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July 19,2019 18107649 <br /> 4. uncertainty in tritium precipitation levels <br /> 5. and analytical uncertainty <br /> Due to the latitudinal effects on tritium levels in precipitation, the levels of tritium currently in precipitation could be <br /> higher at the Site,which would further reduce the calculated percentage of modern groundwater estimated for the <br /> Site wells. For example, if a modern precipitation value of 10 TU is used in the calculation, the percentage of pre- <br /> 1952 water would range from 86%to 98%for the Group 2 and 3 wells. This possibility is supported by the tritium <br /> levels reported for the sample collected from the most upgradient well MW-1 (8.5±0.45 TU). Additionally, typical <br /> analytical 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 /> Based on the flow regime for the site discussed in Section 3.2, water would be expected to be older(higher <br /> percentage of pre-1952) in Group 3 wells than Group 2 wells. However; it is possible that fraction of post-1952 <br /> water calculated for MW-6 and MW-7 represents residual drilling water. As noted previously, at the time of the <br /> tritium testing a number of constituent concentrations had not stabilized since these monitoring wells were <br /> installed (Appendix D). These trending concentrations indicate that the wells were likely not in equilibrium with the <br /> I surrounding formation and may not have been reflective of groundwater conditions prior to drilling. These trends <br /> may be the result of disturbance to the system created by drilling and well installation. <br /> ' Tritium concentrations in MW-2, MW-3 and MW-4 similarly indicate a small percentage of influence from post- <br /> 1952 water. However, given the age of the wells and groundwater chemistry, there is no basis to suspect <br /> influence from drilling water. As such, in addition to uncertainty in tritium precipitation levels and analytical <br /> uncertainty(discussed above), groundwater in wells MW-2 and MW-3 may be influenced by post-1952 water from <br /> the former A2 pit given their location and the hydrogeologic conceptual model. The percentages of pre-1952 water <br /> presented in Table 12 are minimum estimates assuming the post-1952 percentage of the water has tritium levels <br /> similar to modern precipitation. However, recharge was likely greater during the open pit period (pre-2001)given <br /> the pit was not filled or covered during this time. Prior to backfilling of Area A2, precipitation ponded in the open pit <br /> and during this time the pit likely acted as a local recharge area and the water collected in the pit created an <br /> increased hydraulic head for recharge. Similarly, although no ponded water was documented at the dry CKD fill <br /> area (Figure 1), it may have acted as an area of local recharge near MW-4 before it was backfilled. Additionally, <br /> some recharge from 1952 to 1978 may have occurred through surface infiltration. While there is no way to <br /> 1 differentiate between these potential sources and uncertainties, the percentage of pre-1952 waters are almost <br /> certainly higher than the range shown in Table 12, indicating a small contribution, if any, from recent recharge <br /> water, such as that from CKD leachate. <br /> 5.0 CKD LEACHATE CHARACTERISTICS <br /> Golder collected and analyzed native materials and CKD(Golder 2014). These materials included: <br /> One sample of silt from the screened interval of well MW-5; <br /> a Four samples of limestone from the screened intervals of wells MW-6 and MW-7; <br /> ra Two samples of sandstone from below the screened intervals of wells MW-6 and MW-7; and <br /> a Two samples of CKD from a borehole drilled in area A2. <br /> „+�, GOLDER 10 <br />