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Method is a graphical solution for estimating hydraulic conductivity using the D50 grain size, <br />uniformity coefficient, and soil density. The Hazen Method is a simple calculation relating the <br />D10 size of the soil to hydraulic conductivity. Using the available data, both of these methods <br />resulted in very similar estimates of hydraulic conductivity at each borehole. Results estimate <br />that the bulk hydraulic conductivity for the alluvium is 85 feet/day. This value is within the <br />range of reported hydraulic conductivity values for unconsolidated clean to silty sands presented <br />in Freeze and Cherry (1979). In addition, although pump test data was not available for the Big <br />Thompson River alluvium, the calculated hydraulic conductivity value is also within the range of <br />values determined from pump test data collected along other Front Range South Platte River <br />tributaries (Wilson, 1965). <br />Ground water velocities across the site will likely vary with localized changes in hydraulic <br />conductivity, hydraulic gradient, and porosity. However, ground water average lineaz velocities <br />across the site are estimated to be 1.7 feetJday. This estimate was based upon the previously <br />discussed hydraulic conductivity value, water table gradient, and an average porosity of <br />25 percent. <br />Proposed Pit Dewatering 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 Two Rivers site. Dewatering operations <br />would 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 entire pit is estimated to be approximately 5,000 feet. Although the average <br />depth of overbwden and alluvial material at the proposed site is approximately 31 feet, only the <br />bottom 22 feet is saturated with alluvial ground water. Therefore, the surface area of [he alluvial <br />aquifer exposed by the trench is 110,000 ftZ. Darcy's Law approximates flow into the trench <br />based on the exposed surface area and the hydraulic properties of the aquifer. Using Darcy's <br />Law, the estimated flow into the trench from the surrounding alluvial aquifer is approximately <br />245 gallons per minute (gpm). Flow within the trench will be slightly greater dwing the initial <br />dewatering. The added flow will be from ground water inside the pit draining into the trench and <br />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 Two Rivers site. The mathematical approaches developed with <br />the use of these assumptions can approximate the response of the alluvial aquifer near the site. <br />Among other aspects, the satwated alluvium near the site is fairly consistent in material and <br />thickness, the hydraulic gradient is very low, and the specific yield is high. <br />Additional simplifying assumptions were made in estimating the pit dewatering affects. The <br />influence of the rivers was neglected for the estimate. The Big Thompson and Little Thompson <br />rivers will likely eliminate dewatering affects east of the pit and minimize affects north and south <br />of the pit. Surface recharge was also neglected since values would fluctuate wnsiderably <br />throughout the year. For instance, surface recharge from irrigation would likely reduce the <br />3 <br />