R2023-227 Resolution Authorizing Farmland Real Estate Purchase in Dewitt County Pursuant to the Water Consultant's Recommendations (Intera)

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X17 ' EXECUTIVE SUMMARY Layne GeoSciences, Inc. was retained by the City of Decatur (City), Illinois, to perform a groundwater modeling study of the underlying aquifers in east-central Illinois. The City owns two wellfields in the area: the DeWitt wellfield comprising of eight wells, and the Cisco wellfield consisting of two wells. Both wellfields are intended to be operated only occasionally to supplement the City's surface water source. The DeWitt wellfield has a design capacity of 24 MGD for 90 days of operation. However this capacity has never been verified. The specific goal of the study was therefore to utilize the groundwater model to determine the safe operating capacities of the DeWitt and Cisco wellfields. The area is underlain by three unconsolidated formations, each representing a different hydrogeologic unit. From the top, these are the Wedron, Glasford, and the Banner Formation. Of the three, the Banner Formation is the most productive unit. Specifically, the thick sand and gravel deposits in the lower portions of the Banner Formation, referred to as the Mahomet aquifer, is most prolific aquifer in the area. Both the DeWitt and the Cisco wellfields obtain water from this unit. The emphasis of this study is therefore on the Mahomet aquifer. The groundwater model was first calibrated to current-day hydrologic conditions in the Mahomet aquifer. The calibrated model was then validated to the observed response (of the Mahomet aquifer) recorded during the 30-day pump test. An excellent match was obtained between the observed and model simulated aquifer drawdowns. Results of model calibration indicate that fairly confining conditions exist at the DeWitt wellfield thereby restricting recharge from infiltration. In the larger study area, extending from Paxton in the east to Lincoln in the west, approximately 35 MGD enters the Mahomet aquifer as recharge from the overlying Glasford aquifer. Approximately 27 MGD of this recharge is withdrawn for various municipal, industrial, agricultural, and domestic purposes. The remaining 8 MGD exits the study area along the western boundaries of the model. A series of management simulations were conducted with the calibrated model in order to estimate the safe operating capacities of the DeWitt and Cisco wellfields. Simulation results indicate that the wellfield capacity may be limited by excessive drawdowns in the area. It is recommended that pumpage be distributed between the DeWitt and Cisco wellfields in order to minimize drawdowns and maximize wellfield yields. Consideration may be given to increasing wellfield capacity (up to 6 MGD) in the vicinity of the Cisco wellfield. The probable safe operating yield of the DeWitt and Cisco wellfields on an equivalent average daily basis is 7-8.5 MGD combined pumpage. The corresponding total annual withdrawal of 2.55 to 3.10 billion gallons is the recommended annual pumpage for planning and management purposes. The groundwater model can be utilized in various ways to manage groundwater resources in the area on an as-needed basis. For example, the safe operating capacities for non-annual periods of operation can also be determined by the model.