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July 19,2019 18107649 <br /> Additionally, tritium in precipitation varies with latitude and is generally greater at higher latitudes. This relationship <br /> is demonstrated in Figure 12 which presents the Albuquerque(-350 N)data alongside historic data from Denver <br /> (-401 N) and more recent data from Tucson, Arizona(-321 N). The seasonal aggregate samples collected <br /> between 2008 and 2012 near Tucson, Arizona ranged from 3.0 to 8.1 TU (University of Arizona, 2015). Due to the <br /> latitudinal relationship, tritium levels in precipitation at the Site(-401 N)are likely closer to the upper end of these <br /> values, if not higher. Additional data for tritium levels in precipitation near the Site is provided in Cowie et al. <br /> (2014), in which 21 precipitation samples were collected in the San Juan Mountains of Colorado between January <br /> 2010 to October 2011 with a reported mean value of 6.2 TU. <br /> Based on the available regional data and consideration of the factors described above, current levels of tritium in <br /> precipitation for the Site are conservatively estimated to be between 6.2 TU and 8.1 TU. Tritium levels in <br /> groundwater that are greater than current precipitation values represent waters that have recharged between the <br /> ' onset of nuclear testing in the 1950s and the present. Tritium levels in groundwater that are between <br /> approximately 0.4 TU and current precipitation values (6.2 to 8.1 TU)would represent a mixture of pre-1952 and <br /> post-1952 recharge. <br /> Based on these assumptions and calculations, the groundwater sampled in MWA (8.5±0.45 TU)recharged <br /> relatively recently. However, tritium levels for groundwater measured in the other Site wells range between 0.6 TU <br /> and 1.7 TU and represent mostly pre-1952 water with some post-1952 water. The potential mixture for pre-1952 <br /> and post-1952 waters is a non-unique solution, as a range of combinations may be realistic. Additionally, the <br /> values may represent a continuum of recharge to groundwater over time form the open mine pit,from the onset of <br /> mining in the 1930s until the A2 pit was backfilled in 2001. <br /> For the purposes of this discussion, a simple calculation was performed to only assess a two-component system, <br /> assuming mixing of a modern water and a pre-1952 water. This mixing evaluation was conducted to determine <br /> the percentage of pre-1952 (0.4 TU)water that would need to be mixed with current post-1952 recharge(6.2 TU <br /> to 8.1 TU for modern precipitation)water to obtain the reported tritium concentration of the samples collected form <br /> each well (Table 12). Mixing due to recharge is considered in this approach and the assumption is implicitly made <br /> that parallel flow paths would be similar(i.e. no dispersion or mixing into zones with different hydraulic properties). <br /> These are reasonable assumptions in this hydrogeologic regime. <br /> Well MW-2 and the more recently installed wells MW-6 and MW-7 have a calculated percentage of pre-1952 <br /> water between approximately 80% and 90%. Well MW-4 has the lowest percentage of estimated pre-1952 water <br /> at 78% to 83%, whereas MW-3 has the highest at 97%. The results presented in Table 12 are conservative in that <br /> 1 they are calculated with the assumption that the post-1952 groundwater has the tritium levels equal to modern <br /> precipitation. If however, any part of the post-1952 portion of the water recharged prior to present day, tritium <br /> levels of the post-1952 portion of the water would be greater and a higher percentage of pre-1952 water would be <br /> calculated. For example, if a tritium level of approximately 12.6 TU representing precipitation from 1975 is used in <br /> the calculation for MW-2, the percentage of pre-1952 water calculated would be 93%, and this percentage is even <br /> greater(97%) if water from 1965 is used in the calculation. <br /> The calculations indicate that the Group 2 and Group 3 waters are predominantly older than 1952. Possible <br /> explanations for the presence of a small percentage of post-1952 water in the Site wells include: <br /> 1. recharge from the open pit until it was backfilled in 2001 <br /> 2. residual drilling water <br /> 3. surface water seepage <br /> �, COLDER 9 <br />