Continuous Resistivity Profiling Data from the Upper Neuse River Estuary, North Carolina, 2004-2005

Cross, VeeAnn A.; Bratton, John F.; Bergeron, Emile; Meunier, Jeff K.; Crusius, John; Koopmans, Dirk

Abstract:

Introduction: The Neuse River Estuary in North Carolina has suffered impacts of eutrophication in recent years. As part of a larger project to better constrain nutrient budgets in the estuary, field investigations were performed to study occurrence and discharge of fresh and brackish ground water and nutrients beneath the estuary itself (fig. 1). A Continuous Resistivity Profiling (CRP) system (Manheim and others, 2004) was used to map the depth of the freshwater-saltwater interface (FSI) in sub-estuarine groundwater. This study area serves as a typological representation of a submarine groundwater environment characteristic of a shallow estuary in a wide coastal plain that has not experienced glaciation. Similar settings extend from New Jersey to Georgia, and along the Gulf of Mexico in the U.S. This report archives 29 lines of data collected during 2004 and 2005 surveys representing almost 210 km of survey lines. These data are further explained in the Data Processing section of the report and previews available of the processed data are available. Eutrophication background -- The Neuse River Estuary suffers from frequent summer fish kills, especially of menhaden, attributed directly or indirectly to natural and anthropogenic eutrophication. Some evidence indicates that these events have become more frequent in recent years. An increasing excess of nutrients is likely derived from accelerating agricultural and residential development of the watershed over the last few decades. Impacts have included declines in dissolved oxygen in stratified estuary water due to consumption by decaying algal biomass, as well as blooms of the toxic dinoflagellate, Pfiesteria piscicida (Burkholder and others, 2005; but see Drgon and others, 2005). Several groups have attempted to develop well-constrained nutrient budgets for the estuary (e.g., Christian and Thomas 2003), and typically have used estimates of direct groundwater delivery in the range of 5% of the total nutrient load. None of the estimates, however, has incorporated specific field measurements in the estuary to determine the actual contribution from direct groundwater discharge. Particular geological features of the estuary suggest that it may receive substantial direct discharge. Background on submarine groundwater discharge -- Contributions of groundwater to flow in rivers have historically been estimated using hydrograph separation (e.g., Sloto and Crouse 1996), but no similar method exists for estuaries. Most efforts have relied on flow modeling based on extrapolation of onshore data on recharge and groundwater flow velocities into the offshore, often assuming that most discharge occurs within a few meters of the shore. Radioisotopic tracer techniques have recently been developed to permit estimation of direct submarine groundwater discharge (SGD) to estuaries and coastal embayments. These approaches can be useful for calculating SGD for regional budgets, and can also be used to broadly identify discharge hotspots (Burnett and Dulaiova 2003; Crusius and others, 2004, 2005a, 2005b). They do not, however, provide subsurface information on the geological controls on the occurrence of freshened groundwater beneath brackish to saline surface water and the style of discharge (i.e., focused vs. diffuse). Geographic and geologic setting -- The Neuse River Estuary (fig. 1) is a drowned river valley located in the Tidewater Region, or the Outer Atlantic Coastal Plain, of North Carolina. It is a tributary of the large Albemarle-Pamlico estuarine system. The estuary has a distinct ‘V‘ shape, with the upper limb oriented NW-SE and the lower limb oriented SW-NE. The hinge is located where the estuary narrows and crosses the Minnesott sand ridge. This feature is a subaerial barrier island complex correlative with the regional Bogue-Suffolk Scarp that separates two marine terraces of different ages to the east and west. Stratigraphic units of Eocene to Pliocene age underlie

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USGS Numbered Series

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79519

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Cross, VeeAnn A.; Bratton, John F.; Bergeron, Emile; Meunier, Jeff K.; Crusius, John; Koopmans, Dirk

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ENGLISH

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CD-ROM; raw and processed data files

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Open-File Report

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OFR

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2005-1306

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Continuous Resistivity Profiling Data from the Upper Neuse River Estuary, North Carolina, 2004-2005; 2006; OFR; 2005-1306; Cross, VeeAnn A.; Bratton, John F.; Bergeron, Emile; Meunier, Jeff K.; Crusius, John; Koopmans, Dirk