Simulated water sources and effects of pumping on surface and ground water, Sagamore and Monomoy flow lenses, Cape Cod, Massachusetts

The sandy sediments underlying Cape Cod, Massachusetts, compose an important aquifer that is the sole source of water for a region undergoing rapid development. Population increases and urbanization on Cape Cod lead to two primary environmental effects that relate directly to water supply: (1) adverse effects of land use on the quality of water in the aquifer and (2) increases in pumping that can adversely affect environmentally sensitive surface waters, such as ponds and streams. These considerations are particularly important on the Sagamore and Monomoy flow lenses, which underlie the largest and most populous areas on Cape Cod.

Numerical models of the two flow lenses were developed to simulate ground-water-flow conditions in the aquifer and to (1) delineate areas at the water table contributing water to wells and (2) estimate the effects of pumping and natural changes in recharge on surface waters. About 350 million gallons per day (Mgal/d) of water recharges the aquifer at the water table in this area; most water (about 65 percent) discharges at the coast and most of the remaining water (about 28 percent) discharges into streams. A total of about 24.9 Mgal/d, or about 7 percent, of water in the aquifer is withdrawn for water supply; most pumped water is returned to the hydrologic system as return flow creating a state of near mass balance in the aquifer. Areas at the water table that contribute water directly to production wells total about 17 square miles; some water (about 10 percent) pumped from the wells flows through ponds prior to reaching the wells. Current (2003) steady-state pumping reduces simulated ground-water levels in some areas by more than 4 feet; projected (2020) pumping may reduce water levels by an additional 3 feet or more in these same areas. Current (2003) and future (2020) pumping reduces total streamflow by about 4 and 9 cubic feet per second (ft3/s), corresponding to about 5 percent and 9 percent, respectively, of total streamflow.

Natural recharge varies with time, over both monthly and multiyear time scales. Monthly changes in recharge cause pond levels to vary between 1 and 2 feet in an average year; annual changes in recharge, which can be much larger than monthly variations, can cause pond levels to vary by more than 10 feet in some areas over a period of years. Streamflow, which also changes in response to changes in recharge, varies by a factor of two over an average year and can vary more over multiyear periods. On average, monthly pumping ranges from 15.8 Mgal/d in March to 45.3 Mgal/d in August. Pumping and the distribution of return flow can seasonally affect the hydrologic system by lowering ground-water and pond levels and by depleting streamflows, particularly in the summer months. Maximum drawdowns in March and August exceed 3 feet and 6 feet, respectively, for current (2003) pumping. Simulated drawdowns from projected (2020) pumping, relative to water levels representing 2003 pumping conditions, exceed 2 feet in March and 5 feet in August. Current (2003) and future (2020) pumping can decrease pond levels in some areas by more than 3 feet; drawdown generally is largest during the month of August of an average year. Over multiyear periods, seasonal pumping can lower pond levels in some areas by more than 4 feet; the effects of seasonal pumping are largest during periods of reduced recharge. Monthly streamflow depletion varies in individual streams but can exceed 2 ft3/s in some streams.

The combined effects of seasonal pumping and drought can reduce pond levels by more than 10 feet below average levels. Water levels in Mary Dunn Pond, which is in an area of large current and projected pumping, are predicted (2020) to decline during drought conditions by about 10.6 feet: about 6.9 feet from lower recharge, about 2.3 feet from current (2003) pumping, and about 1.4 feet from additional future (2020) pumping. The results indicate that pumping generally does not cause substantial streamflow depletion and that the primary effect of pumping is on water levels in ponds. Natural changes in recharge account for most of the variation in pond levels; however, pumping can cause substantial declines in the levels of ponds near pumping wells. Also, the effects of pumping and recharge can combine to cause drawdowns of more than 10 feet in some areas.