Bratton Et Al. 2004

1021-1034 GW Ocean Issue 04 9/3/04 2:41 PM Page 1021 Ground Water Beneath Coastal Bays of the Delmarva Peninsula: Ages and Nutrients by John F. Bratton1, John Karl Böhlke2, Frank T. Manheim3, and David E. Krantz4 Abstract To complement a large-scale geophysical investigation of occurrence and discharge of fresh water beneath Delaware, Maryland, and Virginia (Delmarva) coastal bays, we measured (1) salinity and nutrient concentrations in ground water samples from several offshore coring sites and (2) a suite of chemical and isotopic parameters, includ- ing age tracers, in ground water samples from a Delaware site. Samples were collected in a variety of Holocene and Plio-Pleistocene sediments in nearshore and offshore areas of the bays. Ground waters that were significantly fresher than overlying waters were found in plumes up to at least 15 m thick extending to more than 500 m offshore in some areas. Steep salinity and nutrient gradients occur within a few meters of the sediment surface in most locations studied. The zone of transition from deeper fresher waters to shallower brackish waters is generally thin near shore, but thickens and becomes more gradual offshore. Ground water ages at the Delaware site were mostly < 50 yr in both fresh waters and brackish waters up to 22 m below the bay bottom. Water chemistry and age data indicate that fresh water plumes beneath the estuary are active extensions of the surficial aquifer carrying nitrate from recharge areas on land, whereas brackish ground water surrounding the fresh water plumes is recharged beneath the estuary and con- tains ammonium and phosphate released by diagenesis of shallow estuarine sediments. Denitrification affects some of the fresh water nitrate before it mixes with brackish ground water or discharges to surface water. Introduction Pavlik 1990), and Massachusetts (Valiela and Teal 1979; Fresh or brackish ground water has been shown to Charette et al. 2001). exist in submarine sediments on a range of scales and in a An issue of particular environmental concern along variety of geologic settings (Moore 1996; Church 1996). much of the Atlantic Coast, especially in the Delmarva Areas on the Atlantic coast of the United States where this Peninsula, is the relative contribution of submarine ground phenomenon has been studied most intensively include the water discharge to the total flux of nutrients (especially outer continental shelf (Hathaway et al. 1979; Kohout et al. nitrate) to coastal bays and estuaries, because eutrophication 1988), as well as areas closer to shore in Florida (Corbett et is the primary environmental problem in many of these set- al. 2000a; Corbett et al. 2000b; Swarzenski et al. 2001), tings. Bachman and Phillips (1996) and Dillow and Greene South Carolina (Moore and Shaw 1998), New Jersey (1999) estimated nitrate loads from ground water discharg- (Paulachok et al. 1985), New York (Bokuniewicz and ing to streams and directly to the shallow Maryland coastal bays (Newport, Sinepuxent, Isle of Wight, and parts of Chincoteague and Assawoman bays) (Figure 1) using large datasets of nitrate concentrations from wells and streams, and a qualitative flow-net analysis and water budget. Andres 1U.S. Geological Survey, 384 Woods Hole Rd., Woods Hole, MA 02543; (508) 457–2254; fax (508) 457–2310; jbratton@usgs. (1987, 1992), Cerco et al. (1994), and authors cited in these gov publications made similar estimates for Indian River Bay 2U.S. Geological Survey, 431 National Center, 12201 Sunrise and Rehoboth Bay in Delaware, but used different tech- Valley Dr., Reston, VA 20192 niques. In these studies, assumptions were made concerning 3 U.S. Geological Survey, retired the location of ground water discharge and the microbial 4Department of Earth, Ecological, and Environmental Sci- ences, The University of Toledo, 2801 West Bancroft St., MS 604, transformation of nitrogen species en route to discharge. Toledo, OH 43606 These assumptions are as follows. (1) All ground water Copyright © 2004 by the National Ground Water Association. recharged in the watershed discharges directly into streams Vol. 42, No. 7—GROUND WATER—Oceans Issue 2004 (pages 1021–1034) 1021 1021-1034 GW Ocean Issue 04 9/3/04 2:41 PM Page 1022 Figure 1. Location maps for Delmarva coastal bays and sketch maps showing coring and sampling locations at the Indian River Bay (Holts Landing) and Chincoteague Bay (Public Landing) study sites. or coastal bays (i.e., no ground water passes under bays and We provide new data here on subbay hydrostrati- barriers and discharges directly into the Atlantic Ocean). 