Streambed Sediments of Virginia Eastern Shore Streams Are Poised for Pore-Water Denitrification

Beneath Virginia Eastern Shore (VAES) streams are extensive biologically active zones poised for denitrification, which have the potential to decrease high groundwater nitrate loads to coastal receiving waters. Leached fertilizer from dense agricultural land use on the VAES has resulted in nitrate concentrations exceeding the Federal drinking water standard of 10 mg N03‘-N L'1 in the underlying, unconfined Columbia aquifer. In addition to human health concerns, large nitrate loads to surface waters can disrupt ecosystems through eutrophication. A part of the riparian buffer system that has recently received attention as a site of substantial denitrification, given the right conditions, is streambed sediment. For this research, four streams on the VAES were selected for a regional study to evaluate the streambed conditions of carbon availability, vertical pore-water velocity, and chemical analysis of pore-water. Many studies cite low carbon supply as a limitation to denitrification at depth in sediment, however an abundant supply of carbon was found throughout streambed sediment at all four streams. At each stream, abundant carbon (total organic matter) existed in 70-900f the sediment. At three streams the pore-water at the maximum sample depth of ~60 cm had median dissolved oxygen (DO) concentrations ranging from 0.82-1.30 mg L'1,' the depleted DO was taken to indicate microbial activity at depth. The fourth stream had higher DO concentrations in pore water. Two of the streams had low pore-water nitrate concentrations throughout the sediments, while the other two streams, Coal Kiln and Parker Creek, had a transverse pattern of higher nitrate and DO concentrations along the sides of the channel sediments. Coal Kiln and Parker Creek iii had positive correlations of DO and nitrate, likely indicating denitrification was inhibited by D0, which is expected as denitrification is an anaerobic microbial process. Prior studies of the Columbia aquifer on the VAES have reported nitrate concentrations more than double the Federal drinking water standard. If the elevated nitrate concentrations are widespread throughout the Columbia aquifer on the VAES, then there is a large amount of nitrate removal along the flow path of groundwater discharging to streams. In the sediments of the four streams, porewater velocity was slow enough for microbial processes and carbon was abundant to greater depths than previously reported. Throughout the streambed sediments of two streams and in the center channel sediments of the other two streams, the low DO and low nitrate concentrations suggest denitrification at a greater depth than measured. Overall, discharging groundwater diluted stream-water nitrate concentrations at three of the streams. The streambed sediments of the four VAES streams studied have similar characteristics and appear to be an important part of the riparian buffer system.

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