Standards

Why Water Quality Standards

• Protect water resources for uses such as fishing, swimming and other recreation, and sustaining fish, bugs, plants, and other aquatic life
• Are a measure to identify polluted waters or healthy waters in need of protection
• Guide the limits set on what regulated facilities can discharge to surface water

The federal Clean Water Act requires states to designate beneficial uses for all waters and develop water quality standards to protect each use.

States either develop their own criteria or implement federal criteria for evaluating water quality. These criteria must accurately reflect the latest scientific knowledge. They are based solely on data and scientific judgments about pollutant concentrations and their effects on the environment, aquatic life, and human health. Montana water quality criteria include both numeric and narrative criteria. Water quality criteria for each use class are detailed in the Montana Code Annotated.

• Montana Water Quality Use Class Viewer (GIS)
• Montana Water Quality Use Class Viewer (PDF)

Note: Montana does not designate use classes for Tribal Nations.

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Numeric Criteria

Many of Montana’s water quality criteria are numeric; that is, the criteria define precise, measurable concentrations of pollutants that, if exceeded, would harm the use. Montana’s numeric water quality criteria are published in Circular DEQ-7.

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Narrative Criteria

Some of Montana’s water quality criteria are narrative (i.e., statements instead of specific quantities) such as general prohibitions to prevent undesirable conditions and criteria that describe desired water quality in terms of allowable ranges and maximums (e.g., water pH and temperature) or in terms of specific variation from naturally-occurring conditions (e.g., water turbidity and color). Naturally-occurring conditions are determined by reviewing available historical data for a waterbody or by comparing conditions with a reference waterbody that is unaltered or otherwise in its most natural condition.

Montana’s water quality standards and details of how water quality standards are implemented in point-source discharge permitting are contained in Administrative Rules of Montana.

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Nondegradation

Nondegradation protections maintain high quality waters from deterioration. Montana’s Nondegradation policy provides three levels of protection for surface waters:

1. Existing uses of the water body must be maintained and protected.
2. High water quality (water quality better than the applicable water quality standard) must be maintained unless a lowering of water quality is necessary to accommodate important economic and social development.
3. Exceptional characteristics of specific waters designated as outstanding, very sensitive, or unique resources -- called outstanding resource value water -- must be maintained and protected.

Nonanthropogenic Standards

Yellowstone River Arsenic Standards

• Demonstration of Nonanthropogenic Arsenic Levels
• Derivation of Nonanthropogenic Arsenic Standards
• Addendum to Derivation of Nonanthropogenic Arsenic Standards

Nonanthropogenic Standards - External Entity Guidance

The Montana Department of Environmental Quality (DEQ) has developed guidance for cases where an entity external to the DEQ is interested in pursuing a modification to an existing water quality standard because the existing standard may be more stringent than the nonanthropogenic condition, as provided for in 75-5-222(1), MCA. A draft version of this guidance was presented to the Nonanthropogenic Standards Workgroup on May 8, 2020 for comment. The final version and the material presented at the May 8th Workgroup meeting are provided below:

• Guidance Document
• May 8, 2020 Meeting Presentation

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Nutrient Standards Documents

Criteria Technical Reports - Large Rivers

• Using a Computer Water Quality Model to Derive Numeric Nutrient Criteria: Lower Yellowstone River (2013)
• Using a Computer Water Quality Model to Derive Numeric Nutrient Criteria: Lower Yellowstone River - Appendix C
• Results from the Deployment of Nutrient-diffusing Substrates in the Upper Missouri River - A project in support of the development of numeric nutrient standards for the upper Missouri River using a computer water-quality model (2014)

Criteria Technical Reports - Wadeable Streams

• Analysis of Daily Patterns of Dissolved Oxygen Change in Flowing Waters of Montana (2023)
• Dissolved Oxygen Spatial Analysis Technical Report - Eastern Montana (2021)
• Eutrophication Thresholds Associated with Benthic Macroinvertebrate Condition in Montana Streams (2023)
  • Report Appendices (zip folder - file will automatically download)
  • Report Addendum (2024)

• Whole-stream Nitrogen and Phosphorus Addition Study (2016)
• Scientific and Technical Basis of the Numeric Nutrient Criteria for Montana's Wadeable Streams and Rivers - Update 1 (2013)
• Scientific and Technical Basis of the Numeric Nutrient Criteria for Montana's Wadeable Streams and Rivers (2008)
• Peer Review Memorandum of 2008 document
• Updated Statistical Analyses of Water Quality Data, Compliance Tools, and Change Point Assessment for Montana Rivers and Streams (2008)
• Seasonally Stratified Water Quality Analysis for Montana Rivers and Streams – Final Report (2005)
• Reference Study Report (2005)
• Nutrient and algal biomass criteria recommendations for Northeastern Montana (2004) 

Economic Consideration Reports

• Supplemental Economic Analysis of Meeting Base Numeric Nutrient Standards (2017)
• Demonstration of Substantial and Widespread Economic Impacts to Montana that would Result if Base Numeric Nutrient Standards had to be Met by Entities in the Private Sector in 2011/2012 (2012)
• Demonstration of Substantial and Widespread Economic Impacts to Montana That Would Result if Base Numeric Nutrient Standards had to be Met in 2011/2012 (2012)
• Scoping Evaluation of Economic Impact Assessment Methodologies for Water Quality Standards (2007)
• Wastewater Treatment Performance and Cost Data to Support an Affordability Analysis for Water Quality Standards (2006)

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Site-Specific Water Column Selenium Standard for Lake Koocanusa

• Water Pollution Advisory Council Meeting Materials (Sep 11, 2020)
• Board of Environmental Review Meeting Materials (Sep 24, 2020)
• Technical Support Document for the Derivation of a Site-Specific Water Column Standard for Lake Koocanusa, MT
• Errata Sheet for Technical Support Document for the Derivation of a Site-Specific Water Column Standard for Lake Koocanusa, MT

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Method for Estimating Attenuation of Nutrients from Septic Systems (MEANSS)

• MEANSS Technical Support Document

Overview of Triennial Review

The review and revision of water quality standards is an ongoing process. The Montana Water Quality Act and the federal Clean Water Act require Montana to hold a public hearing to review and, as appropriate, modify water quality standards at least every three years. This process is known as water quality standards triennial review.

