Groundwater Geology

of DeWitt, Piatt, and Northern Macon Counties, Illinois

David R. Larson, Beverly L. Herzog and Timothy H. Larson

Illinois State Geological Survey Environmental Geology 155, 2003, 35 p.

ABSTRACT

The Mahomet aquifer is one of the largest sources of groundwater in Illinois. This aquifer, which occupies the lower part of the buried Mahomet Bedrock Valley, arcs to the south across east-central Illinois from the Indiana state line westward to where it joins the Sankoty aquifer. Central Piatt and DeWitt Counties overlie a portion of the Mahomet aquifer. The characteristics of the Mahomet aquifer are understood sufficiently well to know that enough groundwater is available to meet foreseeable water needs arising within central Piatt and DeWitt Counties. In addition, shallower aquifers occur throughout both counties, scattered at various depths below the land surface. These aquifers are generally discontinuous, limited in areal extent, and relatively thin. Although their extent, distribution, and characteristics are less well understood than those of the Mahomet aquifer, the shallower aquifers are a significant resource in Piatt and DeWitt Counties. They are the source of water for numerous towns and most of the rural households, especially those located outside the boundaries of the Mahomet aquifer.

The Mahomet Valley Water Authority (MVWA) includes all of Piatt County and all but the southeast corner of DeWitt County. The MVWA determined that mapping the distribution and thickness of aquifers in its area of jurisdiction was an integral part of groundwater resource management. To accomplish this purpose, the MVWA began a cooperative project with the Illinois State Geological Survey (ISGS) in 1994 to map the aquifers located in Piatt and DeWitt Counties. Maps and cross sections were developed using data from 51 stratigraphic-control boreholes, sample sets from 15 boreholes, and approximately 100 of the best drillers' logs available. The maps in this report are based on the data obtained from nearly 3,500 drillers' logs of water wells and other boreholes. This study also incorporates data on depth to bedrock obtained from a shallow seismic reflection survey in the Farmer City-Mansfield area and data on aquifer depth and thickness from an extensive electrical earth resistivity (EER) survey in the southern half of Piatt County.

The maps included in this report show the thickness and extent of the Mahomet and four shallower aquifers in the area. The top of the Mahomet aquifer is more variable than previously thought. Maximum aquifer thickness reported in the drillers' logs is 174 feet. Based on the difference between elevations of the top of the Mahomet aquifer and the bedrock surface, aquifer thickness could locally be as much as 190 feet. Shallower aquifers above the Mahomet are widespread and are most common in areas not underlain by the Mahomet aquifer. Maps of these aquifers indicate that they are sufficiently thick in many parts of the study area to be reliable sources of supply for domestic wells.