2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 309-16
Presentation Time: 12:45 PM


COSANS, Cassandra L., HERMAN, Janet S. and MILLS, Aaron L., Dept. of Environmental Sciences, University of Virginia, PO Box 400123, Charlottesville, VA 22904-4123

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.