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COLORADO DEPARTMEM OF PUBLIC HEALTH AND ENV/RONMENT, Water Quafiry Control Division Rationale -Page ]7, Permit No. CO-OOD0221 Alternate Limitation Burden of Proof Rewirements: In conformance with 40 CFR 434.63, the permittee has the • burden of proof when requesting relief from total suspended solids (!SS), fora! iron and/or settleable solids limitations, as appropriate. The intent of ibis regulation is to afi'ord relief only when needed due to a volume of water generated by a specific storm event, it is not imended to be automatically applied to all discharges caused by precipitation, nor to be used to discharge vo[runes in excess of that generated by the storm event. On this basis, relief shall be granted only when necessary and shall wt be granted when the perminee has control over the discharge, enabling them to meet the primary limitations. The permittee should endeavor to meet the primary limitations whenever possible. All maws! dewatering of ponds that are equipped with automatic dewatering systems must meet TSS and total iron (imitatioru. If a pond has only manual dewatering capability, relief is available only to the euem required ro regain necessary stability and capacity. The Division shall have final authority in determining whether relief is granted. As pan of this determination, the Division shall evaluate whether the permittee could have controlled the discharge in such a manner that primary limitations could have been met. I) For rainfall, to waive TSS and total irors limitations, it is necessary to demonstrate that discharge occurred within 48 hours after measurable precipitation has stopped. To waive settleable solids limitations, it is necessary to prove that discharge occurred within 48 hours after precipitation greater than the 10-year, 24-hour evem has stopped. 2) For snowmelt to waive TSS and total iron limitations, it is necessary to demonstrate that discharge occurred within 48 hours after pond inflow has stopped. To waive settleable solids limitations, it is necessary to prove that discharge occurred within 48 hours after pond inflow volume greater than the 10-year, 24-hour evem has stopped. c) Salinirv Regulations: In compliance with section 6.9.2 (12) of the Colorado Discharge Permit System Regulations, the perntittee shall monitor for total dissolved solids on a quarterly basis. Samples shall be taken at the effluem discharge point(s). Salinity requirements are included in Pan l.B.l.e: of the permit. • d) Whole Effluent Tozicirv lRrEl7 Testing: Acute WET testing is required for outfalls 002, 003, 005, 006, 010 and 011; and chronic WET testing is required for outfalls 004 and 008. See following discussion for WET testing for all of these outfalls. (Also see Pans I.A.4. and I.A.S. of the permit.) 1) Purpose o(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 of pollutants "in amounls, concentrations or combinations which are harmful to the beneficial uses or toxic to humans, animals, plants, or aquatic life" as required by Section 3.1.11 (l) of the Basic Standards and Methodologies (or Surface Waters. 2) Instream Waste Concemration (IWCI: Where monitoring or limitations jot WET are deemed appropriate by the Division, chronic instream dilution as represented by the chronic IWC is critical in determining whether acute ar chronic conditions shall apply. For those discharges where the chronic IWC is greater than (>) 9.1`70, chronic conditions apply, where the /WC is less than or equal to (5) 9. / acute conditions apply. The chronic /WC is determined using the following equation: /li'C = (Facility Flow (FF)/(Stream Chronic Low Flow (anwa!) + FF)J X 100`70 Flows and Tti'C calculations for outfalls 002, 003, 005, 006, 010 and OII are irrelevant because, in ~ addition to the IWC value for determining whether chronic or acute WET testing requirements are applicable, the classification of the receiving stream must be considered. According to the Colorado Water Oualirv Control Division Biomonitoring Guidance Document, dated July 1, 1943, where fhe receiving water is classified aquatic lije, class 2 without all of the appropriate aquatic life wmeric standards, acute WET testing is required, regardless of dilution. Since the receiving waters for outfalls 002, 003, 005, 006, 010 • and 011 are classified as Class 2 Aquatic Lije use, but without a!! of the appropriate aquatic life wmeric standards, the Division has determined that acute WET resting is applicable jot these outfalls (also see jollowing paragraph). Since segmem 12 is classified this way, outfalLs 002, 003, 005, 006, O10 and OI1 are subject to acute WET testing requirements.