Simulated Hydrologic Responses of the Quashnet River Stream-Aquifer System, to Proposed Ground-Water Withdrawals, Cape Cod, Massachusetts

Barlow, Paul; Hess, Kathryn

This report describes efforts by the U.S. Geological Survey, in cooperation with the Mashpee Water District, to simulate the response of the Quashnet River stream-aquifer system, Cape Cod, Massachusetts, to ground-water withdrawals that have been proposed within 3,800 ft of the river. The study was prompted in response to concern over possible reductions in streamflow and degradation of the sea-run brown trout habitat of the river resulting from the proposed withdrawals. A two-layer steady-state and transient finite-difference ground-water-flow model with a stream-routing component was developed to simulate the stream-aquifer system. Steady-state pumping rates of O.5,1.0, and 2.0 million gallons per day were simulated in five scenarios at three well sites 1,500 to 3,800 fi west of the river. No infiltration of water from the river to the aquifer was induced in any of the simulations; the river received ground-water discharge along its entire length for all withdrawal scenarios tested. Streamflow depletions in the river resulting from the simulated withdrawals range from 1.3 to 3.8 percent of the calculated, steady-state, prepumping streamflow for the site that is the upstream limit of active trout spawning, and from 3.8 to 14 percent for the gaging station, 1.6 and 0.36 miles upstream from the mouth of the river, respectively. One withdrawal scenario was investigated further by use of the transient model to investigate the effects of pumping on seasonal fluctuations of streamflow of the river; simulated ground-water withdrawal was 1.0 million gallons per day at a site 1,500 ft west of the river. For this scenario, the calculated steady-state depletion at the streamflow gage is 7.5 percent. Monthly mean streamflow depletions calculated by use of the transient model range from 6 to 8percent of the measured monthly mean streamflows at this site. This small range in depletions can be attributed to the relatively steady flow of the Quashnet River throughout the year. This constancy results from the large infiltration and storage capacity of the glacial outwash deposit which covers all of the Quashnet River basin and which tends to damp the effect of climatic variability on streamflow. Calculated monthly mean streamflows at the gaging station from late spring through fall meet the stream-discharge guidelines established by the Massachusetts Division of Fisheries and Wildlife. Guidelines for winter through early spring are not met for this pumping rate; however, these guideline currently are not met for existing (nonpumping) streamflow conditions. A particle-trackingpostprocessort o the flow model was used to delineate steady-state contributing areas of the river and the simulated wells. A contributing area consists of that area over which recharge entering the aquifer across the water table eventually discharges to the river or the well. The contributing area of the river for the condition of no pumping lies primarily to the west of the river. In each withdrawal scenario, the contributing area of the pumping well also lies west of the river. Although the simulated cone of depression produced by pumping from each well extends areally across the river, the contributing area of each well does not include the river. Thus, ground-water withdrawals will reduce the flow in the river; but no river water will be diverted to pumped wells under the withdrawal scenarios investigated.