10, and 8 times, respectively. Samples from the TMZ site indicate that seasonal, fortnightly and tidal events (Geyer et al, this session) resuspend large quantities of highly contaminated sediment into overlying oxic waters in the Hudson River Estuary. Desorption of contaminant metals from the fine-grained, reactive, redox sensitive phases may have a significant contribution to the dissolved pool.