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Dr. Angela Bellantoni <br /> RE:Evans#2 Pit Hydrogeologic Evaluation Review <br /> May 27, 2014 <br /> As noted in the review comments, some samples are at the boundary or outside of the optimum <br /> applicability of the Hazen method, but that is why several different methods were used for the <br /> estimate. Although not as well known, the Prugh method is used in the general construction <br /> industry to quickly estimate flow to dewatering trenches and does not appear to have been <br /> misused in the analysis. The Kozeny-Carman method is also an applicable method for <br /> estimating hydraulic conductivity and provides similar results to the methods used in the report. <br /> Although use of the Kozeny-Carman method results in higher hydraulic conductivities relative to <br /> the other methods for some samples (as noted in the review comments), for other samples the <br /> method provides lower results than those included in the report. <br /> Based on published information and the site specific estimates, it is likely that hydraulic <br /> conductivity varies on and adjacent to the Evans #2 Pit site. Specific areas within the dewatering <br /> radius of influence may exceed the average value of 480 feet/day which was used in the analysis, <br /> but the location and/or extent of these areas are not known. Published information provides a <br /> similar alluvial aquifer average for the lower Arkansas River of 530 feet/day, and without <br /> additional site specific alluvial aquifer data, the use of higher hydraulic conductivity values is <br /> difficult to justify for the given analytical solution. <br /> For the analytical solution, general approximations for recharge within the dewatering radius of <br /> influence were made. A comprehensive and detailed water budget evaluation was beyond the <br /> scope of the analysis. Although the dewatering analytical solution results are sensitive to the <br /> recharge input value, the analysis cannot compensate for seasonal and annual variations in <br /> recharge. Droughts, seasonal precipitation patterns and river flows, crop rotations, and changing <br /> irrigation practices all have the potential to influence recharge within the dewatering radius of <br /> influence. In addition to precipitation and center-pivot or furrow irrigation, recharge directly <br /> from the Arkansas River and the Excelsior Ditch and its laterals likely influence the dewatering <br /> radius of influence. Even with a comprehensive water budget, compensating for specific <br /> recharge elements and seasonal and annual recharge fluctuations are beyond the input capability <br /> of the analytical solution used in the analysis. The assumed recharge values used in the analytical <br /> solution provide a general approximation for the dewatering radius of influence to help identify <br /> the aerial extent of proposed groundwater monitoring. <br /> The analytical solution used in the dewatering impact analysis, and the limited alluvial aquifer <br /> information available, provide a reasonable estimate for the Evans #2 Pit current and future <br /> dewatering radius of influence. The analysis does not provide the detail and accuracy, nor does <br /> the report attempt, to quantify dewatering impacts on or adjacent to the Evans #2 Pit site. The <br /> report emphasizes the need for, and use of, a groundwater monitoring plan for further evaluation <br /> of existing conditions and interpretation of potential dewatering impacts. <br /> If you have any questions regarding this application and adequacy comment responses, please <br /> call me directly at(970) 227-2803. <br /> Sincerely, <br /> Blue Ea h So n , L C <br /> If <br /> William Schenderlein, P.E. <br /> ti 1 Page 3 of 3 <br />