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Summary Information for Field Activity 2005-014-FA

Area of Operations: Neuse River Estuary, North Carolina, United States, North America, North Atlantic

Dates: May 01, 2005 to May 12, 2005

Chief scientist: John Bratton

   Reide Corbett, East Carolina University

Objectives: Supplemental field investigation: radon mapping, electrical resistivity survey, and surface water and ground water sampling.

Type of Activity: Radon mapping, resistivity survey, water sampling;Radon mapping, resistivity survey, water sampling

Information to be derived: Electrical resistivity data

Summary: Resistivity data compiled in OFR 2005-1306 and presented at GSA (abstract below). Radon data presented at Estuarine Research Federation meeting in Norfolk, VA, October 2005 (abstract below). 2005 Salt Lake City Annual Meeting (October 16-19, 2005), Paper No. 211-13: DELINEATION OF NEARSHORE FRESHWATER-SALTWATER RELATIONSHIPS IN SUBMARINE GROUND WATER USING CONTINUOUS RESISTIVITY PROFILING AND PIEZOMETER TRANSECTS IN THE NEUSE RIVER ESTUARY BRATTON, John F., Coastal and Marine Geology Program, U.S. Geological Survey, 384 Woods Hole Rd, Woods Hole, MA 02543-1598, CRUSIUS, John, USGS, Woods Hole, MA 02543, CROSS, VeeAnn A., USGS, Woods Hole, MA 02543, and KOOPMANS, Dirk J., ETI Professionals, Inc, USGS, Woods Hole, MA 02543. The Neuse River Estuary (NC), a broad V-shaped water body (avg. ~70 km x 6.5 km x 3.6 m) located on the southwestern end of Pamlico Sound, suffers from severe eutrophication. Several water quality models have recently been developed to aid in management of nutrient loading to the estuary. To constrain model estimates of the fraction of nutrients delivered by direct groundwater discharge, field measurements were made in April 2004 and May 2005. Continuous resistivity profiling (CRP) was used to measure electrical resistivity of sediments, a property that is sensitive to differences in salinity of submarine ground water. The 2004 and 2005 surveys used floating 100-m and 50-m CRP streamers, respectively. A total of ~200 km of data was collected in the upstream half of the estuary and processed using AGI EarthImager 2D software. Penetration was ~20-27 m below the sea floor (mbsf) for the 100-m streamer, and ~12-14 mbsf for the 50-m streamer. At four transect sites extending up to 70 m from shore, piezometers were hand-driven to depths of up to 4 mbsf in water depths of up to 2.5 m to collect groundwater samples for measurement of salinity, nutrients, and radon and radium isotopes. Data from CRP surveys indicated that high-resistivity (fresher) ground water is present at depths of ~3-5 mbsf in a zone ~100 m wide parallel to shore that becomes narrower downstream as the estuary widens and becomes more saline. This is consistent with piezometer samples that yielded salinities of <1 psu 35-50 m from shore at some locations. At several piezometer sites, groundwater samples were more saline than overlying waters, suggesting that shallow ground water reflects average annual salinities while surface water salinity varies seasonally. The depth to fresher ground water increases offshore, gradually at first and then sharply. In some upstream areas, fresher water reappears extending more than 1 km offshore at a consistent depth of 3-5 mbsf. A brackish zone more than 10 m thick separates the nearshore and offshore fresher zones. Changes in underlying geology may be partially responsible for this. These survey results will be used in combination with measurements of radon and radium in surface water, as well as seepage meter measurements to calculate the quantity of ground water and the associated nutrient load being delivered to the estuary. ERF 2005 meeting abstract: Submarine groundwater discharge to the Neuse River Estuary (NC) determined from continuous radon measurements; Author(s) Crusius, J., U.S. Geological Survey, Bratton, J. F., U.S. Geological Survey, Koopmans, D., U.S. Geological Survey, Spruill, T., U.S. Geological Survey, Corbett, D. R., East Carolina University; Type Poster and Summary; Session SPS-04 - Identifying, Assessing, and Managing Human and Climatically-Induced Change of Estuarine Ecosystems. In many coastal