1. a) Reasonable potential analysis is conducted on a parameter-by-parameter basis. In instances where a reasonable potential to exceed a water quality standard for a substance does exist, it does not imply that a reasonable potential for all parameters present in the effluent exists or that WQBELs for all parameters are required.
2. b) The assignment of values for WQBELs is dependent on the application of dilution or mixing zones. The process used for permit review will be conducted in a stepwise approach with the first step being a direct comparison of the Projected Effluent Quality (PEQ) to the applicable water quality standard, criteria or value. If the PEQ is less than or equal to the applicable standard, criteria or value, the Agency will conclude that no potential to exceed exists, that the analysis for that parameter is completed and no WQBEL will be established in the permit unless otherwise warranted under Section 352.430. If the PEQ exceeds the applicable standard, criteria or value, the analysis shall proceed to consideration of mixing and dilution pursuant to Section 352.422.

3. c) Exclusions from reasonable potential analysis. This procedure is a statistically based evaluation of the need for WQBEL for toxic substances based on the scientific approaches to toxicity assessment contained within 40 CFR 9, 122, 123, 131, and 132. This procedure is either not amenable to or appropriate for certain pollutants and parameters included in the Lake Michigan Basin water quality standards at 35 Ill. Adm. Code 302.Subpart E. Therefore this procedure shall not be used to establish permit limits for the following substances:

   Alkalinity

   Ammonia

   Bacteria

   Chlorine

   Color

   Dissolved Oxygen

   Dissolved Solids

   pH

   Phosphorus

   Temperature

   Total and Suspended Solids

   Turbidity

   Sulfate

   Biochemical Oxygen Demand (BOD)

   Radioactivity

   Boron

The need for a WQBEL is based on the potential of a given parameter to cause or contribute to a violation of the applicable water quality standard, criteria, or value. In certain circumstances, this may entail application of a mixing zone to the discharge before comparing the effluent concentration of a substance to the water quality standard, criteria, or value. The Agency shall conduct an analysis of the reasonable potential for a given effluent to exceed or contribute to excursions above water quality standards that may occur in the receiving body during the NPDES permit review. This reasonable potential analysis is based on statistical analysis of the effluent and the following factors: