Recharge and pumping hydraulics in a till drumlin above fractured bedrock (Massachusetts, USA)

摘要

Recharge and pumping hydraulics of a till-mantled bedrock are analyzed with existing closed-form theory and 12 years of monthly water levels in cluster wells from Scituate Hill, a glacial till drumlin in eastern Massachusetts (USA). The weathered brown till atop Scituate Hill is an unconfined aquifer, delivering steady recharge and a seasonally varying recharge-head fluctuation to the unweathered gray till aquitard beneath it. The water-table fluctuations generate no seasonally varying flow field in the gray till, due to the relatively low hydraulic diffusivity of the brown till. Nearby irrigation pumps drilled into the underlying Dedham Granite in 2011 have introduced seasonal drawdown, and the gray till leaks into the fractured bedrock aquifer. The leakage reflects the moderate diffusivity of the gray till and the relatively high hydraulic diffusivity of the fractured bedrock. Both seasonal disturbances are mildly attenuated across the gray till, so that the Dedham Granite senses recharge, while the water table in Scituate Hill is drawn down by irrigation pumping. Steady and seasonal gray till data are accordingly used to calibrate the transmissivity and storativity of the fractured bedrock and specific yield of the brown till, with physically plausible values.