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Water: Water Quality Standards Academy

Basic Course: Supplemental Topics - TMDL Development: The Basic Calculation

Listing Impaired Waters and Developing TMDLs

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A TMDL is a calculation defined by this basic formula:


That is, a TMDL is the sum of the following.

  • Waste load allocation (WLA).  Total amount of pollutant from existing point sources (e.g., sewage treatment plant, industrial facility, stormwater).
  • Load allocation (LA). Total amount of pollutant from existing nonpoint sources and natural background (e.g., farm runoff, atmospheric mercury).
  • Margin of safety (MOS). This is used to take into account lack of knowledge concerning the relationship between effluent limitations and water quality. It can be expressed as an explicit factor (e.g., percent of total, such as 10%) or an implicit factor (e.g., conservative assumption in modeling).

Summary of TMDL Development Activities

  • Selection of the pollutant to consider.
  • Estimation of the water body's assimilative capacity (i.e., loading capacity).
  • Estimation of the pollutant loading from all sources to the water body.
  • Analysis of current pollutant load and determination of needed reductions to meet assimilative capacity.
  • Allocation (with a margin of safety) of the allowable pollutant load among the different pollutant sources in a manner that water quality standards are achieved.
Resource. For more information on TMDL development activities, see the Overview of Impaired Waters and Total Maximum Daily Loads Program.

For informational purposes only–Not official statements of EPA policy.

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