Total Maximum Daily Load

A Total Maximum Daily Load (TMDL) is a regulatory term in the U.S. Clean Water Act, describing a plan for restoring impaired waters that identifies the maximum amount of a pollutant that a body of water can receive while still meeting water quality standards.

The Clean Water Act requires that state environmental agencies complete TMDLs for impaired waters and that the United States Environmental Protection Agency (EPA) review and approve / disapprove those TMDLs. Because both state and federal governments are involved in completing TMDLs, the TMDL program is an example of cooperative federalism. If a state doesn't take action to develop TMDLs, or if EPA disapproves state-developed TMDLs, the EPA is responsible for issuing TMDLs. EPA published regulations in 1992 establishing TMDL procedures. Application of TMDLs has broadened significantly in the last decade to include many watershed-scale efforts, including the Chesapeake Bay TMDL. TMDLs identify all point source and nonpoint source pollutants within a watershed.

The Clean Water Act requires states to compile lists of water bodies that do not fully support beneficial uses such as aquatic life, fisheries, drinking water, recreation, industry, or agriculture; and to prioritize those water bodies for TMDL development. These inventories are known as 303(d) Lists and characterize waters as fully supporting, impaired, or in some cases threatened for beneficial uses.

Beneficial use determinations must have sufficient credible water quality data for TMDL planning.

Throughout the U.S., data are often lacking adequate spatial or temporal coverage to reliably establish the sources and magnitude of water quality degradation.

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