A Serendipitous, Long-term Infiltration Experiment: ...

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XVI International Conference on Computational Methods in Water Resources (CMWR-XVI) Ingeniørhuset

A Serendipitous, Long-term Infiltration Experiment: Water and Radionuclide Circulation Beneath the CAMBRIC Trench at the Nevada Test Site.
Author:Reed Maxwell <maxwell5@llnl.gov> (LLNL)
Andrew Tompson <afbt@llnl.gov> (LLNL)
Steven Carle <carle@oyster.llnl.gov> (LLNL)
Mavrik Zavarin <zavarin1@llnl.gov> (LLNL)
Stefan Kollet <kollet2@llnl.gov> (LLNL)
Presenter:Reed Maxwell <maxwell5@llnl.gov> (LLNL)
Date: 2006-06-18     Track: General Sessions     Session: General

Underground atomic weapons testing at the Nevada Test Site introduced numerous radionuclides that may be used to characterize subsurface hydrologic transport processes in arid climates. Beginning in 1975, groundwater adjacent to the CAMBRIC test, conducted beneath Frenchman Flat in 1965, was pumped steadily for 16 years to elicit experimental information on the migration of residual radioactivity through the saturated zone. Radionuclides in the pumping well effluent, including tritium, 36Cl, and 85Kr, were extensively monitored prior to their discharge into an unlined ditch flowing toward a dry lake bed over a kilometer away. We have applied a large (6km x 6km x 1km) and highly resolved (4 m) variably saturated flow model to investigate infiltration into the 220-m vadose zone underlying the ditch as well as subsequent groundwater recharge and well recirculation processes. A Lagrangian particle-tracking model has been used to compute flow pathways and estimate radionuclide travel and residence times in various parts of the system based upon the flow model. Results are consistent with rising tritium levels observed in a monitoring well since 1991. They suggest that recirculation of the ditch effluent through the vadose zone, into groundwater, and back to the test cavity and pumping well are responsible for diluted, tritium-based groundwater age dates observed in 2000 at these locations, as well as for increased tailing effects observed in the pumping well elution curves. Altogether, the models and experimental observations provide an improved basis to understand both historical and future movements of test-related radionuclides in groundwater near CAMBRIC. This work was conducted under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory (LLNL) under contract W-7405-Eng-48. This work was funded by the UGTA program of the U. S. Department of Energy.