Investigation of the Mahomet Aquifer System Continues

ISGS staff drill a Mahomet aquifer monitoring well.The Mahomet aquifer and the shallower aquifers that overlie it constitute the Mahomet aquifer system, the principal groundwater resource of east-central Illinois. The ISGS recently completed test drilling at two sites west of Champaign. Both sites are located over the Mahomet aquifer. Using the wireline method, the initial borehole at each site was drilled through the entire thickness of unconsolidated sediment and a few feet into the underlying bedrock. The continuous core that was collected from the initial boreholes was described in the field by ISGS geologists and boxed for transport to the Quaternary laboratory for more detailed description and analysis. A natural gamma geophysical log of the first borehole at each site was obtained. These geophysical logs complement the descriptive logs of the boreholes and greatly assist the decisions made regarding the number of observation wells to install at the site and depths of the screens. Three observation wells were installed in separate boreholes at the first drill site. Two of the wells were installed in the Mahomet aquifer, one near the top of the aquifer and the other near the base of the aquifer. The third well was completed in a sand unit within the Glasford Formation. At the second drill site, two observation wells were installed in separate boreholes: one near the top of the Mahomet aquifer and the other near the aquifer's base. The observation wells were pumped 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. In support of the drilling at the second site, a seismic survey was conducted along a line from about three miles east of the drill site to nine miles west of it.

The data acquired will support ongoing geological and hydrogeological research of the Mahomet aquifer and shallower aquifers. These data will improve the understanding of the physical and hydraulic characteristics of the sediments within the Mahomet Bedrock Valley as well as groundwater availability and flow in the area. With continued development of east-central Illinois' groundwater resources, such understanding is integral to addressing water supply issues, as is currently being accomplished through the Governor's Water-Supply Planning Initiative.