Water Quality Standards


Water quality standards describe the desired condition of a waterbody and the levels of constituents in the water required to achieve a particular designated use, such as recreation, water supply, aquatic life, and agriculture. Colorado is authorized by EPA to establish water quality standards, which form the legal basis for controlling pollutants entering waterways. 

Surface water quality standards consist of three core components: classified uses, criteria, and antidegradation designations. Use classifications are based on each waterbody’s current or future suitability for the use. Criteria are then assigned to protect the uses. Antidegradation requirements protect existing uses and high-quality waters from degradation. In Colorado, these components are generally implemented on a segment-by-segment basis. 

Colorado is divided into seven major river basins, which correspond to Water Quality Control Commission Regulation Nos. 32-38. Use classifications, criteria, and antidegradation designations are assigned to individual surface waterbody segments contained in the basin regulations.

The Water Quality Control Commission adopts water quality classifications, standards, and implementation processes for surface waters of the state into Regulation Nos. 31-38, listed below. The commission also adopts water quality standards for groundwater into Regulation Nos. 41-42, listed below. The commission routinely reviews the regulations and can adopt changes through a public hearing process. Any changes adopted by the commission are reviewed by EPA. 

All commission regulations can be found on the commission’s regulation website. All commission regulations, including a library of previous versions of the commission’s regulations, can also be found on the Secretary of State’s website.

Water Quality Control Commission Regulations with Water Quality Standards:

  • Statewide Water Quality Control Commission surface water standards (Regulation No. 31)
  • Water Quality Control Commission standards applied by river basin (each basin regulation includes an appendix with water quality standards by waterbody segment):
    • Arkansas (Regulation No. 32)
    • Upper Colorado and North Platte (Regulation No. 33)
    • San Juan and Dolores (Regulation No. 34)
    • Gunnison and Lower Dolores (Regulation No. 35)
    • Rio Grande (Regulation No. 36)
    • Lower Colorado (Regulation No. 37)
    • South Platte, Laramie, Republican, and Smoky Hill (Regulation No. 38)
  • Statewide and site-specific Water Quality Control Commission groundwater standards (Regulation Nos. 41 and 42, respectively)
  • Calculator for hardness-based water quality standards (download as Excel to use)

For information about ongoing and recently completing Water Quality Control Commission hearings, please visit the hearing web page.

For information about participating in Water Quality Control Commission rulemaking hearings, please visit Public Participation Web page.

For information about the Water Quality 10-Year Roadmap, to sign up for email notifications, and to learn about ways to get involved, please visit Water Quality 10-Year Roadmap web page.

As part of the 2017 nutrients rulemaking hearing, the Water Quality Control Division developed plans for managing nutrients and developing new and updated water quality standards between 2017 and 2027. These plans received support from the 2017 nutrients work group and the Water Quality Control Commission and are contained in Implementation Policy CW 8 Colorado Nutrient Management Plan and 10-Year Water Quality Roadmap. Water quality standards included in the Water Quality 10-Year Roadmap are: streams nutrients, lakes nutrients, ammonia, cadmium, arsenic, selenium, and temperature.

For information about the Water Quality 10-Year Roadmap, to sign up for email notifications, and to learn about ways to get involved, please visit Water Quality 10-Year Roadmap web page.

The commission’s provisions at Regulation 31.7(4) allow adoption of a discharger-specific variance (DSV), which is a temporary standard that represents the highest feasible degree of protection of a classified use, while temporarily authorizing alternative effluent limits (AELs) for a specific pollutant and specific point source discharge where compliance with the water quality-based effluent limits (WQBELs) is not feasible. An initial AEL ensures the protection of currently attained ambient water quality from the onset of the variance, and a final AEL represents the highest attainable condition that is feasible to achieve during the term of the DSV. The term of each DSV is set by the commission on a case-by-case basis, to be only as long as necessary to achieve the highest attainable condition. DSVs are required to be reevaluated at a minimum during each basin triennial review for the segment. If the term of the DSV is greater than five years, the DSV must be reviewed at least every five years after EPA’s approval.

The Water Quality Control Commission may adopt site-specific standards for entire segments, individual waterbodies, or portions of waterbodies. Regulation No. 31 describes the types of site-specific standards used in Colorado, which include ambient quality-based, criteria-based, and narrative site-specific standards, as well as the types of information needed to support the adoption of site-specific standards (31.7(1)(b)). 

To ensure that site-specific standards can be reviewed by the Commission in the future, as required by the Colorado Water Quality Control Act, site-specific standards may be accompanied by longevity plans. A longevity plan is a site-specific plan that describes the data and other relevant information that need to be collected and analyzed to facilitate review of each site-specific standard.

A use-attainability analysis (UAA) is an assessment of the factors affecting the attainment of aquatic life uses or other beneficial uses, which may include physical, chemical, biological, and economic factors. A UAA is required to remove or downgrade a classified use.

  • UAA library (will be updated as historical records are located)

Temporary modifications are a Colorado-specific regulatory tool that provide dischargers short-term regulatory relief from the underlying water quality standard for a particular parameter (e.g., temperature) to allow time to address uncertainty. Temporary modifications can be adopted by the Water Quality Control Commission for an entire waterbody or a portion of a waterbody. Temporary modifications can be adopted when the standard for the parameter is not attained in the waterbody, there is demonstrated or predicted non-compliance with the water quality-based effluent limit (WQBEL) for the parameter, and there is significant uncertainty regarding the water quality standard for the parameter necessary to protect the use and/or the extent to which existing quality of the parameter is the result of natural or irreversible human-induced conditions (as described in Regulation No. 31 at 31.7(3)). The commission determines the length of temporary modifications on a case-by-case basis, with the goal to keep temporary modifications as short as possible. 

A plan to resolve the uncertainty (PTRU) must accompany a temporary modification. Additionally, proponents of temporary modifications are required to provide annual progress updates to the division regarding PTRU progress. All temporary modifications must be reevaluated by the commission not less than once every three years. Other requirements for adopting, extending, and reviewing temporary modifications are included in Regulation No. 31 at 31.7(3).