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COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Amendment No. S-Rationale. Page 5, Permit No. CO-0000132 If, upon review of the results of this additional monitoring, it is determined that a reasonable potential to exceed the WQS exists, the permit will be reopened to add eflIuent limitations and any other appropriate conditions, subject to public notice provisions. d) Whole EfRuent Toxicirv (R'E'I) Testing: For outfa[[ 015, chronic WET testing is required. (See Part 1.A.5. of the permit.) I) Purpose ojWET Testing: The Water Qua[iry Control Division has established the use of WET testing as a method for identifying and controlling toxic discharges from wastewater treatment facilities. WET testing is being utilized as a means to ensure that there are no discharges of pollutants "in amounts, concentrarions or combinations which are harmful to the beneficial uses or toxic to humans, animals, plants, or aquatic life" as required by Section 31.11 of the Basic Standards and Methodoloeies for Surface Waters. 2) Instream Waste Concentration (IWC): Where monitoring or limitations for WET are deemed appropriate by the Division, chronic instream dilution as represented by the chronic IA'C is critical in determining whether acute or chronic conditions shall apply. For those discharges where the chronic TWC is greater than (>) 9.1 %, chronic conditions apply, where the IWC is less than or equal to (s) 9.1 acute conditions apply. The chronic IWC is determined using the following equation: IWC =[Facility Flow (FF)/(Stream Chronic Low Flow (annual) + FF)] X 100% The flows and corresponding IWC for the appropriate discharge point are: OutfaU Chronic Low Flow, (cfs) Facility DesiAii Flow,: (cls) IA'C, (%) 015 0 0* 100% * This flow has not been determined at this time, however it is nonessential to this calculation because the law flow is zero. The IWC for this permit is 1003'0, which represents a wastewater concentration of 100% effluent to 09'0 receiving stream. Thus, chronic WET testing is applicable jot this outfall. 3) Chronic WET Monitoring: Mine drainage from coo! mines in Colorado has shown toxicity to aquatic life in some cases. On ihis basis, the potential for toxicity exists for this new outfall. The permittee will be required to conduct routine monitoring for chronic toxicity using fathead minnows. The permittee conduced an Aquatic Impairmen[ S[udy in 1995 for outfall 011 to de[ermine whether [he cause of toxicity to Ceriodaphnia sp., which was determined ro be total dissolved solids (TDS), was impacting the receiving water. The Division and EPA reviewed this study and determined that there was no discernible impact. It was further determined that future testing using Ceriodaphnia sp. would be of little value. Thus, WET testing of Ceriodaphnia sp. was waived for outfatl 011. The permittee indicates that the water quality of outfal1015 is similar to that of outfall 011 and that toxicity would be exhibited only by TDS. The Division has reviewed the available quality data and agrees with the permittee. On this 6arrs, the waiver of Ceriodaphnia sp. testing is extended to ourfall 015. If the six-rime analysis required under this permit amendment indicates that toxic levek of metals are present or if routine monitoring shows that the quality of outfal! 015 is significantly different from outfal[ OII, the permit may be reopened to add WET testing for Ceriodaphnia sp. The results of the testing are to be reported on Division approved forms. The permittee will be required to conduct two types of statistical derivations on the data, one looking for any statistically significant difference in toxicity between the control and the ef]Iuent concentrations and the second identifying the /Cu. should one exist. Both sets of calculations will look at the full range of toxicity (lethality, growth and reproduction).