The two widely recognized tills of southern New England were deposited during two late Pleistocene continental glaciations. The surface (upper) till consists of relatively sandy tills deposited during the late Wisconsin glaciation and includes compact subglacial lodgement and meltout units and a thin overlying supraglacial meltout (ablation) unit. The drumlin (lower) till is the locally preserved till deposited during the Illinoian glaciation and consists chiefly of a compact subglacial lodgement unit. These tills are highly variable in texture, composition, thickness, and structural features, reflecting the composition of the local bedrock and older surftcial materials from which they were derived and the different modes of deposition. The hydraulic properties of tills in this region are also variable because of the differences in texture, composition, and structural features that result from different provenance and genesis. Data on hydraulic properties at 92 sites were compiled from readily available sources. The horizontal hydraulic conductivities of tills derived from crystalline (metamorphic and igneous) rocks range from 1.4 x 10' to 2.3 x 10' centimeters per second, whereas the vertical hydraulic conductivities of tills derived from these rock types range from 4.7 x 10' to 3.4 x 10' centimeters per second. The porosities and specific yields of 15 undisturbed till samples, also composed of crystalline-rock detritus, range from 22.1 to 40.6 percent and from 3.9 to 31.2, respectively. The horizontal hydraulic conductivities of tills derived from the Mesozoic (Triassic and Jurassic) sedimentary rocks of central Connecticut and west-central Massachusetts range from 2.8 x 10' to 1.2 x 10' centimeters per second, whereas the vertical hydraulic conductivities range from 1.8 x 10' to 1.2 x 10" centimeters per second. The porosity of 58 samples of till derived from these sedimentary rocks ranges from 18 to 40.1 percent. Acknowledgment Mr. Rudy Chlanda of the SCS provided valuable assistance by reviewing, collating, and copying relevant SCS data for Massachusetts.

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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.

Ground-water withdrawals by municipal wells in the Mattapoisett River valley are expected to triple in the next two decades. State and local concern about the long-term impacts of these increased withdrawals on ground-water levels and streamflow made it necessary to assess the ground-water resources of the valley and to develop a digital ground-waterflow model for management purposes.The model was calibrated under steady-state and transient conditions and simulates ground-water withdrawals by wells, leakage through streambeds, and leakage from the bordering till. Calculated results of the model are most sensitive to decreases in the values of model parameters, particularly streambed and aquifer hydraulic conductivity. Transient-responsetime tests of the model indicate that changes in long-term recharge rates would have to last at least 1 year for steady-state predictions to be realized.Ten pumping scenarios representing current and proposed withdrawals from the valley were simulated with conditions of reduced recharge. Under conditions simulating 1965 average annual recharge, predicted water levels in the aquifer are as much as 9 feet lower than average annual levels. At the highest withdrawal rates, the predicted drawdown in four wells exceeds the estimated available drawdown. For all pumping scenarios, at least 10 percent of the available ground water in the aquifer discharges to the Mattapoisett River. Under conditions representative of the 7-day 10-year low flow of the river, predicted water levels decline as much as 19 feet; moreover, at the highest withdrawal rates, available drawdown is exceeded in five wells. Simulated withdrawals in six scenarios use all of the available ground-water discharge. If this drought condition should occur and streamflow is not supplemented by surface water, the predictive results indicate that the downstream half of the river will stop flowing under most pumping plans.Test drilling and seismic refraction surveys conducted to aid model development indicate that the bedrock surface generally is flat except for a deep, narrow channel in the center of the valley. Continuous stream-stage data and baseflow data for the Mattapoisett River were used to increase previous estimates of flow duration, 7-day 2-year, and 7-day 10-year low flow. Water quality in both the aquifer and river may be characterized as slightly acidic and low in dissolved solids.