Potential hydrologic impacts of ground-water withdrawal from the Cape Cod National Seashore, Truro, Massachusetts

The hydrologic impacts of continuous ground-water withdrawals at 0.75, 1.0, and 1.24 Mgal/d (million gallons per day) from a test-well site in the Cape Cod National Seashore, Truro, Massachusetts, were evaluated with a three-dimensional finite-difference steady-state-flow digital model. The digital model was prepared during an earlier study and is only briefly described. Continuous withdrawal of more than 1.0 Mgal/d from a well screened from 10 to 40 feet below sea level at the test site will result in upward movement of the freshwater-saltwater interface, and most likely saltwater will eventually contaminate the well. Pumping from a shallower well will decrease the potential for the movement of saltwater into the well, but the water table may be drawn down to the well screen. It is unlikely that movement of the freshwater-saltwater interface in response to pumping from the test site at the simulated rates will result in saltwater contamination of the shallow domestic supply wells in Truro. For the simulated pumping schemes, the water-table decline below average (1963-76) levels did not exceed 0.6 foot except near the pumping wells. Continuous withdrawal at the average year-round rate and the average summer rate will decrease freshwater discharge to the wetland and ocean along the northeastern boundary of the aquifer. (USGS)