Improving Wilmes Lake, One Dose at a Time

Wilmes Lake is a 30-acre shallow lake within the South Washington Watershed District (SWWD), located on the east side of the Minneapolis/St. Paul metro area in Minnesota. With a rapidly developing suburban watershed of over 3,200 acres, Wilmes Lake suffers from excessive nutrient loading and routinely exhibits annual total phosphorus concentrations of 0.06 to 0.09 mg/L, Wilmes Lake has been listed on the Minnesota Section 303(d) impaired waters list since 2006. SWWD has established an annual total phosphorus reduction goal of 153 pounds for Wilmes Lake as it attempts to make substantive water quality and clarity improvements. Several small stormwater management projects have been constructed in partnership with the City of Woodbury (MN) and modest in-lake phosphorus reductions have been observed in recent years. SWWD completed a regional Best Management Practice (BMP) feasibility study in 2019 (Hinds, 2019) which identified a stormwater treatment facility with in-line chemical dosing as the most cost-effective and impactful option to reduce phosphorus loads to Wilmes Lake. HR Green joined SWWD's team in 2020 to complete an alternatives assessment (HR Green, 2020) to determine the preferred chemical dosing and concept design for the stormwater treatment facility within an adjacent city park. This presentation will focus on the processes behind the alternatives assessment, key features of the design, a unique SWWD/City partnership, expected outcomes, cost effectiveness, as well as construction and permitting hurdles. Using SWWD's water quality and flow data for Wilmes Lake, information from a similar local treatment facility (Pilgrim, 2005), a survey of park features and input from SWWD and City staff, HR Green developed a facility concept that pumps water from the primary influent channel of Wilmes Lake to the chemical dosing station. The team reviewed the use of both aluminum sulfate (alum) and ferric chloride for phosphorus reduction treatment. Alum was chosen based on reduced corrosion potential and operation and maintenance needs. The potential introduction of additional chloride residue was another unique concern in our northern climate where road salt use in the winter already threatens lakes, streams and groundwater supplies. The proposed facility consists of a lift station, force main, chemical treatment building and settling basin. Once alum is mixed into the stormwater, the resulting flocculent settles out in a constructed basin before treated (clean) stormwater drains to the lake. With treatment flows of one to three cubic feet per second (cfs) during open water seasons, the facility is expected to remove between 95 and 283 pounds of phosphorus per year in addition to other associated pollutants, achieving SWWD's overall phosphorus reduction goal for Wilmes Lake. Construction of this project required approximately $4.5 million in capital funding, which was solely provided by SWWD. This is reflective of both the project's importance to the watershed district and its expected impact. SWWD has entered into an agreement with the City of Woodbury whereby the City's experienced public works staff will operate and maintain the facility. SWWD has budgeted approximately $180,000 per year to fund facility operation (staff, chemical, electricity, maintenance, etc.). Over an initial 25-year expected life span, the project is conservatively expected to achieve a cost/benefit ratio of $1,900/lb P/year, which is favorable compared to other types of stormwater BMPs in Minnesota's northern environment. For a system like this, expected operation and maintenance activities include refilling alum storage tanks, removing flocculent solids from the settling basin, removing accumulated sediment at the lift station sumps, maintaining chemical feed hoses, pumps, and electronics, and conducting routine sampling activities. HR Green reviewed the operations and maintenance plan with SWWD and the City during design to manage expectations and costs associated with the facility. Collaborative conversations empowered the project team to design and specify system components that improved the accessibility, feasibility and ease of operation and maintenance protocols for City staff. This also results in reduced O&M cost to SWWD over time. Construction began in the summer of 2023 and is expected to be complete in the summer of 2024. While many conversations and considerations were addressed during preliminary design, there are always lessons learned on every project. The presenters will share what worked well and what might be done differently next time. Some examples include the coordination on funding, operations and maintenance, and data security as well as considering costs for aesthetics and other site features.