Watershed-Level Optimization of BMP Selection for Cost-Effective Pollutant Load Reduction in the Lower Fox River Basin and Green Bay, Wisconsin

Lower Green Bay is impaired by excessive phosphorus and sediment loading from the Lower Fox River (LFR), which leads to algae growth, oxygen depletion, submerged aquatic vegetation, and water clarity problems. Studies have shown that phosphorus load reductions up to 50% will be necessary to achieve noticeable improvements in algae production and water clarity in the LFR Basin and Green Bay Area of Concern (AOC). The Wisconsin Department of Natural Resources has given the LFR Basin and Green Bay AOC a high priority for the development of a total maximum daily load (TMDL) to address the impairments.As part of an integrated watershed approach demonstration project sponsored by the U.S. Environmental Protection Agency (EPA), the Cadmus Group, Inc. (Cadmus) was brought on to design an optimization framework for identifying the optimal combination of watershed management practices for restoring water quality in the LFR Basin and Green Bay AOC through the reduction of phosphorus and sediment loading. The optimization analysis was accomplished using the Soil & Water Assessment Tool (SWAT) in conjunction with an Optimization Model (OptiMod) to compare a total of 416 agricultural best management practice (BMP) scenarios, along with their implementation costs, and identify the Optimal Scenario (i.e., cost-effective scenario) of BMPs that achieves the greatest phosphorus load reduction. Urban and construction stormwater BMPs were not included in this demonstration analysis, but will be included in future analyses (i.e., for the TMDL). Although not evaluated within the optimization model, potential phosphorus load reductions and costs associated with permitted point source facility upgrades were estimated as part of this demonstration project.The 1993 Lower Green Bay Remedial Action Plan (RAP) phosphorus load reduction goal of 50% was used as the overall reduction target for this demonstration project; this target is not the final TMDL target. Implementation of the Optimal Scenario of agricultural BMPs in the LFR Basin results in an estimated phosphorus load reduction of about 50,000 kg/yr (21%). Potential point source facility upgrades in the LFR Basin results in an estimated phosphorus load reduction of 45,045 kg/yr (19%). These potential reductions combined results in an estimated 40% decrease in phosphorus loading to Lower Green Bay (from 238,912 kg to 143,700 kg per year). While the 50% reduction goal was not achieved during this demonstration, potential load reductions from urban stormwater BMPs were not considered. Also, as revealed during the analysis, the list of agricultural BMPs should be expanded upon to meet the 50% reduction goal. The cost of implementing all of the agricultural BMPs associated with the Optimal Scenario in the LFR Basin is 6.9 million per year, or about 138 per kilogram of total phosphorus reduced from agricultural nonpoint sources. The total estimated cost associated with point source facility upgrades in the LFR Basin is 10.8 million a year (on average), or about 240 per kilogram of total phosphorus reduced from point sources. The total cost of implementing the Optimal Scenario of agricultural BMPs and upgrading point source facilities in the entire LFR Basin is estimated to be 17.7 million per year, or 186 per kg of total phosphorus reduced.As the analysis shows, applying a 50% reduction to all source categories (i.e., both point sources and nonpoint sources) may not be the most cost-effective strategy, as agricultural BMPs achieve the greatest phosphorus load reductions at the lowest cost. Cadmus is in the process of developing a point source optimization tool, which will be incorporated into OptiMod and used to assess the cost-effectiveness of implementing BMPs for nonpoint sources vs. upgrading point source facilities.