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A U.S. Geological Survey (USGS) team from Massachusetts, Virginia, and North Carolina, along with a graduate student and a boat driver from East Carolina University (ECU), spent the week of April 19-23 on the Neuse River Estuary in North Carolina, conducting studies related to submarine ground-water discharge. Such discharge may be one of the processes that deliver excess nutrients to the estuary, where the nutrients can trigger algal blooms that deplete the water of dissolved oxygen and cause other disruptions to the environment. The Neuse River Estuary has been the site of many fishkills in recent years, several of which have been associated with the occurrence of low-dissolved-oxygen events.
The joint field operation in April was supported by the USGS Eastern Regional Director's Office to address the USGS science priority "Water Availability for Human and Ecological Needs" through the USGS Eastern Region Integrated Science Fund. Work performed was intended to link previous modeling studies of regional aquifers, plus site-specific work at Marine Corps Air Station Cherry Point, to the surface-water quality of the estuary, concentrating on the delivery of nutrients to the estuary from ground water.
The specific focus was to map out the submarine hydrogeologic framework and to quantify discharge. The work was carried out by three teams; one team worked from the research vessel Beeliner, conducting continuous electrical-resistivity profiling and using a new system for near-continuous and simultaneous measurement of dissolved radon. The electrical-resistivity profiling allowed the scientists to map fresh and saline water layers below the bottom of the estuary, because bottom sediment permeated by saline water is a better conductor of electricity than bottom sediment permeated by fresh water. Radon measurements allowed the scientists to detect ground-water discharge into the estuary, because dissolved radona naturally occurring gas produced by the radioactive decay of uranium and other trace elements in common mineralsis far more concentrated in ground water than in surface water. Surface water (fresh or salty) that contains significant radon concentrations indicates recent or ongoing contributions from discharging ground water.
A second team worked from shore, setting up continuous-radon-measurement devices at several stations in the estuary to detect changes over tidal cycles. This team also collected ground-water samples from wells and from a cross-shore transect, using drive-point piezometerstemporary wells that enabled the scientists to collect ground-water samples from discrete depths.
The third team measured hydraulic heads in wells and offshore piezometers. Members of this team also deployed seepage meters to measure discharge from the sediment at multiple sites around the estuary and along selected shore-to-channel transects.
Significant early results include the detection of a region of high ground-water discharge at one beach location and electrical-resistivity imaging of a freshened plume of ground water beneath the central part of the estuary, possibly associated with a former channel of the river that is now buried and filled with permeable sediment. Data from one site suggest that tidal variation may have influenced ground-water discharge there. Future efforts will focus on integrating results from all three teams as processed data and analytical results become available.
Participants in the April work included Tim Spruill, Beth Wrege, Eric Sadorf, and Erik Staub (USGS, Raleigh, NC); Jeff Meunier (USGS, Reston, VA); Eric Diaddorio and Erin Must (ECU); Matt Allen (Woods Hole Oceanographic Institution); and John Crusius, Emile Bergeron, Dirk Koopmans, and John Bratton (USGS, Woods Hole, MA). A Woods Hole-ECU team will be back out on the Neuse River Estuary this summer, collecting seismic-reflection data as part of a related project on the geologic framework of coastal North Carolina. Additional submarine ground-water surveys and sampling are planned for low-flow conditions during fiscal year 2005.
in this issue:
North Carolina Submarine Groundwater
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