Mahomet Aquifer System Investigation: 2008

The field investigation of the Mahomet aquifer system, comprising the Mahomet aquifer and overlying shallower aquifers, continued during summer 2008. The investigation included drilling test holes, installing observation wells, and acquiring geophysical data.

Fourteen sites were drilled in 2008. Of these, eight are located in Champaign County, three in Piatt County, and one each in McLean, Ford, and Vermilion Counties. All of the boreholes were drilled using the wireline method. The core that was collected was described in the field by Illinois State Geological Survey geologists and subsequently boxed for transport to the Quaternary laboratory for more detailed description and analysis. A natural gamma log of the deepest borehole at the site was obtained to complement the descriptive log of the borehole and to assist the geologists with determining the number of observation wells to install at the site and the depths at which the wells screens should be set.

Twenty-four observation wells were installed at 13 sites. Because aquifer material was not encountered at a southwest Champaign County site, a well screen was not installed. However, the borehole was cased with 2-inch polyvinyl chloride pipe so that downhole geophysical data could be acquired. This site is near the confluence of the Pesotum and Mahomet Bedrock Valleys.

A single observation well was installed at four of the 13 sites, two observation wells at seven sites, and three observation wells at two sites. The observation wells were developed to ensure good hydraulic connection to the adjacent aquifer. The water level in these wells will be measured periodically, and water samples will be collected for analysis. With the completion of this drilling program, nine additional observation wells completed in the Mahomet aquifer are available for water-level measurements. The other 15 observation wells are completed in aquifers, many of these probably in the Glasford Formation.

Seismic data were collected from 14 miles of transects in southwestern Champaign County near Sadorus and in northeastern Champaign County near the Middle Fork County Forest Preserve. Resistivity data were collected by ISGS geophysicists from four miles of transects at the Middle Fork Forest Preserve. Additionally, downhole seismic data were acquired by ISGS geophysicists from several of the observation wells.