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COLORADO DEPARTMENT OFPUBL/C HEALTNAND ENVIRONMENT, Water Quality Contra[Division Rationale-Page 6, Permit No. CO-0017!46 Utilizing the assimilative capacities contained irs Appendix A, an analysis must be performed to determine whether to include the calculated WQBELs in the permit. The guidelines for performing a reasonable potential analysis are outlined in the Division's document, Determination of the Requirement ro Include Water Oualiri Standards-Based Limits in CDPS Permits Based ors Reasonable Potential Procedural Guidance dated December 2002. This guidance documen[ 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 reasonablepotential requires a minimum often data points. For the Snowcap WWTF, ten data points were not available. Therefore, a qualitative evaluation of the data was conducted. The data indicate that for theparameters with the highest effluent concentrations, panc~ularly zinc and lead, the WQBELs were 6 to 8 times less that of the maximum effluent concentrations. Because a quantitative analysis should not be conducted for smaller data sets, and because data indicate that the effluent quality is 6 to 8 times less than the applicable water quality-based effluent limits, a qualitative decision has been made to not impose the water quality-based effluent limits for these pollutants at this time. e. Antide~radation: According to the Classi cations and Numeric Standards for Lower Colorado River Basin, stream segment COLCLC02 is Undestgnated. Thus, an antidegradatian review may be conducted for this segment if new or increased impacu are found to occur. However, the ratio of the flow of the Colorado River to the Snowcap WWTF design flow is 258.•1 at low J/ows. Sectton 31.8 (3)(c) specifies that the discharge of pollutants should not be considered to result in signhcant degradation of the reviewable waters if the flow rate is greater than 100:1 dilution at low flow. Thus, condition 31.8(3)(c) of the regulations is met and no further antidegradation evaluation is necessary. f. Colorado Mixing Zone Regulations.• Pursuant to section 31./0 of The Basic Standards arsd Methodologies for Surface Water, a miring zone determination is required for this permitting action. The Colorado Mixing Zone Imolementation Guidance, dated April 2002, idenhfies 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 document provides far certain exclusions from further analysis under the regulation, based on site-specific conditions. The guidance document provides a mandatory, stepwise decision-making process for determining [f the penrtit limits will nor 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 of the chronic low flow to the design flow is 258:1, the permittee is eligible for an exclusion from further analysis under the regulation. g. Saliniri Regulations: In compliance with the Colorado River Saliniri Standards and the Colorado Discharge Permit Svstem Regulations. the permittee shall monitor for total dissolved solids on a quarterly basis. Samples shall betaken at effluent discharge potnts. In accordance with section 61.8(2)(1)(i)(A) of the Colorado Discharge Permit Svstem Regulations, the permittee submitted a report on February 26, 1993, documenting infeasibility of treating jar salinity. The Salinity Regulations allow for the waiver ojTDS limitations upon submittal of such a report that demonstrates that achievement ojzero salt loading or, in the event that is not achievable, discharge of less than one tors per day, is rsot economically feasible. Therefore, limits will not be placed on this facility; only monitoring will be required at each outfall. h. Whole Elliuent ToziciN (WETS Testing: For Outfalls 001, 002 and 016, acute WET testing is required. (See Pan I.A of the permit.) i. Puroose of WET Testing: The Water Quality Control Division has established the use of WET testing as a method for idenhfytng and controlling toxic discharges from wastewater treatment facilities. WET testing it being utilized as a means to ensure that there are no discharges of pollutants "[n amounts, concentrations or combinations which are harmful to the beneficial uses or toxic to humans, animals, plants, or aquatic life" as required by Section 31.1 / (1) of the Basic Standards and Methodologies for Surface Waters. ii. In-Stream Waste Concentration fIWCI: Where monitoring or limitations for WET are deemed appropriate 6y 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 Oualiri Control Division Biomonitoring Guidance Document, dated July 1, 1993, for those discharges where the chronic IWC is greater than 9. I % and the receiving stream has a Class 1 Aquatic Life use or Class 2 Aquatic Life use with all of the nppropriate aquatic life numeric standards, chronic conditions apply. Where the chronic IWC is less than or Last Revised: t(73noos