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r~ <br />~~ <br /> <br /> <br />Typically, most of the runoff occurs in the Spring as a result of <br />snowmelt, or in the Summer due to high intensity connective storms. Due to <br />the fact that no flow records are available upstream from the ore sorter, <br />estimates of peak rates of runoff for the 100 year-6 hour storm were made <br />using empirically derived techniques. (King and Wilkes, 1977) Estimates of <br />peak flood discharges required consideration of such factors as amo»nt and <br />duration of rainfall, size and shape of watershed, soil conditions, and <br />vegetation. <br />Cotter has concluded that calculations for a probable maximum flood <br />are unnecessary for the following reasons: <br />1. Approximately 45-90% of waste is sorter reject, containing less <br /> <br />than 0.015% U. The remaining 50-55% of the waste stockpile consist<., of barren <br />mine development rock. <br />2. Since the surface area of the waste rock is small, tests show, <br /> <br />leaching due to precipitation and/or runoff is minimal. Water quality samples <br />collected upstream from the settling pond discharge and downstream from the <br />waste dump verify this. (See Table 2.2-2) <br />3. The approximate area of the Ralston Creek watershed, including the <br />area downstream from the ore sorter, is 46 square miles, or 29,440 acres. (The <br />additional 8 square miles was added to account for the effects of dilution <br />from the entire watershed.) The area of the waste dump is approximately 3 <br />acres. This would create an effective dilution of 104 times for Ra-226 concen- <br />trations in Ralston Creek that would result from a large (e.g., prob+rble <br />maximum) storm. <br />2-5 <br />