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Page 3 <br />Mr. Carl Mount <br />March 9, 2007 <br />~-~.:/~ <br />CIVIL RES`~URCES,LLC <br />The west side underdrain collects water in a 4 inch diameter perforated ADS pipe covered with a filter <br />sleeve. The pipe run is 400.0 feet in length and has a flat grade day lighting at elevation 4849.0' into an <br />existing drainage swale. <br />This pipe is sized to capture potential groundwater flows above the pipe invert and thereby limit groundwater <br />mounding. Groundwater flows to this pipe were calculated based on Darcy's Equation using the hydraulic <br />gradient of 0.0027 feet/foot and a hydraulic conductivity (K) of 450 feet/day, from WWE. The total bedrock to <br />phreatic surface potential groundwater flow is 39 gallons per minute (gpm) (0.09 cubic feet per second <br />(cfs)). At the maximum allowable mounding elevation, the average head above the underdrain pipe is 0.9 <br />feet. The required head to carry the 0.09 cfs is .5 feet of head using the Hazen-Williams friction coefficient. <br />The perforated pipe will accept all the groundwater inflow as the pipe will inflow a calculated 70 gpm per <br />linear foot of pipe with 0.9 feet of head, per ADS Technical Note 2.105. This flow will exit the pipe at 1.2 ft/s <br />onto riprap and surface flow to Little Dry Creek. <br />Total bedrock to phreatic surface groundwater flow 39 gpm <br />Inflow rate of pipe eg r foot of pipe at 0.9 feet of head 70 gpm <br />Pipe flow capacity at 0.9 feet of head 58 gpm <br />Conclusion <br />The potential for groundwater upslope mounding due to the construction of the Heit Pit slurry wall is limited <br />due to the existing slurry wall of the Koenig Pit to the south and the Little Dry Creek drainage to the west. . <br />Removing the Heit property from irrigation will reduce recharge to the local alluvium. The proposed south <br />and west underdrains adequately address the potential minimal groundwater mounding resulting from slurry <br />wall construction on the site. These underdrains will outfall into an existing drainage channel and surface <br />flow to Little Dry Creek. <br />As per the mine permit, water monitoring will continue during the operation and dewatering of the mine to <br />determine if mounding is occurring. <br />Please call if you have any questions. <br />Sincerely, <br />CIVIL RESOURCES, LLC <br />~~ <br />Brad L. Hagen, P.E. <br />Encl: Fgures and Attachments 1 & 2 <br />J:ISW Inveshnent Group :123We1t PropertylUnderdrainlTech Rev #3.doc <br />