SIMULATION OF GROUNDWATER/LAKE WATER INTERACTION FOR SHELL LAKE, WASHBURN COUNTY, WI

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

Seepage lakes, which lack perennial inlet and outlet streams, commonly exhibit large historical fluctuations in water levels. Shell Lake, the largest inland seepage lake in Wisconsin and located in the headwaters of the St. Croix River Basin, experienced high water levels in the 1990s, which prompted application and approval of a diversion system to transfer water from Shell Lake to the Yellow River. The diversion system was operated intermittently from late-2003 to mid-2005 following a record high water level in May, 2003. Understanding effects of the diversion on water levels in Shell Lake and adjacent lakes requires an understanding of the hydraulic connection among the lakes and the groundwater-flow system.

The USGS is developing a groundwater-flow model for the area around Shell Lake to improve the understanding of the hydrologic system and the expected response in water levels of lakes throughout the area to diversions from Shell Lake. Geologic samples, streamflows, and measured groundwater levels illustrate a complex local groundwater-flow system consisting of several layered aquifers and confining units above a regional flow system. For example, preliminary calibration of the groundwater-flow model indicates that a "hole" in an otherwise laterally extensive deep regional confining unit provides a path for some groundwater to flow beneath the lake rather than discharge to the lake. Annual fluctuations in lake levels throughout the area appear to be influenced primarily by climatic forcing (precipitation, runoff and evaporation) and secondarily by groundwater inflow. The model also demonstrates that the diversion was very important to the water levels of Shell Lake. The diversion of approximately three billion gallons from Shell Lake is estimated to have reduced the current level (4 years later) of Shell Lake by about 3 feet. Lake level declines in adjacent lakes due to the Shell Lake diversion are less because of the layered confining units that appear to limit groundwater and lake water interaction.