IMPROVED HYDROGEOLOGIC CHARACTERIZATION OF AQUIFERS IN SOUTHEASTERN MINNESOTA

Cities rely on groundwater models to determine wellhead protection areas for their public water supplies. The accuracy of these models at predicting groundwater travel times depends on accurate characterization of aquifer porosity and permeability. In Minnesota, issues related to wellhead protection and aquifer characterization are closely tied to the state well code, which explicitly defines bedrock aquifers according to lithostratigraphic nomenclature. The result has been that aquifers are most commonly characterized as homogeneous and isotropic porous media, and wellhead protection areas are modeled based on these assumptions.

Borehole geophysics, including gamma, spontaneous potential, resistivity, caliper and (EM) flowmeter logs, packer tests, outcrop and core analyses, and downhole video have allowed us to improve the characterization of two important lower Paleozoic aquifers in southeastern Minnesota—the Prairie du Chien-Jordan, and the Franconia-Ironton/Galesville. Our results demonstrate that bedding-parallel fractures are important hydraulic conduits in both carbonate and siliciclastic strata. Intervals of fine siliciclastics and carbonate rock that has minimal secondary pore development serve as confining units. These aquifers are best characterized as having dual porosity where relatively rapid advective flow occurs within fractures, and storage occurs within the matrix of the rock.

Furthermore, they are not single aquifers as commonly regarded. The Prairie du Chien-Jordan Aquifer is divided into an upper Shakopee Aquifer, which is dominated by flow through secondary pores, and a lower Jordan Aquifer, which is dominated by intergranular flow. These units are separated from one another by an Oneota confining unit consisting largely of dolostone. The Franconia-Ironton/Galesville Aquifer is divided into an upper Franconia Aquifer, which is dominated by fracture flow, and a lower Ironton-Galesville Aquifer, which is dominated by intergranular flow. These units are separated by interbedded fine siliciclastic and carbonate strata of the mid to lower Franconia Formation. These results differ substantially from commonly used hydrogeologic characterizations of the same lithostratigraphic units, and have important implications for wellhead protection efforts.