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1 <br />increased sediment loads to receiving streams. This would be caused by <br />surface disturbance which increases soil loss. Unnatural sediment loads to <br />receiving streams could be detrimental in the following ways: <br />a. Fish reproduction, growth rate, and life span could be reduced. <br />b. Stream temperature could be increased. <br />c. Stream channel capacity could be reduced. <br />d. Drainage structures and downstream irrigation systems could be clogged. <br />e. Alluvial water tables could be raised. <br />f. Floodplains could be altered. <br />g. Sediment may carry toxic substances that could change water quality. <br />To minimize the potential impacts of sediment loads at this mine, the operator <br />constructed a sediment control system. This system treats all mine discharges <br />and surface disturbed runoff so that total suspended solids and/or settleable <br />solids are reduced to levels of 70 mg/1 and 0.5 mg/1, respectively. <br />The sediment control system was augmented in 1988 when iron-rich seepage <br />from the No. 1 Mine Portal and airshaft began. At that time, two treatment <br />ponds with a combined capacity of 175,000 gallons and afour-day detention <br />time were installed. Water in the treatment ponds was initially treated with <br />sodium hydroxide to enhance iron precipitation before discharging to North <br />Thompson Creek. However, in 1489, an additional treatment pond, referred <br />to as the long pond, was constructed to increase the residence time of the <br />seepage water so that it could be discharged without chemical treatment. The <br />water running into the long pond from the mine water discharges of Mine No. <br />1 and Mine No. 3 has an iron content of about 100 ppm, whereas the water <br />discharging into North Thompson Creek has an iron content of less than 3.6 <br />ppm, which is the maximum allowed by the NPDES permit. <br />Existing surface water monitoring data taken at the mine over the past five <br />years indicates that suspended sediment levels downstream of the mine are <br />equal to or lower than levels at the upstream monitoring site when upstream <br />levels are high. When upstream levels are very low, downstream levels, <br />although slightly higher, are still very low. This shows, in part, that the <br />sediment control system is functioning. <br />The sediment ponds themselves have the potential to affect water quantity. <br />Ponds may impound water that otherwise would be available to the receiving <br />stream. The total amount of water to be impounded at the mine area is only <br />5.6 acre-feet. This amount is 0.0005 percent of the total flow in Thompson <br />Creek (12,100 acre-feet/year). However, the Division still requires the <br />operator to dewater the ponds within 72 hours as a mitigation measure to <br />water rights holders downstream. <br />is <br />