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COLORADO.DEPARTMENT OF HEA~, Water Quality Control Division • Amenrlmeiu No. 2 -Rationale -Page 1. Permit No. CO-0000591 ~II. PURPOSE OF AMEND~NENT By letter dated January 14, 1994, the permiaee requested relief from whole effluem toxicity (WET) testing until its aquatic impairmem study is complete. The Divrsion is amenable to this request. The monitoring waiver will extend through December 31, 1994. At this trine, the Division is reopening all permits whose whole efjiuem rosiciry (WET) requiremems are not now based on the current Stott regulation. This permit is one of those with outdated WET requrremems. Therefore, the permit u being updated to include WET limitations, monitoring and other requirements based on section 6.9.1(5) of the Regulations For the State Discharge Permit System, as amended August 2, 1993, and the Colorado Water Ounlirv Control Division Bromonitorine Guidance Document, dated July 1, /993. For thrs facility, acute WET testing rs required (See Parts I.A and !.B of the permit.) aJ Purvose oFWET Testine: The Water Quality Control Division hat established the use of WET testing as a method for identifying and controlling toxic discharges from wastewater treatment facilities. WET testing is being utilr~ed as a meant to ensure that !here are no discharges ojpollutants 'in amounts, concentrations ar rnmbirwtions which are harmful to the beneficial uses or rosic to humans, animals, plains, or aquatic life' as required by Section 3.1.11 (IJ of the Basic Standards artd Methodologies for Surface Waters. bJ Instrennt Waste Concentration f/WCI: Where monitoring or limitations for WET are deemed appropriate by the Division, chronic inseream dilution as represented by the chronic IWC is critical in determining whether acute or chronic conditions shall apply. For those dscharget where the chronic IWC it greater than (> J 9.1 %, chronic conditions apply, where the IWC is less than ar equal to (SJ 9.I acute conditions apply. The chronic IWC is determined using the following equation: • lWC =[Facility Flow (FFJ/(Stream Chronic Low Flow (annual) + FF.JJ X 100% The flows and corresponding IWCfor the appropriate discharge poir[t are: Disehtirge Point.:;, :: Chranir:Low Flow;:.::. Facility: Design Flow; .' 'IWC, (`,6) :::::.; ; ;.. . 01! 0.33 ]00 the IWCjor this permit is ]00%, which represents a wastewater concenrrarian of 100% ejJIuer[t to 0% receiving stream. However, in addition to the IWC value for determining whether chronic or acute WET testing requirements are applicable, the classification of the receiving strewn must Ge considered. According [a the Colorado Wnrer Ounlirv Cortrral Division Biomonirorin¢ Guidance Document, dated July 1, 1993, where rite receiving water it classed aquatic life, class 2 without al! of the appropriate aquatic life numeric standards, acute WET resting is required, regardless of dilution. Since Sanborn Creek is classed as Class 2 Aquatic Lije use, but does not have all of the appropriate aquatic Lije numeric standards, the Division has determined chat acute WET testing is applicable far tl+is permit (also see following paragraph). cJ Acute WET Limits: Because this facility discharges tonic metals in potentially toxic amowtts, the Division believes there is reasonable potential jar the discharge to interfere with attainment of applicable water quality class cations or standards. On this basis, the acute limit has been incorporated into the permit and 6eeanes . effective July 1, 1995. Prior to the acute WET limit becoming effective, the permirtee is required ra conduct routine monitoring, the resuhs of which are ro be reported as an LCso !f the LCsa S !00% effluent, the permittee is required to conduct the automturc compliance schedule ar identified in Part I.B. of the permit.