MAHOMET AQUIFER

The Mahomet Aquifer is a major regional groundwater resource composed of saturated glacial sand and gravel extending beneath fourteen counties in east‑central Illinois. It lies within the Mahomet Bedrock Valley and is deeply buried beneath layers of fine‑grained glacial sediments across much of its extent.

Major sand-and-gravel aquifers in Illinois typically occur within 300 feet of the land surface and can produce significant quantities of potable water. These aquifers are commonly separated from shallower groundwater units by less permeable glacial tills or lake sediments.

The Mahomet Aquifer formed as glacial meltwater flowed westward through the Mahomet Bedrock Valley during the Pre‑Illinoian glacial episode, depositing thick sequences of sand and gravel. This valley is part of the larger Teays‑Mahomet Bedrock Valley System, which once carried water from the Appalachian region toward the ancient Mississippi River.

Over multiple glaciation cycles, erosion and deposition created overlapping sand and gravel units that now make up the aquifer system. The Mahomet Sand Member forms the primary aquifer unit within the Banner Formation.

Recharge to the aquifer varies across the region. In the eastern portion, the aquifer is confined beneath low‑permeability glacial sediments, resulting in low recharge rates. Toward the west, where the aquifer is unconfined near the Illinois River, recharge is significantly higher due to the exposure of permeable sands.

Groundwater flow patterns within the aquifer have been mapped using water‑level measurements from monitoring wells. These data show several sub‑regional flow systems, including a high potentiometric surface near Paxton in Ford County and a cone of depression near Champaign caused by municipal pumping.

References:

Illinois State Geological Survey (ISGS), 1996, Major sand and gravel aquifers: Illinois State Geological Survey, 1:500,000, at Major Sand and Gravel Aquifers | clearinghouse.isgs.illinois.edu . Retrieved December 17, 2024.

Killey, M. M. (2007). Illinois’ ice age legacy. Champaign, IL: Illinois State Geological Survey. Geoscience Education Series, 14, 74 p. http://hdl.handle.net/2142/45085

Locke, R., Roadcap, G., Stumpf, A., Leetaru, H., Kelly, W., & Winkel, R. (2018). An Introductory Guide to the Mahomet Aquifer and Natural Gas Storage in East-Central Illinois. Prairie Research Institute, Champaign, IL, 18 p. An Introductory Guide to the Mahomet Aquifer and Natural Gas Storage in East-Central Illinois | IDEALS

Mehnert, E. & Hackley, Keith & Larson, T. & Panno, Samuel & Pugin, A. & Wehrmann, Allen & Holm, Thomas & Roadcap, G. & Wilson, Steven & Warner, Kelly. (2004). The Mahomet Aquifer: Recent Advances in our Knowledge. (PDF) The Mahomet Aquifer: Recent Advances in our Knowledge

Roadcap, G. S., Knapp, H. V., Wehrmann, H. A., & Larson, D. R. (2011). Meeting east-central Illinois water needs to 2050: Potential impacts on the Mahomet aquifer and surface reservoirs. Champaign, IL: Illinois State Water Survey Contract Report 2011-08, 188 p. http://hdl.handle.net/2142/39869

Roadcap, G. S., & Wilson, S. D. (2001). The impact of emergency pumpage at the Decatur wellfields on the Mahomet aquifer: Model review and recommendations. Champaign, IL: Illinois State Water Survey Contract Report 2001-11, 68 p. http://www.sws.uiuc.edu/pubs/pubdetail.asp?CallNumber=ISWS+CR+2001%2D11

Soller, D. R., Price, S. D., Kempton, J. P., & Berg, R. C. (1999). Three-dimensional geologic maps of Quaternary sediments in east-central Illinois. Reston, VA: United States Geological Survey, Map I-2669. http://pubs.usgs.gov/i-maps/i-2669

