GEOLOGIC CHARACTERIZATION, MAPPING AND EVALUATION OF AQUIFERS IN THREE SOUTHWESTERN ILLINOIS COUNTIES

The Metro-East region of Illinois is comprised of Madison, Monroe and St. Claire Counties and is part of the St. Louis, Missouri metropolitan area. Tremendous growth, with increasing urbanization and suburban expansion, has occurred in this region. Previous groundwater studies of the area are dated or out-of-print and unavailable for use by the public and other potential users. This study evaluates the groundwater resources of the Metro-East region and provides a regional framework for detailed hydrogeologic mapping. The region was differentiated by primary aquifer lithologic type. These areas were then examined for groundwater resource availability and contamination potential. Map products included a summary of groundwater use, isopachs of unconsolidated aquifers, depth to aquifer materials, and aquifer contamination potential. Lithologic data and drillers' descriptions from logs of water well and other borings were classified by texture and/or rock type. Descriptions were coded using two and three letter combinations. This coding facilitated our mapping efforts by giving us the ability to easily manipulate and visualize a large data set. It also permitted an effective way of synthesizing a wide variety of earth material descriptions into a manageable format. Earth material summaries will enable us to apply hydrologic values, such as hydraulic conductivity, to the geologic materials for modeling purposes. The aquifer resources of the region are varied. Quaternary sands and gravels underlie the Mississippi River floodplain. These thick, extensive deposits provide copious amounts of water for various purposes. On the uplands to the east, shallow and relatively discontinuous alluvial and outwash sand and gravel deposits are found within stream and creek valleys. Karstic, Mississippian-aged limestone aquifers provide small to moderate quantities of water and are present primarily in the southern half of the region. Pennsylvanian-aged bedrock units of mostly shale typically yield very limited quantities of water. Most upland regions have limited groundwater resource availability, and large-diameter, bored wells are prevalent.