New Field Work Adds to the Knowledge of the Mahomet Aquifer

The Mahomet aquifer and the shallower aquifers in the Glasford Formation that overlie it are the focus of a continuing field investigation. As part of the project, six observation wells have been installed at four sites. Two of wells are in the Mahomet aquifer; the other four are in Glasford Formation sand and gravel aquifers. Continuous core was collected at two of the sites; drill cuttings were collected from the drilling fluid at other sites. The deepest borehole at four of the sites was logged using gamma-ray logging. As of the end of 2007, a total of 17 additional observation wells have been installed in the Mahomet aquifer and the overlying aquifers in the Glasford Formation. In addition to the drilling and installation of observation wells, resistivity data have been collected along a quarter-mile line at the River Bend Champaign County Forest Preserve in Mahomet.

Preliminary analysis of the results showed the presence of three units within a depth of 150 feet. These units seem to correlate with alluvial sand and silt, glacial outwash, and glacial till. transect. The units were seen in the core that was collected at a site along the Mahomet Forest preserve transect. Resistivity data were also collected along two transects at Lodge Park, Piatt County Forest Preserve, north of Monticello, to identify a suitable site for installation of observation wells to measure hydraulic head in Illinoisan age (Glasford) sediments and the pre Illinois Mahomet aquifer. Seismic data were collected along a transect over the Pesotum Bedrock Valley. Analysis of these data better characterized the sediments infilling the valley and the bedrock surface along the seismic profile. Two observation wells were installed at a site near the center of the seismic transect. The borehole sediment description and the downhole gamma log data helped with interpretation of data from the seismic profile. Although the Pesotum Bedrock Valley is filled predominantly with fine grained sediment, the additional data suggest a more complex geology. Many different sediment types, representing several glacial episodes and at least one pre-glacial episode, were recovered from the borehole. These materials were lumped into three general units to aid in interpretation of seismic data. Unit 1 includes glacial till and several sand and gravel layers. Unit 2 includes mainly hard glacial till of uniform texture. Unit 3 consists of layers of gravel, sand, and silt and clay that directly overlie weathered bedrock. Aquifer materials found in the lower part of unit 3 are at about the same elevation as the Mahomet aquifer. The geologic and hydrogeologic relationships between the Pesotum Bedrock Valley and the Mahomet aquifer will be further explored.