REGIONAL GROUNDWATER MAPPING IN SOUTHWESTERN ILLINOIS – LARGE AND SMALL SCALE APPLICATIONS

The geology and aquifers of three southwestern Illinois counties, Madison, Monroe and St. Clair, are being studied as part of a regional evaluation of groundwater resources. Thick, Quaternary sands and gravels underlie the Mississippi River floodplain. On the uplands to the east, shallow and relatively discontinuous glacio-fluvial and other sand and gravel deposits are found within stream 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, found along the eastern half of the region, are mostly shale and yield very limited quantities of water.

Regional mapping is being conducted at small scale (1:100,000). Coincident to this mapping project, a comprehensive suite of large scale (1:24,000) quadrangle maps are being prepared as part of a statewide program. Small scale maps provide an overview of the region while the large scale maps provide detail for specific mapping applications such as defining shallow aquifers and aquifer vulnerability.

To better understand the geology and hydrology of a portion of the mapping area, a comprehensive, detailed study of alluvial and glacio-fluvial sediments is being conducted in the area of an abandoned meander channel of the Mississippi River in Madison County. This study, incorporating detailed sedimentology, surficial and down-hole geophysics, and surface and subsurface hydrology, will provide a basis for the examination of similar depositional sequences throughout the broad Mississippi River floodplain of southwestern Illinois. Better predictive geologic interpretation and a sophisticated understanding of the large-scale groundwater flow system will follow. The valley fill sequence of the study area has proven more varied than had been previously interpreted from logs of borings in the area. Multiple thin, fine-grained layers are present within the upper 30 feet of the alluvial sequence and have been found up to a depth of 50 feet in some locations. The underlying sand layers are highly variable in texture. Gravels are only encountered just overlying the bedrock surface in few locations. The bedrock surface is relatively flat, though surface geophysics has indicated a shallow swale running generally north to south.