2019-03-05_REVISION - M1977348 (5)

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March 1,2019 18107649 ' It is likely that a percentage of post-1952 water calculated for MW-6 and MW-7 represents residual drilling water. As noted previously, at the time of the tritium testing a number of constituent concentrations had not stabilized since these monitoring wells were installed (Appendix D). These trending concentrations indicate that the wells were likely not in equilibrium with the surrounding formation and may not have been reflective of groundwater conditions prior to drilling. These trends may be the result of disturbance to the system created by drilling and well installation. Both the constituent concentrations and tritium results indicate that the majority of groundwater ' sampled is representative of formation water and the influence of drilling water at these locations is considered minor. ' Tritium concentrations in MW-2, MW-3 and MW-4 similarly indicate a small percentage of influence from post- 1952 water. However, given the age of the wells and groundwater chemistry, there is no basis to suspect influence from drilling water. As such, groundwater in wells MW-2 and MW-3 may be influenced by post-1952 water from the former A2 pit given their location and the hydrogeology. The percentages of pre-1952 water presented in Table 11 are minimum estimates assuming the Post-1953 percentage of the water has tritium levels similar to modern precipitation. However, recharge was likely greater during the open pit period (pre-2001) given ' the pit was not filled or covered during this time. Prior to backfilling of Area A2, precipitation ponded in the open pit and during this time the pit likely acted as a local recharge area and the water collected in the pit created an increased hydraulic head for recharge. Similarly, although no ponded water was documented at the dry CKD fill area (Figure 4), it may have acted as an area of local recharge near MW-4 before it was backfilled. Additionally, some recharge from 1952 to 1978 may have occurred through surface infiltration. While there is no way to differentiate between these potential sources, the percentage of pre-1952 waters are almost certainly higher than ' the range shown in Table 11, indicating a small contribution, if any, from recent recharge water, such as that from the CKD. ' 5.0 CKD LEACHATE CHARACTERISTICS Golder collected and analyzed native materials and CKD (Golder 2014). These materials included: ■ One sample of silt from the screened interval of well MW-5; ■ Four samples of limestone from the screened intervals of wells MW-6 and MW-7; s Two samples of sandstone from below the screened intervals of wells MW-6 and MW-7; and ■ Two samples of CKD from a borehole drilled in area A2. tAdditionally, analytical testing of the CKD material was conducted by Secor and Holnam (Secor 1998). The results of the solids analysis are discussed below. 5.1 Bulk Testing ' Solid-phase chemical analysis was performed on the soil and rock samples (Golder 2014). The chemical analysis was performed in a two-step process including digestion of the sample in acid to release the elements into the solution phase (EPA Method M3010A)followed by analysis of the elements in the resulting digestion by ' inductively coupled plasma mass spectrometry(ICP-MS; EPA Method M6020). The results from solid-phase chemical analysis were used to make an inference regarding elements of potential environmental concern, although it should be understood that a high concentration of a particular element does not necessarily imply that this element will indeed be mobilized in concentrations that may lead to environmental impacts. The elemental ' 4 GOLDEP 7