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USGS Coastal and Marine Geology Program

Submarine Hydrogeological Data from Cape Cod National Seashore

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Summary and Conclusions

The data collected in this study show that the submarine ground-water-flow paths at Cape Cod National Seashore are more complicated than expected. Results of CRP surveys show areas both in estuaries and along open Atlantic Ocean shorelines where submarine ground water of reduced salinity is present. The geometry of resistivity anomalies suggests that there are connections between submarine anomalies and onshore flow systems. Specific areas where features indicating the presence of low-salinity ground water are present include part of Pleasant Bay in Chatham, the ocean shoreline along the bluffs north of Nauset Marsh adjacent to Eastham and Wellfleet, areas beneath Nauset Marsh including Salt Pond Bay, and areas beneath Salt Pond and the channel connecting the pond to the open marsh. The deeper part of the pond itself does not appear to be an area of active discharge. Shallow anomalies around the shore of the pond and beneath the channel are consistent with active discharge of submarine ground water in these locations. Such discharge is also likely in similar settings.

Data collected by EC probing, piezometer sampling, and vibracoring are all consistent with the CRP data in the areas investigated, but produce information on subsurface conditions with greater resolution, especially in the vertical dimension. Analytical data (Figures 12a and 12b) show that submarine ground water is enriched in nutrients relative to background concentrations, especially in ammonium, although an elevated nitrate/nitrite concentration was measured in a deep sample collected at the location closest to the northern shore of Salt Pond. Ammonium concentrations (Figure 12a) tend to be highest in the most saline ground water. Total dissolved nitrogen (TDN) results indicate that dissolved organic nitrogen is present at high concentrations in some samples, often but not always in association with elevated ammonium concentrations. Results for phosphorus species (phosphate and total dissolved phosphorus, Figure 12b) are most similar to ammonium patterns, with the highest concentrations in brackish to saline samples. Total dissolved phosphorus (TDP) analyses show that phosphorus in most of the ground water is in the inorganic form, except in some of the samples with higher concentrations (about a third of the total analyzed).

Fine-grained lacustrine and estuarine deposits in Salt Pond and Salt Pond Bay act as confining units, allowing low-salinity ground water to flow beneath them for hundreds of meters before discharging. The patterns of submarine flow and discharge observed in the Salt Pond-Nauset Marsh system are likely common in other similar settings on Cape Cod, and along other coastlines formed by glacial processes and subsequent sea-level rise.

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