A Serendipitous, Long-Term Infiltration Experiment: Water and Tritium Circulation Beneath the CAMBRIC Ditch at the Nevada Test Site

Underground nuclear weapons testing at the Nevada Test Site introduced numerous radionuclides that may be used to characterize subsurface hydrologic transport processes in arid climates. A sixteen year pumping experiment designed to examine radionuclide migration away from the CAMBRIC nuclear test, conducted in groundwater beneath Frenchman Flat in 1965, 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, tailored specifically for large scale and efficient calculations. Simulations have been used to estimate radionuclide 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 ditch and pumping well previously suggested by isotopic age dating information; match travel time estimates for radionuclides moving between the ditch, the water table, and monitoring 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 accurate interpretations and forecasts of contaminant migration processes.