A water quality standards (WQS) 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 upon one of the following factors:

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

A WQS variance serves as the applicable water quality criteria for implementing the National Pollution Discharge Elimination System permit program or when issuing Clean Water Act 401 certifications. Current water quality shall not be adversely affected during the term of the variance.

A WQS variance must meet the federal requirements in 40 CFR Section 131.14 and state requirements in 10 CSR 20-7.031(12).

Site-Specific Variance

Multi-Discharger Variance

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 CWC-MDV-1-17 has been incorporated into Missouri’s Water Quality Standards at 10 CSR 20-7.031(12). It was approved by EPA on Dec. 26, 2019 making the 20-year term of the MDV from Dec. 26, 2019 through Dec. 25, 2039.

MDV Resources

Multiple Discharger Variance Framework – Ammonia


Public Notices/ Public Comments

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