Modifications to water quality criteria are made when the base water quality criteria established in Wyoming’s Surface Water Quality Standards are either not attainable or sufficiently protective of the designated use. These changes are typically referred to as site-specific criteria and are tailored to a particular waterbody or waterbody segment. Development of site-specific criteria may be appropriate when the:

• Physical or chemical characteristics of the waterbody alter the toxicity of a pollutant;
• Aquatic life in a waterbody have different sensitivities to a pollutant than those used to establish the base water quality criterion for that pollutant; or
• Background levels of a pollutant are naturally elevated relative to the base water quality criterion.

In all cases, site-specific criteria developed for a waterbody, whether based on natural background conditions, recalculation of existing criteria, toxicological studies, or other scientifically defensible methods; must protect all existing uses, designated uses, the highest attainable use, as well as downstream surface water quality standards. All proposed site-specific criteria require formal rulemaking to amend Wyoming’s Surface Water Quality Standards (Chapter 1, Wyoming Water Quality Rules).

The document “Options to Modify Water Quality Standards” available in the Standards Resources and Guidance tab above, describes considerations for when changes to water quality criteria may be appropriate. For more information contact Eric Hargett (eric.hargett@wyo.gov) or 307-777-7079.