The Impacts of Variations in Groundwater-Stream Hydrologic Connectivity on Nitrogen Fluxes on the Eastern Shore of Virginia

Abstract:

Nitrogen (N) pollution of natural waters leads to severe environmental problems including eutrophication. A large portion of the N load is stored in groundwater before being discharged through base flow into streams. Understanding the groundwater-stream connection is crucial for determining the N flux to coastal waters. Nitrate in the groundwater and streamwater was studied in 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 collected 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. One site, Tommy’s Ditch, consistently had the highest concentrations, but also had the lowest groundwater NO₃^- concentrations. Another site, Cobb Mill Creek, had groundwater NO₃^- concentrations which were dramatically larger than the groundwater at the other streams (8.5 to 13.3 mg NO₃^--N L^-1 compared to <0.1 to 3 mg NO₃^--N L^-1). The notable differences in the N concentration of water at Cobb Mill Creek compared to the other streams may be related to their respective hydrologic settings. The relatively great topographic relief at Cobb Mill Creek may favor deep groundwater circulation, and such a flowpath could cause groundwater to bypass favorable zones for dentirfication, resulting in higher N concentrations persisting at very shallow depths under the stream channel.