Hydrologic Resources Management Program and Underground Test Area Project FY 2000 Progress Report Page: 91 of 156
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Chapter 7
A Chlorine-36 Study of Regional Groundwater
Flow in Southern Nevada
Jean E. Moran and Timothy P. Rose
Analytical and Nuclear Chemistry Division, Lawrence Livermore National Laboratory
Executive Summary
Chlorine-36 data for groundwater samples from southern Nevada were evaluated as a
means of constraining hydrogeologic processes in regional flow systems in the vicinity of
the Nevada Test Site (NTS). Data from more than 50 locations were used to investigate
the 36C1/Cl systematics in two settings: (1) the Death Valley regional carbonate aquifer
system, including the Pahranagat Valley, Yucca and Frenchman Flats, Ash Meadows,
and the Spring Mountains, and (2) the carbonate, volcanic, and alluvial aquifers in Yucca
and Frenchman Flat.
End-member 36C/Cl compositions were identified for both the initial (recharge) and
chemically evolved components in the carbonate aquifer. The initial cosmogenic
component during recharge is characterized by high 36C1/Cl and low Cl values, whereas
more chemically-evolved carbonate aquifer groundwaters are typically low in 36C1/Cl and
high in Cl. Laboratory studies were performed to determine the characteristics of
leachable chloride in carbonate and volcanic tuff samples from Nevada. Leachable
chloride concentrations were high in limestone from Army Well #1 (up to -75 mg/L). In
contrast, less than 1 mg/L of chloride was leached from a zeolitized volcanic tuff from
well UE-7f in Yucca Flat. The measured 36C/Cl ratios for both rock types were low.
Groundwater 36C1/Cl values are observed to evolve along flow paths in the Death Valley
regional carbonate aquifer due to water-rock interaction processes. Initial 36C1/CI ratios
in Pahranagat Valley samples are similar to that of modern recharge waters in southern
Nevada. In contrast, carbonate aquifer groundwaters beneath Yucca Flat exhibit highly
evolved compositions with comparatively low 36Cl/Cl ratios and high Cl concentrations.
Groundwater from Army Well #1 and Ash Meadows springs have higher 36C1/Cl ratios
than most Yucca Flat carbonate groundwaters, possibly reflecting mixing with less
evolved groundwater originating in the Spring Mountains. Waters from different
hydrostratigraphic units at the NTS fall into dissimilar groups based on 36C1/Cl and Cl
values. Most significantly, water from Tertiary Volcanic Aquifers has a 36C1 signature
that is distinct from water from the Lower Carbonate Aquifer, providing a means of
examining vertical transport between these units. In combination with other geochemical
and hydrogeologic data sets, the 36C1 end members defined here provide a means of
constraining aquifer residence times. This study develops the necessary framework for
future interpretive studies of 36C1 in southern Nevada groundwaters. A study of vertical
interactions between hydrostratigraphic units in Frenchman Flat is planned for FY01.79
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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/m1/91/: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.