Insights into Nitrogen Fluxes: Quantifying Variations in Groundwater-Steam Hydrologic Connectivity

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. Nitrate in the groundwater and streamwater was studied at four low-relief, gaining coastal streams on the Delmarva Peninsula of Virginia with the goal of identifying factors important in controlling N flux. Saturated hydraulic conductivity (K_s) was measured by rising and falling head tests in piezometers installed at depths of 60, 100, and 150 cm below the sediment surface and in 70-cm-long sediment cores taken at each stream. Percent organic matter was determined along the sediment cores through weight-loss on ignition. Percent organic matter was inversely related to K_s, but both were extremely variable among the different samples at each stream, differing as much within a single stream as among streams. K_s was between <0.006 and 0.24 cm/sec, while organic matter content ranged from 0.3 to 42%. Nitrate concentrations in the streamwater were fairly similar among the streams and were always less than 8 mg NO₃^--N L^-1, although Tommy’s Ditch consistently had the highest concentrations. Tommy’s Ditch also had the lowest groundwater NO₃^- concentrations. Cobb Mill Creek had groundwater NO₃^- concentrations that ranged from 8.5 to 13.3 mg NO₃^--N L^-1, which were dramatically larger than the groundwater at the other streams (range of <0.1 to 3 mg NO₃^--N L^-1). The notable differences in the N content of water at Cobb Mill Creek compared to the other streams is likely a direct result of their respective hydrologic settings. The relatively great topographic relief at Cobb Mill Creek favors deep groundwater circulation, and such a flowpath results large hydraulic gradients and higher N concentrations persisting at very shallow depths under the stream channel.