Hydrologic Resources Management Program and Underground Test Area Project FY 2000 Progress Report Page: 99 of 156
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host formation. Other wells in the Yucca Flat LCA (e.g., U-3cn#5, UE-10j, and ER-6-l)
have comparatively lower Cl concentrations and higher 36C1/Cl ratios. These samples
may contain a component of younger groundwater, possibly derived from downward
leakage from one of the shallower aquifer units in Yucca Flat, although this idea is
currently speculative. Alternatively, these waters may have been transported along
shorter flowpaths, with shorter residence times in the carbonate aquifer. The 36C1/Cl ratio
in Army Well #1 water can be interpreted as a mixture of LCA groundwater from Yucca
Flat and groundwater originating from the Spring Mountains. This interpretation is
consistent with previous geochemical models for regional flow in this area. Our samples
from the Spring Mountains have significantly higher 36C1/Cl ratios than LCA water from
NTS wells, which accounts for the increase in the 36Cl/Cl ratio in Army Well #1. The
36C1 signature of Army Well #1 water is very close to that measured in the Ash Meadows
spring samples, and there is likely little change in the 36C1 content of the water between
Mercury Valley and Ash Meadows.
4.3 Aquifer residence times
It is notable that groundwater samples with low 36C/Cl ratios commonly show high 4He
abundances (Figure 6), suggesting a correlation related to aquifer residence times.
Detectable buildup of 4He in groundwater occurs over 1,000- to 10,000-year time scales
due to decay of U and Th in the aquifer matrix. We observe that some of the samples that
have resided in the LCA long enough for their 36C1/Cl ratios to decrease to values less
than ~ 5 x 10-13 (due to water-rock interaction) also exhibit significant buildup of 4He.
In general, low 36Cl/Cl ratios and high Cl concentrations characterize groundwaters with
long residence times, as observed for groundwaters from the LCA. For example, the UE-
10j well is screened over three discrete intervals in the LCA, and the 36C1/Cl ratio
decreases with depth while the Cl concentration increases over the same interval (see
Table 1). The UE-10j samples also show an increase in 4He with depth. This trend may
be the result of longer flowpaths for the deeper section of the LCA, allowing more
interaction with the aquifer material.
Several independent pieces of geochemical evidence suggest that groundwater ages in the
regional carbonate aquifer are generally less than 10,000 years (e.g. Thomas et al., 1996;
Davisson et al., 1999; Rose and Davisson, 2001). However, a residence time of 10,000
years is insufficient for radioactive decay of 36C1 to have an effect on the 36C/Cl ratio.
Furthermore, buildup of 36C1 due to neutron capture on 35C1 in the groundwater is also
negligible over 10,000 years unless U concentrations are extremely high. Hence, the
primary "aging" effect we expect to see in LCA groundwaters from southern Nevada is a
progressive enrichment in Cl coupled with decreasing 36C/Cl ratios. More careful
consideration of the relationships between 4He and 36C1/Cl sources and processes are
needed before these observed correlations can be used to measure quantitative residence
times.87
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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/m1/99/?rotate=90: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.