Hydrogeology of glacial deposits in a preglacial bedrock valley, Waukesha County, Wisconsin

This report describes the areal extent, thickness, and hydraulic properties of glacial deposits in a preglacial bedrock valley south of the city of Waukesha in southeastern Wisconsin. In the 40- square-mile study area, the preglacial bedrock valley underlies an area across which the Fox River flows. A previous regional study of the area indicated that extensive glacial sand and gravel deposits may exist in the preglacial bedrock valley. New test-hole, well-construction, and seismic data collected from 1986 through 1991 showed that the preglacial bedrock valley immediately south of the city of Waukesha is narrower and shallower than previously thought. However, these data indicate that thicknesses of saturated glacial deposits in excess of 250 feet exist in a 1- to 2-mile-wide part of the valley in the southern part of the study area. Test-hole logs indicate that clean sand and gravel deposits are present in a shallow part of the preglacial bedrock valley. Fifty to sixty feet of silty and clayey sand and gravel deposits appear to underlie varying thicknesses of relatively impermeable clay till in the center of the study area. Ground water flows from upland areas on the eastern and western sides of the Fox River and discharges to the Fox River and wetlands adjacent to the river.

Results of a 6.5-hour aquifer test indicate that the silty sand and gravel deposits have an average transmissivity of about 140 feet squared per day and an average storage coefficient of about 1.2x10"3 at one location. The horizontal hydraulic conductivity of these deposits averages about 4 feet per day. Analysis of drawdown indicates that these deposits are part of a leaky confined-aquifer system and that some water is derived from storage in an overlying clay layer. The transmissivity value determined from this aquifer test and a lack of clean sand and gravel encountered in other test holes indicate that glacial deposits at these sites may not yield enough water for a large municipal water supply. Sand and gravel deposits, capable of development as a municipal supply, may be present in the southern part of the study area. However, additional test holes are needed to determine whether adequate sand and gravel deposits underlie this area.