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EPA regulates the discharge of pollutants from point and nonpoint sources under the Clean Water Act (CWA), which requires states to identify affected waters and develop total maximum daily loads (TMDLs) to establish water quality-based effluent limitations for use in National Pollutant Discharge Elimination System (NPDES) permitting.
The CWA establishes a process for states to identify waters within their boundaries where implementing technology-based controls isn’t enough to achieve water quality standards. States establish a priority ranking of these waters and, for the priority waters, develop 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 (WLAs).
Effluent limits in NPDES permits must be consistent with the assumptions used to derive the WLAs. 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 WLAs and effluent limits. This analysis could be done for an entire watershed or separately for each individual discharge.
Permit writers have to consider the potential impact of every proposed surface water discharge on the quality of the receiving water. If TMDLs are not sufficient to meet the water quality standards in the receiving water, the CWA and NPDES regulations require that the permit writer develop more stringent, water quality-based effluent limits.
The regulations governing the TMDL program are found at 40 CFR 130.7. Once a state has identified its impaired waters, it must: