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What is a TMDL

Page history last edited by Jordan Tollefson 9 years ago

What is a TMDL?


TMDL stands for Total Maximum Daily Load and refers to the maximum amount of a pollutant a waterbody can receive and still meet water quality standards (think of a TMDL as a loading rate). A TMDL can also be defined as a reduction in pollutant loading that results in meeting water quality standards.


Why Does DEQ Develop TMDLs?


The state of Montana monitors its waters and conducts water quality assessments to determine if waterbodies are supporting their designated uses. Waterbodies in the state of Montana have been classified to designate what beneficial uses they must support. The water quality in Montana streams typically need to be maintained suitable to support the uses of: agriculture, industrial uses, recreation, drinking water, as well as support of fish and aquatic life. Waters that are determined not to be supporting their designated uses are called impaired and are placed on Montana’s list of impaired waters. Impaired waterbodies and their associated probable causes and sources of impairment are published within Montana’s biennial Water Quality Integrated Report


Montana’s state law, and the federal Clean Water Act that was established by Congress in 1972, require development of total maximum daily loads (TMDLs) for all waterbodies impaired by a pollutant. Examples of pollutants include metals (e.g., arsenic, lead, copper, mercury), nutrients (nitrogen and phosphorus), sediment, temperature, and pathogens (E. coli).  


The Steps of TMDL Development


TMDL development includes four main steps:

  • Further characterizing the impaired waterbody’s existing water quality conditions and comparing those conditions to Montana’s water quality standards. During this step, measurable target values are set to help evaluate the stream’s condition in relation to the applicable water quality standards.
  • Determining the significant sources and quantifying the magnitude of the pollutant contribution from each of those sources
  • Determining the total allowable load of the pollutant that the waterbody can recieve (i.e., the TMDL)
  • Allocating the total allowable pollutant load into individual loads for each significant source (referred to as load allocations for nonpoint sources and wasteload allocations for point sources). You can also think of this step as determining the amount of reduction needed from each source in order to satisfy the TMDL.


For each TMDL project, a document is published that includes information and results from each of these four steps. The document also typically includes recommended land management activities for improving water quality in the TMDL project area, and a monitoring strategy to evaluate progress toward attainment of water quality standards. Completed documents can be found on DEQ's Final TMDL Documents webpage. Information on land management practices that can improve water quality can be found on DEQ's Nonpoint Source webpage and Nonpoint Source wiki site.


Schematic of the TMDL Concept: 




This schematic demonstrates the definition of a TMDL. The box on the left represents the existing, or current, load of a particular pollutant in a stream and all of the identified sources of that pollutant. The boxes in the middle and on the right represent the TMDL, or the allowable load that the stream can receive and still meet water quality standards. The load is comprised of both a natural load and the allowable load from human sources. In the figure, LA stands for load allocation, and WLA stands for wasteload allocation.



Additional Information & Resources


Downloadable TMDL Pamphlet: Understanding the TMDL Process  (Size: 0.5MB). If you would like a hard copy, please contact Christina Staten (406-444-2836).


DEQ's TMDL Webpage

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