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1 <br />' ARCADISGERAGHTY&MILLER <br />1 <br />Technical Support <br />Document for the <br />CDPS Permit <br />Application <br />' 26) Discharge quality: Discharge from the permitted industrial wastewater treatment unit will meet limits <br />as described by Colorado Water Quality Control Commission Regulation Number 36, Classification <br />and Numeric Standards for the Rio Grande Basin. These water quality standards are provided in Table <br />I. Specific Table Value Standards (TVS) are calculated for Rio Grande Basin stream segment 28, <br />which include the mainstem of the Rito Seco. TVS calculations require input of hardness data for the <br />water at the point of compliance. Water quality standards were evaluated for RS-2 under pre-mining <br />' conditions. Hardness data collected from 1987 up to (but not including) 1990, when the mine opened, <br />are used in the calculations to reflect pre-mine conditions downgradient of the mine deposit. The <br />average hardness value calculated is 63.2 mg/L with a standard deviation of 9.5. This results in a 95 <br />percent confidence interval range of 57 to 69.4 mg/L. In accordance with the regulation, the lower 95 <br />percent confidence interval hardness value is used in the TVS calculation (57 mg/L). <br />' Discharge limits were then calculated under low Flow conditions (average of January and February <br />1999 Flows) using the low flow analytical data collected during January 1999 at station RS-3A. <br />Hardness values were used as calculated from the calcium and magnesium data (the single sampling <br />' event does not allow for the statistical evaluation of the confidence limit). The discharge limits were <br />calculated with consideration for dilution from the receiving stream in accordance with the following <br />calculation: <br />Q I C I (Rito Seco) + Q2C2 (Effluent) =Q3C3 (Water Quality Standard) <br />Where: Q =Flow in liters per second <br />C =Concentration in ug/I or mg/I <br /> <br /> <br />1 <br /> <br />1 <br /> <br /> <br />Q I C 1 represents the water quality and loading at a location upstream of the effluent discharge in the <br />Rito Seco. Q3C3 represents the water quality and loading of the combined Flow in the stream <br />downstream of the effluent discharge. The discharge Flow rate is estimated to be 400 gallons per <br />minute (25.2 liter/sec) under initial West Pit dewa[ering conditions. The equation is solved for C2 <br />which is the allowable discharge limit. Table 2 presents the effluent limits for a discharge above RS- <br />3A. In several instances, the discharge limit is defaulted to the in-stream water quality standard for <br />Segment 28 where the calculated standard is negative due [o the inherent difference between the <br />standard and the method detection level. <br />The water quality of the influent wastestream is provided in Tables 3, 4 and 5. These tables represent <br />the measured water quality for wells in the West Pit backfill area (BF-3, BF-4,and BF-5), the seeps, and <br />the curtain wells. The backfill wells will comprise the major portion of the waste stream (more than <br />75%). Based on these discharge limits, we anticipate that a mixing zone, as contemplated in Regulation <br />31 may be developed during the preparation of the permit terms and conditions. <br />27) Dioxin Testing: Not applicable, no dioxins or TCDD were ever used at the site <br />unit, not yet operational. <br />28) Whole Effluent Toxicity testing: No data are available because the nanofiltration system is a new <br />8/ I 0 <br /> <br />