U.S. Geological Survey Open-File Report 2005-1306
Continuous Resistivity Profiling Data from the Upper Neuse River Estuary, North Carolina, 2004-2005
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 the estuary and dip gradually to the northeast (DuBar and others, 1974). The upper estuary crosses the updip pinchouts or outcrops of units that host several confined aquifers and associated confining units. The unconfined surficial aquifer is hosted by Quaternary marine and estuarine deposits.
There are no significant astronomical tides in the Neuse River Estuary because the Albemarle-Pamlico estuary system is only connected to the ocean by four narrow inlets through the Outer Banks barrier island complex. Wind-induced seiches, however, can produce semi-diurnal water level fluctuations of up to about 1 m in the Neuse River Estuary (Leuttich and others, 2002). Average annual rainfall is 130-140 cm in the Outer Coastal Plain. Annual discharge to the estuary from the Neuse River averages 150 m3/s, with peak flows in March and low flows in October (typically 20% of the March flow). The Neuse contributes about 17% of the total freshwater input to the Albemarle-Pamlico estuary system. The estuary was heavily impacted by six hurricanes and tropical storms from 1998 through 2004, which created exceptionally high freshwater and sediment inputs, and caused significant shoreline erosion. The strength of estuarine circulation and stratification varies seasonally, with surface waters of the entire estuary dropping to salinities below 5 psu in the spring, and often exceeding 15 psu well upstream during the late summer and early fall. When present, the pycnocline/halocline occurs at a depth of 2-4 m in the center of the estuary. Average annual residence time for water in the Neuse River Estuary is 68 days (Robbins and Bales 1995), but this can be reduced to less than one week during major storms.
Previous hydrostratigraphic work and complementary studies
Previous studies of the regional aquifer system of the coastal plain in North Carolina were synthesized by Winner and Coble (1996). Detailed investigations of the hydrogeology of Marine Corps Air Station (MCAS) Cherry Point, which is located on the south shore of the Neuse River Estuary, have been reported by Daniel and others (1996), Cardinell (1999), and Wrege and Jen (2004). These studies have involved stratigraphic borings, well installations, and seismic surveys onshore and offshore. Significant results have included delineation of incised paleochannels that may act as conduits for groundwater flow, and that may create interconnections between confined aquifers, and, in some cases, with the surficial aquifer.
Parallel investigations designed to complement the study described here have included direct sampling of submarine groundwater and surface water for natural radionuclides that can serve as tracers of groundwater discharge (Crusius and others, 2005b), and short-term deployments of sea floor seepage meters at various sites in the Neuse River Estuary to directly measure discharge of groundwater (Spruill and others, 2005). In addition, a cooperative project to map the regional coastal sedimentary system of northern North Carolina was begun in 1999 (Thieler and others, 2001), and included seismic surveys of the Neuse River Estuary in 2004.
Primary objectives of this study
The study described here was designed to test whether substantial fresh groundwater is present under the Neuse River Estuary using electrical resistivity surveying techniques. Onshore evidence of filled paleochannels that may act as conduits suggests the potential for offshore flow and discharge of groundwater. Although the volume of SGD into the Neuse River Estuary may be small relative to the input from the Neuse River itself (average flow = 4.7 × 108 m³/yr), elevated concentrations of nitrogen species in groundwater relative to surface water may make the role of SGD in the nutrient budget of the estuary much larger than previously appreciated.
Hypotheses and implications
Based on existing onshore data and patterns observed in published studies conducted in similar settings, it is likely that regional confining units and incisions through them by lowstand fluvial systems have created complicated offshore groundwater flow and discharge geometries beneath the Neuse River Estuary. A general assumption is that the freshwater-saltwater interface in the subsurface would be found farther offshore where intact confining units or paleochannel conduits are present, where greater hydraulic heads are present in coastal aquifers due to greater shoreline relief (areas of coastal bluffs), or where surface water salinity is lower. Submarine groundwater discharge is likely larger than previously estimated due to insufficient incorporation of the sub-estuarine geologic framework into assumptions about discharge. This implies that nutrient budgets previously calculated for the estuary system are incorrect. It also suggests that particular areas of the Neuse River Estuary that are prone to exceptionally harsh environmental conditions and impacts (e.g., especially low dissolved oxygen, unusually intense blooms of harmful algae, recurring fish kills), may be affected by the presence of SGD hotspots in the vicinity.
Specific management actions that may be impacted by the findings of this study include: 1) future adaptations of the total maximum daily load (TMDL) criteria for nitrogen discharge to the estuary and monitoring targets set in 2002, as well as refinements of the water quality models developed for the estuary, 2) future development of water supplies in confined aquifers adjacent to the estuary, and 3) selection of appropriate remedial actions to address contaminant plumes originating from sites adjacent to the estuary (e.g., MCAS-Cherry Point) and impacting groundwater at various depths. For a number of reasons, water quality management within the Neuse River Estuary is relatively advanced at this time. Because of this, the scientific and management approaches used in the estuary are serving as models for addressing coastal nutrient issues elsewhere in the U.S., and beyond.