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Colorado Division of Mining Reclamation and Safety February 17, 2015 Eric Scott 5 1400159 Tritium concentrations in the atmosphere peaked in 1963 (Clark and Fritz, 1997). Since 1963, tritium concentrations in precipitation have decreased because of the discontinuation of weapons testing, exchange of water with the oceans, and the short half -life of tritium. Clark and Fritz (1997) presented a qualitative interpretation of tritium levels for continental regions (Table 1) as of 1997. Table 1: Typical Tritium Concentrations in Groundwater Tritium Concentration Qualitative Assessment < 0.8 TU Sub modern- recharged prior to 1952 0.8 to —4 TU Mixture between sub modern and recent ages 5 to 15 TU Modern < 5 to 10 ears 15 to 30 TU Some bomb 3H present > 30 TU Lar e constituent of recharge from 1960s or 1970s > 50 TU Mainly 1960's recharge Source: Clark and Fritz (IYYf) The tritium value for modern (or current) precipitation and recharge is affected by many factors. The factors include time, costal proximity, seasonal variation and latitude. The time has affected levels of tritium in precipitation due to isotope decay and exchange of waters within the hydrologic cycle. The exchange of waters is more prevalent in coastal regions, so the levels of tritium in precipitation are lower near the oceans. Tritium concentrations in precipitation typically have strong seasonal variations; therefore, estimates of tritium in precipitation typically are not based on any one storm event but are often aggregate -based to account for seasonal and individual storm variation, all of which would contribute to groundwater. Additionally, tritium in precipitation varies with latitude and is generally greater at higher latitudes. This relationship is demonstrated in Figure 3 which presents the Albuquerque (--350 N) data alongside historic data from Denver (--400 N) and more recent data from Tucson, Arizona (--320 N). The seasonal aggregate samples collected between 2008 and 2012 near Tucson, Arizona ranged from 3.0 and 8.1 TU (University of Arizona, 2015). Due to the latitudinal relationship, tritium levels in precipitation at the Site (--400 N) are likely closer to the upper end of these values, if not higher. Additional data for tritium levels in precipitation near the Site is provided in Cowie et al. (2014), in which 21 precipitation samples were collected in the San Juan Mountains of Colorado between January 2010 to October 2011 with a reported mean value of 6.2 TU. ' Golder is \14 \1400159 \0100 \0 1 2 \t m- boettchedimestone uar rynl- 17feb15 \1400159 tm- boettcherlimestone ua F ry_ fnl- 17feb15.docx GlAssociates