SIMULATION OF THE GROUND-WATER-FLOW SYSTEM IN PIERCE, POLK, AND ST. CROIX COUNTIES, WISCONSIN

Paul Juckem, U.S. Geological Survey, Wisconsin Water Science Center

A regional three-dimensional ground-water-flow model and three associated demonstration inset models were developed to simulate the ground-water-flow systems in Pierce, Polk, and St. Croix Counties, Wis. The objectives of the regional model were to improve understanding of the ground-water-flow system and to develop a tool suitable for evaluating the effects of potential water-management programs.

The regional ground-water-flow model simulates the major hydrogeologic features of the area, including bedrock and surficial aquifers, ground-water/surface-water interactions, and ground-water withdrawals from high-capacity wells. Results from the regional model indicate that about 82 percent of ground water in the three counties is from recharge within the counties; 15 percent is from surface-water sources, consisting primarily of recirculated ground-water seepage in areas with abrupt surface-water-level changes, such as near waterfalls, dams, and the downgradient side of reservoirs and lakes; and 4 percent is from inflow across the county boundaries. Ground-water flow out of the counties is to streams (85 percent), outflow across county boundaries (14 percent), and pumping wells (1 percent). These results demonstrate that the primary source of ground water withdrawn by pumping wells is water that recharges within the counties and would otherwise discharge to local streams and lakes.

Under current conditions, the St. Croix and Mississippi Rivers are ground-water discharge locations (gaining reaches) and appear to function as “fully penetrating” hydraulic boundaries such that ground water does not cross between Wisconsin and Minnesota beneath them. Being hydraulic boundaries, however, they can change in response to water withdrawals. Tributary rivers act as “partially penetrating” hydraulic boundaries such that ground water can flow underneath them through the deep sandstone aquifers. The model also demonstrates the effects of development on ground water in the study area. Water-level declines since predevelopment (predating withdrawal wells) are most pronounced where pumping is greatest and flow between layered aquifers is impeded by confining units or faults. The maximum simulated water-level decline is about 40 feet in the deep Mount Simon aquifer below the city of Hudson, Wis.

Three inset models were extracted from the regional model to demonstrate the process and additional capabilities of the USGS MODFLOW code. Although the inset models were designed to provide information about the ground-water-flow system, results from the inset models are presented for demonstration purposes only and are not sufficiently detailed or calibrated to be used for decision-making purposes without refinement. Simulation of ground-water/lake-water interaction around Twin Lakes near Roberts, in St. Croix County, Wis., showed that ground water represents approximately 5 to 20 percent of the overall lake-water budget. Ground-water contributing areas to streams in western Pierce County are generally similar in size as the surface-water contributing areas but do not necessarily correspond to the same land area. Transient streamflow simulations of Osceola Creek in Polk County demonstrate how stream base flow can be influenced not only by seasonal precipitation and recharge variability but also by systematic changes to the system, such as ground-water withdrawal from wells.