MGS Stream Photo

Water quality standards are the foundation for a wide range of programs under the federal Clean Water Act (CWA). The goal for water quality standards is protecting water uses through applying criteria. For information about when water quality standard are reviewed and, as appropriate, modified and adopted, visit Water Quality Standards Review

A water quality standards variance is a time-limited designated use and criterion change for a specific pollutant, allowing deviation from meeting a water quality-based effluent limit for dischargers. Federal and state regulations allow the flexibility to adopt a variance based on one of the following factors:

  • Naturally occurring pollutant concentrations
  • Natural, ephemeral, intermittent or low flow conditions
  • Human caused conditions cannot be remedied or would cause more environmental damage to correct than to leave in place
  • Dams, diversions or other hydrologic modifications
  • Physical conditions related to natural features preclude aquatic life uses
  • Controls more stringent than needed to meet technology-based limits cause substantial and widespread economic and social impact
  • Actions needed to facilitate lake, wetland or stream restoration through dam removal or other significant reconfiguration activities preclude attaining the designated use and criterion while the actions are being implemented

A variance must meet the federal requirements in Code of Federal Regulations 40 C.F.R. § 131.14 and state requirements in Code of State Regulations 10 CSR 20-7.031(12). A variance serves as the applicable water quality criteria for implementing the National Pollution Discharge Elimination System permit program or when issuing Clean Water Act Section 401 Water Quality Certifications. Current water quality must not be adversely affected during the term of the variance.

Public Notices/ Public Comments

There are no currently active Water Quality Standards Variance Public Notices/ Public Comments.

Site-Specific Variances

Multi-Discharger Variances

The multiple discharger variance (MDV) is a variance from the total ammonia nitrogen water quality standard. It allows multiple communities to be covered under one variance. In a permit incorporating the MDV, typical ammonia effluent limits that are based on the water quality standard would be replaced with the highest attainable conditions of the current treatment facility during the term of the variance. This provides communities with the time and flexibility to make water quality improvements in a cost-effective manner.

The Multiple Discharger Variance Framework: Total Ammonia Nitrogen has been incorporated into Missouri’s Water Quality Standards at 10 CSR 20-7.031(12). EPA approved the framework Dec. 26, 2019, making the 20-year term of the MDV from Dec. 26, 2019 through Dec. 25, 2039.

Framework – Ammonia