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COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Divisiors Rationale -Page 6, Permit No. CO-0044776 Where: Qr = Upstream low flow (30E3) from Partlll.8.2 (36 cfs) Qz =Average daily effluent flow (mine drainage design capacity - 0.144 MGD or 0.22 cfs) Q, =Combined downstream flow (Q, + Q~ Mr = Upstream background pollutant concentration from /II.B.1 MZ = Unknown; Maximum allowable effluent pollutant concentration calculated using mass balance equation M~ =Maximum downstream allowable pollutant concentration (stream standard) A summary of the mass balance calculations for total recoverable iron is s/town in Table Vl-3. No other pollutants regulated by water quality standards are known or believed to ezisr in the [nine drainage discharge. !f new information to the contrary becomes available at a future date, the permit may be reopened to add appropriate limitations and/or other conditions, subject to public notice provisions. Table VI-3 -- Surnmarv of Water Oua[ity Based Mass Balance Calculations Upstream WQS-Based Ambient Mean Stream Standard, Effluent Concerstratian, Parameter Concentration Chronic Chronic (mg/Aj (MI) (M3) (hf2) Total Recoverable Iron 1.7 1.0 (negative)* * Because the result is a negative number, the Basic Standards artd Methodologies for Surface Water (5 CCR 1002-31) state that tl:e limitation shall be specified as equal to the standard. As stated above, this evaluation was performed to determine the applicable limitation for mine drainage discharge to the Nortlt Fork of the Gunnison River. However, the discharge goes directly !o Deer Trail Ditch, an irrigation conveyance rltat is privately owned and operated and is not stare waters. The ditch meets the North Fork of the Gunnison River at a point approximately one mile below the point of the permittee's discharges. Since the entire flow front the mine drainage discharge is not expected to reach the river undiluted by other flows during low flow periods, this [imitation will not be applied. 71te federal effluent guideline-based limitation for total iron of 3.0 mg/P will 6e applied. According to reference doctements available to rite Division, this limitatiar: will protect all potential agricultural uses for which Deer Trail Ditclt water may be consumed. /t is expected that any remaining flow that rnigitt eventually reach the river would not impact the standard for [he river. d) Antidegradation: Since the receiving water is unclassified, an antidegradation review is not required pursuant to section 37.8(1) of The Basic Standards and Methodologies for Surface Water. e) Salinity Regulations: /n compliance with the Colorado River Salinity Standards and tl:e Colorado Discharee Permit Svstem Regulations the permittee shall monitor for total dissolved solids oa a quarterly basis. Samples shall be taken at the effluent disc/tnrge paints. Snlittity requirements are included in Part /.B.2. of the perntit. f) Whole E(lluent Toxicity (WET) Testing: For outfall 002, acute WET testing is required. (See Part I.A. of the permit.). TJtis applies only to discharges from this otttfall that include a mine drainage component. 1) Purpose of WET Testing: 77te lYater Quality Control Division has established the use of WET testing as a method for identifying ar:d controlling toxic discharges front wastewater treatment facilities. WET resting is being utilized as a means to ensure that there are rto discharges of pollutants "in amounts, concentrations or combinatioru whit!: are harmful to the beneficial uses or toxic to humans, animals, plants, or aquatic life" as required by Section 31.11 of the Basic Standards and Metltodoloeies for Surface Waters.