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U.S. Geological Survey Open-File Report 2011-1040

Continuous Resistivity Profiling Data From Great South Bay, Long Island, New York


Introduction

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Click on figure for larger image.

Thumbnail image for figure 1, map of Long Island, New York indicating glacial features and link to larger image.

Figure 1.Maps showing Long Island, New York. The lower map shows the location of Great South Bay adjacent to the glacial outwash plains of southern Long Island, New York. The bay is bounded on the south by a long barrier island, Fire Island, that includes the land area of Fire Island National Seashore as indicated in the upper map. The ridges that form the backbone of Long Island are glacial moraines. The base image in the lower map was acquired from Stony Brook University (accessed June 2011 at http://www.geo.sunysb.edu/reports/dem_2/).

Great South Bay is a shallow coastal lagoon bounded to the south by a long barrier island (Fire Island), and by the shore of Long Island, New York, to the north (fig. 1). The average depth in the bay is 1.3 meters (m) and the bay floor is underlain by sandy glacial outwash (Bennington and Hanson, 2010) and finer grained estuarine and marine deposits. The Fire Island National Seashore (FIIS) is located on the barrier island, along with many small private communities and New York State parks that attract more than two million visitors each year. Fire Island faces competing ecological and human water use needs with this number of visitors — clean groundwater and surface water versus seasonally intensive wastewater disposal and recreation impacts. Nearly 40 percent of the FIIS lies within Great South Bay and adjoining smaller bays (Bokuniewicz and others, 1993; Hinga, 2005), so park managers have to make choices that consider both onshore and offshore components of ecosystem health and human use. Existing data detailing the interconnected onshore and offshore hydrogeology of the shallow aquifer system on Fire Island are limited. In particular, the role of submarine groundwater discharge from either Fire Island or Long Island in the delivery of excess nutrients, a primary cause of eutrophication and harmful algal blooms in Great South Bay (Cerrato and others, 2004), has not been studied in detail using newer survey, sampling, and analytical methods (Crusius and others, 2005; Zhao and others, 2011). Applying these approaches will provide more detailed information regarding the Long Island coastal groundwater system.  This report contains results of electrical resistivity surveys in Great South Bay that can detect low-salinity submarine groundwater and represents one element of a larger study. The results will improve the ability of resource managers in communities and agencies with interests in Great South Bay to make scientifically informed decisions about management of water quality that affect marine habitat and resources.

The problem of coastal nutrient overenrichment is one of the foremost water quality problems along much of the Atlantic coast and in similar settings worldwide (Howarth and Marino, 2006). Among the most harmful effects of eutrophication are algal blooms. These blooms result in decreased dissolved oxygen and reductions in water clarity that combine to increase mortality of clams and fish and loss of seagrass habitat as ecosystems shift to macroalgal and phytoplankton-dominated productivity (Short and Burdick, 1996; Hauxwell and others, 2003). In addition, toxic algal blooms have become more frequent.  In Great South Bay, blooms of the brown tide organism Aureococcus anophagefferens have been a particular problem (Clark and others, 2006; Gobler and others, 2011). Investigations by Capone and Bautista (1985), Capone and Slater (1990), Schubert (1998, 1999), Gobler and Sañudo-Wilhelm (2001), and Monti and Scorca (2003) estimated direct groundwater discharge rates and, in some cases, nitrogen loads to shallow Long Island estuaries using large datasets of nitrate concentrations from wells, streams, and sediment cores, along with water budgets or flow models developed from estimates of recharge, runoff, and evapotranspiration. These studies estimated that direct groundwater discharge contributed 10 to 20 percent or more of the total freshwater input to Great South Bay (Monti and Scorca, 2003) and similar estuaries. Despite the relatively low fresh water contribution to Great South Bay from groundwater relative to runoff inputs from rivers and streams, the role of runoff in the delivery of nutrients may be greater relative to other fresh water inputs than previously estimated due to the elevated concentrations of nitrate in Long Island groundwater (Monti and Scorca, 2003). Direct measurements of key parts of the whole groundwater flow and discharge system spanning the land-bay interface at regionally representative locations are needed to verify the accuracy of these previous studies.

Buried stream valleys, filled tidal inlets, and drowned salt marshes often control whether water from land discharges at the shoreline or flows farther offshore before discharging (Manheim and others, 2004; Cross and others, 2008, 2010, 2011). These subsurface geologic features affect the length of time required for groundwater to make it to coastal surface water and the amount of nutrients and other dissolved constituents that the groundwater contains when it finally discharges (Bratton and others, 2004, 2009a). Significant bypassing of shoreline discharge zones may also produce underestimates of total discharge when measurement techniques such as seafloor seepage meter deployments are used (Michael and others, 2005). In addition, the geometry of the submarine groundwater flow system is important to consider when evaluating proposals for construction activities, such as dredging of navigation channels and harbors or modifying shorelines with bulkheads. Such disturbances can create conduits for focused discharge of nutrient-bearing groundwater with unanticipated and undesirable results, such as localized blooms of macroalgae, complications with construction or maintenance, or saltwater intrusion into nearshore surficial or confined aquifers (Foyle and others, 2002; Stieglitz and others, 2007).

The geophysical components of this project were designed to examine and define in greater detail the underlying geology comprising the subsurface land-sea and fresh-saline groundwater interfaces.  Better knowledge of this transition zone would contribute to improved model estimates of fresh groundwater discharge and nutrient flux to Great South Bay in particular and would apply to other back-barrier estuaries as well (Valiela and others, 1992, 1997; Kroeger and others, 2006; Bratton, 2010). The study also extends recent onshore groundwater investigations on Fire Island (Schubert and others, 2010) and Long Island (Abbene, 2010) into the offshore. Improving our understanding and capability to provide more accurate information regarding coastal groundwater systems will inform management decisions about drinking-water supply and wastewater disposal for human residents and visitors to Fire Island and about sewering of unsewered areas on Long Island in towns such as Patchogue.


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