Mapping the Base of the Cambrian Aquifer through Geophysical Modeling of Precambrian Topography, Southern Wisconsin

Background/ need: Wisconsin’s “sandstone aquifer”, composed of Cambrian-age sandstone and dolomite formations, extends over most of the state, serving as the principal bedrock aquifer for large portions of southern and western Wisconsin. This aquifer also supplies groundwater to many highcapacity wells in eastern Wisconsin that extend below the Maquoketa aquitard. The lower boundary to this groundwater system is the Precambrian basement. Basement topography exerts strong control on aquifer thickness, the volume of groundwater stored in the system, and the geometry of a “no-flow” boundary to the aquifer. Accurate interpretation of the aquifer thickness supports assessment of the effects of pumping on groundwater levels and surface water features, as well as simulations of well-head protection areas. Additionally, in regions where nitrate exceedances limit the potability of shallow groundwater, efforts to find low-radium groundwater in the deeper sandstone aquifer are of increasing importance. However, published maps of the Precambrian basement rely heavily on the relatively few drill hole locations where the Precambrian surface elevation is known. The existing Precambrian topographic map of southern Wisconsin (Smith, 1978) is based on outcrop and well data but does not incorporate geologic detail evident in available gravity and aeromagnetic data.
Objectives: Produce a geologically reasonable interpretation of Precambrian basement topography across southern Wisconsin through modeling of existing gravity, aeromagnetic, and well data. Display results in map format at a minimum scale of 1:100,000. Provide map and accompanying report through the Wisconsin Geological and Natural History Survey (WGNHS) in PDF and GIS format.