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from 20 to 30 feet. Similarly, Nelson et al. (1989) suggest that the alluvial groundwater gradient <br />is about 0.002 (0.2 feet decline per 100 feet of horizontal distance) from northwest to southeast <br />across the site. <br />Groundwater information collected at the time of the May 2006 subsurface investigation (Deere <br />and Ault 2006) indicates that the alluvial groundwater elevation on the site is between 4514 and <br />4517 amsl with a saturated thickness ranging from about 23 feet on the north side of the property <br />to 27 feet in the south near the river. However, groundwater surface elevations on the western <br />portion of the site were likely influenced at the time of the investigation by the Evans #2 Pit <br />dewatering operations. Based on results from the subsurface investigation, published reference <br />material, and well data from the SEO database, typical groundwater elevations were estimated <br />for the site and are presented as surface contours in Figure 2. The alluvial groundwater contours <br />in Figure 2 are considered typical, may not correlate exactly to the bore hole groundwater <br />elevations collected at the time of the 2006 subsurface investigation, and would be expected to <br />fluctuate seasonally by approximately 2 feet. <br />Grain -size distributions from the alluvial samples collected during two of the bore hole drillings <br />(Deere and Ault 2006) and a bulk sample from all of the drillings were used to estimate the <br />hydraulic conductivity for the aquifer (Table 2). Calculated hydraulic conductivity values from <br />three different methods were compared. The Hazen Method (Fetter 1994) is a simple calculation <br />relating the D10 size of the soil to hydraulic conductivity. Shepherd (1989) used data sets to <br />produce idealized graphic relationships that relate hydraulic conductivity to the mean grain <br />diameter for different sediment types. Finally, the Beyer Equation (Kresic 2006) calculates <br />hydraulic conductivity using the D10 particle size and uniformity coefficient of poorly sorted <br />porous media. Using data available from the exploration borings, these methods resulted in a <br />range of hydraulic conductivity estimates. Results from the individual bore holes estimate that <br />the hydraulic conductivity of the alluvium is between 73 and 477 feet/day. The bulk sample <br />estimates hydraulic conductivity at approximately 480 feet/day. <br />Data from exploration borings immediately east of the property (Deere and Ault 2007) were also <br />used to estimate and compare alluvial hydraulic conductivity near the site. Estimates based on <br />five individual bore hole samples range between 128 feet/day and 431 feet/day. In addition, <br />Wilson (1965) reported a hydraulic conductivity value of 670 feet/day from a well nearly five <br />miles upstream from the site and 910 feet/day from a well about three miles downstream of the <br />property. The Colorado Geologic Survey (2003) reported measured hydraulic conductivities <br />between 70 to 1,200 feet/day for the lower Arkansas River alluvial aquifer with an average value <br />of 530 feet/day. <br />Based on available information, a bulk hydraulic conductivity value of 480 feet/day was used for <br />potential dewatering impact evaluations. This value is at the upper end of the range from <br />individual bore hole samples collected on -site and near the site, but is the calculated hydraulic <br />conductivity of the on -site bulk sample. The value is also similar to published references of <br />Arkansas River alluvial aquifer bulk hydraulic conductivity values. <br />3.0 POTENTIAL DEWATERING IMPACTS <br />To estimate potential impacts from mine dewatering operations, the steady -state two - <br />dimensional analytical solution of Marinelli and Niccoli (2000) was used for each phase of the <br />Cblue earth Page 5 of 11 <br />SOLUTIONS <br />