On the evaluation of groundwater contamination from underground nuclear tests

Increasing concern about radioactive contamination of groundwater from underground nuclear tests has reinforced the need for a basic understanding of how the radionuclide inventories of such tests enter and migrate through groundwater. As a basis for studying these processes, the physically and thermally disturbed geologic environment produced by such tests and its relation to the post-test distribution of radionuclides is discussed from a conceptual perspective. These concepts are used to support the development of a reactive transport model to evaluate the nature and extent of radionuclide contamination within alluvium surrounding a specific underground nuclear test at the Nevada Test Site (NTS). Simulations are focused on determining the abundance and chemical nature of radionuclides that are introduced into groundwater, as well as the rate and extent of radionuclide migration and reaction in groundwater surrounding the working point of the test. Transport simulations based upon a streamline-based numerical model are used to illustrate the nature of radionuclide elution out of the near-field environment and illustrate the conceptual modeling process. The numerical approach allowed for relatively complex flow and chemical reactions to be considered in a computationally efficient manner. The results are particularly sensitive to the rate of melt glass dissolution, distribution of reactive minerals in the alluvium, and overall groundwater flow configuration. They provide a rational basis from which defensible migration assessments over larger spatial and temporal scales can proceed.