INVESTIGATING SUSTAINABLE DEVELOPMENT OF GROUNDWATER RESOURCES FOR A LOWER CAPE COD FRESH-WATER LENS USING SEAWAT

Municipalities on Lower Cape Cod are under increasing pressure to develop sustainable, high quality water supplies. This pressure comes in part from continuing urban development, and in part from state regulatory requirements for redundant water supplies. The current domestic-well and non-community water supply wells that pump tens to hundreds of gallons per day each from the shallow, nitrate-laden portion of the aquifer will be replaced by million-gallon-per-day well fields that concentrate pumping in intermediate-depth high quality zones. Maintaining the large production wells within a thin fresh-water lens is possible, but the location, depth and withdrawal rate must be carefully selected because saltwater upconing can threaten the water supply if a well is placed too near the fresh-water / saltwater interface (SWI) and the pumping rate is too large.

As part of the new source investigation for a proposed well field site for the town of Eastham, Massachusetts, the numerical groundwater flow and density-dependent transport code SEAWAT was used to evaluate the optimum depth and pumping rate for a large production well field. The SEAWAT model originally developed by the USGS for the entire Lower Cape Cod was modified to incorporate site-specific geologic data that led to significant changes in the conceptual hydrogeologic model of the Nauset fresh-water lens in this area. The model was then calibrated to data collected from aquifer tests performed for various candidate production zones for the proposed well site.

Results of this SEAWAT modeling application indicate that properly evaluating the site-specific conditions, including geologic depositional environment, and aquifer properties from the water table down to the SWI, especially vertical anisotropy, is crucial to determining the sustainable aquifer yield. This same approach was applied in the Pamet fresh-water lens near Truro, Massachusetts where the SEAWAT model results indicated that although upconing is predicted to occur, the drinking water quality is maintained for a long period of time due to well screen mixing.