R2026-125 Resolution Authorizing Agreement with INTERA Incorporated to Provide Professional Engineering Services to the City of Decatur for the Alternate Water Supplies Update, City Project 2024-05

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Exhibit A SCOPE OF WORK Introduction Over the last two years,the City of Decatur, IL(City) has discussed several commercial endeavors that would potentially increase water demand on Lake Decatur beyond the demand projected in INTERA's 2019 report to the City (Additional Water-Supplies: Local Solutions to Manage Drought Risk). In that report, a demand increase was projected, bringing the total lake demand (potable demand at the City's South Water Treatment Plant (SWTP) plus non-potable demand at ADM's North Water Treatment Plant (NWTP) from 32.3 million gallons per day (MGD) in 2017 to 38.5 MGD in 2050—a modest 6 MGD increase over approximately 30 years. Our analysis showed that during a worst-case drought, approximately 3,100 MG would be needed over a 10-month drought period to meet the projected demand. A selection of readily available water-supply options, in combination, were found to be capable of meeting the supply gap, most of which have been used in previous droughts. These include: 1) the DeWitt County wellfield (630 MG), 2) the former Vulcan Pit or "water mine" (250 MG), and 3) Lake Tokorozawa (900 MG).The most significant measure, however,was to allow the lake level to drop below 607' (ADM's NWTP west intake elevation) to the SWTP intake elevation of 604' (1,670 MG). Since the 2019 assessment was completed, the City has taken several steps to improve their drought resiliency. Pipeline was laid to provide a permanent connection between the former Vulcan Pit to the SWTP raw water intake building, allowing water from the pit to be used on- demand and coincidentally with treatment plant sludge decant water. When implemented, planned improvements to the existing wellfield will improve efficiency in delivering water to address lake level drops before drought impacts are fully realized. Additional farm property was purchased northwest of the DeWitt wellfield, potentially increasing the total yield from the Mahomet Aquifer should new wells be developed there. Rounding out current improvements to the City's water supply portfolio, the City purchased the Harristown Vulcan gravel pit several miles downstream from Decatur. Our analysis completed earlier this year determined that the pit can provide 900- 1,000 MG. Additional demands could increase the drought supply gap beyond 3,100 MG. These new demands may be met with a combination of potable and non-potable supplies, some of which were not evaluated in the 2019 report. The timing of the water supply needs will be highly dependent on the pace and type of development that occurs. Multiple variable growth scenarios need to be evaluated to prepare for a range of potential futures. To ensure that the level and rate of investment in new water supplies is appropriately matched to needs, an update to the 2019 adaptive management plan for implementing projects is recommended. In addition, the recent drought alerted City officials to Lake Decatur's drought sensitivity and the need to update the 2012 Low Lake Level/Drought Action Plan. An assessment of operating rules and action triggers can improve the balance between reliability and cost. This can ensure droughts are detected in time while minimizing the cost of corrective action. We propose to 1) assess the impact of additional water demand on Lake Decatur's drought resiliency and supply gap, 2) review previously evaluated and new water supply projects, 3) Last Revised:2/03/2022