During triennial review, the department invites all interested persons to submit comments regarding any aspect of Montana’s water quality standards, including suggested revisions and the basis for the suggested revisions, during a public comment period and public hearing. The department evaluates the need for new or revised water quality standards and identifies water quality standards program priorities.

2020/2023 Triennial Review

The department conducted public hearings for the purpose of reviewing water quality standards in 2020 and 2023. The department submitted the results of the 2020 and 2023 triennial reviews to the U.S. Environmental Protection Agency: Montana Water Quality Standards Triennial Review Responsiveness Summary, 2020/2023.

2026 Triennial Review

The department will hold a triennial review public hearing in the spring of 2026. Notice of the hearing and instructions for submitting comments will be posted on DEQ's Public Participation page.

In 2025, the Montana Legislature passed House Bill 664 which repealed the State of Montana’s base numeric nutrient standards and directed the department to delete all references to department circular DEQ-12A, base numeric nutrient standards, and nutrient standards variances.

In October 2025, the U.S. Environmental Protection Agency approved House Bill 664 as consistent with the Clean Water Act and its implementing regulations and meeting requirements to protect designated uses (i.e., protection of aquatic life and recreation).

The department will implement the narrative nutrient criteria at ARM 17.30.637(1)(e) using the best available science.

On July 2, 2025, DEQ received a petition for rulemaking to modify the Lake Koocanusa water column standard and the definition of steady state pursuant to 2-4-315, MCA. On September 2, 2025, DEQ denied the petition after consideration of the body of record evidence, including submitted petition materials and the public comments received. DEQ’s full response to the Board of County Commissioners of Lincoln County and related materials are provided below:

• DEQ's Response to the Board of County Commissioners of Lincoln County
• Public Comments Received
• Submitted Petition and Appendices

Background:  On December 11, 2020 the Board of Environmental Review (board), voted to adopt the Department of Environmental Quality’s (department) proposed selenium water quality standard for Lake Koocanusa into state law and determined the proposed standard was no more stringent than federal guidelines. Therefore, there was no requirement for the completion of written findings as described in MCA 75-5-203. The multi-part standard includes fish tissue and water column components with the following numeric values: 15.1 mg/kg dry weight (dw) egg/ovary, 11.3 mg/kg dw muscle, 8.5 mg/kg dw whole body, and 0.8 μg/L total dissolved selenium in Lake Koocanusa and 3.1 μg/L in the Kootenai River mainstem. See ARM 17.30.632. The frequency and duration of the fish tissue standards are instantaneous measurements, not to be exceeded. The water column standard is computed as a 30-day average and shall not be exceeded more than once in three years, on average.

The standards were adopted into state law on December 25, 2020 and codified in ARM 17.30.632. The water quality standards were approved by the U.S. Environmental Protection Agency (EPA) on February 25, 2021. Two petitions were filed to the board calling for a review of the stringency determination for the site-specific water column standard for Lake Koocanusa in ARM 17.30.632(7)(a). On February 25, 2022 the board reversed its previous stringency determination and determined that the site-specific water column standard for Lake Koocanusa is more stringent than comparable federal guidelines. The department, therefore, complied with MCA 75-5-203 by making the written findings set forth in 75-5-203(2), MCA. 

Associated Materials:

• Final Stringency Written Findings
• Stringency Findings Response to Comments

Lake Koocanusa Selenium Updates:

• 2019 Lake Koocanusa Selenium Update - Fact Sheet
• 2021 Lake Koocanusa Selenium Update - Fact Sheet
• 2022 Lake Koocanusa Selenium Update - Fact Sheet
• 2023 Lake Koocanusa Selenium Update - Fact Sheet

 Items Related to 2020 Rulemaking:

• Lake Koocanusa and Kootenai River Se EPA Action Letter
• Water Pollution Advisory Council Meeting Materials (Sep 11, 2020)
• Board of Environmental Review Meeting Materials (Sep 24, 2020)
• Lake Koocanusa & Kootenai River Public Info Session, September 18, 2020
• MAR Notice No. 17-414
• Technical Support Document for the Derivation of a Site-Specific Water Column Selenium Standard for Lake Koocanusa, MT
• Errata Sheet for Technical Support Document for the Derivation of a Site-Specific Water Column Selenium Standard for Lake Koocanusa, MT
• Information on Public Comment and Public Hearing
• Board of Environmental Review Meeting Materials including DEQ presentation (Dec 10, 2021)
• Board of Environmental Review Rulemaking Record

Copies of any of these documents may also be obtained by contacting Lauren Sweeney at (406) 444-5226 or lauren.sweeney@mt.gov

The Department of Environmental Quality continues to work with the British Columbia Ministry of Environment and Climate Change Strategy (BC-ENV) and the Lake Koocanusa Monitoring and Research Working Group (LKMRWG). The LKMRWG is a bi-national, multi-stakeholder work group comprised of three committees; Steering Committee, Monitoring and Research Committee (MRC), and the Selenium Technical Subcommittee (SeTSC). The department and BC-ENV co-manage a wiki website for items related to the LKRMWG including; meeting agendas, meeting summaries, data, literature, technical reports, sampling and analysis plans, and more. Lake Koocanusa Wiki Site