THREE-DIMENSIONAL GEOLOGIC MODEL OF THE HADLEY BURIED VALLEY IN WILL COUNTY, ILLINOIS

Growth in water demand within the Chicago Metropolitan area, together with declining water levels in deep bedrock wells and strictly regulated use of Lake Michigan water, have led to a regional awareness of the need for science-based, sustainable water-supply management. The Illinois State Geological Survey (ISGS) has been a primary scientific resource to local and regional decision-makers in the area, providing technical expertise, reports, and maps regarding the distribution and character of aquifers. The ISGS geologic mapping efforts in Will County address mapping priorities through the production of large-scale, 3-D geologic and hydrogeologic maps and framework models of glacial deposits. Will County is within the southwestern and southern metropolitan Chicago area, a region with high population and industrial growth, critical groundwater issues, and a nationally significant intermodal freight and passenger transportation corridor. Recent geological investigations have revealed two large bedrock valleys east of Joliet roughly coincide with the present valleys of Spring Creek and Hickory Creek and a third, called the Hadley buried valley, forms a connecting channel between the two. The three channels surround a bedrock island upland which rises 30 meters above the bedrock valleys on all sides. The valleys are about a mile wide, have relatively steep sides, and average more than 30 meters in depth. The longest valley is that along Spring Creek which extends at least 20 km eastward from Joliet. Numerous tributaries of various sizes have been delineated largely on the basis of a resistivity survey. Flat uplands appear to be absent, so even where closely spaced well logs are present, the elevation of bedrock is not likely to be the same. The valley fill consists largely of sand with significant amounts of gravel. Locally in Joliet, the gravel beds are cemented to form a conglomerate which is exposed at the surface along Spring and Hickory Creeks and which has been reported in the field. Sediment distribution in the eastern part of the Hadley buried valley seems to be irregular and contains a large proportion of sand and silt. Thick deposits of sand in this part of the valley, coupled with a greater depth to bedrock, caused difficulties during an exploratory drilling program. This study discusses the methodology for developing a detailed 3-D geologic map and hydrogeologic framework of this area of bedrock valleys in Will County.