Stumpf, A. J., & W.S. Dey, (eds.) (2012). Understanding the Mahomet aquifer: Geological, geophysical, and hydrogeological studies in Champaign County and adjacent areas. Champaign, IL: Illinois State Geological Survey, draft report to Illinois American Water, contract no. 2007-02899. http://hdl.handle.net/2142/95787

Other resources:

Brown, S. E., Thomason, J. F., & Mwakanyamale, K. E. (2018). The future of science of the Mahomet aquifer: Champaign. Illinois State Geological Survey, Circular 594. The future of science of the Mahomet aquifer | IDEALS

Burch, Stephen. (2008). Development of an Observation Well Network in the Mahomet Aquifer of East-Central Illinois. Illinois State water Survey Data/Case Study 2008-01, Champaign, IL. Development of an Observation Well Network in the Mahomet Aquifer of East-Central Illinois | IDEALS

Gibb, James P. Groundwater Availability In Ford County. Urbana: Illinois State Water Survey, 1970. Public groundwater supplies in Ford County. Urbana, IL : Illinois State Water Survey , 19 p. ; 28 cm. Illinois State Water Survey. Bulletin ; 60-8.

Hackley, K.C., 2002. A chemical and isotopic investigation of the groundwater in the Mahomet Bedrock Valley Aquifer: age, recharge and geochemical evolution of the groundwater, Ph.D. thesis, University of Illinois at Urbana-Champaign, Department of Geology, 152 p. A Chemical and Isotopic Investigation of the Groundwater in the Mahomet Bedrock Valley Aquifer: Age, Recharge and Geochemical Evolution of the Groundwater | IDEALS

Kempton, J. P., Johnson, W. H., Heigold, P. C., & Cartwright, K. (1991). Mahomet Bedrock Valley in east-central Illinois. In W. N. Melhorn & J. P. Kempton (Eds.), Geology and hydrogeology of the Teays-Mahomet Bedrock Valley system. Geological Society of America, Special Paper, 258, 91–124. http://dx.doi.org/10.1130/SPE258-p91

Panno, S.V., K.C. Hackley, K. Cartwright and C.L. Liu, 1994. Hydrochemistry of the Mahomet Bedrock Valley Aquifer, east-cental Illinois: Indicators of recharge and ground-water flow, Ground Water, 32(4): 591-604. Hydrochemistry of the Mahomet Bedrock Valley Aquifer, East‐Central Illinois: Indicators of Recharge and Ground‐Water Flow - Panno - 1994 - Groundwater - Wiley Online Library

Rickels, E.S.; Stumpf, A.J.; Malone, D.H.; Shields,W.E. Surficial geology of the Saybrook 7.5-min Quadrangle, Mclean County, Illinois, USA. J. Maps 2017, 13, 191–195. Full article: Surficial geology of the Saybrook 7.5-minute Quadrangle, Mclean County, Illinois, USA

Watson, Andrew & Peterson, Eric & Malone, David & Tranel, Lisa. (2020). Geology and Aquifer Sensitivity of Quaternary Glacial Deposits Overlying a Portion of the Mahomet Buried Bedrock Valley Aquifer System. Hydrology. 7. 10.3390/hydrology7040069 (PDF) Geology and Aquifer Sensitivity of Quaternary Glacial Deposits Overlying a Portion of the Mahomet Buried Bedrock Valley Aquifer System.

Wirth, H.; Peterson, E.W.; Malone, D.H. Surficial geology of the 7.5 Minute Gibson City West Quadrangle, Champaign, Ford, and McLean Counties, Illinois; Illinois State Geological Survey: Champaign, IL, USA, 2018; Style_Template_1

Wirth, H, Surficial geology of the 7.5 Minute Gibson City West Quadrangle, Champaign, Ford, and McLean Counties, Illinois, Master of Science Thesis, Department of Geography, Geology, and the Environment, Illinois State University, 2019 Microsoft Word - Wirth_Thesis_2019_Final.docx