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March 1,2019 18107649 ' <br /> Additionally, tritium in precipitation varies with latitude and is generally greater at higher latitudes. This relationship ' <br /> is demonstrated in Figure 6 which presents the Albuquerque (-351 N)data alongside historic data from Denver <br /> (-401 N) and more recent data from Tucson, Arizona (--320 N). The seasonal aggregate samples collected <br /> between 2008 and 2012 near Tucson, Arizona ranged from 3.0 and 8.1 TU (University of Arizona, 2015). Due to ' <br /> the latitudinal relationship, tritium levels in precipitation at the Site (-400 N) are likely closer to the upper end of <br /> these 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 t <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 MW-1 (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 a modern water and a pre-1952 water, To provide a lower bound to the estimation of pre-1952 versus <br /> post-1952 recharge the estimated range of 6.2 TU to 8.1 TU for modern precipitation, a percentage of pre-1952 , <br /> (pre-bomb)water was calculated for each well (Table 11). Well MW-2 and the more recently installed wells MW-6 <br /> and MW-7 have a calculated percentage of pre-1952 water between approximately 80% and 90%. Well MW-4 <br /> has the lowest percentage of estimated pre-1952 water at 78% to 83%, whereas MW-3 has the highest at 97%. ' <br /> The results presented in Table 11 are conservative in that they are calculated with the assumption that the <br /> post-1952 has the tritium levels equal to modern precipitation. If however, any part of the post-1952 portion of the <br /> water recharged prior to present day, tritium levels of the post-1952 portion of the water would be greater and a ' <br /> higher percentage of pre-1952 water would be calculated. For example, if a tritium level of approximately 12.6 TU <br /> representing precipitation from 1975 is used in the calculation for MW-2, the percentage of pre-1952 water <br /> calculated would be 93%, and this percentage is even 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 includes recharge from ' <br /> the open pit until it was backfilled in 2001, residual drilling water, surface water seepage, uncertainty in tritium <br /> precipitation levels, and analytical uncertainty. Due to the latitudinal effects on tritium levels in precipitation, the <br /> levels of tritium currently in precipitation could be higher at the Site, which would further reduce the calculated ' <br /> percentage of modern groundwater estimated for the Site wells. For example, if a modern precipitation value of <br /> 10 TU is used in the calculation, the percentage of pre-1952 water would range from 86% to 98%for the Group 2 <br /> and 3 wells. Additionally, typical analytical uncertainty associated with the tritium measurements is approximately ' <br /> 0.3 TU. The 0.3 TU uncertainty affects the calculation of the percent of pre-1952 water by up to 5%. <br /> ;, GOLDER 6 ' <br />