Applied Ecosystem Services, LLC

The Environmental Issues Doctor


Total maximum daily loads, TMDLs, are the tool used by environmental regulators to address nonpoint source pollution under the Clean Water Act.

The CWA’s policies and goals are that, “It is the national policy that programs for the control of nonpoint sources of pollution be developed and implemented in an expeditious manner so as to enable the goals of this Act to be met through the control of both point and nonpoint sources of pollution.”

When nonpoint sources of contaminants and pollution adversely impact specific designated uses the regulator must assess water quality status before a corrective TMDL can be created.

A total maximum daily load is the quantity of a pollutant determined by the regulatory agency to be the greatest amount a defined waterbody’s assessment unit can absorb each day without adversely impacting a designated use. The agency partitions this quantity into waste load allocations for point source dischargers and load allocations for nonpoint source dischargers.

Assessment units can be a stream or river reach, a tributary subbasin, or an entire river network.

This tutorial presents the history of water quality protection and TMDLs under the Clean Water Act and explains why regulators find it difficult to quantify ambient conditions relative to designated use standards. These issues make it difficult to determine a TMDL and effectively allocate discharge limits to point and nonpoint dischargers.

This work was originally published on the Applied Ecosystem Services, LLC web site at

It is offered under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International license. In short, you may copy and redistribute the material in any medium or format as long as you credit Dr. Richard Shepard as the author. You may not use the material for commercial purposes, and you may not distribute modified versions.

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