Water Quality Standards Regulations: Montana

Presented are water quality standards in effect for Clean Water Act (CWA) purposes for this state.

EPA is posting these standards as a convenience to users and has made a reasonable effort to assure their accuracy. For each document listed under "State Standards in Effect for CWA Purposes", EPA has made a reasonable effort to identify parts of the standards that are disapproved or are otherwise not in effect for CWA purposes. Additionally, please note the documents may also contain provisions that EPA does not have the authority and duty to act on under the Clean Water Act because the provisions are not water quality standards.

• What is a New or Revised Water Quality Standard Under CWA 303(c)(3)? - Frequently Asked Questions (pdf) (108.04 KB)

• State Standards in Effect for CWA Purposes
• Federally Proposed or Promulgated Standards

State Standards in Effect for CWA Purposes

• Water Quality Standards Provisions in Montana's Water Quality Act, Title 75 - Environmental Protection, Chapter 5 - Water Quality (75.5.101–75.5.327) (pdf) (517.11 KB)
  These are the water quality standards enacted by Montana State legislature.
• Mixing Zones in Surface and Ground Water - Sub-chapter 5 (17.30.501–17.30.518) (pdf) (513.1 KB)
  This subchapter contains rules governing the granting of mixing zones.
• Surface Water Quality Standards and Procedures - Sub-chapter 6 (17.30.601–17.30.670) (pdf) (2.12 MB)
  This subchapter contains water use classifications for waters of the state.
• Nondegradation of Water Quality - Sub-chapter 7 (17.30.701–17.30.717) (pdf) (526.8 KB)
  This subchapter contains the implementation procedures for implementing the nondegradation policy contained in 75-5-303 of Montana's Water Quality Act.
• Circular DEQ-7, Montana Numeric Water Quality Standards (pdf) (1.6 MB)
  This document contains numeric water quality criteria for Montana surface and ground waters.

Montana's water quality standards are applicable to "waters of the State". In some limited instances, the definition adopted by Montana for "waters of the State" as part of its water quality standards may result in water quality standards that are more comprehensive than required under section 303(c) of the Clean Water Act and the implementing federal regulations. Specifically, Montana has adopted additional provisions related to groundwater that are outside the scope of the Clean Water Act and implementing federal regulations. EPA has included the State's water quality standards in their entirety here for context and as supplementary information for the reader.

Montana water quality standards apply to all waters within the State of Montana, with the exception of those waters that are within Indian Country, as defined in 18 U.S.C. Section 1151. EPA's approval of Montana's water quality standards was not intended as an action to approve or disapprove water quality standards applying to waters within Indian Country. EPA, or eligible Indian Tribes, as appropriate, will retain responsibilities for water quality standards within Indian Country.

Action Letters on State WQS Submissions

• EPA’s action on Montana’s water quality standards in Sections 1, 2(1), and 5 of House Bill 664 (pdf) (320.75 KB, Action Date: 10/3/2025) EPA approved revisions to Montana’s water quality standards in House Bill 664 which the state submitted for EPA review under the Clean Water Act. Among the provisions in House Bill 664, the legislation removed Montana’s numeric nutrient water quality standards applicable for certain state waterbodies. The EPA action on October 3, 2025 approved removal of these numeric nutrient water quality standards consistent with the Clean Water Act and EPA’s water quality standards regulation.

  Montana’s existing narrative criteria remain in place to protect against harmful effects of nutrient pollution in waterbodies state-wide. Narrative criteria describe the desired conditions for safe designated water uses and healthy aquatic life. Montana has the scientific information available to utilize the narrative criteria for its Clean Water Act programs. These programs include assessing water quality and issuing discharge permits that protect aquatic life and water recreation.

Federally Proposed or Promulgated Standards

• None at this time.