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COLORADO DEPARTMENT OF HEALIN, Water Quality Control Division Amendment No. I -Rationale -Page 3. Permit No. CO-0027146 The basis jor the limitmionsfor outfall015 shown in Table V-1 is unchanged from that jor the other mine drainage outfalLr already in place. Total suspended solids and !oral iron limitations are based on Federal Best Available Technology (BAT! because the new outfa!! will disthharge mica water from the same source abettdy addressed through outjall 002. Ijrhis was a new source, the limitations would have been based on Federal New Source Performance Standards (NSPSJ. The nezl section addresses WET requiremems in depth. VI. WET TESTING REQU/REMENTS For this facility, acute WET testing is required jor outfalls 001, 002, x'004 and 015. (See Parts LA and LB ojtht permit.) aJ Purtwse of WET Terrine: Tl+e Water Quality Control Division has established the use ojWET testing ar a method jor idem tying and controlling toxic discharges from wastewater rrearmem facilities. WET testing is being utilized as a means to ensure that there are no disdtargu of pollmams 'in amounts, concrmrations or combinations which are harmful to the beneficial user or toxic to humans, animals, plants, or aquatic hfe • as required by Section 3.1.11 (1J of the Basic Standards and Methodaloeies jor Surface Waters. bJ Instream Waste Concentration (1WC1: Where monitoring or limitations jor WET are deemed appropriate 6y the Division, chronic instream dilution as represemed by the chronic IR'C is critical in determining whether acute or chronic conditions shall apply. For those discwrges where the chronic IWC is greater than (>J 9.1 %, chronic conditions apply, where the IWC is less than or equal to (SJ 9.1 acute conditions apply. 71te chronic !WC is determined using the jollowing equation: IWC =[Facility Flow (FFJ/(Stream Chronic Low Flow (annual! + FFJJ X 100% The flows and conesponding 1WC jor the appropriate discharge point are: 001 +002 1086 0.50* 0.05 004 1086** 0.67 0.1 015 0 0.28 100 • Because outfal4 001 and 001 arc jean the same rourcr and go to the same retciving wnur, the Combined drrign flow u cared here. • • The Division has pmiously utablirhed that, although our dutharge goer to Cod Canyon Crcek, due ro the prarimiry to the conJlutntr with the Colorado River and the lack of krorn attud ar poundo! user far the wafer in the remaining rtrefeh of Cad Canyon Crcek, the Disirion it using 0u law flow of the Caforado Rivcrfor this deurmifwlion. The IWCfor outfa!!s 001/002 is 0.05%, which represems a wastewmer canremrmion aj0.05% e,Bluem to 99.95% rereiving stream. The /WCfor outfall 004 is 0.1 %, which represents a wastewater concemrmian of 0.1 % e,~luent to 99.9% receiving stream, the 1WCfor outfaU 0l5 is 100%, which represems a wastewater concentration oj100% efjluem to 0% receiving stream. However, in addition to the /WC value jor determining whether a4ronic or acute WET testing requiremems are applicable, the class flcation of the receiving stream must be considered. According to the Colorado Water Oualiry Control Division Biomonitorirte Guidance Document, dated July 1, 1993, chronic WET testing is required for jaciiities where the receiving stream has a Class 1 Aquatic Lije use or Class 2 Aquatic Life use with all of the appropriate aquatic life numeric standards; otherwise, acute WET testing is required. Since the receiving waterfor omfal! 015 is classified as Class 2 Aquatic Life use, but does not have aU of the approprime aquatic life numeric standards, the Division has determined that acute WET testing is applicable for this permit (see following paragraph!.