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COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division Rationale -Page 7, Permit No. CO-0043648 Since the mean hardness at both of the stations for which data was summarized (AG 1 and AG 1.5) is greater than 400 mg/l, which is the highest value allowed for use in metals standards calculations, the value of 400 mg/l will be used to do the calculations. The resulting standards will then apply at all locations along Arequa Gulch. 71te following table summarizes the metals standards for all uses. Table ///-1 Metals Standards Summary -Arequa Gulch, Segmetu 21 of the Upper Arkansas Sub-basin of the Arkansas River Basin, AU Concenttntians Are ug/I Parameter 'Aquatic Ljfe Use.. .. Agrieu(tuml,.;Ilse:-:.: . . ,(1)usolvtd `Method?) (To(al'RecavenoL'k, 7Ktlhudf: . Acute Chronic Chronic .:. Arsenic 50 50 - Cadmium 19 3.4 10 Chromium /// SO SO - Chromium V/ 16 l1 100 Copper 65 39 200 Iron - 1000* - Lend 899 28 100 Manganese - l000* - Mercury ~ - 0.01 * - Nickel 2645 274 200 Selenium 135 17 20 Silver 22 - - Zinc 379 343 2000 Aquatic Life Use standards are based upon the dissolved method of analysis, except for iron and manganese standards, which are based upon the total recoverable method, and the mercury standard, which is based upon the torn/ method. Allowable lnsrream Total Ammonia Due to the fact that there will be no significant sources of anvnonia associated with the active mining operation, it will not be a pollutant of concern umil reclamation of the leach heap is begun, or it is determined that cyanide removal is required for any unanticipated discharges. At that time, however, ammonia will be created as a by-product of the chemical reactions that will be used in the destruction of cyanide. Since the leach heap liners will be breached as part of the final reclamation of the site, it will then be possible for any anvtwnia that retwins within the heap to be leached into the underdrain system, and then enter Arequa Gulch. And, even before the liner is breached, it is possible that ammonia may tench Arequa Gulch through leaks or spilLr. Since arrurwnin limits will not be required prior to the next permit renewal, it is not necessary to discuss arnrnonia stream standards in great detail in this rationale. However it should be noted that the Division standnrdly uses the Colorado Ammonia Model in order to evaluate ammonia ejJiuent limitation. This nwdel requires 3 to 4 years worth of weekly instream pH and temperature measurements. While it is possible that the next permit will be written early enough to include provisions for the collection of this amount ojdata prior to the need for ammonia limits, the permittee should independently track the progress of the mining project and prepare for the collection of pH and temperature data approximately 4 to 5 years before the data will be needed. At that time, the Division should be consulted with regard to the specific monitoring procedures that should be followed.