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~OLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division 4ationate-Pnge 6, Permit No. CO-0038776 e. An~odegada~tion: Since the North Fork of the Gunnison River Stream segment COGUNF02) is Undesignoted, an anti egd radation review is applicable pursuant to Section 31.8 of a Basic Standards and Methodologies for Surface Water. However, the ratio of the flow of the North Fork of the unCi noon IFiver tote ounrazn oa an ear oa WW1rFs combined design flows (see Appendix A) u more than 150:1 at low flows. Section 31.8 (3)(c) specifies that the discharge of pollutanu should not be considered to result in significant de radation of the reviewable waters f the flow rate is greater than 100:1 dilution at law ,flow. Thus, condition 31.8(3 c) of the requlotions is met and no further anhdegradation evaluation is necessary for discharges ro the North Fork aft e (~iunnuon River. Stream segment COGUNF06 is designated as Use Protected. Because the applicable receiving waters are designated as Use Protected no antidegradation review is necessary in accordance with the regulations. Thus, anodegradation review requirements have been met for the discharges to Sylvester Gulch, West Sylvester Gulch, the Dry Fork of Minnesota Creek and Lone Pine Gulch. j. Colorado Mixin Zone Re lotions: Pursuant to section 31.10 of The Basic Standards and Methodologies for Surface ales a mixing zone etermination is required for this permitting action. a ora o~ ne m ementatioit i ante dated April 2002, idenrifies the process for determining the meanin imit on t e area impacte y a is~ 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-sped c conditions. The ~ttidance document provides a mandatory, stepwise decision-making process for determining i the permit limiu will not be affected by this regulotion. Exclusion, based on Extreme Mixin Ratios, may be anted i the ratio of the design flow to the chronic low flow 30E3)) u grealer than 2:1 or if the ratio o~the chronic low~w to t e design flow is gseater than 20:1. Since the ratio of the low flow to the design flow is more than 150.1 for discharges to the North Fork of the Gunnison River (stream segment COGUNFO2), the permittee is eligible for an exclusion from further analysis under the regulation. Additionally, since the ratio of the design flow to the low flow is greater than 2: l for discharges to tributaries to the North Fork of the Gunnison River (stream segment COGUNF06), the permittee is eligible for an exclusion from further analysis under the regulation. On this basis, all outfalls under ihu permi[ are eligible for exclusion. g. Salini Re lotions: An evaluation of the disci est ine exceeds the threshold of 1 ton/d Standards. In conformance with section 61.8 permittee has submitted required repporu. Ther Colorado Dsscharge Permit Svstem Reeulations. amp[es shalt be taken at all authorized effluent of total dissolved solids indicates that the Mountain Coal SO tons/year of salinity as set forth in the Colorado Rivf ~)/A/ of the Colorado Discharge Permit vstem ego a ittee shall monitor for total dissolti points except internal Outfa11007. h. Whole Effluent Toxicity (WET) Testing the For Outfalls 004, 011, Ol Z, 013, 016, 017 at this facility, acute WET testing is required. (See Part I A of the permit.) i. Pu ose o~~WET Testing~: The Water ppuuality Control Division has established the use o WET testing as a method for i enti ing anacontrolfng toxic ducTtarges from wastewater treatment facilities. WE testing is being utilized as a means to ensure that there are no discharges of pollutanu "in amounu, concenfrations or combinations which are harmfiil to the beneficial uses or toxic to humans, animals, plants, or aquatic life" as required by Sectiors 31.11 (I) of the Basic Standards and Methoifolo¢ies for Surface Waters. ii. In-Stream Waste Concentration /IWC): Where the Division deems monitoring or limitations for WET apps cc ronic~ i uh'on~nted by the chronic IWC is critical in determining whether acute or conditions shall apply. Accordingh to the Colorado Water Quality Control Division Biomonitortne G Document dated July 1, 1993, (or t ose disc ar esc sonic is greater t an a an t e r stream as a Class I Aquatic Life us_ or Class ~ Aquatic Life use with all of the appropriate aquatic life standards, chronic conditions apply. Where the ehronu IWC u less than or equal to 9.7, or the stream is not e as described above, acute con thous apply. The chronic IN'C is determined using the following equation: IWC = [Facility Flow. (FF)/(Stream Chronic Low Flow (annual) + FF)J X 100% The flows and corresponding IWC for the appropriate discharge point are: uc arge otnt tonic ow ow, c s act :ty esign ow, c s o I an 011 0 (Minimal)** 100* -012 0 (Minimal)** 100* 013 0 (Minimal)** 100* 017 43 4.5 ! 00 * 'vecause al tneproxtmt al these two atscnar es, [hey are eonstnerea as n smgie impact io me recemng water. * 'These oatfalls na y do wt discharge. ecause the chronic low ffows are zero, a facility flow is not repaired to determine the IWC. It will always equa1700% The IWC for Outfalls 004 and OI6 u 0.5%, which represents a wastewater concentration of 0.5% effluent to 99.5% receiving stream. The IWC for putfalls 011, 012, 013, and 017 is 100%, which represents a wastewater concentration 0 100% effluent to 0% receiving stream. Since the receiving water segment to which Outfalls 011, 012, 013 and 017 discharge now has all of the appropriate aguaric life numeric standards, there is no exemption from chronic WET testing on this basis. An additional factor exists for Outfalls 011, 012, and 013 which was not addressed by the WET guidance. Because the IWC is 100% chronic WE teshn should be required. However, these Outfalls discharge very infrequently (typically less frequentfy than once a month~wtth most discharges lasting no more than three days. Thu prevenu the perminee from petformtng statistically valid anal ses for chronic WET testing. Because of this the potential impact on the receiving waters from these three Ou~lls is acute, not chronic. n this bass, it is fogica to require acute WET Zesting to assess the true otential impact on the receiving waters for Outfalls 011, 012, and 073. This determination was made previously andpis carried over in thu renewal permit. Last Revised: 4/19/1004