STREAMBED SEDIMENTS OF THE ATLANTIC COASTAL PLAIN AS SITES FOR DENITRIFICATION: THE ROLE FOR SEDIMENT CARBON CONTENT AND PORE-WATER COMPOSITION AND VELOCITY

This project sought to extend findings of denitrification in streambed sediments of a single, low-relief stream on the Eastern Shore of Virginia (ESVA) sufficient to remove 70-90% of the agriculture-derived NO₃^- to other streams in that region. In these small, low-gradient watersheds, streams gain N from the discharge of contaminated groundwater, but the streambed sediments present favorable conditions to potentially decrease high groundwater NO₃^- loads to surface receiving waters. Four streams were examined for organic-matter, NO₃^-, and O­₂ concentration in the pore water at multiple depths, and vertical pore-water velocity through those sediments. The mean organic carbon content of the streambed sediment ranged from 1.3-7.3% of sediment mass. With a single exception, all sediments contained organic carbon > 0.15% in each 10-cm interval from 0 to 70 cm depth. Dissolved oxygen (DO) concentrations in the pore water at depths of 10, 30, and 60 cm ranged from 0.32–6.44 mg L^-1, with the medians for all four streams ranging from 0.84–2.93 mg L^-1. Two streams exhibited decreasing DO with depth, one stream was uniform, and one stream showed higher DO at greater depth. Nitrate concentrations in the pore water ranged from non-detect to 8.08 mg L^-1, with the medians for all four streams ranging from 0.05–6.01 mg L^-1. In the sediments of all four streams, pore-water velocity was slow enough for rate-limited microbial processes to modify pore-water composition, allowing for NO₃^- removal along the flow path of groundwater discharging to streams. Abundant carbon was found at the maximum sample depth and the low DO and NO₃^- concentrations suggest denitrification occurs at a greater depth than was examined. Conditions appropriate for substantial removal of NO₃^- were present in sediments of all of the streams examined suggesting that denitrification in streambed sediments may be the major mechanism for attenuating groundwater NO₃^- loads to streams. The four streams visited appear to have a biologically active zone with potential for denitrification that extends to a much greater depth in ESVA streams then previously reported, and may provide significant protection from eutrophication to the lagoons and bays receiving water from the agricultural uplands on the Lower Delmarva Peninsula.