HYDROSTRATIGRAPHIC CHARACTERIZATION OF THE SINNIPEE DOLOMITE IN JEFFERSON COUNTY, WISCONSIN

Accurate characterization of aquifer heterogeneities is necessary for many groundwater investigations. The three-dimensional distribution of aquifer heterogeneities is difficult to characterize with sparse borehole data. Sedimentological and stratigraphic models provide a method of incorporating geologic variability into models of fluid flow in sedimentary aquifers. In carbonate aquifers, the porosity and permeability structure is dependent on both matrix properties and the development of secondary porosity. Matrix properties are controlled by lithostratigraphic variability and diagenetic processes; the distribution of secondary fractures is controlled by stress history and mechanical stratigraphy which reflect lithostratigraphic variations. The secondary fractures, which provide high-permeability preferential flowpaths, can be further enhanced by dissolution.

Previous hydrostratigraphic characterizations of fractured dolomite aquifers in Wisconsin have shown that these aquifers have a bi-modal distribution of hydraulic conductivity with low-permeability values for the rock matrix and high-permeability values for the fractures. This study investigated the hydraulic conductivity distribution of Sinnipee dolomites conducted as part of the Jefferson County Statemap project.

Sixteen straddle-packer tests, with an open interval of 4.63 ft, were conducted in a borehole located in a quarry near Sullivan, WI. The tests, designed to sample the range of lithologies and to test specific fracture zones, yielded hydraulic conductivity values that range over six orders of magnitude. Slug tests conducted on unfractured sections of the hole exhibited a typical exponential recovery of water levels and were analyzed using the Hvorslev method. Slug tests conducted in intervals containing fractures commonly exhibited an oscillatory response and were analyzed using the Springer-Gelhar method.

The hydraulic conductivity values measured in this study are similar to the range of values noted by Stocks (1998) for the Sinnipee dolomite in Dodge County. The Sinnipee in eastern Wisconsin exhibits lateral facies changes from north to south (Choi, 1998). We are investigating the lithostratigraphic controls on the hydraulic conductivity distribution within the Sinnipee and whether lithostratigraphy can be used as a predictor of hydrostratigraphy.