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Watershed Management Bureau
Total Maximum Daily Load (TMDL) Program
Definition
The term "total maximum daily load" (TMDL) refers to the calculation of the maximum amount of a pollutant that a waterbody can receive, and attain or maintain water quality standards for its designated use. The TMDL calculation allows for the determination of a carefully identified allowable pollutant load equivalent to the sum of the waste load allocations (WLA) for point sources, load allocations (LA) for nonpoint sources and naturally occurring background sources, and an allocation of that load among the pollutant's sources. The TMDL calculation is required to account for seasonal variations and it must include a margin of safety (MOS) that accounts for uncertainty and any lack of knowledge concerning the relationship between effluent limitations and water quality. Therefore the TMDL equation can be expressed as:
TMDL = WLA + LA + MOS
This amount is then allocated between all sources of that pollutant. TMDLs may be expressed in terms of either mass per time, toxicity, concentration, narrative description, or other appropriate measure that relate to a State's water quality standard. All TMDLs are subject to public review and comment and must be submitted to the United States Environmental Protection Agency (EPA) for review and approval.
In the broader sense of the term, a TMDL refers to a detailed plan that identifies the pollutant reductions a waterbody needs to meet New Hampshire's water quality standards and develops a strategy to implement those reductions is order to restore the water quality. The general process by which TMDLs are developed includes identifying the problem pollutant, establishing of the water quality goals or target values needed to achieve water quality standards, identifying of the specific sources contributing the pollutant of concern, and then assigning a specific load allocation to each of the sources. Follow-up monitoring is needed to ensure that the implemented TMDL results in the attainment of the targeted water quality standard.
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