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Fieldwork

Integrated Science Team Deploys New Tools to Study Submarine Ground Water in North Carolina


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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.

Neuse River: Neuse River Estuary in North Carolina, showing survey tracklines. [larger version] index map
Neuse River Estuary in North Carolina, showing survey tracklines.

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 radon—a naturally occurring gas produced by the radioactive decay of uranium and other trace elements in common minerals—is 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 piezometers—temporary 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.

Electrical-resistivity streamer being towed behind the survey vessel An offshore ground-water sample being collected by using the drive-point piezometer system.
Above left: Electrical-resistivity streamer being towed behind the survey vessel.

Above right: An offshore ground-water sample being collected by using the drive-point piezometer system. The research vessel Beeliner is in the background, making high-resolution radon measurements while anchored in shallow water.

Water-spray chamber for degassing of radon from surface water  three attached devices for measuring radon at offset intervals
Radon measurement: Water-spray chamber for degassing of radon from surface water (left), and three attached devices for measuring radon at offset intervals (right). This arrangement provides dissolved-radon measurements about every 10 minutes, or every half-mile for a research vessel traveling at 3 mph.

Plot suggesting an inverse relation between radon concentration and tidal height.
Above - Radon Concentration: Plot suggesting an inverse relation between radon concentration (a proxy for ground-water discharge) and tidal height. Although other interpretations are possible, the most likely explanation is that ground-water discharge is higher when tide level is low. m, meters; dpm, disintegrations, or number of atoms of radon decaying, per minute.

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.


Related Sound Waves Stories
Submarine Ground-Water Discharge and Its Influence on Coastal Processes and Ecosystems
June 2004
Studying Submarine Ground Water in Rhode Island Under Arctic Conditions
March 2004
Studying Underwater Water in the Land of Misty—Chincoteague Bay, Maryland
October 2003
Progress in Delineating Submarine Ground-Water Discharge in Delmarva Coastal Bays
June 2002

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in this issue: Fieldwork
cover story:
Benthic Habitats in Glacier Bay

North Carolina Submarine Groundwater

Research Mucus-Hosted Microbial Communities

Gulf of Maine Mapping Initiative

Forensic Geology Assists Investigation

Submarine Groundwater Discharge

Outreach
USGS Participates in Marine Quest X

Meetings
Caribbean Tsunami Hazard Workshop

Awards
USGS wins Blue Pencil, Gold Screen Awards

Publications
June Publications List


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Updated April 15, 2014 @ 01:53 PM (JSS)