Ground-water resources of the Mattapoisett River aquifer, Plymouth County, Massachusetts: Summary for water-resource managers

Abstract

Proposed increases in municipal pumpage in the Mattapoisett River valley will triple ground-water withdrawals in the next two decades. Because of growing State and local concern about the long-term effects of these withdrawals on ground-water levels and streamflow, a computer ground-water-flow model was developed to assist in water-resource management. An executive summary of the modeling work, as well as the mathematical and hydrologic principles used in the hydrogeologic study and the development of the ground-water-flow model are presented in nontechnical terms accompanied by a detailed glossary.Monthly ground-water-level measurements, continuous streamflow data, and measurements of low flow on Mattapoisett River were used to develop the steady-state ground-waterflow model. The model simulates a high-yielding sand and gravel aquifer which fills a bedrock channel as much as 110 feet deep. Recharge to the aquifer is from precipitation and from water entering the aquifer from the less permeable material adjacent to it. Ground water flows horizontally and discharges to the river through the streambed. Water in the aquifer and in the river is soft and slightly acidic. Water levels calculated by the model were within 4 feet of observed levels over 90 percent of the model area, calculated ground-water flow to the river closely matched measured flow, and inflows to the system balanced outflows to within 0.02 percent.Ten scenarios to represent the current and proposed pumping demands in the valley were simulated using drought conditions. Under conditions simulating the driest year of record, predicted water levels in the aquifer were as much as 9 feet lower than average. Under severely dry conditions simulating only enough recharge to keep the river flowing with no pumping, predicted water levels were as much as 19 feet lower than average. During the greatest pumping demands, predicted water level in five wells was low enough to cause the wells to fail. Simulated pumping demands in 6 out of 10 scenarios used all the available ground-water discharge to the river. Under severely dry conditions, if there were no additional streamflow entering the river from ponds in the valley, the results indicated that the southern half of the river would dry up under most pumping plans.