PRELIMINARY HYDROGEOLOGIC CHARACTERIZATION OF THE MINK RIVER ESTUARY, DOOR COUNTY, WISCONSIN

The Mink River Estuary (MRE), an (~1.5 km²) embayment of Lake Michigan located in the Door Peninsula of Wisconsin, is one of the most pristine freshwater estuaries in the United States, and home to several endangered species. The MRE is dominated by several springs in its upper reaches, and groundwater quantity and quality are critical to the health of the estuary. In a cooperative project funded by the Wisconsin Coastal Management Program we are establishing a groundwater monitoring network around the estuary, characterizing the local hydrogeology, and evaluating mixing between the Mink River and Lake Michigan.

Data collected during the summers of 2010 and 2011 by two hydrogeology field courses have greatly improved our understanding of the MRE. A survey of surface-water chemistry revealed a distinct mixing pattern between groundwater discharging in discrete spring complexes and Lake Michigan waters. Stream discharge measurements and in-stream mini-piezometers yielded mixed results concerning groundwater/surface-water interactions. In the northwest arm of the Mink River, both stream discharge and mini-piezometer data suggest that groundwater discharges from discrete springs near the edges of the marsh but that the Mink River does not gain flow along significant reaches. In contrast, mini-piezometers installed in the northeast arm showed consistently upward hydraulic gradients. Heat-pulse flow logs from three bedrock monitoring wells indicate upward flow from fractures near the base of the wells (~90 ft depth) to fractures near the bedrock surface (~depth 20 - 40 ft).

Based on preliminary field reconnaissance, we hypothesized that depth to bedrock might control the distribution of springs within the MRE. Cores collected by hand auguring (12 feet depth) and by vibracoring (18 ft depth) indicate that peat overlies marl deposits near the northwest spring complex. Two EM31 surveys within the tussock-sedge marsh show systematic changes in earth conductivity. These changes may be related to differences in peat thickness and/or depth to rock. Additional coring and additional geophysical surveys are needed to determine the distribution of depth to rock within the marsh and to evaluate whether spring location is related to bedrock depth.