General Mitchell Airport TMDL Analysis

General Mitchell International Airport (GMIA) is assigned TSS (total suspended solids) and TP (total phosphorus) wasteload allocations (WLA), as well as a Water Quality Based Effluent Limit (WQBEL) to meet Total Maximum Daily Load (TMDL) requirements established by the Wisconsin Department of Natural Resources (WDNR) and the Environmental Protection Agency (EPA) for the Milwaukee River.

R/M created a comprehensive summary and operational evaluation report that included:

  • Evaluation of United States Geological Survey (USGS) collected effluent data and summary of findings

  • Identification of possible pollutant source

  • Reduction measures

  • Identification of possible operational improvements

  • Plan and schedule for implementation.

The biggest challenge of this project was the modeling software, WinSLAMM, not being tailored to modeling an airport. Airports were not utilized in the development of the WinSLAMM standard land use files, and the software does not allow for pollutant reductions to be accurately modeled for the existing conditions at General Mitchell.

R/M overcame this by modeling the existing conditions as General Mitchell’s permit required, but also by creating a nontraditional alternative to solve this challenge. The alternative created would allow for a full year of monitoring data to be collected to assess against the modeled data to determine the best path forward for determining compliance.

A unique detail of the project was that a portion of the project was to review USGS monitoring data that had been collected periodically since 1996. R/M was able to meet and discuss with a Research Hydrologist with the USGS to gain insight into airport operations and sampling conditions. This allowed R/M to have a better understanding of existing conditions and nuances of data that could potentially affect General Mitchell’s analysis results.

Ultimately, it was determined that GMIA was meeting TSS and TP compliance in all outfalls except TSS at Outfall 007. To set GMIA up for future success, it was determined that collecting an accurate representation of TSS and TP discharges will benefit both GMIA and the regulating agencies to appropriately reflect storm water runoff pollution and reductions within the Milwaukee River Basin.