Modified from Living on the Land 2001: Stewardship for Small Acreages Project Leader: Susan Donaldson, University of Nevada, Cooperative Extension

"...a calculation of the maximum amount of a pollutant that a water body can receive and still meet water quality standards."

A TMDL or Total Maximum Daily Load is a calculation of the maximum amount of a pollutant that a water body can receive and still meet water quality standards. The Clean Water Act, section 303, establishes the TMDL program.

Water quality standards are set by states, territories, and tribes. They identify the uses for each water body, for example, drinking water supply, contact recreation (swimming), and aquatic life support (fishing), and the scientific criteria to support those uses. The standards cannot be set below EPA standards – they can be more restrictive, but not less.

creekA TMDL is the sum of the allowable loads of a single pollutant from all contributing point and nonpoint sources plus a margin of safety. The calculation must include a margin of safety to ensure that the water body can be used for the purposes the State has designated. The calculation must also account for seasonal variation in water quality.

Useful link:
EPA's TMDL Program: http://www.epa.gov/owow/tmdl/atlas/index.html