Water: Water Quality Standards Academy
Basic Course: Supplemental Topics (NPDES Permit 18)
- Listing Impaired Waters and Developing TMDLs
- Monitoring & Assessment
- NPDES Permit Program
- Point Source Control
- Permitting Authority
- Types of Permits
- Categories of Permits
- Permit Components
- Key Considerations
- Technology-Based Determination
- Water Quality-Based Determination
- WQBELs Process
- Step 1: Identify WQSs
- Step 2: Assess Concentration
- Pollutants of Concern
- Critical Conditions
- Dilution/Mixing Allowance
- Step 3: Establish Need
- Step 4: Calculate Limits
- Human Health Ambient Water Quality Criteria
- Aquatic Life Criteria
Establishing WQBELs: Step 4—Calculate Parameter-Specific WQBELs
If the assessment steps in the four-step sequence establish the need for a water quality-based effluent limit for a pollution of concern, then the permit writer will need to calculate a water quality-based effluent limit that is fully protective of the applicable water quality standards.
The first task in this process is to use the water quality model to establish a waste load allocation (WLA), which represents the maximum allowable pollutant concentration in the effluent that, after accounting for allowable dilution/mixing, will not cause an exceedance of the instream water quality criterion.
Once a WLA is established for each of the applicable criteria, the permit writer would use the mathematical approach provided in EPA’s Technical Support Document for Water Quality-Based Toxics Control—or other approved State/Tribal implementation procedures—for calculating the average monthly limit (AML) and maximum daily limit (MDL). These WQBELs establish performance expectations that should ensure that the WLA is not exceeded.
Resource. The methodology for calculating a point source-specific WLA is presented in EPA’s Technical Support Document for Water Quality-Based Toxics Control (March 1991).
Resource. EPA provides classroom training on NPDES permit writing. EPA’s NPDES Permit Writers' Training Course website provides date and location information. PowerPoint presentations for the classroom sessions are also available on the website.
Resource. See also EPA’s NPDES Permit Writers Management Tools webpage.