Agenda - Public Works Committee - 10/19/2021

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Executive Summary The primary source of drinking water in the Northwest Metro is the Tunnel City-Wonewoc (TCW) aquifer. Concerns with groundwater use in the region include naturally occurring manganese, a potential health concern, and aquifer drawdown due to increased future demands. It is possible that groundwater may not be able to meet all of the future drinking water demands. The primary objective of this study is to understand the relative costs and implementation considerations of different approaches to long term water supply within the study area. The study area includes the Northwest Metro cities of Corcoran, Dayton, Ramsey and Rogers. The study will be referenced to support future planning of metro area water supplies and water sustainability practices. As cities face increased demands on their water supplies in the future, this report provides concept level options for consideration. This report meets the requirements of Minnesota Statutes, section 473, subdivision 1565, which calls for the Council to "carry out planning activities addressing the water supply needs of the metropolitan area". Special funding for this project was provided through the Clean Water Fund. This study evaluates four approaches to meet future water demands in the study area: • Approach 1: Regional Surface Water Treatment Plant • Approach 2: Regional Groundwater Treatment Plant • Approach 3: Conjunctive Use of Surface Water and Groundwater • Approach 4: Status Quo— Individual Lime Softening Water Treatment Plants This study provides communities concept level costs and considerations for various water supply approaches. It is not meant to prescribe specific solutions for implementation. Rather these approaches serve as examples to stimulate future planning that could involve a hybrid of the alternatives identified in this study or in combination with water conservation measures and other sustainability approaches. Study Area Community Information The communities in the study area represent different levels of development. All the communities have significant growth forecasted for 2040, with the study area population estimated to grow by 250%. The average day water demand for the study area is expected to increase from 3.3 million gallons per day (MGD)to 7.8 MGD in 2040. The maximum day demand for the four communities is estimated to increase from 7.4 MGD to 21.8 MGD. Community projections for ultimate buildout conditions for the study area predict an average day demand of 29 MGD and maximum day demand of 73 MGD. Regional Water System Capacity In planning for a regional water system with a single, centrally treated supply, the maximum day demand is the criteria used to establish the capacity of most system components. For approaches 1 and 2 with a central water treatment plant, the design capacity is assumed to be 25 MGD for 2040 and 75 MGD for ultimate conditions. For Approach 3, the central water treatment system capacity is based on the average day demand and peak day demands are served by each community's well system. The 2040 water treatment plant (WTP) capacity is assumed to be 12 MGD and the capacity serving ultimate demands would be 40 MGD. Phased Approach For water distribution systems, the trunk watermain is typically sized for the ultimate system capacity. To provide reliability of service, typically there is a "loop" trunk system. Given the different levels of development for the four communities and the distances involved, a looped trunk watermain sized for the ultimate capacity is a significant investment that would be underutilized for a long time. Page—ES-1 I METROPOLITAN COUNCIL