WHERE IS THE DEEP GROUNDWATER DIVIDE IN SOUTHEASTERN WISCONSIN?

Applying science to problems of public interest often requires translation of complex scientific findings into simple concepts that legislators, regulators, and the public can understand. There is currently significant societal interest in the historic and current positions of a deep groundwater divide in southeastern Wisconsin. The potentiometric divide in the deep Cambrian sandstone aquifer represents a hydrogeologic boundary between groundwater flowing east toward Lake Michigan and west toward the Mississippi River drainage basin. The groundwater divide does not coincide with regional surface-water or topographic divides.

Our current understanding of the groundwater divide is based on estimates of historic groundwater use and regional groundwater flow models. Prior to development, in the region west of Milwaukee, the divide was probably located in western Waukesha County. Today, pumping from the deep aquifer in southeast Wisconsin has caused the divide to migrate westward several miles into Jefferson County, and the pumping centers, rather than Lake Michigan, have become the main discharge points for the regional flow system. Modeling shows that the divide is actually a complex three-dimensional surface that varies in position in relation to depth and time. Although current models give a reasonable estimate of the positions of the divide through time, regulators need to understand that the divide is not fixed in space and that different models and different analyses using the same model might yield multiple scientifically defensible estimates of the divide location.

During recent legislative deliberations over the international Great Lakes Compact, the divide has gained legal and political significance as a potential regional groundwater boundary because the Compact bans future withdrawals of water from the Great Lakes Basin. On the basis of some legal interpretations still under debate, groundwater east of the divide might be considered waters of the Great Lakes, giving local communities rights to Lake Michigan water even though they lie outside the Great Lakes surface-water basin. Accordingly, establishing the position of the divide could be important for regional water management.