Total Maximum Daily Load (TMDL) is essentially a pollution budget that the EPA has established for communities. TMDL iscomprised of point and non-point source pollution. For example, a leaky sewage pipe is a source of point pollution because pollution coming from the pipe can be clearly identified.
Non-point source pollution comes from many different places. Each time it rains, water runs along the ground picking up a variety of pollutants, including uncollected dog waste. Dog waste’s affect on the amount of pollution in the water, therefore, is considered a source of non-point pollution because it comes from many different places.
The following definition, along with a history of TMDL and how it is calculated and used, can be found on the EPA’s website.
TMDL or Total Maximum Daily Load is a calculation of the maximum amount of a pollutant that a body of water can receive and still meet water quality standards.
Water quality standards are set by states, territories, and tribes. They differ for each waterbody’s designated use such as drinking water supply, contact recreation (swimming), and aquatic life support (fishing). Each has a different, and corresponding, TMDL.
A TMDL is the sum of the allowable loads of a single pollutant from all contributing point and non-point sources. The calculation must include a margin of safety to ensure that the waterbody can be used for the purposes the state has designated. The calculation must also account for seasonal variation in water quality.
The Clean Water Act, section 303, establishes the water quality standards and TMDL programs.