TRACE METAL CYCLING WITHIN THE SUBTERRANEAN ESTUARY OF STONY BROOK HARBOR

In coastal aquifers, the mixing of fresh groundwater with seawater occurs in a nearshore area termed the subterranean estuary (STE). The STE is a zone where geochemical reactions controlling the flux of trace metals from groundwater to surface water occur over a short temporal and spatial span. The influence of changing dissolved oxygen conditions between seawater and groundwater (i.e redox conditions) displays a unique environment for trace metal speciation that is not adequately understood. However, few studies have addressed how geochemical transformations in the STE may alter the final trace metal composition of groundwater discharged into coastal waters.

This study investigates trace metal flux and distribution in a STE where oxic groundwater interacts with suboxic seawater within a coastal aquifer at Stony Brook Harbor, Long Island, NY. Submarine groundwater is discharged (SGD) through the Upper Glacial Aquifer, which consist of highly permeable glacial deposits of a fine to medium grain quartz sand. A series of piezometer wells were also installed at a transect perpendicular to the shore line at the approximate mean high tide mark to roughly 15 meters below low tide, with depth intervals ranging 0.5-1.0m. A Thermo Finnegan ICP-MS was used to test individual samples for trace metal intensities and then compared to the salinity, pH, and dissolved oxygen gradients in order to identify correlations to these more easily obtained variables.

Results show that the saline transition zone within the STE dictates the zone of maximum concentration for Manganese and Vanadium. In contrast with other studies of trace metals in the STE, total dissolved Barium concentrations are anti-correlated with Manganese concentrations indicating a varying rate of mobilization for the two trace metals. Iron displays a positive correlation with a change in the pH of the porewater, whereas Manganese is anti-correlated with the dissolved oxygen content. The position of an intertidal S.Alternaflora marsh enriches shallow STE water with organic carbon, which may control the movement of trace metals in two ways: 1) by facilitating microbial activity that drives porewater to suboxic levels and 2) by providing a sorption surface (in the case of particulate organic carbon) that removes trace metals from porewater and binds them in the sediment.