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the proposed American Tunnet bulkhead site) is 146 days. Assuming a constant <br />inflow rate of 1230 gpm (the American Tunnel flow rate plus slightly more than the <br />historical average flow from the Terry Tunnel) yields 1 10 days. In actuality, the rate <br />of inflow will decrease as the water level rises in the flooding workings. In a more <br />conservative case, the time required to completely flood the mine workings would be <br />slightly over two years if the average inflow rate is half of the present American <br />Tunnel flow rate, and that the equilibrium water level is actually at the surface in <br />Sunnyside Basin. <br />6.2 Estimate Using a Ground-Water F/ow Mode/ <br />The estimated schedules of the flooding of mine workings discussed in section 6.1 <br />do not consider changes in inflow rate with time, nor porosity of the fractured <br />volcanic rocks. To overcome these simplifications anumerical ground-water flow <br />model was developed. The U.S. Geological Survey MODFLOW code was employed <br />to create a simple model of the mine drainage via the American Tunnel. <br />The model employed a number of simplifying assumptions including <br />initial hydraulic head prior to mine dewatering is constant at an elevation of <br />11,370 feet msl, <br />hydraulic conductivity is constant at 0.15 ft/day (the geometric mean of the <br />results of flow testing in the American Tunnel}, <br />the fractured volcanic rocks are only permeable down to an elevation of 9668 <br />feet msl (1000 feet below the American Tunnel Level), <br />the storativity is constant at 0.01, <br />there is no recharge from above or below, and <br />all inflow to the mine is from constant head boundaries at a distance of over <br />four miles from the mine workings. <br />sunnylwp\treci\2251 \Mar1993.flpt 36 <br />N:I SIRI011 HYDRO-SERRCH <br />