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~~ <br />~~ ! • III IIIIIIIIIIIII III <br />sss <br />STATE OF COLORADO <br />DIVISION OF MINERALS AND GEOLOGY <br />Department of Natural Resources <br />1313 Sherman Sl., Room 215 <br />Denver, Colorado 80203 <br />Phone: (3031 866-3567 <br />FAX: (3031832-8106 <br />DATE: May 22, 1997 <br />TO: Bruce Humphries, Berhan Keffelew, Harry Posey <br />FROM: Jim Stevens ~L~~ <br />II~~ <br />DEPARTMENT OF <br />NATURAL <br />RESOURCES <br />Roy Romer <br />Governor <br />lames 5. Lochhead <br />E~ecmive Director <br />Michael B. Long <br />Division Direclar <br />RE: Possible improvements in forthcoming hydrologic <br />modeling of Cresson Project <br />After review of the two HCI studies provided by CC&V to demonstrate <br />the probable lack of significant impact by the mine on the quality <br />and quantity of the discharge from the Carlton Tunnel, I believe <br />the future modeling proposed for the operation would be more <br />convincing if the following improvements in approach were made: <br />1) Utilize a more accurate estimate of the area of the caldera <br />drained by the Carlton Tunnel. <br />The previous studies assumed the entire caldera was drained by <br />the Carlton Tunnel and then assigned a proportion of its <br />discharge to the mine based on the relative area of the mine. <br />That may have resulted in a significant underestimate of the <br />relative contribution of recharge thru the mine to the Carlton <br />Tunnel discharge and less than conservative modeling of the <br />possible effects of that recharge. <br />2) Utilize a more accurate estimate of the volume of the water <br />infiltrating thru the mine. <br />The previous studies based their estimates of the contribution <br />of the mine to the Carlton Tunnel discharge on the area of the <br />mine itself without considering the adjacent areas which drain <br />into the mine and contribute in varying degrees to the total <br />groundwater recharge occurring thru the mine. <br />3) Utilize a range of precipitation values, not simply an <br />average, as input for the modeling. <br />The two HCI studies made use of two different values of <br />average precipitation to be expected at the site; neither <br />value was determined to result in acid discharge from the <br />Carlton Tunnel. Use of average precipitation values, however, <br />does not represent the potential of the operation to impact <br />the Carlton Tunnel discharge as well as would the use of a <br />