WHERE SHALE MEETS STREAMBED: GROUNDWATER MANAGEMENT INFORMED BY AQUITARD HYDROGEOLOGY

Aquitards, lithologic units of relatively low hydraulic conductivity, play an outsize role in subsurface hydrology. Aquitards both restrict and direct groundwater flow and influence local and regional patterns of recharge and discharge. Although these low-conductivity horizons lead to large vertical hydraulic gradients, relatively small volumetric fluxes are transmitted across aquitards to adjacent aquifers. Subsurface heterogeneities, including geologic windows and cross-connecting wells (wells open to multiple aquifers and aquitards), provide pathways for much larger fluxes across aquitards.

In Wisconsin, hydrogeologists at the Department of Natural Resources consider aquitard integrity and effects of multi-aquifer wells during evaluation of applications for high-capacity wells and associated groundwater withdrawals. Evaluations include estimating impacts of proposed pumping on nearby streams and wells. In 2023, the Department reviewed an application for a 1000-ft well to support snow making operations at a ski area in the Driftless Area of western Wisconsin. The upper portion of the well would be drilled through 500 feet of layered dolomite and sandstone, with grouted casing extending across a local siltstone / shale aquitard within the St Lawrence and Lone Rock Formations. This aquitard directs discharge to high elevation springs and sustains flow to the upper reaches of a class 2 trout stream. The well would be open to the underlying Wonewoc sandstone, which forms a regional unconfined aquifer that provides baseflow to lower reaches of the stream. To ensure sufficient well yield for snow making, the borehole would extend through the Eau Claire aquitard and access groundwater from both the Wonewoc and the underlying, confined Mount Simon sandstone. Under ambient, non-pumping conditions, we estimate this cross-connecting well would divert about 0.1 cfs from the Wonewoc to the Mount Simon, less than 1% of the stream’s mean August stream flow. By considering the hydrologic function of local and regional aquitards and the effect of well construction on aquitard integrity, the review process can support preservation of baseflow along the entire stream length.