What is a "TMDL"

A Total Maximum Daily Load (TMDL) is the maximum amount of pollution that a waterbody can assimilate without violating state water quality standards.
Setting TMDLs has been required for years. It was mandated by Section 303(d) of the Clean Water Act (passed in 1972). At that time people had a more limited idea of what constituted pollution than we have now. They were picturing a big pipe spewing stuff into the river.
Now we know a whole variety of sources and activities can degrade water. In addition to setting a TMDL the responsibility for reducing pollution among both point sources (pipes) and diffuse sources is also assigned. Diffuse "sources" include, but are not limited to run-off, leaking underground storage tanks, unconfined aquifers, septic systems, stream channel alteration, and damage to a riparian area.
1. Identify waters that do not meet water quality standards. In this process we identify the particular pollutant(s) causing the water not to meet standards.
2. Prioritize waters that do not meet standards for TMDL development (for example, waters with high naturally occurring "pollution" will fall to the bottom of the list).
3. Establish TMDLs (set the amount of pollutant that needs to be reduced and assign responsibilities) for priority waters to meet state water quality standards. A separate TMDL is set to address each pollutant with concentrations over the standards.
4. Develop strategies for reducing water pollution and assess progress made during implementation of the strategy.