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Water: Water Quality Standards Academy

Basic Course: Supplemental Topics (NPDES Permit 15)

NPDES Permit Program

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If the State/Tribal water quality standards allow for consideration of dilution/mixing, then the permit writer needs to select an appropriate water quality model for predicting the impact of the effluent in terms of the receiving water’s assimilative capacity.

Since the majority of such analytical situations involves a single set of conditions for projecting the effluent’s impact, often the permit writer may use a steady-state water quality model.


Key Point. Because a steady-state water quality model is run on a single set of conditions, those conditions should be the critical conditions for protection of the receiving water.

Critical conditions generally fall into the following framework:

  • Effluent critical conditions
    • Effluent flow
    • Effluent pollutant concentrations (i.e., for pollutants of concern)

  • Receiving water critical conditions
    • Receiving water flow (if applicable)
    • Background pollutant concentrations (i.e., for pollutants of concern)
    • Other characteristics of the receiving water (e.g., temperature, pH, reaction rates)

Key Point. A State/Tribe’s water quality standards (or permitting procedures) should identify the appropriate critical conditions. If not, the permit writer should look at other permits and follow past practice.


Learn More. Further explanation based on the critical condition for flow in a receiving water that is a river or stream. Proceed to the Learn More Topic. » (Note: This link launches a pop-up window.)

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

For informational purposes only–Not official statements of EPA policy.

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