2) graphic relationships, ground water flow systems, and All discharge to the bays comes from the surficial aquifer. chemical transformations of dissolved nutrients prior to (3) Nitrate concentrations in discharging water are the same discharge. Ground water sampling from cores and drive- as those measured in onshore wells (i.e., no denitrification point samplers permits calibration of geophysical data from takes place in anoxic aquifers and shallow sediment pore these sites. By measuring salinity, nutrient concentrations, waters prior to discharge). Our investigation evaluates and isotopic composition of nitrogen species, dissolved gases, refines some of these assumptions. The ultimate aim of this and age indicators in ground waters, we test multiple work is to complement the modeling approaches by better hypotheses about the geometry, geochemistry, and style of constraining the general conceptual framework within submarine discharge of nutrients into the Delmarva coastal which model estimates are made of nutrient loading from bays. The results of this investigation have broader appli- discharge of ground water to coastal bays. cation to coastal nutrient studies in other regions, as well as Previous geophysical and coring studies (Belknap et to marine nutrient and elemental cycling on a global scale. al. 1994; Kerhin et al. 1999) have shown that filled paleo- channels underlie the axes of many coastal bays found on the Delmarva Peninsula. Rivers eroded these channels during low stands of sea level during previous glacial periods. Methods Stillstands of sea level, both below and above modern sea Samples of ground waters (also referred to as pore level, have also left ridges of beach sand parallel to the waters or pore fluids in shallow submarine sediments) were modern shoreline exposed at the ground surface or buried collected in two coastal bays during this investigation— in the shallow subsurface. Both sand ridges and paleochan- Chincoteague Bay, Maryland (adjacent to Public Landing), nels are likely to influence submarine transport and dis- and Indian River Bay, Delaware (adjacent to Holts Landing charge of ground water in the coastal zone. Streaming and State Park on White Neck) (Figure 1). Coring was done first downhole resistivity investigations (Manheim et al. this in Chincoteague Bay in June 2000 to test the capabilities of issue; Krantz et al. this issue) detected subsurface occur- a hovercraft-mounted vibracoring rig (Hoverprobe 2000, rence of fresh and brackish ground water associated with Hovertechnics Inc., Eau Claire, Michigan) and to provide an such features underlying Delmarva coastal bays. offshore transect of subsurface samples perpendicular to 1022 J.F. Bratton et al. GROUND WATER 42, no. 7: 1021–1034 1021-1034 GW Ocean Issue 04 9/3/04 2:41 PM Page 1023 both the shoreline and an onshore scarp located ~1.5 km ridometer data were in good agreement with refractometer inland. The second study area, cored in October 2001, results, so only refractometer values are reported here. adjoined Holts Landing State Park along the southern shore Pore-fluid samples for nutrient analyses were filtered of Indian River Bay in Delaware. Sampling locations there (0.2 µm pore size) in the field, as previously described, and were selected in order to (1) verify results of previous resis- stored frozen prior to analysis at the University of + tivity surveys by targeting paired low-resistivity (brackish) Delaware. Dissolved ammonium (NH4 ), nitrate + nitrite – – 3– and high-resistivity (fresh) sites, as well as to (2) determine (NO3 + NO2 , abbreviated NO2+3), phosphate (PO4 ), and the influence of paleotopography on ground water flow by reactive silicate (Si(OH)4, abbreviated Si) were determined choosing nearshore and offshore targets in a known incised by automated colorimetry using an O/I Analytical Flow paleovalley imaged in previous seismic surveys. The Indian Solutionா IV Analyzer (O.I. Analytical, College Station, + River Bay cores were collected using a vibracoring rig sim- Texas). Concentrations of NH4 were determined by the ilar to that mounted on the Hoverprobe 2000, but in this case phenol hypochlorite method (Glibert and Loder 1977; a trailer-mounted rig was secured to the deck of a shallow- Grasshoff and Johansen 1972). NO2+3 concentrations were draft barge. Coring locations are shown on the site maps in determined by the sulphanilamide/N(1-napthyl) ethylene – – Figure 1. diamine method after cadmium reduction of NO3 to NO2 3– (Glibert and Loder 1977). PO4 was determined by the Sediment Squeezing and Analysis phospho-molybdenum blue method and Si by the silico- Most subsurface fluid samples were obtained using molybdate blue method (Strickland and Parsons 1972). + 3– manual or hydraulic press methods to separate fluids from Typical precisions for analyses of NH4 , NO2+3, PO4 , and saturated sediments. To extract pore fluid with minimal dry- Si were ~ ± 0.2 to 1, ± 1 to 5, ± 0.02 to 0.05, and ± 0.5 to 1 ing, subsamples were collected either from cores through µmol/L, respectively (Savidge 2001). small holes cut in the core liners prior to longitudinal splitting or from exposed core faces shortly after the splitting of Drive-Point Sampling and Analysis core sections. Saturated samples were either pressed imme- At some coring locations, submarine ground water diately or placed in sealed containers until they could be samples were collected from discrete depths below the bay pressed.

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