HR: 14:55h
AN: H23J-06    [Abstracts]
TI: The role of the streambed zone on the fate of nitrate in the Morgan Creek, MD watershed
AU: * Essaid, H I
EM: hiessaid@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025,
AU: Brayton, M
EM: mbrayton@usgs.gov
AF: U.S. Geological Survey, 5522 Research Park Dr., Baltimore, MD 21228,
AU: Hancock, T C
EM: thancock@usgs.gov
AF: U.S. Geological Survey, 1730 East Parham Rd., Richmond, VA 23228,
AU: Denver, J
EM: jmdenver
AF: U.S. Geological Survey, 1289 McD Dr., Dover, DE 19901,
AB: Understanding the watershed-scale fate of nitrate introduced through agricultural practices requires an understanding of the physical and chemical processes taking place during ground water and surface water interaction within the highly reactive streambed zone. The Morgan Creek, MD watershed is one of several agricultural watersheds being studied as part of the Agricultural Chemicals Sources, Transport, and Fate Topical Study of the U.S. Geological Survey's National Water-Quality Assessment Program. Approximately half of the annual streamflow of Morgan Creek, MD is attributed to ground-water inflow. This ground water enters the creek by flowing through the streambed, as well as discharging through seeps at the edge of the streambed. Comparison of baseflow stream-water chemistry to ground-water chemistry shows that concentrations of relatively non-reactive elements such as silica and magnesium change very little as water flows through the streambed zone. However, there is a substantial decrease in nitrate concentrations as ground water passes through the organic-rich, low-oxygen denitrifying zone of the streambed and discharges to the creek. Preliminary mass balance analysis suggests that nearly 80% of the nitrate present in near-stream ground water is removed as the ground water discharges to the stream. Although the majority of fertilizer-applied nitrate is believed to be removed in the soil and shallow unsaturated zones by the combined processes of crop uptake and harvest, volatilization, and denitrification, the attenuation of nitrate concentrations in the streambed zone significantly reduces nitrate loads in the creek. Thus, quantifying the watershed-scale mass budget of nitrate requires characterizing and quantifying ground-water discharge and nitrate concentration changes in the streambed zone.
DE: 1830 Groundwater/surface water interaction
DE: 1831 Groundwater quality
DE: 1871 Surface water quality
DE: 1879 Watershed
SC: Hydrology [H]
MN: 2007 Fall Meeting