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What is a TMDL

Over the years, the quality of some Illinois lakes, rivers, and streams has been impaired by pollutants from a variety of sources. However, since the signing of the federal Clean Water Act (CWA) in 1972, water quality has improved greatly, primarily through regulation of point source discharges. Although great strides have been made in restoring our state waters, there are still degraded lakes, streams, and rivers that need attention. Restoring their quality is crucial in maintaining a healthy environment and ensuring the sustainability of these waters for all to use and enjoy.

  • TMDL is short for Total Maximum Daily Load. It determines the greatest amount of a given pollutant that a water body can receive without violating water quality standards and designated uses.                                      
  • TMDLs set pollution reduction goals that are necessary to improve the quality of impaired waters.
  • A TMDL takes a watershed approach in determining the pollutant load that can be allowed in a given lake or stream. By taking a watershed approach, a TMDL considers all potential sources of pollutants, both point and non-point sources. It also takes into account a margin of safety, which reflects scientific uncertainty and future growth. The effects of seasonal variation are also included.
  • A TMDL is calculated using the following equation:
  • TMDL = WLA + LA + MOS + RC

  • Where:

    • WLA = Waste Load Allocation (i.e. loading from point sources);
    • LA = Load Allocation (i.e. loadings from nonpoint sources including natural background);
    • MOS = Margin of Safety; and
    • RC = Reserve Capacity

    In addition, the TMDL load calculation will take into account the Seasonal Variation (SV) of pollutant loading and hydrology for all seasons.