CONCEPTUALIZATION AND CONSTRUCTION OF HYDROSTRATIGRAPHIC LAYERS FOR SIMULATING GROUND-WATER FLOW IN PIERCE, POLK AND ST. CROIX COUNTIES, WISCONSIN.

Paul Juckem, U.S. Geological Survey - Wisconsin Water Science Center
Charles Dunning, U.S. Geological Survey - Wisconsin Water Science Center
Kenneth Bradbury, Wisconsin Geological and Natural History Survey, University of Wisconsin-Extension
David LePain, Wisconsin Geological and Natural History Survey, University of Wisconsin-Extension

Land-use impacts on water resources are important concerns in west central Wisconsin, particularly in Pierce, Polk, and St. Croix Counties where demands for agricultural and urban development are strong. An understanding of ground-water movement in this geologically complex region is needed to evaluate the effects of land-use change on water resources. Towards this end, the U.S. Geological Survey and Wisconsin Geological and Natural History Survey are working cooperatively with the three counties to construct numerical models for simulating the three-dimensional ground-water-flow system and associated surface-water/ground-water interactions.

Simulation of ground-water movement in the region requires development of a conceptual model of the important hydrogeologic units and implementation of the conceptual model into a numerical ground-water flow model. In the three-county area, ground-water flow is primarily through unlithified glacial material and thick layers of Lower Paleozoic sandstone and dolomite bedrock overlying relatively impermeable Precambrian crystalline rocks. Three features of the Lower Paleozoic rocks complicate understanding and simulation of ground-water flow in the region. First, in some locations faulting has produced large vertical offsets in stratigraphic contact elevations. Second, all sedimentary rock units thin toward the northeast and are entirely absent in northern portions of the study area, where the underlying crystalline rocks are exposed. Lastly, karst-related conduits in upper carbonate aquifers can substantially perturb local flow directions, but identification and simulation of these underground features can be difficult.

Existing data were input into a Geographic Information System and used to determine layer top and bottom elevations for the numerical model based on the conceptual model. The resultant layers capture the regional sedimentary dip, while also incorporating local variations, such as faulting. The next phase of the study will focus on evaluating properties of the individual rock units with the numerical model by comparing simulated to measured water levels and stream flows. Analyzing existing data with the numerical model will help to evaluate potential karst-related influences on the regional flow system.

Suggested Reading

Runkle, A.C., R.B. Tipping, E.C. Alexander, J.A. Green, J.H. Mossler, and S.C. Alexander. 2003. Hydrogeology of the Paleozoic Bedrock in Southeastern Minnesota: Minnesota Geological Survey Report of Investigations 61m 105 p., 2 pls.

Young, H.L. and S.M. Hindall. 1973. Water Resources of Wisconsin - St. Croix River Basin. US Geological Survey Hydrologic Investigations Atlas HA-451, 4 pls.