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Colorado Division of Mining Reclamation and Safety February 17, 2015 <br />Eric Scott 7 1400159 <br />The calculations indicate that the Group 2 and Group 3 waters predominantly older than 1952. Possible <br />explanations for the presence of a small percentage of post -1952 water in the Site wells includes <br />recharge from the open pit in until it was backfilled in 2001, residual drilling water, surface water seepage, <br />uncertainty in tritium precipitation levels, and analytical uncertainty. Due to the latitudinal effects on tritium <br />levels in precipitation, the levels of tritium currently in precipitation could be higher at the Site, which <br />would further reduce the calculated percentage of modern groundwater estimated for the Site wells. For <br />example, if a modern precipitation value of 10 TU is used in the calculation, the percentage of pre -1952 <br />water would range from 86% to 98% for the Group 2 and 3 wells. Additionally, typical analytical <br />uncertainty associated with the tritium measurements is approximately 0.3 TU. The 0.3 TU uncertainty <br />affects the calculation of the percent of pre -1952 water by up to 5 %. <br />It is likely that a percentage of post -1952 water calculated for MW -6 and MW -7 represents residual <br />drilling water. As noted previously, a number of constituent concentrations have not stabilized since <br />these monitoring wells were installed (Attachment A). These trending concentrations indicate that the <br />wells are likely not in equilibrium with the surrounding formation and may not be reflective of groundwater <br />conditions prior to drilling. These trends may be the result of disturbance to the system created by drilling <br />and well installation. Both the constituent concentrations and tritium results indicate that the majority of <br />groundwater sampled is representative of formation water and the influence of drilling water at these <br />locations is considered minor. <br />Tritium concentrations in MW -2, MW -3 and MW -4 similarly indicate a small percentage of influence from <br />post -1952 water. However, given the age of the wells and groundwater chemistry, there is no basis to <br />suspect influence from drilling water. As such, groundwater in wells MW -2 and MW -3 may be influenced <br />by post -1952 water from the former A2 pit given their location and the hydrogeology. The percentages of <br />pre -1952 water presented in Table 1 are minimum estimate assuming the Post -1953 percentage of the <br />water has tritium levels similar to modern precipitation. However, recharge was likely greater during the <br />open pit period (pre -2001) given the pit was not filled or covered during this time. Prior to backfilling of <br />Area A2, precipitation ponded in the open pit and during this time the pit likely acted as a local recharge <br />area and the water collected in the pit created an increased hydraulic head for recharge. Similarly, <br />although no ponded water was documented the dry CKD fill area (Figure 1) it may have acted as an area <br />of local recharge near MW -4 before it was backfilled. Additionally, some recharge from 1952 to 1978 may <br />also have occurred through surface infiltration. While there is no way to differentiate between these <br />potential sources, the percentage of pre -1952 waters are almost certainly higher than the range shown in <br />Table 1, indicating a small contribution, if any, from recent recharge water, such as that from the CKD. <br />°o <br />Golder <br />14 14 1 9 \0100 \0122 \tm- boettchedimestone ua fnl- 17feb15 \1400159 tm- boettcherlimestone uar fnl- 17feb15.docx Associates <br />i.\ \ 00 5 9 RY_ Q rY_ <br />