R2026-200 Resolution Authorizing a Sponsored Service Agreement with the Board of Trustees of the University of Illinois to Assess Sustainable Yield of the Mahomet Aquifer at the Decatur Wellfields

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Council Memo Public Works Department No. 2026-81 DATE: 10/19/2026 TO: Honorable Mayor Julie Moore Wolfe and City Council Members FROM: Melissa Hon, City Manager Matt Newell, Director of Public Works Robert Weil, Water Production Manager SUBJECT: Resolution Authorizing a Sponsored Service Agreement with the Board of Trustees of the University of Illinois to Assess Sustainable Yield of the Mahomet Aquifer at the Decatur Wellfields SUMMARY RECOMMENDATION: It is recommended that the City Council authorize the Mayor to sign, and the City Clerk to attest, a Resolution authorizing a Sponsored Service Agreement with the Board of Trustees of the University of Illinois to Assess Sustainable Yield of the Mahomet Aquifer at the Decatur well fields in the amount of$72,948.00. BACKGROUND: After the 1988 drought, the City of Decatur made a strategic decision to install wells into the Mahomet Aquifer in DeWitt County as an emergency water supply. Since that time,the well field has become the most important alternate water supply in the City's drought action plan. During the 2025/26 drought,the City pumped about 456 million gallons (MG) from the DeWitt well field, of which an estimated 70%, or 320 MG, reached Lake Decatur via Friends Creek and the Sangamon River. It is believed that the lake would have been about 4 inches lower during the drought without the use of emergency supplies from the DeWitt well field. Despite its importance, the maximum sustainable yield from DeWitt well field is not well documented. The current Alternate Water Supply Plan assumes a net yield of 630 MG based on historical evidence on impact to surrounding wells,but some research by the Illinois State Water Survey(ISWS) indicates that the yield could be higher. The drawdown of the aquifer was last studied in a 2001 report which stated, without supporting evidence,that the well field should be able to support a sustained withdrawal of 7.5 milliongallonsper day (MGD). INTERA is currently reviewing options for additional water supplies to provide drought resilience and has recommended that the 2001 yield estimate be reviewed for accuracy and also review its effect on the new Lane property and the Cisco well field. The ISWS has strong, well established,Mahomet Aquifer expertise and was requested to propose a modeling study. The proposed study will include withdrawals from the DeWitt well field,the newly acquired field near Lane, and the City's Cisco well field. This will detail the interaction between them and settle on an expected sustained withdrawal rate. The City of Decatur has enjoyed a long and cooperative relationship with ISWS, and we have worked together to gain knowledge of the Mahomet aquifer. The City has installed monitoring wells to measure aquifer drawdown as the wells are operated. ISWS has used a number of advanced technologies to study the aquifer, including helicopter-based time domain electronics used to develop 3D digital maps of groundwater basins.