828 underground nuclear tests were conducted at the Nevada National Security Site during the period of January 1951 through September 1992. Approximately 1/3 of these tests were conducted at or below the water table, depositing significant quantities of radionuclides that are potentially susceptible to transport via groundwater flow. The DOE funds a major program, the UGTA project, to understand the nature and extent of contamination in the saturated zone. The strategy for achieving this objective is dependent upon an iterative approach of field characterization followed by groundwater flow and contaminant transport modeling. The ultimate objective is to predict, with a high degree of confidence, where and how rapidly groundwater may transport contaminants and at what concentration. DRI supports this effort through a variety of projects that utilize geochemistry, geophysics, groundwater hydraulics, geostatistics, and groundwater modeling in the achievement of the research objectives.
DRI’s UGTA research endeavors falls into the following areas:
- Groundwater Recharge using the chloride mass balance approach as well as an integrated soil physics and vadose-zone modeling approach.
- Well hydraulics and aquifer testing. This includes the application and interpretation of a variety of geophysical tools for determining groundwater flow (as a function of depth) within boreholes under stressed (pumping) and unstressed (ambient) conditions.
- Laboratory investigations of parameters that affect the transport of radionuclides (sorption, matrix diffusion, glass surface area) as well as the measurement of dissolved organic carbon-14, Oxygen-18, deuterium, and carbon-13.
- Geochemical modeling of isotopic and ionic tracers. Kinetic and mass-balance models utilize measures of naturally occurring tracers within the groundwater to constrain the results of groundwater flow models based upon potentiometric heads and the distribution of hydraulic conductivity.
- Modeling groundwater flow and transport. This project has previously focused on representing the spatial variability of hydrogeologic parameters and the development of efficient semi-analytical tools and is now moving into decision support analysis and numerical flow and transport modeling.
In addition, various personnel from DRI belong to and support the UGTA Technical Working Group. This group makes recommendations to DOE relative to scientific and technical studies that promote the effective closure of Corrective Actions Units (CAUs) on the Nevada National Security Site (NNSS) and ensure the continuing protection of public health.
The results of DRI’s effort are collaborative in nature and are often published jointly with the USGS, Lawrence Livermore and Los Alamos National laboratories as well various environmental contractors. These reports provide the basis for subsequent numerical modeling and support the Department of Energy in the decision making process requisite to the closure of sites impacted by underground nuclear testing.