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Water: Water Quality Standards Academy

Basic Course: Supplemental Topics (NPDES Permit 17)

NPDES Permit Program

« Page 17 of 20 »

Establishing WQBELs: Step 3—Determine the Need for Parameter-Specific WQBELs

This step in assessing the need for water quality-based effluent limits involves running the model (or models, as necessary) using inputs for the critical conditions and the dilution/mixing allowances identified in the preceding step.

  • If: The model output predicts that discharges could result in concentrations of pollutants of concern in the receiving water that exceed applicable criteria in the State/Tribe’s water quality standards,
  • Then: A “reasonable potential” for exceedance has been established and water quality-based effluent limits must be written into the permit.
Key Point. The so-called Reasonable Potential Test for determining whether WQBELs must be included in a permit is based on the regulation, which states: “Limitations must control all pollutants or pollutant parameters (either conventional, nonconventional, or toxic pollutants) which the Director determines are or may be discharged at a level which will cause, have the reasonable potential to cause, or contribute to an excursion above any State water quality standard.” (122.44(d)(1)(i)) Exit EPA Disclaimer
Resource. Details on the methodology for calculating “reasonable potential” are presented in EPA’s Technical Support Document for Water Quality-Based Toxics Control (March 1991). The methodology focuses on projecting maximum expected value (i.e., the highest concentration that would be expected from a point source discharge if the effluent were monitored continuously for a very long time). (The link for downloading the document is TSD document (PDF) (335 pp., 26.6 MB, About PDF).)
Key Point. In the “fact sheet” that accompanies most permits, the permit writer should document the need for any WQBELs by presenting relevant data and assumptions.

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

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