Basic Definition. A Total Maximum
Daily Load (TMDL) is the maximum amount of pollution that a waterbody can assimilate
without violating state water quality standards.
Background. 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.
Today. 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.
- Identify waters that do not meet water quality standards. In this process, the state
identifies the particular pollutant(s) causing the water not to meet standards.
- 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),
- 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
- Develop strategies for reducing water pollution and assess
progress made during implementation of the strategy. This is when a watershed partnership
most likely will want to get involved. If the partnership has already developed a plan of
action, it should be shared with the state. In fact, several states have incorporated
watershed partnership plans in the state's strategy for specific TMDLs.
What you can do.