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COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water QuaJiry Control Division
<br />Rationale -Page 10, Permit No. CO-0032115
<br />For snowmelt, to waive 7SS and rota! iron limitations, it is necessary to prove that discharge directly resulteri
<br />from snowmelt and occurred through automatic dewatering devices within 48 hours after poM inflow has
<br />stopped.
<br />In addition, to waive settleable solids limitations, it is necessary to prove that an event greater than the
<br />magnitude of the applicable 10-year, 24-hour event occurred.
<br />d) Antidegradation: Since the receiving waters are Use Protected, an antidegradation review is not required
<br />pursuant to section 3. l.8(2) (b) of The Basic StaMards and Methodologies (or Surface Water.
<br />e) Whole Efl7uent TosiciN (WE!) Testing• For outjalJs 001, 002, 005, 009, OII, 0/3, OIS, 0!6 and 017, acute
<br />WEI'testing is required. (See Parts I.A. /.(b), l.A.l.(c) and l.A.2. ojrhe permit.)
<br />1) Purpose of WET Testing: The Water Quality Control Division has established the use of WET testing as a
<br />method for identifying and controlling tonic discharges from wastewater treatment facilities. WET resting is
<br />being utilized as a means to ensure that there are no discharges ojpol[utants "in amounts, concentrations or
<br />combiwrions which are harmful to the beneficial uses or toxic to humans, animals, plants, or aquatic life"
<br />as required by Section 3.1.11 (1 J ojrhe Basic Standards and Methodologies for Surface Waters.
<br />2) Instream Waste Concentration (1WCl: Where monitoring or limitations for WEl are deemed appropriate by
<br />the Division, chronic irrstream dilution as represented by the chronic 1WC is critical in determining whether
<br />acute or chronic conditions shall apply. For those discharges where the chronic IWC is greater than {>)
<br />9.1 %, chronic conditions apply, where the JWC is less than or equal to (<_) 9.1 acute conditions apply.
<br />The chronic /WC is determined using the following equation:
<br />/WC = [Facility Flow (FF)/(Stream Chronic Low Flow (annual) + FF)] X I00%
<br />The flows and corresponding IWC for the appropriate discharge point follow.
<br />Discharge Point . Chronic Low Flow, Facility Design Flow, IWC,. (%)
<br />30E3, (cfs) (cis)
<br />002, ODS, 0 varies !00
<br />011, O13,
<br />OI6. 017
<br />The IWC for these outfatts is 100k, which reprtsems a wastewater concentration of 10090 effluent to 0%
<br />receiving stream.
<br />However, in addition to the IWC value for deternuning whether chronic or acute WET testing requirements
<br />are applicable, the classifrcatiort ojrhe receiving stream must be considered. According to the Colorado
<br />Water OualiN Control Division Biomonitoring Guidance Document. dared July 1, 1993, where the receiving
<br />water is classified aquatic life, class 1 without a!! ojrhe appropriate aquatic life numeric standards, acute
<br />WET testing is required, regardless ojdilution. Since the receiving water is classified as Class 2 Aquatic
<br />Life use, but does not have all ojrhe appropriate aquatic life numeric standards, [he Division has
<br />determined that acute WET testing is applicable for this permit (also see following paragraph).
<br />3) Acute WET Limitations: The Division has found that discharges of underground mine drainage, preparation
<br />plant water and spoils spring water from coal mines at similar sites have exhibited effluent toziciry. On this
<br />basis, the Division believes there is reasonable potential for the discharge to interfere with attainment of
<br />applicable water quality classifications or standards. Because of this condition, the acute toziciry limit has
<br />been incorporated into the permit and becomes effective immediately. Prior to the acute WET limit
<br />becoming effective, the permittee is required to conduct routine monitoring, the results of which are to 6e
<br />reported as an LCD !f the LC,~ 5 100% effluent, the perntittee is required to conduct the automatic
<br />compliance schedule as identified in Pan /.A.2. ojrhe permit.
<br />4) Outfafl 002 Monitoring Freauertcv: 77te permittee requested and was granted a reduction in frequency for
<br />outjall 002. Because all WET tests for this outjall have passed with minimal toxicity, this reduction has
<br />been carried over to this renewal permit, The monitoring frequency for this outjall is semiannual with
<br />fathead minnow as the single species for the first haljojeach year and Ceriodaohnia sp. as the single
<br />species for the second half of each year.
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