2001-12-11_PERMIT FILE - C1980007A (4)

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COLORADO DEPART,HENT OF PUBLlC HEALTH AND E~WIRONME•YT, Water Quality Control Division Amendmem No. 4 - Raioonafe. Page S, Permit No. CO-0038Titi. B. Whole Effluent Toxicity Limitation Changes /n the June, 1997, CWQCC hearing mentioned in section IV. A. 2. of this rationale, the standards for segment 6 were amended to add Lvater supply based standards. However, this does not change the determination of applicable WET requirements for otttfal/s 011 and 013. The Colorado Water Ortalirv Control Division BiamonitarirtQ Guidance meat dated Jttly 1, 1993. specifies that where the receiving water is classified aquatic life, class 2 without all ojthe appropriate a tt ri li a numeric standards, acute WET testing is required. As stated, these neLV standards are tivater supply based, not aquatic life based. Zherejore no change is necessary. Since new outfall 017 also discharges to segment 6, acute WET requirements apply ro that outfall alto. The determination of }vhether acute or chronic WET resting applies to outfalls 004 and 0!2 and netiv outja11016 is based solely on the chronic Instream Waste Concentration (IWC). The previous determination to apply acute limitations to outfall 004 was made when discharge flow ivas estimated to be much lolver and rsewer sources were not included in t/tis permit. T1:at resulted in a low 1WC that placed outfal! 004 in the range for acute WET testing. The latest information from the permittee indicates that the rata! flotiv from outfalls 004 and 016 could reach 2, 000 gpm (4.5 cfs). TTte chronic 1WC is determined using the following equation: 1WC =[Facility Flow (FF)/(Stream Chronic Lotiv Flow (annual) + FF)J X 100% Zhe flo}vs and corresponding IWC for the appropriate discharge points are: Discharge Point Chronic Low Flow, Facility Design Flow, TR'C, (%) 30E3, (cjs) (cfs) L 012 varies 100 004, 016 (combined) 4l 4.5 9.9 For those discharges where t/re chronic IWC is greater than (>) 9.1 %, chronic conditions apply, where the IWC is less than or equal to (s) 9.1 acute conditions apply. The IWC for ourfal! 0/ 2 is 100%, which represents a wastewater concentration of 100% effluent to 0% receiving stream. The IA'Cfor ourfalls 004 and 016 (combined because they are very close together) is 9.990, which represents a wastewater concentration of 9.9% effluent to 90.1 % receiving stream. Thos, chronic WET testing is applicable for these three ourfalls. Chronic WET Limitations: Mine drainage {or mine water) has been sho}vn at similar coal mining facilities to exhibit whole effluent toxicity. For this reason, the Division believes there is reasonable potential for these discharges to interfere with anainment of applicable water quality classifications or standards. Because of this condition, the chronic limit hat been incorporated into the permit for outfalLr 004 and 016 (it was already applied to outfall 012) and becomes effective immediately. 73~e permiaee is required to conduct routine monitoring. The results of the testing are to be reported on Division approved forms. the permirtee wi116e required to conduct a statistical derivation an the data, looking for any statistically significant difference in tociciry between the control and the effluent concentrations. Tl:is set of calculations will look at the full range of toxicity (lethality, growth and reproduction). 1f a level of chronic tosiciry occurs, such that there is a statistically significant difference in the lethality (at the 95°a confidence level) benveen the control and any effluent concentration less than or equal to the Instream Waste Concentration (IWC), the permittee will be required to fo11o}v the automatic compliance schedule identified in Pan I. B. 4. (c) of the permit, if the observed tociciry is due to organism lethality. Only esceedance of the limitation specified in Pan 1..q.1. (e) tivill trigger the requirement for conducting the automatic compliance schedule identified in Part 1. B. ~. (c) of the permit. !f the tosiciry is due to differences in the growth of the fathead minnows or the reproduction of the Ceriodaphnia, no immediate action on the pan of the permittee tivill be required. However, • this incident, along with other WET data, will be eva(uared by the Division and may form the basis for reopening the permit and including additional WET limits or other requirements. Since chronic WET testing is now required for outfall 004, its inclusion under Part I.B.3. of the permit -Acute WET Testing no longer applies. lTris pan tivill be modified accordingly.