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;OLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Wafer Qualiry Control Division :ationnle-Page 6, Permit No. CO-0038776 ( ggmm )) ,,qqnn e anti egradation revieweist app cable pursuantkto Section 31R8vof The BasiceStandards and Methodologies foraSurface Water. However, the ratio of the ,/low of the North Fork of the unnison ever tote ountai~ )~ )a ~ n fiBear oa T1~P! 's combined design,Jlows (see Appendix A) is more than 150:1 at low flows. Section 31.8 3 c s ecl es that the discharge of pollutants should not be considered to result in significant ~ egr~ dah'on of ~ reviewable waters if the flaw rate is greater than 100:1 dilution at low flow. Thus, condition 31.8 3 c o the re [attons is met and no further antidegradation evaluation is necessary for discharges to the North Fork o t e unison ever. Stream segment COGUNF06 is designated as Use Protected. Because the applicable receiving waters aggrere designated as Use Protected, no antidegradation review is necessary in accordance with the regullch, the Dku ForkioeMinnesotarCreek requirements have been met for the discharges to Sylvester Gulch, West Sylvester CCrruis ry f and Lone Pine Gulch. f. Colorado Mizin Zone Re lotions: Pursuant to section 31.10 of The Basic Standards and Methodolo ies or Su ace ater a mixing zone etermination is required for this permitting action. e o ora o ixirz one m ementation i ante dated April 2002, identifies the process for determining the meanin unit on t e area impacte y a isc arge to surface water where standards may be exceeded (i. e., regulatory mixing zone . This guidance document provides for certain exclusions from further analysis under the regulation, based on site-specific conditions. The uidanee document provides a mandatory, stepwise decision-making vrocess for determinin i the permit limits will not bgge a~~{{ected b this regulotion. Exclusion, based on Extreme Mixin IZatios, may be anted the ratio of the design flow to ike chro in c low flow 30E3)) it eater than 2:1 or if the ratio o,~the chronic low f~ow to t e design flow is gseater than 20:1. Since the silo of the low~w to the design Jlow is more than 150:1 for discharges to the North Fork of the Gunnison River (streamysegment CO((iiUNF02), the permiiee is eligible for an exclusion from further analysis under the toe~le tNorth F~ k o{the Gunnison River (streaml~egfimenttCOGUNF06), th~permilfeenis ~el~gible foraaneexclusiontfrom further analysis under the regulation. On this basis, ail outfalls under this permit are eligible for exclusion. g. Salmi Re lotions: An evaluation of the discharge o(total dissolved solids indicates that the Mountain Coal Company est ine'exceeds the threshold of 1 ton/da or 50 tons/year of salinity as set forth in the Colorado River Salmi ef,.ndnrdc In conformance with section 61.8~f(1)(i)(A) of the Colorado,Dischargn Permita~ Sianna asound the ittee shall monitor points except inter h. Whole Effluent Toxieity (WET) Tesring For Outfalls 004, 011, 012, 013, 016, 017 at this facility, acute WET testing is required. (See Part IA of the permit.) i. Pur ose o WET Tesfin :The Water ppuality Control Division has established the use of WET testing as a method for i3enti ing an contro trig toxic discAarges from wastewater treatment facilities. WET testing is being utilized as a means to ensure that there are no discharges of pollutants "in amounu, concentrations or combmanons which are the~Ba~.ric Standards and Methodologies fo~ Surface Wate s:plants, ar aquatic lifge" as required by Section 31.11 (1) of ii. In Strea~~'ast? ~nutanaasorea~ented_byrthe c ~onicolWC is crncal inndetermininglwhelher mute oPrchromc ~_-.....1 n:.,;~;nn Rimm~nitnrinP GUidanCe Document dated /uTy-1, 1993, or those auenar es where one ~..,..,••....• ~ ~ e~---- --- stream liar a Class 1 Aqua!ie if use or Class .dquatic L~e use with all of the appropriate aquatic i e standards, chronic conditions apply. P ye the chronic IWC u less than or equal to 9.I or the stream is not as described above, acute con ittons a 1 . The chronic IWC is determined using the following equation: IN'C = [Facility Flow (FF)/(Stream Chronic Low Flow (annual) + FF)J X 100% The flows and corresponding IWCfor the appropriate discharge point are: IIOWand 0l6' ~ O' (Minimal)** 100# 011 0 (Minimal)** 100 017 0 * (Minimal)** 100 013 * 5 100 4 017 ecause a t eproxfmi o ese (w 7 43 o tsc ar es, t ey are cons! er as a sing . __ . Lb___.....ue .x.,..,o- hw Wows ar . a impact tot a recefmng water. e zero. a famliryllow is not required to determine the IH'C It will always equril IOD% The IR'C for Outfalls 004 and 016 is 0.5%, which represents a wastewater concentration of 0.5% effluent to 99.5% receiving stream. The IWC for Outfalis 011, 012, 013, and 017 is 100%, which reppresents a wastewater concentration 0 100% effluent to 0% receivirg stream. Since the receiving water segment to w7iich Outfalls 011, 012, 013 and 017 discharge now has all of the appropriate aquatic life numeric standards, there is no exemption from chronic WET testing on this basis. An addirional factor exists for Outfalls 011, 011, and 013 which was not addressed by the WET guidance. Beeauscalle IWC is 100%, chronic WE7'tesrin should be required. However, these Outfalls discharge very infrequently (typ' . y less frequently than once a month with most discharges lasting no more than three days. Thu prevents the permttee from performing statistically valid anal ses for chronic WET' testingg. Because of this the potential impact on the receiving waters from these three Outfalls is acute, not chronic. On this basis, it is [ogica to require acute WET was made previously andpis carried oPe cinothis renewalip~rmaters for Outfalls 011, 012, and 013. This determination tract Reveced: 6/!92004