SIMULATING THE EFFECT OF GROUNDWATER WITHDRAWALS FROM A GLACIAL DRIFT AQUIFER ON STREAM AND WETLAND HYDROLOGY

A groundwater simulation model of a glacial sediment aquifer in Eastern Massachusetts is employed to determine groundwater management strategies that avoid ecological damage to wetlands and streams and impairment to use of a state recreational lake potentially affected by a proposed large groundwater withdrawal. The complexity of the thin glacial aquifer makes groundwater flow paths uncertain and hydraulic responses in surface water and wetland difficult to predict. So the groundwater simulation model is being used to determine if pumping schedules can be properly managed to minimize any adverse effects of withdrawals. A key parameter in determining pumping schedules is the response time – the length of time between a change in pumping and maximum hydraulic response in a nearby stream or wetland. Response times are location specific and determined primarily by aquifer hydrogeology. The aquifer in our study consists of thin, generally unconfined, highly permeable sand and gravel glacial drift deposits with low storage capacity, providing fast response times between pumping wells and surface water bodies. Fast response times are advantageous for managing impacts of pumping, because pumps can then be turned on during wet periods or periods of high stream flow when impact on surface water is minimal. Successful management strategies for minimizing impacts of pumping during dry months are a priority for different interest groups active in the study area. Use of a numerical model of the hydrologic system is critical to quantify the hydrologic effects in enough detail to construct effective management strategies. Recent results from the study are presented showing the interplay between uncertainty in the model and uncertainty in predictions of low stream flows and wetland water levels.