2019-03-05_REVISION - M1977348 (5)

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r ' March 1,2019 18107649 ' A similar trend was observed for MW-4 following installation in 1999 and 2000 for constituents such as chloride and TDS. This stabilization process may indicate an initial influence of drilling water on water quality. The highest concentrations of barium are found in samples collected from the Group 3 wells (MW-4, MW-6 and ' MW-7). These wells are located greater than 500 feet from CKD disposal area A2. Group 2 wells (MW-2 and MW-3) are located less than 100 feet from the CKD disposal area A2 and have lower observed concentrations of barium. Likewise, observed TDS concentrations are higher in samples collected from the Group 3 wells than the Group 2 wells. This suggests barium and TDS concentrations reflect influence by native host materials rather than influence from CKD leachate. If the CKD deposited in area A2 was an ongoing source for barium and TDS loading ' to the groundwater, higher concentrations would be expected closer to CKD disposal area A2 in the Group 2 wells. This is particularly the case given that MW-2 and MW-3 are screened across the Niobrara/Codell contact; groundwater would be expected to flow along this contact in the Niobrara based on the understanding of Site ' hydrogeology(Secor 1998, Golder 2013, Golder 2014). Although TDS and barium concentrations in MW-4 increased during the 1998 and 1999 sampling, the concentrations have been stable since groundwater sampling resumed in 2010. ' 4.2 Groundwater Age Dating To assess the recharge age for the water monitored in the Site wells relative to CKD disposal, groundwater samples were collected on August 27, 2014 for tritium analysis from all of the Site wells except MW-5. Well MW-5 was not tested for tritium because of its proximity to a non-backfilled pit at the Site which likely acts as a localized recharge zone. Laboratory analysis for tritium was conducted by the Environmental Isotope Laboratory at the t University of Arizona. The laboratory results are presented in Appendix D. Appendix D includes results for "MW-20,"which was a blind duplicate for MW-6 submitted to the laboratory. Tritium (3H) is the radiogenic isotope of hydrogen and decays with a half-life of 12.23 years to helium (3 He). Tritium concentrations are measured in tritium units (TU), where 1 TU is equal to 1 tritium atom per 1018 hydrogen atoms. Although small amounts of tritium occur naturally through cosmic ray interactions with ' atmospheric gases, tritium was introduced to the environment in significant concentrations through nuclear weapons testing, which began in 1952. Tritium concentrations in precipitation prior to weapons testing are not well known but likely did not exceed 2 to 8 TU (Rupert and Plummer, 2004). Therefore, due to tritium decay, ' pre-1952 waters would currently have tritium concentrations near 0.4 TU (less than the 0.6 TU detection limit used in this study). ' 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 10) as of 1997. 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. 1 ' J GOLDER 5