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Prepared by: Bolton & Menk, Inc. PROPOSED IMPROVEMENTS 2026 PMP Street & Utility Improvements ǀ 25X.137753 Page 11 The Phase 2 improvement is proposed for design and construction as part of the 2026 PMP project and includes installation of a stormwater BMP to treat stormwater runoff from the Boston Scientific Campus and Cummings Park before it reaches Karth Lake. The stormwater BMP retention pond would be located at the southeast side of the lake, where the Phase 1 improvement outlets under the trail. No water from the site is currently treated causing pollutant concerns for Karth Lake and Valentine Lake, which receives the pumped flow leaving Karth Lake. Valentine Lake is impaired for chloride and nutrients and will benefit from any water quality benefits to Karth Lake. Two BMP options, a wet pond and an infiltration basin, were explored to treat this stormwater runoff. With the proximity to Karth Lake, groundwater elevation will likely be within 3 feet of the final infiltration depth and make infiltration infeasible. A wet pond is likely the best option for this site. This location receives approximately 11 acres of drainage area, with 5.5 acres of that being impervious. According to Rice Creek Watershed Districts rule of 1.1 inches of runoff over the contributing impervious area, the water quality volume that would be required if the entire area was developed or reconstructed today is listed in the table below: Storage Aspirations Stage Volume Required (Ac-Ft) Volume (Cu-Ft) Dead Pool 0.452 19674 Live/WQV 0.499 21746 Total 0.951 41420 In order to accommodate that volume of pond, it is estimated that the pond itself will be approximately ¼ acre in size and its impacts will extend beyond that to tie into existing grades and for associated storm sewer installations. However, there isn’t sufficient space between the Phase 1 outlet and Karth Lake to accommodate that area of pond. Therefore, it is proposed to design the pond to the maximum size feasible with the space available. Based on the scale of the proposed stormwater pond and the proposed location, it should be noted that significant tree removals will be necessary to accommodate its construction. This improvement was originally identified as part of the Karth Lake Management Action Plan. See Figure 1 in Appendix A for the proposed stormwater BMP retention pond location. Karth Lake Pump System The Karth Lake Improvement District (KLID) consists of Karth Lake’s lakefront property owners. The KLID modeled the pump operations and has made a recommendation to revise operations to limit the lake to a 6-inch bounce level between 935.0 and 935.5 to help alleviate shoreline erosion by allowing vegetative growth to maintain and stabilize the shoreline. Three options to implement this improvement to the pump system were evaluated as part of this feasibility report. The KLID had previously conducted their own research and coordinated with Joy Loughry, Water Monitoring and Surveys Unit Supervisor for the MnDNR which were all considered in development of these options. Option 1 Maintain the existing ~350 GPM deployable pump and install a cloud-connected water logging device to more accurately determine when to deploy and remove the pump from Karth Lake. DRAFT