Mahomet Aquifer Mapping Reveals Surprises and Gains International Collaboration

Geologist Bill Dey loads core into a box for transport.Bill Dey of the Illinois State Geological Survey and George Roadcap of the Illinois State Water Survey (ISWS) met with staff from Illinois American Water (IAW) in early November 2008 to select a drilling location by the company's water tower in north Champaign. The ISGS mobile drill rig was used to drill the hole November 18-20, 2008. Continuous core was collected during drilling, and core recovery was over 80%. A downhole gamma log was collected following drilling. Within the 305 feet of Quaternary sediments overlying the bedrock, only two sand units were encountered that were thick enough to permit installation of an observation well. Each observation well was installed with a 10-foot screen in contact with the sand unit and 10 feet of casing extending below the screen. Neither unit was the Mahomet Sand. A 10-foot-thick sand unit was found 105 to 115 feet below the land surface, which most likely is late Illinois age material in the upper Glasford Formation. The water level measured in the well on December 11, 2008, was below the bottom of the screen. A second sand unit was encountered from 175 to 180 feet below the land surface. This sand is most likely in the lower Glasford Formation and could be part of a lower Glasford aquifer. Drillers' logs for water wells in the area report sand at or near this depth. The water level in the well was measured at 143 feet below the land surface on December 11, 2008. Sediments interpreted to be the top of pre-Illinois Banner Formation were encountered at 205 feet below the land surface. Sand and gravel units representing the Mahomet aquifer were not encountered at this site. Ages and designation of the units will be verified following more detailed description of the core and core sample analysis of particle size distribution and clay mineralogy.

Mapping of the potentiometric surface of the Mahomet aquifer by the ISWS shows a steeper hydraulic gradient in northern Champaign County than in other parts of the aquifer. The absence of the Mahomet aquifer at the drill site confirms that the edge of the aquifer is in the area but farther west than previously mapped. It is not well understood if a relationship exists between the somewhat narrower aquifer and the steeper hydraulic gradient in the potentiometric surface of the Mahomet aquifer. The absence of the Mahomet aquifer at this point highlights the uncertainty that exists about the distribution and thickness of the aquifer along its edge in the area directly north of Champaign-Urbana and elsewhere in east-central Illinois. Additional study is warranted, and an increased emphasis is being placed on data acquisition and mapping in this area in 2009. Martin Ross and Lisa Atkinson from the University of Waterloo will be assisting in the mapping effort.