DELINEATION OF NEARSHORE FRESHWATER-SALTWATER RELATIONSHIPS IN SUBMARINE GROUND WATER USING CONTINUOUS RESISTIVITY PROFILING AND PIEZOMETER TRANSECTS IN THE NEUSE RIVER ESTUARY

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 ground-water 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 seafloor (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 ground-water 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, ground-water 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.