Hydrologic Resources Management Program and Underground Test Area Project FY 2000 Progress Report Page: 24 of 156
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The 36C1/Cl ratio in.RNM-1 groundwater is 1.06 x 1012. In comparison, environmental
36C1/Cl ratios measured at 8 wells in Frenchman Flat (alluvial and volcanic aquifers)
range from 5.3 x 10-1 to 8.4 x 10-13 (data from Rose et al., 1997; and LLNL unpublished
results). Given that RNM-1 groundwater is only slightly enriched in 36C1 relative to
environmental samples from the same area, it would appear that most of the 36C1
originally present in the Cambric test cavity was mobilized during pumping at RNM-2S.
Uranium isotopic values measured in RNM-1 groundwater are consistent with natural
abundances (i.e. 235U/238U = 0.007).
RNM-2S
Groundwater samples were collected from RNM-2S on 11 October 1999, and again on
14 June 2000. In 1999, the well was pumped at nearly 600 gpm for 5 days prior to
sampling, allowing time for the system to reach a steady-state. Thompson (2000) reports
a tritium activity of 1.9 x 105 pCi/L for samples collected on 11 October 1999 (1.3 x 106
pCi/L at to = 14 May 65), and notes this value is the same as the concentration observed
when continuous pumping was ended in 1991. Well RNM-2S was re-sampled eight
months later in June 2000. The tritium activity of this sample, measured by the helium
accumulation method, was 5.84 x 105 pCi/L (corrected to the 14 June 2000 sample date).
This is equivalent to an activity of 4.21 x 106 pCi/L at the Cambric zero time.
Helium isotopic abundances were measured for RNM-2S groundwaters collected in both
1999 and 2000. The two sets of analyses are fairly consistent and indicate a strong
enrichment in 3He derived from tritium decay (R/Ra ~ 2.5 x 103). Calculated 3H-3He ages
are 19.2 and 19.9 years for the 1999 and 2000 samples, respectively. As observed for the
RNM-1 sample, these ages are substantially younger than the age of the Cambric test,
implying helium loss from the system. A possible explanation for the loss of helium is
that some of the water pumped from RNM-2S during the 1975-1991 radionuclide
migration experiment has been recycled back into the system. Partial degassing of
helium would likely occur at the surface, and during re-infiltration of the water through
the vadose zone. Observed variations in 8D and 8180 values at RNM-1 and RNM-2S
tend to support this recycling hypothesis.
Groundwater samples from eight different wells perforating the alluvial and volcanic
aquifers in central Frenchman Flat have a composite average 8D and 6180 value of
-107 %o and -13.8%0, respectively (Rose et al., 1997; and LLNL unpublished data). This
average does not include RNM-1 and RNM-2S, which show comparative enrichments in
8D (-104 to -105%o) and 6180 (-12.7 to -13.0%o). On a plot of 6D versus 8180 (Figure 1),
the RNM-1 and RNM-2S data plot along a line projected from the Frenchman Flat
average value, with a slope of 2.8. This variation indicates the RNM-1 and RNM-2S
groundwaters have undergone enrichment in heavy isotopes (2H and 180) due to
evaporation, as would occur if water were brought to the surface for a period of time.
Note that the data point for UE-5n also plots along the same evaporation trajectory.
These data suggest that some of the groundwater now present at RNM-1 and RNM-2S
was recycled from Cambric ditch.12
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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/m1/24/: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.