THE RESPONSE OF A REGIONAL AQUIFER TO PROLONGED OVER PUMPING: THE SOUTHEASTERN WISCONSIN DEEP SANDSTONE AQUIFER

Regional aquifer systems provide vast supplies of water to many important industrial and irrigation centers. The large storage capacity of these aquifers often leads to over pumping, which causes declining water levels and deteriorating water quality. The Cambrian-Ordovician sandstone aquifer system of southeastern Wisconsin is a good illustration of the effect of over pumping.

Pumpage from the sandstone aquifer has created a large cone of depression centered in eastern Waukesha County (SEWRPC 2002). Now, the historically good water quality is experiencing rising levels of total dissolved solids. In response to these conditions, a sophisticated groundwater model of the flow system of southeastern Wisconsin was created. The Southeast Wisconsin Regional Groundwater Model was constructed by the Southeast Wisconsin Regional Planning Commission with assistance from the United States Geological Survey and the Wisconsin Geological and Natural History Survey. The SEWRPC model is a three-dimensional MODFLOW model of the shallow and deep ground water flow system of southeastern Wisconsin. This tool is making possible detailed analysis of the consequences of prolonged draw down on the sandstone aquifer.

Several alternative water withdrawal scenarios were considered using the model to determine the effect a change from groundwater withdrawal to direct surface water withdrawal from Lake Michigan would have on the aquifer. These alternatives include: current groundwater pumping rate today, increase the groundwater pumping rate 50% in Waukesha County, and eliminate pumping from the sandstone aquifer in Waukesha County.

The groundwater model demonstrates that the deep aquifer below most of Waukesha County historically flows to the east into the Great Lakes. Water now pumped from the deep aquifer currently comes from aquifer storage and from induced recharge principally from an expanding western recharge area. A portion of recharge now comes from the east reversing the direction of the historic groundwater flow. Increasing aquifer pumping will increase the rate of drawdown and the induced recharge from all directions. Switching from a deep groundwater source to direct withdrawal from Lake Michigan will allow the groundwater to return to a condition similar to predevelopment conditions over about 20 years.