2009-03-06_PERMIT FILE - C1981012A (6)

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COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Rationale -Page 16, Permit No. CO-0000906 • • • Water Quality Control Division Biomonitorin Guidance Document host Class 1 Aquatic Life use or Class 2 Aquat c Life use with all of theer than 9.1 "o and the receiving stream Where the chronic /WC is less than or appropriate aquatic life numeric standards, chronic conditions apply. equal to 9.1 • or the stream is not classified as described above, 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 IWCfor the appropriate discharge point are: Chronic Low Flow, Facility Design Flow, IWC1 (%) Discharge Point 30E3, (efs) (cfs) 4.3 5.6 57% OOIA 001B 4.3 2.0 27% 001C 4.3 0.45 9.5% 0 5.6 100% 009A 0 2.0 100% 009E 0 0.45 100'% 0090 For Outfalls 001A, 001B and 001 C the IWC is greater than 9.1% and therefore chronic conditions apply., For Outfalls 009A, 009E and 009C, the IWC is 100``%, which represents a wastewater concentration of 100%. y However, Outfall 009A discharges to a stream that does not have all the aquatic life numeric standards. According to the Water Quality Control Division Biomonitorin Guidance Document acute WET may apply in such conditions. Therefore, acute WET will apply at Outfall 009 in this permit. iii. Chronic WET Limit at Outfa11001A 001E and 001 C- This facility is expected to discharge metals at concentrations that have been found to have toxic effects to fish and other aquatic life. On this basis, the Division believes there is reasonable potential for the discharge to interfere with attainment of applicable water quality classifications or standards. Because of this condition, the chronic limit has been incorporated into the permit at Outfalls 001A, 001B and 001C and becomes effective immediately. The permittee is required to conduct routine monitoring. 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 effluent concentrations and the second identifying the IC,5, should one exist. Both sets of calculations will look at the full range of toxicity (lethality, growth and reproduction). If a level of chronic toxicity occurs, such that there is a statistically significant difference in the lethality (at the 95% confidence level) between the control and any effluent concentration less than or equal to the In-stream Waste Concentration (IWC) and if the lethality IC,5 < the IWC, the permittee will be required to follow the automatic compliance schedule identified in Part I.A.4. of the permit, if the observed toxicity is due to organism lethality. Only exceedance of the limitation specified in Part I.A. 1. will trigger the requirement fbi- conducting the automatic compliance schedule identified in Part LA. 4. of the permit. If the toxicity is due to differences in the growth of the fathead minnows or the reproduction of the Ceriodaphnia, no immediate action on the part of the permittee will be required. However, this incident, along with other WET data, will be evaluated by the Division and may form the basis for reopening the permit and including additional WET limits or other requirements. iv. Acute WET Limit tit Out Is 009A 009B and 009C- As previously discussed, this facility is expected to discharge metals at concentrations that have been found to have toxic effects to fish and other aquatic life. On this basis, the Division believes there is reasonable potential for the discharge to interfere with attainment of applicable water quality classifications or standards and therefore, will cu become toxicity l imitihas beately. incorporated into the permit. The acute WET limit of LC5o The permittee is required to conduct quarterly monitoring consistent with the frequency specifications in the Colorado Water Quality Control Division Biomonitorin Guidance Document, dated July 1, 1993, the results of