U.S. GEOLOGICAL SURVEY
Water-Resources
Investigations Report 96-4229
Delineation of Areas Contributing Recharge
to Municipal Wells in Three Selected Confined-Glacial
Aquifers in Erie County, New York
By Richard M. Yager, Todd S. Miller, and John Thayer
ABSTRACT
Areas that contribute water to municipal wells screened in three glacial aquifers that supply water to the Village of Alden and the Towns of Collins and Holland in Erie County were delineated through conceptual ground-water-flow models that were based on hydrogeologic information from previous studies. Seismic refraction profiles, stream-discharge measurements, and streambed inspections were conducted to obtain additional information on depth to bedrock and the degree of hydraulic connection between aquifers and streams. Uncertainty in the location and extent of the areas contributing recharge to the municipal wells was examined through a comparison of areas delineated with conceptual models representing alternative hypotheses concerning aquifer boundaries and hydraulic properties; this comparison indicated what additional information would be needed to decrease the uncertainty in the contributing-area delineations.
The Village of Alden's municipal wells are screened in a shallow, confined aquifer overlain by fine-grained sediments deposited during a readvance of glacial ice. The ground-water system was simulated with a two-layer model representing the confined aquifer and the underlying weathered-shale bedrock, and recharge to the confined aquifer was derived from downward leakage through the confining layer and underflow through the weathered bedrock from upgradient areas. MODFLOWP, a parameter-estimation method based on nonlinear regression, was used to calibrate the model to ground-water levels measured in 18 wells and ground-water discharge to Spring Creek. Areas contributing recharge to four municipal wells ranged from 0.045 to 0.07 square miles, and from 7 to 28 percent of this recharge originated as runoff from upland areas upgradient to the south; the remainder was derived from precipitation on areas overlying the aquifer. Decreasing the vertical-leakage rate from 24 to 10 inches per year in an alternative model increased the size of the contributing areas by 40 to 90 percent and increased the percentage of recharge that entered the aquifer from upland areas as underflow and unchanneled runoff by 10 to 40 percent. The Town of Collins municipal well no. 1 is screened in a deep, confined sand and gravel aquifer in a buried bedrock valley eroded by a tributary to the ancestral Allegheny River valley. The confined aquifer is overlain by fine-grained sediments that are, in turn, overlain by an unconfined aquifer composed of alluvial and deltaic sand and gravel with a maximum thickness of 550 feet, except in areas near the valley walls where the aquifer extends to land surface. The confined aquifer is recharged at these exposures by precipitation and infiltration from streams and upland runoff, and at depth by underflow from upgradient parts of the buried bedrock valleys to the north, east and south. Ground-water flow was simulated with a one-layer numerical model representing average steady-state conditions in the confined aquifer. Two alternative models were considered: in model A a large conductance value (100 feet squared per day) was used along the head-dependent flow boundary to represent a large rate of recharge from the uplands, and in model B, a small conductance value (20 feet squared per day) was used to represent less recharge from the uplands. In model A, the areas contributing recharge to the municipal well within the aquifer and upland areas were 0.29 and 0.95 square miles, respectively, and all water pumped from the municipal well was derived from upland areas as underflow or unchanneled runoff. In model B, the contributing areas were 0.87and 2.0 square miles, respectively, and 85 percent of the pumped water was derived from upland areas, while the remainder was derived from underflow through a buried-tributary valley now occupied by Clear Creek. The Town of Holland's municipal well is screened in a deep, confined aquifer within a narrow, glacially scoured bedrock valley bordered to the south by the Lake Escarpment moraine. The confined aquifer is overlain by lacustrine silt and clay deposited in a proglacial lake that formed between the moraine and the receding ice front. Ground-water flow through the confined aquifer was simulated with a two-layer model representing the confined aquifer and overlying glacial sediments, but the model was not calibrated because little information on ground-water levels and flow rates is available. The area contributing recharge to the Holland municipal well from morainal sediments covers an area of 0.063 square miles and pro-vides about 35 percent of the water pumped from the well; the remainder of the recharge to the well flows through a part of the confined aquifer that covers 0.34 square miles within the modeled area and extends southward to upgradient areas outside the modeled area. Decreasing the vertical hydraulic conductivity of the morainal sediments by a factor of 10 in an alternative model resulted in a decrease in the contribution from leakage through the morainal sediments to less than 10 percent of the flow to the well; the remainder of the water pumped from the well was derived mainly from underflow through the southern boundary.
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