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COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division Rationale, Page 3 Permit No. CO-0026981 AMENDMENT NO 1 (Continued): arsenic (0.05 mg/1), chromium (0.1 mg/1 - trivalent, sad 0.025 mg/1 - hexavalent), iron, (total, 1.0 mg/1), manganese, (total, 1.0 mg/1), nickel (0.05 mg/1), selenium (0.02 mg/1), dissolved oxygen (6.0/7.0 mg/1), pH (6.5 - 9.0 s.u.), fecal coliform bacteria (2000 per 100 ml), ammonia (unionized, 0.02 mgfl), residual chlorine (0.003 mg/1), cyanide (free, 0.005 mg/1), sulfide (undissociated, 0.002 mg/1 as S), boron (D.75 mg/1), and nitrite (0.05 mg/1). In determining the changes in this permit's effluent limitations for cadmium, copper, lead, mercury, and zinc, two other revisions were also made during the time of the water quality standards redeterminations. New seasonal design (Q7-10) low flows for the Canyon Creek above the Camp Bird Mine discharge were determined. The effluent flow which is projected to occur for each season under Q7_10 low flow stream conditions was also determined. These two flow values for each season are summarized below. Table 2: Canyon Creek Low Flows and Facility Flows Season: Nov. - April May - July Aug. - October Stream Q7-10 Low Flow, cfs 2.7 6.3 3.9 Discharge Design Flow, cfs 1.5 1.3 3.5 The revised Q7-10 seasonal low flows for Canyon Creek, which are based upon the use of gauging station data for the Uncompahgre River near Ridgeway and also the watershed area ratios for the Canyon Creek area above the discharge and for the area including the Uncompahgre River at the gaging station, were determined by the Division and a consultant far Chipeta Mining Corp. Since the facility discharge design flow and the Q7_lD stream low flow were determined and agreed upon previously by the Division and the permittee, the new design stream Low flows (chronic Q3E30 flow for determining 30-day average limitations, and acute Q1E3 flow for determining the daily maximum limitations) were not used for this amendment. However, these new design stream low flows will be used to evaluate limitations for the next renewal permit. The facility discharge design flow represents an estimate of the facility flow which would occur under stream Q7_10 low flow conditions. This relationship is based upon a correlation that was found between facility discharge flows and the downstream flows. According to a consultant's report for the permittee (referenced following in the ambient data discussion), "the volume of flow discharged from the mine is directly related to the snow pack with the peak discharge of the mine lagging peak flows in the stream by about two months. Because of this phenomena, peak discharges will never occur during minimum stream flows and will always occur on the descending curve of the hydrograph. In addition, the mine discharge will be directly related to the amount of snow pack and precipitation in the basin which also established the flow in Canyon Creek."