What's 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. In the early '70s, they
were picturing the big nasty 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.
Recent lawsuits have brought setting TMDLs on impaired waters to the forefront
in most states. When states set TMDLs, they:
- 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 standards.
After the state sets a TMDL, it develops a strategy to reduce 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.
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