R2025-013 Resolution Authorizing a Four-Year Sponsored Research Agreement with the Board of Trustees of the University of Illinois for Real-Time Mahomet Aquifer Monitoring

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Exhibit A Real-Time Mahomet Aquifer Monitoring in Dewitt County,2025-2028 A Proposal to the City of Decatur November 2024 Daniel R. Hadley, MS, PG Kevin Rennels Center for Groundwater Science Illinois State Water Survey University of Illinois Introduction and Background: The Mahomet aquifer is a vital source of water for east-central Illinois communities, industries, and homeowners.The aquifer was designated as a sole-source aquifer in 2015 by the USEPA since it supplies drinking water to more than half the population in east-central Illinois and there is no viable water supply alternative if contamination were to occur in the aquifer. Water levels in the Mahomet aquifer were impacted by an increase in irrigation in 2011, 2012, and 2013 (particularly in Champaign County), and a major drought in late 2011 and 2012.Withdrawals from the Illinois American Water-Champaign (IAWC)wellfield, as well as the DeWitt wellfield when it is utilized as a backup supply, also impacts Mahomet aquifer water levels. The Illinois State Water Survey(ISWS) maintains an extensive monitoring well network for the Mahomet and overlying Glasford aquifers(Figure 1). Many of the monitoring wells are equipped with transducers and telemetry systems that report water levels to the ISWS on an hourly basis. Long term monitoring of water levels in the Mahomet allows assessment of: 1)the impacts to the aquifer from changing use patterns (new demands from municipalities and/or irrigators), 2) impacts to the aquifer from changing weather and climate patterns such as droughts, and 3)the long-term viability and sustainability of the aquifer. Since 2020,the ISWS has collected hourly water level measurements at two monitoring wells owned by the City of Decatur(OW-1 and OB-W1)(Figure 2).These water levels are live and available to the public at: https://www.isws.illinois.edu/groundwater-science/groundwater- monitoring-well-networks/central-mahomet-decatur. In addition,the ISWS maintains a record of current and historic hand measurements taken by ISWS staff and consultants (Guillou and Associates)at the 13 remaining observation wells. Real-time measurements allow the City of Decatur and the ISWS to respond to any concerns from the public or stakeholders regarding changing water levels near the DeWitt wellfield. Using the City of Decatur's observation wells near the DeWitt wellfield also allows the ISWS to fill in a large "data gap" in this region of the aquifer. Such basic data collection efforts are essential to assessing the sustainability of large-scale water withdrawals in this portion of the aquifer. Analysis of the monitoring data also assist hydrogeologists to better understand how the Mahomet aquifer is recharged and how it interacts with surface water streams.These data also serve as critical calibration targets for groundwater flow models of the Mahomet aquifer, ultimately making the model and predicted future results much more robust. 1