HYDROGEOLOGIC CONTROLS ON SPRINGS IN THE MUKWONAGO RIVER WATERSHED, SE WISCONSIN

To allow assessment of current and future impacts of groundwater withdrawals and suburban development on the Mukwonago River watershed in southeastern WI, this study investigates hydrogeologic controls on groundwater discharge to the area’s springs and wetlands. Because the Mukwonago watershed contains a complex of largely intact wetlands ranging from bogs to calcareous fens as well as globally endangered oak openings, the Wisconsin Department of Natural Resources has identified it as an “outstanding water resource.” Previous studies of groundwater in this watershed focused on local scale flow patterns and geochemistry. These studies produced a conceptual model in which springflow and diffuse discharge to the wetlands are supported by shallow groundwater flow through glacial deposits, with minimal contributions from (or even groundwater loss to) shallow and deeper bedrock aquifers.

An alternative conceptual model, suggested by recent studies of springflow in nearby Dane County, WI, is that high volume springs may be supported by discharge from preferential flow zones in the shallow bedrock. The location of these springs is controlled by the intersection of the preferential flow zones with the steep wall of a buried bedrock valley. A preglacial bedrock valley, the Troy Valley, exists within the Mukwonago watershed and extends northeast towards Waukesha, WI. The northwest side of this valley underlies many of the wetlands in the northern portion of the watershed as well as the single spring complex that contributes approximately 90% of the streamflow in the upstream reaches of the Mukwonago River. The southeast side of the buried valley appears to coincide with another set of wetlands, springs and lakes along the southern margin of the watershed. Major ion and strontium isotope analyses are being used to further constrain sources of water to springs and wetlands in the watershed. A numerical groundwater flow model, developed by telescopic mesh refinement of a regional model, is also being used to assess the sensitivity of spring location and fluxes to the geometry of the buried bedrock valley and the hydraulic properties of the glacial and bedrock stratigraphic units.