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Tongue River Watershed TMDL Development

Page history last edited by Christina Staten 1 year, 2 months ago Saved with comment


Tongue River Home Page          Salinity Model          TMDL Development


Water Quality Data and Assessment          Restoration and Protection 


  Outreach          Contacts


Page Contents


What is a TMDL?

A total maximum daily load (TMDL) is the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards. A TMDL can often be calculated using a simple equation that involves streamflow and pollutant concentration to express the allowable load in pounds per day, although the TMDL can also be expressed as a percent reduction in loading.


TMDLs being written for the Tongue River

An electrical conductivity (EC) TMDL is being written for the segment of the river from Beaver Creek to the Twelve Mile Dam (assessment unit MT42C001_014) (Map 1). The upstream start of this segment is near Brandenberg, MT. The downstream end of this segment is at the Twelve Mile Dam, which is just upstream of Pumpkin Creek. This segment of the river is not currently identified as impaired for salinity on Montana's list of impaired waters (not meeting Montana's water quality standards for salinity for the Tongue River); however, it is currently undergoing an updated water quality assessment and will be listed for salinity impairment in Montana's 2020 Water Quality Integrated ReportSalinity, as defined in Montana's water quality standards, includes measurements of both electrical conductivity (EC) and sodium adsorption ratio (SAR). A TMDL will be written for EC for this segment of the river, and a TMDL may also be written for SAR. 


Map 1: Tongue River Watershed and River Segment with TMDL Development

(Click on map to enlarge)


Why DEQ Writes TMDLs

TMDLs address cumulative water quality impacts within a watershed and incorporate multiple types of pollution sources – both regulated and unregulated. Pollution to our waterbodies is not always caused by permitted surface water discharges (such as from a mine, wastewater treatment facility, or construction site) and water quality problems may not always be solved by adjusting permit limits. Sources of pollution may instead be from unregulated land management activities and/or natural sources, or from a combination of all three types of sources. In all of these cases, a TMDL can be an avenue for developing water quality improvement solutions.


Waterbodies impaired by a pollutant are not fully supporting all of their beneficial uses designated by the state of Montana (such as drinking water, agricultural uses, and support of aquatic life). Under Montana state law and the federal Clean Water Act, TMDLs must be written for waterbodies that do not meet water quality standards due to impairment by a pollutant. There are water quality standards for salinity (EC and SAR) that are specific to the Tongue River, and these standards are set at a level that is protective of the river’s beneficial uses. See the Tongue River Salinity Model page for the definitions of salinity, EC, and SAR, and how the Tongue River’s designated beneficial uses may not be supported if the EC and SAR standards are not met on a regular basis.


Montana’s EC and SAR Water Quality Standards

Montana uses electrical conductivity (EC) and sodium adsorption ratio (SAR) as measurements of salinity. Numeric water quality standards are established for these two parameters; however, if a waterbody does not meet these water quality standards, it is determined to be impaired for "salinity" and placed on the list of impaired waters. Montana adopted EC and SAR criteria in 2006, which the U.S. Environmental Protection Agency recently approved. EC and SAR water quality standards for the Tongue River are:


From November 1 through March 1, the monthly average standard for EC is 1,500 microsiemens per centimeter (µS/cm) and no sample may exceed an EC value of 2,500 µS/cm. The monthly average standard for SAR is 5.0 and no sample may exceed a SAR value of 7.5.


From March 2 through October 31, the monthly average standard for EC is 1,000 µS/cm and no sample may exceed an EC value of 1,500 µS/cm. The monthly average standard for SAR is 3.0 and no sample may exceed a SAR value of 4.5.


Salinity TMDL Development Methods for the Tongue River

The Tongue River salinity water quality model is being used for the source assessment portion of the TMDL development process to determine the significant sources of salt loading to the river. The model is also being used to help determine the TMDL’s allocations to those sources (how much of the maximum allowable load is allocated to, or allowed from, each source).


TMDLs (loads) can be calculated by multiplying streamflow by the allowable pollutant concentration or water quality target. Montana’s EC water quality standard can be used as the water quality target for the TMDL. However, since electrical conductivity is not expressed as a concentration, but instead as microsiemens per centimeter (µS/cm), a load-based TMDL cannot be directly calculated by multiplying the EC value by the Tongue River’s flow. There is a strong correlation between EC and total dissolved solids (TDS) in the Tongue River, with TDS expressed as a mass-based concentration (in milligrams per liter (mg/L)). Therefore, DEQ may be using TDS values that equate to the EC standard in the Tongue River as a TMDL target that can be used to calculate the TMDLs and determine the TMDL allocations.


Keep in mind that the TMDL is not a static number because the allowable load changes with the flow of the Tongue River, even where the TMDL target remains static. The allowable load increases as flow increases, and vice versa.


Steps for Completing the Salinity TMDL Document

Work on this project began in the early 2000s, led by the Montana Operations Office of the U.S. EPA, working in conjunction with Montana DEQ. This included development of an LSPC watershed model for the Tongue River to investigate sources of salt loading to the river. That model is now outdated, and part of this current project has been to develop a new model using a more relevant modeling platform (a SWAT-Salt model) and also using more current water quality data. 


  1. A stakeholder group is formed to work with DEQ in an advisory capacity throughout this project
  2. Salinity model for the Tongue River is set up and calibrated 
  3. Model (allocation) scenarios are developed and evaluated, with stakeholder input 
  4. DEQ drafts the TMDL document - CURRENT PROJECT PHASE
  5. Stakeholders review and comment on the draft document (see table below)
  6. DEQ works with stakeholders to address and incorporate their comments 
  7. A public comment period is held for the draft document
  8. DEQ prepares responses to public comments and makes final document edits
  9. DEQ submits the TMDL document to the U.S. EPA in Denver for approval
  10. DEQ publishes a final TMDL document


Project announcements and updates on the status of the project will be posted on the Tongue River Watershed Project Outreach page.


Draft TMDL Document

DEQ has not yet begun drafting a TMDL document. To work out the details of the TMDL, DEQ is currently working directly with the entities that will be most affected by the TMDL's allocations. Once a document has been drafted, it will be posted on this page for stakeholder review and comment (step 5, above) prior to the formal public comment period. Stakeholders will be notified once the document becomes available for their review.


Draft Tongue River Salinity TMDL Document for Stakeholder Review

Draft Document Section

Section Contents

Date Posted

Submit Comments To:

Not yet available










Tongue River Salinity TMDL Project Contacts


DEQ Contact



Project Coordinator

Christina Staten


(406) 444-2836

Project Modeler*

Eric Regensburger


(406) 444-6714

Salinity TMDL Project Manager

Kristy Fortman


(406) 444-7425

* the project modeler changed from Erik Makus to Eric Regensburger in January 2019

* the TMDL project manager changed from Dean Yashan to Kristy Fortman in July 2019


See the Tongue River Watershed Project Contacts page for a full description of each project team member’s role and for a list of additional contacts.



Click on the Adobe Reader icon if you need to download the free Adobe Acrobat software to view the PDF documents on this page. 


Page Released: July 6, 2016

Last Updated: July 23, 2019