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COLORADO DEPARTMENT OFPUBL/C HEALTHAND ENVIRONMENT, Water Quality Control Division Rationale -Page 6, Permit No. CO-0000132 Outfa11018: Previous evaluations have determined that there is no reasonable potential for a conventional water treatment plant backwash discharge to contribute to an exceedence of any water quality standard pollutant other than total residual chlorine. On this basis, a further reasonable potential analysts for outfa11018 rs not necessary. e. Antidegradation: Since the receiving water is Use Protected an antidegradation review is not required pursuant to Section 31.8(2)(6) ofThe Basic Standards and Methodologies for Surface Water. f. Colorado Mixing Zone Regulations: Pursuant to section 31.10 of The Basic Standards and Methodologies far Surface Water. a mixing zone determination is required for this permitting action. The Colorado Mixing Zone Implementation Guidance, dated April 2002, identifies the process for determining the meaningful limit on the area impacted by a discharge to surface water where standards may be exceeded (i.e., regulatory mixing zone). This guidance documentprovidesfot certain exclusions from further analysis under the regulation, based onsite-speck conditions. The guidance document provides a mandatory, stepwise decision-making process for determining ifthe permit limits will not be affected by this regulation. Exclusion, based on Extreme Mixing Ratios, may be granted if the ratio of the design flow to the chronic low flow (30E3) is greater than 2:1 or if the ratio of the chronic low flow to the design flow is greater than 20:1. Since the ratio ojthe design flow to the low flow is greater than 2: 1, the permihee is eligible for an exclusion from further analysis under the regulation. g. Saliniri Regulattons.• In compliance with the Colorado River Salinity Standards and the Colorado Discharge Permit System Regulations, the permihee shall monitor for total drssolved solids on a rLuarterly basis. Samples shall be taken at all authorized efJluenJ discharge points. h. Whole Eflluenr Toxicity /WETI Testing: For outfall 01 S, acute WET testing is required. (See Part I.A of the permit.) i. Puraose of WET Testing: The Water Quality Control Division has established the use of WET testing as a method for identifying and controlling toxic discharges from wastewater treatment facilities. WET testing is being utilized as a means to ensure that there are no discharges afpollutants "in amounts, concentrations or combinations which are harmful to the beneficial uses or toxic to humans, animals, plants, or aquatic life" as required by Section 31.11 (I) of the Basic Standards and Methodologies for Surface Waters. ii. In-Stream Waste Concentration /lWCI: Where monitoring or limitations for WET are deemed appropriate by the Division, chronic tn-stream dilution as represented by the chronic IWC is critical in determining whether acute or chronic conditions shall apply. According to the Colorado Water Oualitv Control Division Biomonitoring Guidance Document. dated July 1, 1993, for those discharges where the chronic IWC is greater than 9.1 % and the receiving stream has a Class 1 Aquatic Life use or Class 2 Aquatic Life use with all of the appropriate aquatic life numeric standards, chronic conditions apply. Where the chronic IWC is less than or equal to 9.1, or the stream is nor classified as described above, acute conditions apply. The chronic IWC is determined using the following equation: IWC = [Facility Flow (FF)/(Stream Chronic Low Flow (annual) + FF)J X 100% The flows and corresponding IWCfor the appropriate discharge point are: Chronic Low Flow, Facility Design Flow, o Discharge Point 30E3, (cjs) (cfs) ~C' (~) OJS 0 unknown 100% The IWCfor this permit is 100%, which represents a wastewater concentration of 100% effluent to 0% receiving stream. However, if discharge occurs, it is expected to be intermittent and very short term (typically, for less than one day), as was the discharge from previous outfall 011, which originated from the same general source. Thus, the actual impact of such discharge would be acute. Additionally, because any discharge that occurs is not expected to last for three days, it would not be possible to collect the three composite samples required for the chronic WET test. Thus, acute WET testing will be required for outfa11015. If in the future, discharge occurs chronically, the WET criteria may be reevaluated and chronic WET testing may be imposed. Such a permit requirement change would be subject to public notice provisions. The permtttee will be required to submit dacunrcntation whenever discharge that is subject to WET testing occurs regarding the durntion of the actual discharge event. Last Revised: 3/19/2004