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Ground water velocities in the Offpost Study Area vary with local changes in hydraulic <br />conductivity, hydraulic gradient, and porosity. Near the RNIA north bomtdary, estimates of <br />average linear velocities of ground water range front I.5 ft/day to 10 ft/day. Near the RMA <br />northwest boundary, ground water average linear velocities range from 5.5 fUday to 22.5 ft/day. <br />These estimates were based upon the previously discussed hydraulic conductivity values, water <br />table gradients, and an average porosity of 30 percent. Likewise, the estimated linear ground <br />water velocity at the proposed Tanabe site is estimated to be approximately 6.7 ft/day. <br />Proposed Pit Dewaterinr~ Affects <br />The alluvial aquifer parameters discussed in the previous section were used to evaluate the <br />influence of pit dewatering operations at the proposed Tanabe site. Dewatering operations would <br />typically commence with the construction of a perimeter trench that fully penetrates the <br />alluvium. The bottom of the trench is sloped to one or more sumps where the water is collected <br />and pumped from the site. <br />The perimeter of the Phase I and Phase 11 pits is estimated to be approximately 6,700 feet. <br />Although the average depth of overburden and alluvial material at the proposed Tanabe site is <br />approximately 3l feet, only the bottom 9 feet is saturated with alluvial ground water. Therefore, <br />the surface area of the alluvial aquifer exposed by the trench is 60,300 ftZ. Darcy's Law <br />approximates flow into the trench based on the exposed surface area and the hydraulic properties <br />of the aquifer. Using Darcy's Law, the estimated flow into the trench from the surrounding <br />alluvial aquifer is approximately 630 gallons per minute (gpm). Flow within the trench will be <br />slightly greater during the initial dewatering. The added flow will be from ground water inside <br />the pit draining into the trench and drying the alluvial material to be mined. <br />In order to estimate the pit dewatering affects, the Theis assumptions were extended to the <br />alluvial aquifer at the proposed Tanabe site. The mathematical approaches developed with the <br />use of these assumptions can approximate the response of the alluvial aquifer near the site. <br />Among other aspects, the saturated alluvium near the site is fairly consistent in material and <br />thickness, the hydraulic gradient is very low and the specific yield is high, and the nearest large <br />impervious boundary is over a mile away. <br />The approach that was used to estimate the extent of drawdown from dewatering is referred to as <br />"flow to a drainage trench from a line source" and is referenced in Construction Dewatering, <br />New Methods and Applications (Powers 1992). Results were checked with additional <br />approaches as presented in Groundwater (Freeze and Cherry 1979) and Groundwater Hydrology <br />(Todd 1980). Results indicate that drawdown from pit dewatering will extend approximately <br />750 feet from the pit boundary. The saturated thickness of the alluvial aquifer during dewatering <br />is illustrated in Figure 3. <br />3 <br />