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[1] 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 beddingplane 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.

There are few studies on the hydrogeology of sedimentary rock aquitards although they are important controls in regional ground water flow systems. We formulate and test a three-dimensional (3D) conceptual model of ground water flow and hydrochemistry in a fractured sedimentary rock aquitard to show that flow dynamics within the aquitard are more complex than previously believed. Similar conceptual models, based on regional observations and recently emerging principles of mechanical stratigraphy in heterogeneous sedimentary rocks, have previously been applied only to aquifers, but we show that they are potentially applicable to aquitards. The major elements of this conceptual model, which is based on detailed information from two sites in the Maquoketa Formation in southeastern Wisconsin, include orders of magnitude contrast between hydraulic diffusivity (K/S(s)) of fractured zones and relatively intact aquitard rock matrix, laterally extensive bedding-plane fracture zones extending over distances of over 10 km, very low vertical hydraulic conductivity of thick shale-rich intervals of the aquitard, and a vertical hydraulic head profile controlled by a lateral boundary at the aquitard subcrop, where numerous surface water bodies dominate the shallow aquifer system. Results from a 3D numerical flow model based on this conceptual model are consistent with field observations, which did not fit the typical conceptual model of strictly vertical flow through an aquitard. The 3D flow through an aquitard has implications for predicting ground water flow and for planning and protecting water supplies.

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