Total Maximum Daily Load (TMDL)
What does "Total Maximum Daily Load (TMDL)" mean in law?
A Total Maximum Daily Load (TMDL) is a calculation of the maximum amount of a specific pollutant that a water body can receive and still meet applicable water quality standards, as required under Section 303(d) of the Clean Water Act. States must identify water bodies that fail to meet water quality standards even after point sources have implemented technology-based controls, place them on the 303(d) impaired waters list, and develop TMDLs that allocate permissible pollutant loads among point sources (as waste load allocations) and nonpoint sources (as load allocations), with a margin of safety. TMDLs are significant because they represent one of the few mechanisms under the CWA for addressing nonpoint source pollution (such as agricultural runoff), which is otherwise largely unregulated at the federal level. EPA must approve or disapprove state-submitted TMDLs and may establish its own TMDLs if a state fails to act, making the TMDL program a frequent subject of litigation by environmental groups seeking to compel action on impaired waters.
Definition
A Total Maximum Daily Load (TMDL) is a calculation of the maximum amount of a specific pollutant that a water body can receive and still meet applicable water quality standards, as required under Section 303(d) of the Clean Water Act. States must identify water bodies that fail to meet water quality standards even after point sources have implemented technology-based controls, place them on the 303(d) impaired waters list, and develop TMDLs that allocate permissible pollutant loads among point sources (as waste load allocations) and nonpoint sources (as load allocations), with a margin of safety. TMDLs are significant because they represent one of the few mechanisms under the CWA for addressing nonpoint source pollution (such as agricultural runoff), which is otherwise largely unregulated at the federal level. EPA must approve or disapprove state-submitted TMDLs and may establish its own TMDLs if a state fails to act, making the TMDL program a frequent subject of litigation by environmental groups seeking to compel action on impaired waters.
Example
After a lake was listed as impaired for excessive phosphorus, the state developed a TMDL allocating 60% of the allowable phosphorus load to the upstream wastewater treatment plant and 40% to agricultural runoff from surrounding farms.