INSIGHTS INTO NITROGEN FLUXES: QUANTIFYING VARIATIONS IN GROUNDWATER-STREAM CONNECTIONS

Significant nutrient nitrogen (N) that degrades coastal waters comes from groundwater that discharges through base flow in streams; thus, understanding the movement of N from groundwater to streams is essential. Our previous research demonstrated that denitrification in streambed sediments of a single, low-relief, gaining stream on the Delmarva Peninsula in Virginia removed 70-90% of the agriculture-derived nitrate (NO₃^-) from the groundwater. The present research tests the general potential for similar behavior in other streams that might host favorable conditions to reduce high groundwater NO₃^- loads to surface waters. Surface-water N flux in 4 streams during water year 2011 ranged from ~3 to 7 kg NO₃^- per ha watershed area. To elucidate differences in hydraulic properties and biogeochemical factors active in the streambed sediments, piezometers were installed in each stream at ~60, 100, and 150 cm below the sediment-water interface to allow determination of NO₃^- concentration at various depths. Hydraulic head measurements revealed an upward gradient with elevations measured in groundwater piezometers ranging from 4 to 20 cm higher than the stream-surface elevation. Three 70-cm sediment cores were collected at each stream. Porewater samples were extracted from the cores at 10-cm intervals for determination of NO₃^- concentrations. Variability in permeability along the cores was measured using small falling-head permeameters, and percent organic matter was determined by ignition. Overall, NO₃^- tended to increase with depth and organic matter tended to decrease. Sediment characteristics varied among streams with some streams having sandier bed material and others having finer-grained, organic-rich material. Flux tended to increase with head differential suggesting that residence time in the sediments controls NO₃^- discharge to the streams.