TMDL stands for Total Maximum Daily Load and is the maximum amount of an impairing substance or pollutant that a particular water body can assimilate and still meet water quality criteria.
Imagine the pollutants such as nitrogen, phosphorous, sediment are to the Chesapeake Bay as sugar and fat are to the human body. The Bay (or body) can assimilate a certain amount of these, but too much and health is impacted.
The EPA says that the Chesapeake Bay needs a strick "pollution diet" to restore water quality and ecosystem health. TMDLs are the "diet" - setting limits for each pollutant to return the bay to a healthier state.
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The Fedral Clean Water Act (CWA) became law in 1972. This law requires States to set water quality standards for pollutants, nutrients, and other criteria.
Section 305 (b) of the Clean Water Act requires states to assess water quality for all waterbodies within their jurisdiction to determine whether they meet water quality standards.
Section 303(d) of the Clean Water Act requires states to list all water bodies that do not meet water quality standards. If water quality assessment for a waterbody does not meet the standards for one of the criteria a TMDL (Total Maximum Daily Load) is set for that criteria.
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