By 2020, demand for water in the Blackstone River Basin is expected to be 52 million gallons per day, one-third greater than the demand of 39 million gallons per day in 1980. Most of this increase is expected to be supplied by increased withdrawals of ground water from stratified-drift aquifers in the eastern and northern parts of the basin. Increased withdrawals from stratified-drift aquifers along the Blackstone River and in the western part of the basin also are expected.

The eastern and northern parts of the Blackstone River Basin contain numerous small, discontinuous aquifers which, as a group, comprise the largest ground-water resource of the study area. Fifteen aquifers, ranging in areal extent from 0.57 to 4.3 square miles, were identified. These aquifers have maximum saturated thicknesses ranging from less than 10 feet to 105 feet and maximum transmissivities ranging from less than 1,000 to more than 20,000 feet squared per day. Yields of nine study aquifers were estimated by use of digital ground-water-flow models. Yields depend on the hydraulic properties of the aquifer and the amount of streamflow available for depletion by wells. If streamflow is maintained at 98-percent duration, long-term yields from the aquifers that would be expected to be equaled or exceeded 50 percent of the time range from 0.22 to 11 million gallons per day, and long-term yields equaled or exceeded 95 percent of the time range from 0.06 to 1.0 million gallons per day. If streamflow is maintained at 99.5-percent duration, long-term yields equaled or exceeded 50 percent of the time range from 0.22 to 11 million gallons per day, long-term yields equaled or exceeded 95 percent of the time range from 0.04 to 1.4 million gallons per day, and longterm yields equaled or exceeded 98 percent of the time range from 0.02 to 0.39 million gallons per day. Maintaining streamflow at 98-percent duration is a more restrictive criterion than maintaining streamflow at 99.5-percent duration.

The upper Lake Quinsigamond, upper West River, and Stone Brook aquifers are capable of sustaining withdrawals of at least 1 million gallons per day more than their rates in the mid-1980s. The upper Mill River and Auburn aquifers are not capable of sustaining additional withdrawals of 0.25 million gallons per day. Ground-water quality in the Auburn aquifer has been degraded by activities and contaminants associated with urbanization.

A nearly continuous deposit of stratified drift almost 30 miles long and from 400 feet to more than 1 mile wide occupies lowland areas along the southeastern part of the Blackstone River. These deposits were divided into four aquifers ranging in areal extent from 1.8 to 3.5 square miles. These aquifers have maximum saturated thicknesses ranging from 54 to 170 feet and maximum transmissivities ranging from less than 1,500 to more than 20,000 feet squared per day. The Blackstone River receives substantial amounts of treated municipal wastewater. Infiltration of poor-quality surface water has significantly increased the specific conductance and the concentrations of all major ions, ammonia, iron, and manganese in the water pumped from at least two wells near the river. These wells derive about 41 and 48 percent of their yield from infiltrated surface water. At both sites, aquifer heterogeneity controlled the movement of infiltrated water to the wells. At one of these sites, where the flow of infiltrated water was tracked (by use of a digital model) in three dimensions, infiltrated water moved to the well through gravel layers that did not constitute the entire thickness of the aquifer. Changes in stream discharge that resulted in changes in surface-water quality also affected the quality of ground water at that site.

The western part of the Blackstone River Basin contains the smallest aquifers evaluated in the study area. Six aquifers, ranging in areal extent from 0.05 to 1.3 square miles, were identified. The hydraulic properties of most of these aquifers have not been determined, but available data indicate that maximum saturated thicknesses range from 28 to 71 feet and maximum transmissivities range from 2,300 to 15,000 feet squared per day.