A serendipitous, long-term infiltration experiment: water and tritium circulation beneath the CAMBRIC trench at the Nevada Test Site.

Underground nuclear weapons testing at the Nevada Test Site introduced numerous radionuclides that may be used subsequently to characterize subsurface hydrologic transport processes in arid climates. In 1965, a unique, 16-year pumping experiment designed to examine radionuclide migration away from the CAMBRIC nuclear test, conducted in the saturated zone beneath Frenchman Flat, Nevada, USA, gave rise to an unintended second experiment involving radionuclide infiltration through the vadose zone, as induced by seepage of pumping effluents beneath an unlined discharge trench. The combined experiments have been reanalyzed using a detailed, three-dimensional numerical model of transient, variably saturated flow and mass transport in a heterogeneous subsurface, tailored specifically for large-scale and efficient calculations. Simulations have been used to estimate tritium travel and residence times in various parts of the system for comparison with observations in wells. Model predictions of mass transport were able to clearly demonstrate radionuclide recycling behavior between the trench and pumping well previously suggested by isotopic age dating information; match travel time estimates for radionuclides moving between the trench, the water table, and monitoring and pumping wells; and provide more realistic ways in which to interpret the pumping well elution curves. Collectively, the results illustrate the utility of integrating detailed numerical modeling with diverse observational data in developing more accurate interpretations of contaminant migration processes.