Basic Course: Supplemental Topics (NPDES Permit 14)

Characterizing the Effluent & Receiving Water: Establish Whether Dilution/Mixing Is Allowed

With the pollutants of concern in mind, the State/Tribal authority must then look to the water quality standards to see whether the permit can be written to allow for dilution/mixing of the effluent in the receiving water, either at the discharge point or in a mixing zone for the relevant pollutant at issue.

Key Point. Under the CWA, States/Tribes have the discretion to decide whether to include consideration of effluent dilution/mixing as part of their water quality standards. (Sec. 122.44(d)(1)(ii)) Where a States/Tribe’s water quality standards allow the consideration of dilution/mixing for a given pollutant or category of pollutants, the State/Tribe should identify the procedures and conditions under which mixing is allowed. When specific provisions are not written into the standards or implementing procedures, the permit writer may have to rely on existing guidance or practices to account for the mixing analysis.

Step 2 Tasks for
Establishing WQBELs

• Identify Pollutants of Concern
• Establish Whether Dilution/Mixing Is Allowed
• Identify Critical Conditions for Modeling
• Establish Dilution Allowances or Mixing Zones