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COLORADO DEPARTMENT OF FIF-ACTH, Water Quality.Control Division Amendment No. 2 - Rationale -Page 2. Permit No. CO-0000591 //1. PURPOSE OF AMENDMENT By lever dated January 14, 7994, the perminu requested relief from whole efiluem tonicity (WET) testing until its aquatic impairmem study is complete. The Division is amenable to this request. The monitoring waiver will entered through Deoanber 31, 1994. At this rime, the Division is reopening all permits whose whole yBTuem taziriry (WEB requiremenu are not now based on the cwrent State regulation. This permit is one ojthose with outdated WET requirements. Therefore, the permit is being updated to include WET limitations, monitoring and other requirements based on section 6.9.2(5! of the Regulations For the State Discharge Permit System. as amended August 2, 1993, and the Colorado Water Oualiry Control Divrsion Biomonitoring Guidance Doeumem. dated July 1, 1993. For this jaciliry, ague WET testing is required. (Su Parrs I.A and LB of the permit.) aJ Puroase of WET Testing: The Water Quality Camrol Division has established the use of WET testing as a method far idenrEying and controlling tonic discharges from wastewater trtatmem facilities. WET testing is being utilized as a means to ensure that there an no discharges of pollutanu 'in amounts, concentrations or combinations which are harmful to the beneficial uses or tonic to humans, animals, planes, or aquatic life' err required by Section 3.1.11 (IJ of the Basic Standards and Mahodologies for Surface Waters. bJ Instream Waste Conemration /IWCI: Where monitoring or limitations jog WET are deemed appropriate by the Division, chronic instream dilution as represemed by the chronic IWC is criricnl in deeermining whdher acute or chronic conditions shall apply. For those discharges where the dtronic IWC is greater than (>) 9.1 %, chronic conditions apply, where the IWC is less than err equal to (5J 9.7 acute conditions apply. The chronic /WC is determined using the following equation: /R'C = [FariGry Flow (FFJ/(Stream Chronic Low Flow (annual) + FFJJ X 100% The flows and corresponding IWCjor the appropriate discharge poim are: ':::'..`; .:30E3;:.(ifs)..:!.!:!:.:,'::;:;.': j;(cjs) ..::::.:.:.:::.::..:? ::r:; 011 0.33 100 The /WC jar this permit is 100%, which represents a wastewater concentration of 100% effluent to 0% receiving stream. However, in addition to the /WC value for determining whaher chronic or acute WET testing requirements are applicable, the classficarion of the receiving stream must be considered. According to the Colorado Water Oualirv Control Division Biomonirorine Guidance Document. dated July 1, 7993, where t1u receiving water is classified aquatic life, class 2 without all of the appropriate aquatic life numeric standards, acute WET testing is required, regardless of dilution. Since Sanborn Creek is class fled as CGtss 2 Aquatic Life use, but does not have all of the appropriate aquatic GJe numeric standards, the Division has daermined that acute WET testing is applicable for this permit (also see following paragraph). cJ Acute WET Limits: Because this facility discharges tonic metals in poreruially tonic amounts, the Division believes there is reasonable potential for the disdwrge to interfere with attainmem of applicable water quality clars~cations or standards. On this basis, the acute limit has been incorporated into the permit and becomes ejfeaive July 1, 1995. Prior ro the ague WET limit becoming effeait+e, the permittu is required to conduct routine monitoring, the resuGs of which are to be reported as an LCso if the LCso 5 700% ejJluent, the permittee is required to conduct the automatic compliance scfiedule as idem fled in Part I.B. of the permit.