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.OLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, W¢fer Q¢nlity Control Division
<br />4atioaate-Page 6, Permit No. CO-0038776
<br />e. An~odegad~ati~on: Since the North Fork of the Gunnison River (Stream segment COGUNF02) is Undesignated, an
<br />anti ed gradation review is applicable pursuant to Section 31.8 of The Basic Standards and Methodologies for Surface
<br />Water. However, the ratio of the flow of the North Fork of the nnison fiver tot a ountain oC a ear oa
<br />TpYVTFs combined deign flows (see Appendix A) is more than 150:1 at low flows. Section 31.8 (3)(c) specifies that the
<br />discharge of pollutants should not be considered to result in significant deggrradation of the reviewable waters tf the flow
<br />rate is greater than 100:1 dilution at low flow. Thus, condition 31.8(3)(c) of the regulations is met and no further
<br />anSdegradarion evaluation is necessary for dzscharges to the North Fork off thhe Gunnison rver.
<br />Stream segment COGUNF06 is designated as Use Protected. Because the applicable receiving waters are designated as
<br />Use Protected no antidegradatian review u necessary in accordance with the regulotions. Thus, anodegradation review
<br />requirements (cave been met for the discharges to Sylvester Gulch, West Sylvester Grelch, the Dry Fork of Minnesota Creek
<br />and Lone Pine Gulch.
<br />f. Colorado Mixin Zone Re lah'ons: Pursuant to section 31,10 of The Basic Standards and Methodo~lo ie~s for Surface
<br />ater a mixing zone eterminauon is required for this permitting action. ~ o ora o zz:n onZ e Im ementat:on
<br />Wince dated April 2002, identifies the process for determining the meanin imit on t e area impacte y a
<br />rs~ to surface water where standards may be exceeded (i. e., regulatory mzxing zone . This guidance document
<br />provides for certain exclusions from further analysis under the regulation, based on site-specific conditions.
<br />The guidance document provides a mandatory, stepwise decision-making process for determining z the permit limits will
<br />not be affected by this re lotion. Exclusion, based on Extreme Mixin Ratios, may be anted f the ratio ojthe design
<br />flow to the chronu low~w 30E3)) isgr eater than 2:1 or if the ratio o~the chronic low~w to t e design flow zs gneater
<br />than 20:1. Since the ratio of the lowllow to the design flow is more than 150:1 for discharges to the North Fork of the
<br />Gunnison River (stream segment CO(GiUNF02), the permtttee is eligible for an exclusion from further analysis under the
<br />regulation. Addztionally, since the ratio of the design flow to the low h~ow is greater than 2:1 for discharges to tributaries
<br />to the North Fork of the Gunnison River (stream segment COGUNF'06), the permtttee is elzgible for an exclusion from
<br />further analysis under the regulation. On this basis, all outfalls under the permit are eligible for exclusion.
<br />g. Salim Re lotions: An evaluation of the discharge o~total dissolved solids indicates that the Mountain
<br />est fine exceeds the threshold of 1 ton/da or .ISO tons/year of salinity as set forth in the Colorac
<br />Standards. In conformance with section 61.8~)(!)(i)(A) of the Colorado Duchar a Permit sS tem ,
<br />permtttee has submitted required reyorts. Thereffore, m compliance wit t e o ora o rver a ini t.
<br />Colorado Dtschar a Permit S stem Re lotions the permtttee shall monitor for tota isso ve so i on a
<br />amp es s a eta en at a out orize a uent discharge points except internal Outfa11007.
<br />h. Whole Ef/luent Toxicity /WET) Testing
<br />For Outfalls 004, 011, 012, 013, 016, 017 at this facility, acute WET telling is required. (See Part IA of the permit.)
<br />i. Pur ose o WET Testin :The Water Quality Control Division has established the use of WET testing as a method for
<br />z enti ing an contro ing toxic dischharges from wastewater treatment facilities. WE testing is being utilized as a
<br />means to ensure that there are no discharges ojpollutants "in amounu, concentrations or combinations which are
<br />harmful to the beneficial uses or toxic to humans, animals, plants, or aquatic life" as required by Section 31.11 (1) of
<br />the Basic Standards and Methodologies for Surface Waters.
<br />ii. In-Stream Waste Concentration (IWC): Where the Division deems monitoring or limitations for WET ap
<br />comic in-stream i uSon as represented by the chronic IWC is critical in determining whether acute i
<br />conditions shall apply. According to the Colorado Water pua[ity Control Division Biomonitoring
<br />Document dated July 1, 1993, jog those disc angel w ere t e c sonic is greater t an o an t e
<br />stre~ a Class I ,4guatic Life use or Class Z Aquatic L~e use with all of the appropriate aquafrc lif
<br />standards, chronic conduions apply. Where the chronic IWC is less than or equal to 9.7, or the stream is not
<br />as described above, acute con znons apply. The chronic IWC is determined using the following equation:
<br />]A'C = [Facility Flow (FF)/(Stream Chronic Low Flow (annual) + FF)J X 100%
<br />The flows and corresponding IWCfor the appropriate discharge point are:
<br />uc urge aint rontc ow ow, c s act zty eszgn ow, c s o
<br />an
<br />011 0 (Minimal)** 100*
<br />OIZ 0 (Minimal)** 100*
<br />013 0 (Minimal)** 100*
<br />017 43 4.5 100*
<br />`Because of the proximi of inese two aucnarges, tney are connaerea as a sirsgte impact io the receiving water.
<br />•*These outfalLr no y do not discharge because the chronic low flows are zero, a facility flow it not required to determine the fit'C. It will
<br />always egua1100%
<br />The IWC for Outfalls 004 and 016 is 0.5%, which represents a wastewater concentration of 0.5% effluent to 99.5%
<br />receiving stream. The IWCfor Outfalls 011, 011, 013, and 017 is 100% which represents a wastewater concentration
<br />0(100% effluent to 0% receiving stream. Since the receiving water segment to which Outfalls 011, 012, 013 and 017
<br />discharge now has all of the appropnate aquatic life numeric standards, there is no exemption from chronic WET
<br />testing on this basis.
<br />An addifional factor exists for Outfalls O11, 012, and 013 which was not addressed by the WET guidance. Because the
<br />IWC is 100% chronic WET testin should be required. However, these Outfalls discharge very znfrequenily (typically
<br />less frequently than once a month with most discharges lasting no more than three days. Thrs prevents the permtttee
<br />from performing statistically valid analyses for chronic WET resting. Because of this, the potential impact an the
<br />receiving waters from these three Ou alts is acute, not chronic. n this basis, it u logical to require acute WET
<br />testing to assess the truep otential impact on the receiving waters for Outfal[s 011, 012, and 013. This determination
<br />was made previously and is carried over in This renewal permit.
<br />Last Revised: 4/192004
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