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<br />the atmosphere by evapotranspiration. Due to the impermeability of these <br />rocks, snow melt and rainfall infiltration account for only 3 percent of the <br />yearly precipitation. According to the U.S. Weather Bureau, the mean annual <br />precipitation that occurs at this altitude is 20 inches per year. An evaluation <br />of the catchment area above the proposed tailings pond determined that a <br />total of 1.267 acres of ground surface is immediately adjacent to the embank- <br />ment site. Based upon these parameters, the catchment area receives an <br />average of 2.12 acre feet of precipitation annually. 'IYtis amount of precipa- <br />tation over the catchment area results in O.S9 acre feet of surface runoff, 1.17 <br />acre feet of evapotranspiration, and 0.06 acre feet of infiltration. <br />Inaanmch as the catchment area above the tailings pond embankment is <br />limited in size, water runoff inflow from rainfall and melted snow ie not <br />significant. The rate of the surface water runoff ie also quite low, ae the <br />drainage area is on a gentle slope, with only a 20 foot difference in elevation <br />from the top of'the tailings embankment to the crest of the ground behind it. <br />All of the made providing access to this millsite and tailings pond area were <br />constructed many years ago. While some grading was done to improve the <br />surface mnditiona of these roads, no major relocation work will be necessary. <br />These access roads will remain in place after the cessation of milling <br />operations by C.O.M. Inc. <br />Security considerations and safety monitoring of the tailings embankment <br />were also accounted for in the selection of this site. By closing the lower access <br />mad below the tailings pond, and fencing the perimeter of the mill mmples, <br />both the property and the public are protected from one another. Since the <br />tailings pond is located near the millaite, the tailings discharge and decantation <br />lines can be easily observed and closely monitored. <br />Guidelines for the design, construction, operation, and maintenance of mill tail- <br />ings ponds have been established by the U.S. Bureau of Mines, the Colorado <br />Division of Mines, and the Colorado Water Quality Control Commission. Using <br />these guidelines, C.O.M. Inc. designed a tailings pond that was built by employ- <br />ing the upstream method of rnnetruction, operation, and maintenance of the <br />tailings retention structure. This specific tailings dam design was chosen be- <br />cause it provided structural stability, afforded adequate storage capacity, <br />utilized readily available borrow material, and allowed for the construction of <br />an embanlnnent that was built in several stages. Environmental conaidera- <br />tiona were also taken into accotmt, when this particular design was selected for <br />tailings disposition. The design principles for an upstream tailings dam are <br />compatible with the location selected for tailings impoundment, and with the <br />type of milling operation that is being conducted by C.O.M. Inc. <br />The upstream method of tailings dam rnnstruction begins with the construc- <br />tion of a low starter dike, or toe dam, that was built to contain the tailings <br />during the initial stages of deposition. Tailings are discharged off the upstream <br />face of the toe dam at regular intervals from spigots connected to a peripheral <br />discharge line. A favorable gravity separation of the tailings results from this <br />