Introduction

Figure 1: Location map.

In order to test hypotheses about groundwater flow under and into estuaries and the Atlantic Ocean, geophysical surveys, geophysical probing, submarine ground-water sampling, and sediment coring were conducted by U.S. Geological Survey (USGS) scientists at Cape Cod National Seashore (CCNS) in Massachusetts from 2004 through 2006 (Figure 1). Coastal-resource managers at CCNS and elsewhere are concerned about nutrients that are entering coastal waters via submarine ground-water discharge; these nutrients are contributing to eutrophication and harmful algal blooms (Figure 2). The USGS has performed several numerical modeling studies in recent years to provide managers with information necessary to make informed decisions about this issue (Masterson, 2004; Crusius et al., 2005; Colman et al., 2005; Colman and Masterson, 2007; Colman and Masterson, 2008). The research carried out as part of the study described here was designed, in part, to help refine assumptions required by earlier versions of models about the nature of submarine ground-water flow and discharge at CCNS. In addition, the investigation was understaken to characterize the submarine hydrogeology of this site as a location representative of typical conditions along the formerly glaciated parts of the Atlantic coast of the United States.

Figure 2: Bloom of Cladophora sp. macroalgae.

Figure 3: Probe, sample, and core locations.

This study was conducted in four phases, with a variety of field techniques and equipment employed in each phase. Phase 1 consisted of continuous resistivity profiling (CRP) surveys of the entire study area conducted in 2004. Phase 2 consisted of CRP ground-truthing through resistivity-probe measurements and submarine ground-water sampling with hydraulically driven piezometers from a barge in the Salt Pond/Nauset Marsh area in 2005. Phase 3 consisted of supplemental detailed CRP surveys in the Salt Pond/Nauset Marsh area in 2006. Finally, Phase 4 consisted of sediment coring and pore-water extraction in the Salt Pond/Nauset Marsh area later in 2006 to supplement the 2005 sampling. In order to maintain consistency, 2004 CRP survey lines were numbered sequentially with prefixes to indicate study subregion (for example, chatham9 = 2004 survey line #9 in the Chatham/Pleasant Bay area). Geophysical survey lines from 2006 in the Salt Pond/Nauset Marsh area were sequentially numbered L1 through L15. Subsurface sampling and geophysical probe sites occupied in the Salt Pond area in 2005 and 2006 were numbered sequentially with the prefix "SP" followed by the sample year and sequential site number (e.g., SP2006-3 = sampling site #3 in the Salt Pond area sampled in 2006; see Table 1). Note that not all sampling methods or analyses were employed at all sampling sites (Table 1), and that site reoccupations in 2006 for coring did not exactly duplicate 2005 probe and/or piezometer positions, but were usually within 10-20 meters (Figure 3).

In Phase 1 of this investigation, continuous resistivity profiling (CRP) surveys were performed at CCNS with a surface-towed streamer system to take advantage of the relative differences in resistivity associated with sediments saturated with fresh (high-resistivity) versus saline (low-resistivity) ground water. Measurements indicate that Salt Pond, parts of the adjacent Nauset Marsh, Town Cove, Pleasant Bay, and segments of the open ocean coast in the Wellfleet/Eastham area (Figures 1 and 4) are underlain by brackish to fresh ground water extending tens to hundreds of meters offshore. Kettle ponds breached by the ocean, such as Salt Pond, contain thicker zones of saline to brackish ground water in the fine-grained surface sediments in their centers. The area of the Nauset Marsh estuary adjacent to Salt Pond is underlain by a layer of freshened ground water which extends offshore about 500 m. This offshore flow, and similar phenomena at other locations, is related to the presence of fine-grained confining sediments at the surface or in the shallow subsurface that act as aquitards to downward flow of overlying saline water and upward flow of underlying fresh water (Bratton, 2007a). The convoluted shoreline configuration, shallow depth, and fine-grained sediments in this system are common along coasts of the northern United States, Canada, and northern Europe and Asia. This suggests that shallow embayments along formerly glaciated coastlines, such as the ones investigated here on Cape Cod, may commonly be underlain by extensive fresh or brackish ground-water-flow systems.