Prince William Conservation Alliance
Prince William Conservation Alliance About Us PWCA Calendar Blog Resources Support PWCA

What is a Total Maximum Daily Load (TMDL)?

A "TMDL" is the calculation of the maximum amount of a pollutant (such as excessive nitrogen) that a waterbody (such as Cedar Run or the Occoquan Reservoir) can receive and still meet water quality standards.

A TMDL is the maximum sum of a single pollutant allowed into a waterbody from all sources (point and nonpoint). The TMDL calculations are different for different pollutants, and for different streams and lakes/reservoirs. In a TMDL, percentages of the total maximum daily load are allocated to the various pollutant sources (agriculture, runoff from developed land, watewater treatment plants, etc.).

The Clean Water Act requires every state to (1) identify waters not in compliance with water quality standards; (2) establish priorieties for developing TMDL's; (3) develop a list of impaired waters (the 303(d) list, referring to a specific section in the Clean Water Act); (4) develop TMDLs for the identified impaired waters on the 303(d) list. TMDLs were to be implemented through existing pollution reduction regulations and voluntary strategies. No additional implementation measures were included in the Clean Water Act.

In July 2000 the EPA declared revised 303(d) regulations, effective October 1 2001, that required each state to develop an implementation plan as a component of each TMDL. Later the American Canoe Association and the American Littoral Society filed a complaint against the EPA for failure to comply with the provisions of Section 303(d) of the Clean Water Act. This resulted in a Consent Decree that contained a TMDL development schedule through 2010.

Virginia has developed a Ten Year Schedule for completing TMDL, and needs to develop 648 TMDL's on 600 impaired waters by 2010. The assessment of Virginia's waters was based on evaluation of (1) water quality data from government agencies and citizen monitoring; fish and shellfish consumption actions; Virginia Pollutant Discharge Elimination System permit information. Waters defined as 'impaired' do not support, or only partially support, one or more of the five designated uses: (1) a balanced, indigenous population of aquatic life; (2) produce edible fish; (3) produce edible shellfish; (4) safe for recreational uses such as swimming; (5) safe for drinking water. Virginia's waters should be swimmable, fishable and drinkable.


Home | Calendar | About Us | Site Index
Maps | Youth Education |Membership