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Total Maximum Daily Loads (TMDLs)

A TMDL, or Total Maximum Daily Load, can be thought of as the maximum amount of a pollutant that can be released into a stream or river such that the water quality does not decline beyond established standards.  The TMDL allocates the opportunity to release pollutants to different sources, such as waste water treatment plants, while maintaining a limit on total pollutant loading and keeping to established water quality standards.

When a stream’s water quality does not meet its standards it is designated as being “impaired” and a TMDL is required by the federal Clean Water Act to chart a course for reducing the sources of the contamination. In California, the Regional Water Boards enforce the federal Clean Water Act and develop the TMDL which, when adopted, becomes a part of the basic regulations that the regional boards use to regulate water quality.

TMDLs for a pollutant are equal to the assimilative or Loading Capacity (LC) of the receiving water for that pollutant.  TMDLs represent the sum of all individual Wasteload Allocations (WLAs) for point source discharges, Load Allocations (LAs) for nonpoint source discharges, and natural background level.  A TMDL calculation also takes into account any uncertainties by including a margin of safety (MOS).  Seasonal variations and critical conditions are also usually accounted for. 

TMDL implementation programs are typically incorporated into a region’s Water Quality Control Plan (also known as Basin Plan) and consist of several components:

• Description of the nature of the water quality problem;
• Numeric standards that exist for water quality protection;
• Assessment of the various sources of contamination;
• Determination of how much of a particular pollutant can be allowed in a stream without causing the standard to be exceeded;
• Designation of pollutant allocations to specific sources of contamination;  
• Implementation plan measures that will ultimately reduce the levels of the pollutant in the stream so that compliance with the water quality standard is achieved.

For Southern California, the Los Angeles and the San Diego Regional Water Boards have been developing and implementing several TMDL programs targeting many pollutants that directly impact the agricultural community.  Please follow the links to find out more about each individual program that may be of interest to you.

Los Angeles Regional Water Board

• Los Angeles Regional Water Quality Control Board Website
• Upper Santa Clara River Chloride TMDL
• Calleguas Creek Chloride TMDL
• Calleguas Creek Nitrogen TMDL

San Diego Regional Water Board

• Rainbow Creek Nutrient TMDL

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