10-21-2004 Council Work SessionA

Home Browse Search

Each of the in-lake improvement options—in-Iake alum application, hypolimnetic aeration, diffused aeration and artificial circulation are expected to improve the \satcr quality in Stubbs Bay to the same level. However, the annualized cost per decrease in total phosphorus concentration is at least twice as high for hypolimnetic aeration, compared to the other options, while the cost of artificial aeration is approximately two-thirds of the cost for an in lake alum application or diffused aeration. In addition, artificial circulation is the only in- lake improvement option that could realistically involve an option of renting the units, to verify their performance, before purchase. The predicted total phosphorus concentration should be reduced from 62 • g/L to 43 • g/L following implementation of the combination of artificial circulation w ith iron addition, rain water gardens, and additional wet detention ponding in Subwatersheds CLC-3, CLC-4 and SB-2. The predicted chlorophyll-a concentration should be reduced from 41 • g/L to 28 • g/L, and the Secchi depth should increase from 0.66 meters (2.2 feet) to 0.82 meters (2.7 feet) with this improvement option combination. This should increase the maximum depth of aquatic plant colonization from approximately 4.2 to 5.0 feet, according to Canfield et al. (1985). The severe and very severe nuisance algal bloom frequencies, as a percentage of the summer, should be reduced from approximately 100 to 44 percent and from 41 to 18 percent, respectively. The total annualized cost for this improvement option is $8,200 per • g/L reduction in the Stubbs Day total phosphorus concentration. Implementation of the recommended improvement options will ensure tliat the Stubbs Bay phosphorus concentrations will meet the goals in MCWD’s Water Resource Management Plan (1997) and the Hydrologic, Hydraulic, and Pollutant Loading Study (2003). The predicted phosphorus concentration very nearly meets the full use support designation ;ind would fall in the review category for listing eutrophication impairments, based on phosphorus concentrations for the North Central Hardwood Forest Ecoregion (MPCA, 2004). Tlie MPCA intends to further review chlorophyll-a and Secchi depth data, as well as trends and user perception information for lakes in the review category to determine the impairment status. 4.2 Recommendations for Implementation The conclusions of this feasibility analysis support our recommendation to implement the combination of artificial circulation with iron addition, rain water gardens, and additional v\et detention ponding in Subwatersheds CLC-3, CLC-4 and SB-2 (see Figure 6). This combination of improvement options would result in somewhat higher annualized costs, but would result in the best P \23V27\HO\I)raft Stubbs Bay Feasibility Study doc