Section 303(d) of the Clean Water Act calls for each state to identify those waters within its boundaries for which effluent limitations are not stringent enough to implement any water quality standard applicable to such waters. The state also priority ranks those waters, accounting for the severity of the pollution and the uses to be made of the waters. For those identified waters, the state is to establish the total maximum daily load (TMDL) for those pollutants causing the non-attainment of the water quality standards. Such loads are to be established at a level necessary to implement the applicable water quality standard with seasonal variations and a margin of safety, which accounts for uncertainty concerning the relationship between effluent limitations and water quality.
Federal Regulations provide additional guidance on the 303(d) process. 40 CFR 130.7 describes:
- The process for identifying water quality limited segments, requiring TMDLs.
- The process for setting priorities for developing TMDLs, including Wasteload Allocations and Load Allocations.
- The process for establishing TMDLs for those identified segments, including:
- Water quality monitoring
- Modeling
- Data analysis
- Calculation methods
- The list of pollutants to be regulated
- The process of submitting the state's list and priority ranking and established TMDLs to EPA for approval.
- The process of incorporating the approved loads into the state Water Quality Management Plans and NPDES permits.
- The process to involve the public, affected dischargers, government agencies, and local government in 303(d) activities.