The aquifer lies on top ofthe bedrock, and in the eastern three quarters of the area, it is deeply buried below younger fine-grained sediment.

The Mahomet aquifer consists of saturated glacial sand and gravel below fourteen counties, from the Illinois-Indiana state line west to the Illinois River.

Figure courtesy of the Illinois State Geological Survey

An aquifer is a geologic unit that can store and transmit water at rates fast enough to supply reasonable amounts of water to a well.

What is an aquifer?

Water is pumped from wells to provide drinking water, to irrigate farmland, and to use for commercial and industrial purposes.The aquifer provides about 220 million gallons of water per day.

Why is the Mahomet aquifer important?

The Mahomet aquifer supplies water to more than 800,000 people in east-central Illinois. It supplies

most of the drinking water in this area.

The aquifer is rechargedby rain that infiltrates the soil. This water may be used by plants or flow along the one of the flow paths shown in the figure.

The Mahomet Bedrock Valley was part of a much larger river system known as the Teays-Mahomet River System that once drained the Appalachian Mountains westward to the Ancient Mississippi River. The Mahomet Bedrock Valley was subsequently filled in and buried by glacial sediment deposited during the earliest glacial episodes.

Before the first glacial episode in Illinois, river valleys were carved into the bedrock. The deepest and widest valley carved into the bedrock was the Mahomet Bedrock Valley.

How did the Mahomet aquifer form?

As glaciers crossed east-central Illinois, meltwater flowing in front of and under the ice carried a large amount of sediment. Much of the Mahomet Bedrock Valley is filled with sand and gravel deposited by glacial meltwaters. The valley was completely filled in by the end of the Illinois Episode glaciation.

Collectively, these deposits of sand and gravel form the Mahomet aquifer. In some places, the aquifer is 250 feet thick.

As part of a U.S. Geological Survey program to monitor groundwater levels in the nation, scientists from the Prairie Research Institute and U.S. Geological Survey are measuring the water level in observation wells installed in the Mahomet aquifer, including several in the Sangamon River Forest Preserve. The data collected from the wells will inform water management in the future.

Monitoring groundwater levels

How do scientists monitor groundwater?

The water-level data collected from observation wells in the Mahomet aquifer and adjacent aquifers are being used to help determine the effects of increased pumping and climate change on these aquifers.

The change in water level over time is used to determine such effects.

Water levels are recorded manually in the field or remotely using sensors.

This diagram shows the inside of a observation well.

View of well protectors at the Lakeof the Woods Forest Preserve

URL: http://www.epa.gov/r5water/gwdw/mahomet/. Accessed March 11, 2015

Under the federal Safe Drinking Water Act, an aquifer supplying at least 50% of the drinking water consumed in the over-lying area is designated a sole source, meaning no alternative water sources could meet the demand of the area.

On March 11, 2015, the U.S. Environmental Protection Agency designated a portion of the Mahomet Aquifer System as a sole source aquifer. This action adds a level of review for projects using federal funds, which may adversely impact it.

How can we protect the aquifer?

Properly seal abandoned wells to prevent contamination (hire a licensed professional).

Unused or abandoned water wells pose one of the greatest threats to our water supply.

They provide a direct route for pollutants to reach an aquifer.

Protecting our groundwater from contamination is important for managing water resources. Once water is contaminated, it takes decades or longer to clean. Often the damage is permanent.

For more information visit http://www.epa.illinois.gov/topics /drinking-water/private-well-users/abandoned-wells/index

Andrew Stumpf

Edward Mehnert

Lisa Anderson

George Groeschen

Many thanks to the geoscientists who prepared and edited this content,

including

For Further Reading

Kempton, J.P., W.H. Johnson, P.C. Heigold, and K. Cartwright, 1991, Mahomet Bedrock Valley in east-central Illinois: Topography, glacial drift stratigraphy, and hydrogeology, in W.H. Melhorn and J.P. Kempton, eds., Geology and hydrogeology of the Teays-Mahomet Bedrock Valley system: Boulder, Colorado, Geological Society of America, Special Paper 258, p. 91–124. URL: http://dx.doi.org/10.1130/SPE258

Panno, S.V., and H. Korab, 2000, The Mahomet aquifer: The Illinois Steward, v. 9, no. 1, p. 19–21. URL: http://www.mahometaquiferconsortium.org/samsILS.html

Regional Water Supply Planning Committee, 2009, A plan to improve the planning and management of water supplies in east-central Illinois: Mahomet Aquifer Consortium, 92 p. URL: http://www.rwspc.org/documents/ECI-WaterPlan_062909.pdf

Roadcap, G.S., H.V. Knapp, H.A. Wehrmann, and D.R. Larson, 2011, Meeting east-central Illinois water needs to 2050: Potential impacts on the Mahomet aquifer and surface reservoirs: Illinois State Water Survey, Contract Report 2001-08, 188 p. URL: http://www.sws.uiuc.edu/pubs/pubdetail.asp?CallNumber=ISWS+CR+2011-08

Soller, D.R., S.D. Price, J.P. Kempton, and R.C. Berg, 1999, Three-dimensional geologic maps of Quaternary sediments in east-central Illinois: United States Geological Survey, Geologic Investigations Series Map I-2669. URL: http://pubs.usgs.gov/imap/i-2669