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Impaired Waters
Section 303(d) of the Clean Water Act establishes a process for states to identify waters within its boundaries where implementing technology-based controls are inadequate to achieve water quality standards. States establish a priority ranking of these waters and, for the priority waters, develop total maximum daily loads (TMDLs). A TMDL identifies the amount of a specific pollutant or property of a pollutant, from point, nonpoint, and natural background sources, including a margin of safety, that may be discharged to a water body and still ensure that the water body attains water quality standards. The allocations of pollutant loadings to point sources are called wasteload allocations. Effluent limits in NPDES permits must be consistent with such wasteload allocations. Also, in the absence of a TMDL, permitting authorities still must assess the need for effluent limits based on water quality standards and, where necessary, develop appropriate wasteload allocations and effluent limits. This analysis could be done for an entire watershed or separately for each individual discharge.
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