Polluted Water Body: Total Maximum Daily Load (TMDL)


Total Maximum Daily Load (TMDL) is a calculation of the maximum amount of a pollutant—from both point and nonpoint sources of pollution– that a water body can receive and still safely meet water quality standards (WQS), and an allocation of that load among the various sources (e.g. sewage treatment plant, factory effluent) of that pollutant. TMDLs are often described as a water body’s “pollution diet.” State environmental agencies develop TMDLs for waters that fail to meet water quality standards and are considered “Impaired” after regularly scheduled water quality assessments. TMDL calculations are the basis for deriving both required point source pollution permits on any “water quality-based” point source discharges (i.e. National Point Source Discharge Elimination or NPDES) and voluntary nonpoint source controls (supported by the Clean Water Act (CWA) Section 319 program). TMDLs set limits on discharges for a specific pollutant within a specific waterbody segment and establish roadmaps for restoration and protection of water quality standards. 

How can I use TMDLs to protect drinking water sources?

TMDLs are inherently complex given the multitude of factors considered in their derivation (see segments on Water Quality Criteria, Monitoring and Assessment, National Pollution Discharge Elimination System (NPDES), and Designated Uses), nevertheless, stakeholders have sufficient opportunity to inform the priority setting and development of TMDLs through the Water Quality Standards program. It is important for drinking water stakeholders to share high quality source water and water quality data with state environmental agencies, including information from source water quality readings, utility closures, and water intake locations, to help state CWA programs identify pollutant sources and characteristics in relation to drinking water sources, prioritize waterbody monitoring schedules and water impairment listings, calculate water pollution budgets, set numeric water quality targets, and determine effective implementation and restoration activities.

  • Collect and share substantive source water quality information with state Clean Water Act authorities
  • Encourage states to prioritize water quality monitoring of contaminants of concern to public water supplies
  • Encourage source water protections in TMDL implementation plans

Opportunities for involvement

Stakeholders have opportunity to submit information and comments during development of draft TMDLs and draft TMDL implementation plans, in addition to opportunities during other stages of the Water Quality Standards program. Find and/or request the schedule for your state’s plan to develop or update a TMDL and watch for public hearing notices. Find more information at your state environmental agency website, found here.

For further discussion on protecting drinking water through the Clean Water Act, see Opportunities to Protect Drinking Water and Advance Watershed Goals through the Clean Water Act: A Toolkit for State, Interstate, Tribal, and Federal Water Program Managers.