Areas contributing water to the Dry Brook public-supply well in South Hadley, Massachusetts, were delineated using a numerical ground-water flow model developed to simulate flow in the confined sand and gravel aquifer tapped by the supply well. The study area is located along the Connecticut River in Central Massachusetts, about 12 miles north of Springfield, Massachusetts. The geologic framework of the study area is composed of Mesozoic bedrock, overlying late-Pleistocene glacio-lacustrine sediments, and recent alluvial deposits of the Connecticut River floodplain. Dry Brook Hill, immediately south of the supply well, is a large subaqueous lacustrine fan and deltaic deposit formed during the last glacial retreat by deposition of sediments into glacial Lake Hitchcock from a meltwater tunnel which was likely located near where the Connecticut River cuts through the Holyoke Range. These sediments which comprise the aquifer grade from very coarse sand and gravel along the northern flank of the hill to medium sands in the body of the hill and finer-grained sediments along the southern flanks of the hill. The interbedded and overlapping fine-grained deposits of lake bed sediments associated with Dry Brook Hill are composed of varved silts and clays. These fine-grained sediments form a confining bed above the coarse-grained aquifer at the supply well and partially extend out under the Connecticut River adjacent to the supply well.

Ground-water flow in the aquifer supplying water to Dry Brook well was simulated using the USGS ground-water flow model MODFLOW. The Dry Brook aquifer model was calibrated to drawdown data collected in 8 observation wells during an aquifer test conducted using the supply well for ten days at a pumping rate of 914 gallons per minute. In general the highest hydraulic conductivities were in the sand and gravel aquifer in the immediate vicinity of the Dry Brook well. Delineation of areas contributing water to the Dry Brook water-supply well was completed using the MODPATH particle-tracking routine. Results of the contributing area analysis indicated that greatest sources of water to the Dry Brook supply well are recharge from Dry Brook Hill and infiltration of water from the Connecticut River from areas beyond the extent of the confining bed where the aquifer is in good hydraulic connection with the river.