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A TMDL Report can contain multiple TMDL calculations and multiple waterbodies within a watershed.  These reports and calculations can provide an analytical foundation for planning and implementing water quality-based controls to reduce pollution from identified sources.

What is the calculation for a Total Maximum Daily Load?

The TMDL process establishes the allowable amount (i.e. "load") of the pollutant that a waterbody can receive and still meet water quality standards for each designated use. The pollutant load must be established at a level necessary to implement the applicable water quality standards with seasonal variations and a Margin of Safety (MOS) to account for uncertainty.

The TMDL calculation is:

TMDL = WLA + LA + MOS where

  • TMDL is the total allowable pollutant load
  • WLA is the wasteload allocation (allowable pollutant load contributed by KPDES-permitted sources)
  • LA is the load allocation (allowable pollutant load contributed by sources that do not have a KPDES permit)
  • MOS is the margin of safety to account for uncertainty

Which waterbodies require a TMDL?

A TMDL is developed for each impaired waterbody in category 5 (a surface water assessment category). The 303(d) list is a list of all waterbodies in category 5.

Each entry on the 303(d) list is called a pollutant-waterbody combination (PWC). Each PWC consists of a waterbody that is not meeting water quality standards for a specific parameter (identified as a pollutant), which results in the impairment of one or more designated uses. The same waterbody can have multiple entries on the 303(d) list if impaired by more than one pollutant. For example, if a waterbody was impaired for two pollutants (E. coli  and pH), the waterbody name would appear twice on the 303(d) list with an entry for E. coli  and for pH.  A TMDL calculation is developed for each PWC.

Waterbodies with a TMDL Alternative Approach remain on the 303(d) and assigned to Category 5, but are assigned a lower priority for TMDL development. If the TMDL Alternative Approach does not achieve water quality standards, a TMDL is still required.

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