Hydraulic transience and the role of bedding fractures in a bedrock aquitard, southeastern Wisconsin, USA

Abstract

Hydraulic heads in a dolomitic shale bedrock aquitard in Wisconsin, USA, are apparently not in equilibrium with drawdown in the underlying aquifer system caused by pumping for municipal supply over the last century. Measurements of head with depth, downhole geophysical logs, and estimates of hydraulic conductivity indicate very low vertical hydraulic diffusivity, and show that high horizontal permeability within the aquitard due to bedding-plane fracture zones can allow lateral groundwater flow. Unlike the hydrogeological conceptual models used in many investigations, flow in aquitards cannot always be inferred to be primarily one-dimensional and vertical. Failure to account for transient conditions and lateral flow in similar settings of intensive groundwater pumping could lead to error in estimates of aquitard leakage and underlying aquifer properties.