waters submarine groundwater discharge (SGD) is an important vector for delivery of nutrients. However, SGD is frequently diffuse and difficult to quantify. In this work we sought to infer locations and rates of groundwater discharge to the Neuse River Estuary, North Carolina, using measurements of radon in surface water and groundwater. Radon is an excellent tracer of SGD because it: 1) is strongly enriched in groundwater relative to surface water; 2) is non-reactive; 3) is continuously supplied by long-lived parent isotopes; and 4) integrates over a wide area. We carried out continuous measurements of radon during a two-day period of April, 2004 in the surface water offshore of Cherry Point where seismic evidence suggested a buried paleochannel filled with coarse sediments might be a conduit for groundwater discharge. Elevated radon activities observed near this site are consistent with groundwater discharge in this vicinity. Radon activities measured continuously at a nearshore site were roughly inversely proportional to tidal height. Groundwater discharge velocities calculated from the radon data averaged 6 cm/d during a two-day interval of relatively low tide. These values are comparable to estimates based on seepage meters. Additional fieldwork is planned for the spring of 2005. Modified abstract presented at meeting: Submarine groundwater discharge to the Neuse River Estuary (NC) determined from continuous radon measurements: John Crusius; John Bratton; Dirk Koopmans: USGS; Woods Hole Science Center; 384 Woods Hole Road; Woods Hole, MA 02543. Timothy Spruill; U.S. Geological Survey, 3916 Sunset Ridge Road, Raleigh, NC 27607; D. Reide Corbett; Department of Geology; East Carolina University; Greenville, NC 27858 In many coastal waters submarine groundwater discharge (SGD) is an important source of nutrients. However, SGD is frequently diffuse and difficult to quantify. In this work we sought to infer locations and rates of groundwater discharge to the Neuse River Estuary, North Carolina, using measurements of radon in surface water and groundwater. Radon is an excellent tracer of SGD because: 1) it is strongly enriched in groundwater relative to surface water; 2) it behaves conservatively over the full estuarine salinity range; 3) it is continuously supplied by long-lived parent isotopes; and 4) quantification of discharge based on radon measurements integrates over a large area. We carried out measurements of radon spanning a 50 km stretch of the Neuse River during early May, 2005. Radon was measured continuously from a ship using a method similar to that described by Burnett et al. (2001). The highest radon activities (200 Bq m-3 = 12 dpm L-1) were observed in the northern ~20 km of the sampling area (north of New Bern). These radon activities are roughly 30 times higher than can be supported by diffusive inputs from sediments, and are therefore attributed to significant groundwater discharge in this region of the river. A steady-state box model suggests that the radon inventories can be explained by groundwater discharge that is comparable to ~2 to 10% of the river flow. Reducing the uncertainty in this estimate will require reduction in the uncertainty in the radon content of groundwater.

Contact: John Bratton (jbratton@usgs.gov)

InfoBank: uses the identifier B-1-05-NR. Visit this site for more information.

Related Links: http://soundwaves.usgs.gov/2007/10/meetings.html

Platform Info:

Publications resulting from these data

Cross, VeeAnn A., Bratton, John F., Bergeron, Emile, Meunier, Jeff K., Crusius, John, and Koopmans, Dirk,2006, Continuous resistivity profiling data from the Upper Neuse River Estuary, North Carolina, 2004-2005: U.S. Geological Survey Open-File Report 2005-1306, CD-ROM. (Also available at http://pubs.usgs.gov/of/2005/1306/)

Bratton, J.F.,2007, The importance of shallow confining units to submarine groundwater flow, in Sanford, W., Langevin, C., Polemio, M., and Povinec, P., eds., A New Focus on Groundwater-Seawater Interactions: International Association of Hydrological Sciences, v. 312: p. 28-36

Data Types and Locations:

GeochemistryGeophysicsLogsResistivitySamples

 

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