Characterization of the Geology and Hydrogeology of the Rountree Formation in Southwestern Wisconsin

The goal of this project was to better understand the spatial distribution of residual clays of the Rountree Formation in the subsurface in Iowa, Grant, and (to a lesser extent) Lafayette Counties; and to evaluate what, if any, buffering capacity this clay unit provides to the bedrock groundwater system from non-point source surface contamination.

Recent studies in southwest Wisconsin (Grant, Iowa, and Lafayette Counties) have documented widespread pathogen contamination in private water wells. This previous work has sought to better characterize and understand the bedrock geology and bedrock aquifer systems, and to understand the potential causes of contamination.

One specific component of understanding the local geology that may control groundwater quality is improving our understanding of the Rountree Formation. This geologic unit is a red, residual, often-cherty clay that is formed by the chemical weathering of carbonate bedrock. Where it occurs—which is directly related to occurrence of Ordovician carbonates as the capping bedrock unit—it is found on top of the bedrock surface and at times within weathering voids and vugs within the top few meters of the top of the carbonate bedrock. It is not exposed at the surface because the landscape is pervasively blanketed by windblown silt (loess) sourced from the Mississippi River corridor during the last glaciation. Because it only occurs in the subsurface, its distribution and physical characteristics are poorly understood. Anecdotally, it has widely been presumed to be fairly widespread in the subsurface across the landscape and to provide significant buffering capacity to the bedrock. This project provided quantitative data on the distribution of the Rountree Formation across Grant and Iowa Counties, and multi-scale assessment of the buffering capacity of the unit based on its distribution and comparison to water well contamination data.