TMDLs are a requirement of the Clean Water Act
. States must identify waters that are impaired and must develop a Total Maximum Daily Load (TMDL) for those impaired by pollutants. The list of impaired waters requiring a TMDL is called the 303(d) list. Waters are 303(d) listed for specific designated uses; examples of designated uses are:
- Warm water aquatic habitat
- Cold water aquatic habitat
- Primary contact recreation
- Secondary contact recreation
- Domestic water supply
- Outstanding state resource water
- Fish consumption (while not a designated use, it is recognized as an implied use per 401 KAR 10:031 Section 2)
A TMDL is a calculation of the maximum amount of a pollutant that a waterbody, such as a stream, lake or spring, can receive and still meet water quality standards.
Technically, TMDL = WLA + LA + MOS where
- TMDL is the allowable load;
- WLA is the wasteload allocation (allowable load for KPDES-permitted sources);
- LA is the load allocation (for sources that do not have a KPDES permit);
- MOS is the margin of safety to account for uncertainty.
A TMDL may also refer to a written report, which includes the location of the impaired waters, watershed information, sampling data, the calculation of the allowable pollutant load and the allocation of this load to sources in the watershed.
Which waterbodies require TMDLs?
TMDLs are required for waterbodies in what is called Category 5, as shown in the following table:
Surface Water Assessment Categories
||All designated uses for water body fully supporting.|
Assessed designated use(s) is/are fully supporting, but not all designated uses assessed
||Segment currently supporting use(s), but 303(d) listed & proposed to EPA for delisting.|
||Segment with an EPA approved or established TMDL for the following use(s) now attaining full support.|
||Designated use(s) has/have not been assessed (insufficient or no data available).|
||Segment with an EPA approved or established TMDL for all listed uses not attaining full support.|
||Nonsupport segment with an approved alternative pollution control plan (e.g. BMP) stringent enough to meet full support level of all uses within a specified time.|
||Segment is not meeting full support of assessed use(s), but this is not attributable to a pollutant or combination of pollutants.|
||Segment does not support designated use(s) and is impaired by a pollutant or a combination of pollutants. A TMDL is required.|