EXPANDING OUR UNDERSTANDING OF WISCONSIN’S SILURIAN DOLOMITE AQUIFER

While the geology of the Silurian dolomite in Wisconsin has been well characterized since the earliest geologic survey of the state by T.C. Chamberlin, the hydrogeologic characteristics of this important aquifer have only received significant study in the last 40 years or so – a time frame that overlaps with Ken Bradbury’s career. His contributions to our current understanding of this aquifer began with his PhD, continued over his career, and span a diversity of topics including characterization of the connections between the dolomite aquifer and the surface waters of Green Bay, exploration of how the sedimentology of the dolomite controls aquifer properties, and approaches to wellhead protection for this vulnerable aquifer.

Weideman and Schultz (1915) provided a general description of the water-bearing properties of the bedrock geologic units of Wisconsin, including the Silurian dolomite, but it wasn’t until the work of Sherrill in the 1970's that it was recognized that flow through fractures was important within the Silurian of Door County. Sherrill identified eight horizontal “water-bearing zones” within the aquifer, but he did not correlate these zones with specific stratigraphic features. In the mid-1990's, Ken and others began to integrate sedimentologic and stratigraphic data with detailed hydrogeologic data sets in order to better characterize the hydraulic properties of the Silurian dolomite. Their approach integrated surface and subsurface stratigraphic data, with geophysical logs and hydrogeologic data in order to characterize the hydraulic properties of the Silurian dolomite in northeastern Wisconsin. From their work, it became apparent that both the occurrence of laterally-continuous high-permeability zones and the variation in matrix hydraulic conductivity are largely controlled by the stratigraphy. This approach has been applied to a variety of aquifers in Wisconsin, and the Minnesota Geological Survey has adopted these methods to develop a comprehensive hydrostratigraphy of the Paleozoic aquifers in that state.

The improved hydrostratigraphic understanding of the Silurian dolomite, developed by Ken and others, has been incorporated into numerical models of fluid flow for the purposes of delineating wellhead protection strategies for this important Wisconsin aquifer.