MODELING DRAWDOWN AND RECOVERY IN FRACTURED BEDROCK OVERLAIN BY A LEAKY AQUICLUDE

This presentation describes the results of an analysis of the subsurface hydraulic response of fractured bedrock overlain by a drumlin due to significant offsite groundwater pumping. The drumlin is located in eastern Massachusetts and consists of an upper weathered brown clayey sand till and a lower unweathered gray clayey sand till underlain by fractured bedrock which is part of the Dedham Granite. The hydraulic head was measured every 15 minutes via a large network of spatially distributed monitoring wells installed in the approximately 25 meter thick till together with three bedrock monitoring wells. The till units serve as a leaky aquiclude to the higher permeability underlying fractured bedrock as evidenced by an attenuated drop in hydraulic head upward in the till.

Offsite pumping in a nearby irrigation well installed in the bedrock commenced in May 2012 and ceased in September 2012 and created a sudden and significant drawdown at the site. The Hantush and Jacob (1955) solution for non-steady flow in a leaky aquifer was used to model this drawdown in the bedrock wells as well as the recovery after pumping had ceased. The storativity and transmissivity of the bedrock were estimated by purge tests in the wells. The model was then calibrated using the measured monitoring well data and estimates of storativity and transmissivity to determine the leakance of the overlying till aquiclude. Three extrinsic properties of the fractured bedrock were thus estimated and verified through a calibrated model. The modeling of hydraulic head recovery after the cessation of pumping using the Hantush and Jacob solution is novel.

Hantush, M.S. and Jacob, C.E. (1955). “Non-steady radial flow in an infinite leaky aquifer.” Trans. Am. Geophys. Union, 36(1), 95-100.