Water: Water Quality Standards
[Why are WQS Important?] [WQS Review and Revision] [Role of the Public] [Designated Uses] [Water Quality Criteria] [Antidegradation] [General Policies] [Indian Tribe Participation] [WQS Program History]
States and Tribes may adopt policies and provisions regarding water quality standards implementation, such as mixing zone, variance, and low-flow policies. Such policies are subject to EPA review and approval.
Mixing Zones: States and Tribes may, at their discretion, allow mixing zones for point source discharges. A mixing zone is a defined area surrounding or downstream of a point source discharge where the effluent plume is progressively diluted by the receiving water and numeric criteria otherwise applicable to the segment may be exceeded. Mixing zone procedures describe the methodology for determining the location, size, shape, and in-zone quality of mixing zones.
Variances: As an alternative to removing a designated use, a State or Tribe may wish to include a variance as part of a water quality standard. Variances temporarily relax a water quality standard. They are subject to public review every three years, and may be extended upon expiration. A variance may specify an interim water quality criterion which is applicable for the duration of the variance. Variances can help to assure that further progress toward improving water quality is achieved.
Low Flows: State and Tribal water quality standards may identify policies and procedures to be applied in determining critical low flow conditions. Such procedures are applied, for example, when calculating discharge permit requirements to be included in National Pollutant Discharge Elimination System (NPDES) permits.