A TMDL or Total Maximum Daily Load is a calculation of the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards, and an allocation of that amount to the pollutant's sources.
Water quality standards are set by States, Territories, and Tribes. They identify the uses for each water body, for example, drinking water supply, contact recreation (swimming), and aquatic life support (fishing), and the scientific criteria to support that use.
A TMDL is the sum of the allowable loads of a single pollutant from all contributing point and nonpoint sources. The calculation must include any contributions from natural sources and a margin of safety to ensure that the water body can be used for the purposes SD has designated. The calculation must also account for seasonable variation in water quality.
Section 303(d) of the federal Clean Water Act requires that states develop Total Maximum Daily Loads (TMDLs) for water bodies. TMDLs are calculations of the amount of pollution a water body can receive and still meet the applicable water quality standards. TMDLs are necessary for water bodies that do not meet the water quality standards due to nonpoint source pollution, or might not meet water quality standards with the application of technology-based controls for point sources. In these cases, point sources of pollution would need additional water quality-based controls. Nonpoint sources of pollution are considered on a watershed basis.
Once the state has developed a TMDL, we are required to public notice the TMDL and have it approved by the Environmental Protection Agency (EPA).