Ground-water transport of tritium and krypton 85 from a nuclear detonation cavity

Abstract

Underground nuclear detonations at the Nevada Test Site provide a unique opportunity to study the hydrologic transport of radionuclides in the field. At the Cambric experiment a pumped well 91 m from the detonation cavity was sampled regularly over 16 years and recovered 94% of the tritium but only 42% of the expected 85Kr, both following decay correction. In addition, the elution curves were different for these two ideal tracers of groundwater movement. Modeling is used to determine the fate of the missing 85Kr based on the phenomenology of underground nuclear detonations and the specific hydrogeology of the Cambric site. At Cambric, large amounts of carbon dioxide created by the detonation caused the upward migration of 85Kr through the collapsing chimney and into the unsaturated zone. A numerical model simulated tritium and 85Kr transport to the pumped well using tritium data to determine regional anisotropy in hydraulic conductivity and reduced hydraulic conductivity in the cavity region. The calibrated model reproduces the 85Kr breakthrough data when emplacement of 85Kr by the upward migration of carbon dioxide is included. Nuclear detonations provide long-term tracer tests in the subsurface, but knowledge of the spatial distribution of sources at this specific site is essential in understanding the transport of these two radionuclide tracers.