Total Maximum Daily Load (TMDL)

A total maximum daily load (TMDL) is the maximum amount of a pollutant a water body can receive and still meet water quality standards, the central remedy in US Clean Water Act section 303(d). States list waters that fail standards as impaired, and a TMDL must then be developed for each pollutant. The TMDL is the sum of wasteload allocations to point sources, load allocations to nonpoint sources and background, and a margin of safety: TMDL = sum WLA + sum LA + MOS. Wasteload allocations feed back into NPDES permit limits. Common TMDL pollutants are bacteria, nutrients, sediment, and metals. Implementation relies on permits for point sources and best-management practices for diffuse sources.