Hydrologic Resources Management Program and Underground Test Area Project FY 2000 Progress Report Page: 20 of 156
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activity of 5.1 x 107 pCi/L (corrected to the October 1999 sample date). Groundwater
previously collected from the same depth interval in September 1998 had a field-
measured 3H activity of 7.0 x 107 pCi/L. Laboratory analysis of this sample yielded a 3H
activity of 6.5 x 107 pCi/L (corrected to the Sept. 1998 sample date). Comparison of
results for the 1998 and 1999 samples indicates the 3H concentration in the cavity is
apparently decreasing faster than the 3H-decay rate. This suggests 3H may be migrating
in groundwater, or that groundwater pumping is gradually diluting the cavity fluid.
The 3He/4He ratio measured in 1999 was five orders of magnitude above atmospheric
levels, reflecting the in-growth of 3He derived from tritium decay (3H -> 3He + P', tA =
12.3 y). The calculated 3H-3He age (19.7 years) is younger than the actual age of the
nuclear test (25 years on 02/14/01) and may reflect the partial loss of helium gas from
solution. The Cheshire cavity fluid also contains high concentrations of the gaseous
fission product "Kr (2.77 x 104 pCi/L in the 1999 sample). The 85Kr concentration
decreased slightly since 1998 (3.39 x 104 pCi/L), and like 3H, the rate of decrease is more
rapid than the 85Kr-decay rate (ty, = 10.7 y).
The 14C value of dissolved inorganic carbon (DIC) is 1.54 x 105 percent modern, or about
1500 times greater than pre-nuclear testing atmospheric levels. The measured 14C
activity is slightly lower than the 1998 value of 1.6 x 105 percent modern (Eaton et al.,
1999), and is consistent with the observed decreases in 3H and 8Kr concentrations. It is
notable that the Cheshire groundwater DIC concentration (108 mg/L as HCO3-) and S3C
value (-6.0 %o PDB) is similar to uncontaminated environmental wells on Pahute Mesa.
For comparison, groundwater from the U-20WW has a DIC concentration of 109 mg/L
(as HCO3-) and a S13C value of -6.2 %o (Thomas et al., 1999). U-20WW is located <1
mile northwest of U20n PS1 DDh, and produces water from the same hydrostratigraphic
unit (Calico Hills Fm), but at a slightly shallower depth interval (692 - 996 m; data from
DOE/NV, 1997). In test cavities that remain open to exchange with the surrounding
environment, there is evidently no long-term impact on groundwater DIC concentrations
and stable carbon isotope ratios due to nuclear testing.
The 36C1/Cl ratio for Cheshire groundwater is 1.15 x 10-9, and is approximately four
orders of magnitude above natural environmental levels (-1 x 10-13). The 36C1/Cl ratio is
identical to the value obtained for a 'sample from the same depth interval in 1998 (Eaton
et al., 1999). The 99Tc activity measured in the 1999 Cheshire sample was 22 pCi/L,
compared with a value of 31 pCi/L measured in 1998 from the same depth interval.
Hence, 99Tc appears to be decreasing over time at a proportionally greater rate than is
observed for 3H, 14C, or 8IKr.
The uranium concentration in Cheshire groundwater (2.3 pg/L) falls within the range of
previously reported values for volcanic aquifer groundwaters in this region (Thomas et
al., 1999). The 235U/238U ratio (0.007) indicates the uranium is of natural origin.
Enrichment in the 24U/238U-activity ratio (3.44) reflects the preferential leaching of 24U
from uranium-bearing minerals in the host rock following a-decay of 238U.8
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Davisson, M. L.; Eaton, G. F.; Hakemi, N. L.; Hudson, G. B.; Hutcheon, I. D.; Lau, C. A. et al. Hydrologic Resources Management Program and Underground Test Area Project FY 2000 Progress Report, report, July 1, 2001; California. (https://digital.library.unt.edu/ark:/67531/metadc1411019/m1/20/: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.