What is a TMDL and How Does it Relate to a Watershed Plan?

Afternoon everyone. Last time we met I mentioned TMDLs, and rightly so, someone asked, "What is a TMDL and how does it relate to a Watershed Plan
Valid question, Stephanie! Good old Professor Loam is prepared to answer it.
If a waterbody is impaired, it is placed on the 303(d) list. For each impaired waterbody, a state of tribe must develop an accounting of loads that would result in the waterbody's meeting water quality standards. This is called a Total Maximum Daily Load, or TMDL. 
So, a TMDL is the amount, or load, of a specific pollutant that a waterbody can assimilate and still meet the water quality standards. The load is allocated among the current pollutant sources (point, non point, and background sources), a margin of safety, and sometimes future growth.
The typical steps for developing a TMDL include the following:


  1. Identify linkages between water quality problems and pollutant sources.
  2. Estimate total acceptable loading rate that acheives water quality standards.
  3. Allocate acceptable loading rates between sources.
  4. Package the TMDL for EPA approval.
How does this relate to a Watershed Plan?
Although watershed plans should be holistic and include information on the borad array of attributes, problems, and protection strategies needed in a watershed, plans that include impaired waters should also contain quantified estimates of current problem pollutant loads.


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