Effects of Simulated Ground-Water Pumping and Recharge on Ground-Water Flow in Cape Cod, Marthas Vineyard, and Nantucket Island Basins, Massachusetts

Masterson, John; Barlow, Paul

The management and protection of water resources of Cape Cod, Martha’s Vineyard, and Nantucket Island water-resource planning basins are of concern to Massachusetts State and local officials because ground water is the sole source of drinking water in the basins. Significant growth in the number of summer and permanent residents has increased ground-water use during the last 30 years and placed stresses on the ground-water resources. In particular, there is concern over the extent of long-term declines in ground-water and pond levels and in the quantity of streamflow, as well as in the possibility of saltwater intrusion from the surrounding ocean. The effects of simulated ground-water pumping and recharge on the surface- and ground- water hydrology were assessed for the Cape Cod, Martha’s Vineyard, and Nantucket Island Basins. Five of the six flow cells of the Cape Cod Basin were assessed-the West Cape, East Cape, Eastham, Wellfleet, and Truro flow cells. These effects are reported as (1) changes in water-table altitudes in the three basins, (2) changes in pond altitudes and streamflow for selected ponds and streams of the Cape Cod Basin, (3) changes in the sources and sinks of water in the Cape Cod Basin, and (4) changes in the position of the freshwatersaltwater interface in the West Cape, East Cape, and Truro flow cells of the Cape Cod Basin. Transient, three-dimensional, finitedifference models were developed to simulate freshwater and saltwater flow in the West Cape, East Cape, and Truro flow cells. Model results indicate little change in the position of the freshwater-saltwater interface in the West Cape and East Cape flow cells for ground-water pumping and recharge conditions similar to those that occurred in the basin from predevelopment (assumed to have ended in 1950) to 1989 and for those estimated to occur from 1989 to 2020. Increases in pumping in the Truro flow cell also had a negligible effect on the position of the freshwater-saltwater interface except near the three areas of ground-water pumping in the flow cell. Transient, three-dimensional, finite difference models also were developed to simulate freshwater flow in the West Cape, East Cape, Wellfleet, and Eastham flow cells for the period from predevelopment to the year 2020. Total average declines in the water table at 32 observation wells in the West Cape flow cell and 19 observation wells in the East Cape flow cell are 1.8 and 2.9 feet, respectively, for the simulation period. Water-table altitudes range from 0 to nearly 75 feet in the West Cape flow cell and from 0 to nearly 45 feet in the East Cape flow cell. Declines in the average water levels of ponds during the simulation period are less than those at observation wells because of the greater storage capacity of the ponds than of the surrounding aquifer material. The average depletion in the rate of streamflow at the gaging points of eight of the largest streams in the West Cape and East Cape flow cells simulated in the models in the year 2020 was 14 percent of the model-calculated predevelopment streamflow in the rivers. Total sources and sinks of freshwater to the West Cape and East Cape flow cells increase from predevelopment flow conditions to the year 2020 because the total amount of ground-water pumping and subsequent wastewater-return flow to each system increases with time. The source of groundwater pumpage from the flow cells is freshwater removed from storage in the aquifers and decreased rates of freshwater discharge to streams and saltwater boundaries of the flow cells. Sources and sinks of water to the Eastham and Wellfleet flow cells have not changed and are not projected to change significantly with time because there is no large-scale pumping in the flow cells for public supply.