What is a Total Maximum Daily Load (TMDL)?

• A. The daily discharge limit for the largest facility
• B. The maximum pollutant load that can be discharged by the entire watershed
• C. The sum of the allowable pollutant loadings that a waterbody can receive and still meet water quality standards, including a margin of safety and reserve capacity
• D. The rating of water quality for drinking water

Answer: (C)

A Total Maximum Daily Load sets the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards. This overall limit is the sum of all allowable pollutant loads from all sources and includes a margin of safety to cover uncertainty, as well as reserve capacity for future conditions. In practice, the TMDL is divided into allocations for point sources (wastewater discharges) and nonpoint sources (runoff, natural background) so that the waterbody can meet the standards. It’s not about a single facility’s discharge limit, nor is it just a general watershed rating or a drinking water quality rating.