UTILIZING QUATERNARY MAPPING FOR GROUNDWATER STUDIES IN THE ST. LOUIS METRO EAST AND CHICAGO METRO WEST AREAS

Recent 1:24,000-scale mapping of Quaternary deposits and bedrock topography in northeastern and southwestern Illinois has contributed to our understanding of aquifer systems, including their distribution and connectivity. Such maps are useful for predicting occurrences, modeling, and determining contamination potential of water-bearing sand and gravel deposits.

In southwestern Illinois, near the terminus of the Illinois and pre-Illinois Episode ice sheets, Wisconsin Episode aquifer material occurs in the broad Mississippi Valley, where up to 50 feet of coarse sand and gravel is buried below postglacial fine sand and mud. The vulnerability of this aquifer to contamination is high because of the lack of a continuous overlying aquitard, its relatively shallow depth, and the proximity of a long standing industrial area. High yield sand and gravel aquifers of Illinois Episode age are limited primarily to northeast-southwest trending bedrock valleys, parallel to the predominant ice flow direction. Low yield aquifers are common in upper portions of Illinois Episode deposits over a broad area, but are generally suitable only for individual household or farm use. Other high yield aquifers of this age include ridged-drift deposits, but they may be variable in sorting, interbedded with diamicton units, and have fluctuating water table levels. Pre-Illinois Episode aquifers are rare and difficult to identify, probably due to subsequent fluvial and glacial erosion of such units.

In northeastern Illinois, Quaternary mapping has been completed in Kane County and modeling of groundwater quantity and contamination sensitivity has begun. Surficial and buried aquifers in the county are abundant and consist of Wisconsin Episode outwash and ice-contact deposits, associated with each of five ice margins mapped, as well as older units. In contrast to SW Illinois, the distribution of surficial aquifers is strongly related to the geomorphology (occurring in terraces, kames, eskers, deltas, lowlands and outwash plains). These aquifers commonly lack a thick overlying confining unit for protection from potential contamination. Deeper, well-protected aquifers occur in southwest trending bedrock valleys filled with Illinois Episode outwash (similar to SW Illinois), and are utilized for municipal water supplies.