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_ COLORADO DEPARTMENT OFPUBLICHEALTHAND ENVIRONMENT, Water Quality Control Division Rationale -Page S, Permit No. CO-0038342 Include Water Quality Standards-Based Limits in CDPS Permits Based on Reasonable Potential Procedural Guidance dated December 2002. This guidance document utilizes both quantitative and qualitative approaches to establishing reasonable potential depending on the amount of available data. Per the Procedural Guidance, a quantitative determination of reasonable potential requires a minimum often data points. There were no effluent data for total recoverable (TR) arsenic, potentially dissolved (PD) cadmium, TR trivalent chromium, PD copper, TR iron, PD lead, PD manganese, total mere~ury, PD nickel, PD selenium, PD silver, and PD zinc, and therefore monitoring will be prescribed, as discussed subsequently, prior to the determination of the need for effluent limits. 1. Antidegradation: Since the receiving water is Use Protected an antldegradation review is not required pursuant to Section 31.8(2)(6) of The Basic Standards and Methodologies for Surface Water. 2. Colorado Mixing Zone Regulations: Pursuant to section 31.10 of The Basic Standards and Methodologies for Surface Water, a mixing zone determination is required for this permitting action. The Colorado Mixing Zone Implementation Guidance, dated Apri12002, identifies theprocess 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 document provides for certain exclusions from further analysis under the regulation, based on site-specific conditions. The guidance document provides a mandatory, stepwise decisebn-making process for determining ifthe permit limits will not be affected by this regulation. Exclusion, based on Extreme Mixing Rarios, 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 of the design flow to the low flow is greater than 2:1, the permittee is eligible for an exclusion from further analysis under the regulation. 3. Salinity Regulations: In compliance with the Colorado River Salinity Standards and the Colorado Discharge Permit System Regulations. the permiuee shall monitor for total dissolved solids on a quarterly basis. Samples shall betaken at all authorized effluent discharge points. An evaluation of the discharge of total dissolved solids indicates that the Central Appalachia WWTF does not exceed the threshold of I ton/day or 350 tons/year ofsalinity. To determine TDS loadtngfrom this facility, the average reported TDS values for each quarter for the previous two years were multiplied by the average flow reported for the appropriate quarters, then by 8.39. This was done for outfal1002 (the only outfall that discharged more than one time}. Then, all quarterly TDS values, in pounds per day, were averaged. The result is: 100.5 lb/day or 0.05 tons/day. 4. Whale Effluent Toxicity (WET) Testing: For outfal1002, chronic WET testing is required. (See Part LA of the permit.) i. Purpose ofWET Testing: The Water Quality Control Division has established the use of WET resting 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 of pollutants "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 iIWC): Where monitoring or limitations for WET are deemed appropriate by the Division, chronic in-stream dilution as represented by the chronic IWC is critical in determining whether acute or chronic conditions shall apply. According to the Colorado Water puality 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 Lije use or Class 2 Aquatic Life use with all of the appropriate aquatc life numeric standards, chronic conditions apply. Where the chronic IWC is less than or equal to 9.1, or the stream is not 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 IWC for the appropriate discharge point are: Last Revised. tS/9/2004