2018-03-01_HYDROLOGY - C1981010 (2)

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COLORADO Water Quality Assessment Department of Public :,b • Health b Environment there is only an acute standard, the acute standard should be used. The appropriate standards are used in the following antidegradation analysis. Significance Tests for Temporary Impacts and Dilution The ratio of the chronic (30E3) low flow to the design flow is 6:1, and is less than the 100:1 significance criteria. Therefore, this facility is not exempt from an AD evaluation based on the dilution significance determination test, and the AD evaluation must continue. For the determination of a new or increased impact and for the remaining significance determination tests, additional calculations are necessary. Therefore, at this point in the antidegradation evaluation, the Division will go back to the new or increased impacts test. If there is a new or increased impact, the last two significance tests will be evaluated. New or Increased Impact and Non Impact Limitations (NILS) To determine if there is a new or increased impact to the receiving water, a comparison of the new WQBEL concentrations and loadings verses the concentrations and loadings as of September 30, 2000, needs to occur. If either the new concentration or loading is greater than the September 2000 concentration or loading, then a new or increased impact is determined. Note that the AD Guidance document includes a step in the New or Increased Impact Test that calculates the Non -Impact Limit (NIL). The permittee may choose to retain a NIL if certain conditions are met, and therefore the AD evaluation for that parameter would be complete. As the NIL is typically greater than the ADBAC, and is therefore the chosen limit, the Division will typically conclude the AD evaluation after determining the NIL. Where the NILs are very stringent, or upon request of a permittee, the Division will calculate both the NIL and the AD limitation so that the limitations can be compared and the permittee can determine which of the two limits they would prefer, one which does not allow any increased impact (NIL), or the other which allows an insignificant impact (AD limit). The non -impact limit (NIL) is defined as the limit which results in no increased water quality impact (no increase in load or limit over the September 2000 load or limit). The NIL is calculated as the September 2000 loading, divided by the new design flow, and divided by a conversion factor of 8.34. If there is no change in design flow, then the NIL is equal to the September 2000 permit limitation. If the facility was in place, but did not have a limitation for a particular parameter in the September 2000 permit, the Division may substitute an implicit limitation. Consistent with the First Update to the AD Guidance of April 2002, an implicit limit is determined based on the approach that specifies that the implicit limit is the maximum concentration of the effluent from October 1998 to September 2000, if such data is available. If this data is unavailable, the Division may substitute more recent representative data, if appropriate, on a case by case basis. Note that if there is a change in design flow, the implicit limit/loading is subject to recalculation based on the new design flow. For parameters that are undisclosed by the permittee, and unknown to the Division to be present, an implicit limitation may not be recognized. This facility was in place as a discharger prior to September 30, 2000 and therefore the new or increased impacts test must be conducted. For the existing outfalls (002, 011, 017, 020, 021, and 023) the design flow of the outfalls have decreased collectively from the AD period as shown below (i.e. 21.73 MGD in the AD period versus 11 MGD in this permit renewal). The design flow at each outfall as of September 30, 2000 were as follows: Outfall 002 = 7.06 MGD Outfall 011 = 4.08 MGD Outfall 017 = 1.37 MGD Outfall 020 = 6.23 MGD Outfall 021 = 2.08 MGD Page 29 of 40 4300 Cherry Creek Drive S., Denver, CO 80246-1530 303-692-2000 www.colorado.gov/cdphe/wqcd I ��