Evaluation of the Radiochemistry of Near-Field Water Samples at the Nevada Test Site Applied to the Definition of a Hydrologic Source Term

Abstract

EXECUTIVE SUMMARYFor almost three decades radiochemical data have been collected from water samples that originate from saturated nuclear test cavities, chimney rubble zones, and satellite wells constructed adjacent to underground nuclear tests conducted on the Nevada Test Site (NTS)*_Although these data have been . collected regularly under the auspices of Defense Programs and Environmental Management funded programs, there has been less effort made at interpreting the data to define controls on the dissolved or mobile fraction of radionuclides in groundwater. The need for data analysis have been given more impetus by the contaminant transport in the near-field. These models rely on input parameters a d boundary conditions which can best be obtained through field observations of radionuclide migration adjacent to nuclear tests. The present report is an attempt to synthesize observations and analyze data returned from the near-field over the past ten years and draw general conclusions regarding the behavior of radionuclides in Nevada Test Site groundwaters.. recent effort to construct sophisticated models of hydrologic flow andThe complexity of the hot well problem deserves mention. Most nuclear tests were conducted between -500 and -1200 m below the surface. Besides the obvious residual radioactivity, the post-test environment is characterized by extreme heterogeneity due to the mechanical damage done to the rock units at the working (firing) point. For these reasons, the construction of wells and collection of water samples is a technically complex, expensive, and logistically complicated enterprise. While data exists, it must be interpreted with care and, for these reasons, data interpretations have been limited in scope. In the majority of cases, near-field or "hot" wells were completed opportunistically in post-shot re-entry borings into a nuclear test cavity or chimney, originally drilled to return solid samples diagnostic of device performance. Of the more than eight himdred nuclear tests conducted at the NTS, only fifteen near-field study sites regularly produce water. It must be emphasized that the near-field or "hot well" sampling program is not a monitoring program designed to detect the arrival or movement of radionuclides.. Rather the data from these wells can be used to establish bounds on those radionuclides in solution and to deduce controls on their solubility and mobility. Data returned from near-field wells are affected by well completion techniques and in particular by whether the water is pumped or bailed. Only pumped samples are truly representative of water in the sampling interval, and time-series analyses of pumped cavity water samples have proven particularly important in illuminating the nature of radionuclide migration.The justification for a program to collect near-field groundwater samples can be capsuled in five points. First, hot well sampling and analysis provides an the empirical measure of radionuclide solubility and resulting concentrations of importance to determining risk to ...

Phase I Contaminant Transport Parameters for the Groundwater Flow and Contaminant Transport Model of Corrective Action Unit 97: Yucca Flat/Climax Mine, Nevada Test Site, Nye County, Nevada, Revision 0

“…Samples collected using downhole submersible pumps are more likely to be representative of the bulk fluid composition in the near-field environment. Several of the wells, particularly those that have been converted from post-test re-entry holes, consist of a small diameter (7.3-cm) carbon-steel piezometer tube (Smith, 2002). In most cases, these wells have only been sampled using wireline bailers, although there have been recent attempts to develop pumps that would fit into the small diameter tubing.…”

“…Prompt injection refers to dynamic, early-time movement of radionuclides as plasma or high-temperature gas along zones of weakness in rock adjoining the test cavity (Smith, 2002). Convection in the chimney driven by residual heat from the test can move radionuclides upwards to permeable rock layers.…”

“…This implies that differences in radionuclide production, transport mechanisms, or collapse phenomena produced significantly different radionuclide distributions at BOBAC and JERBOA compared to HYRAX (Smith, 2002). Cs was detected in any cores, although there was a slight increase in 3 H levels over the ambient levels in the very deepest samples (at about 4 cavity radii above the working point).…”

“…Cs ascent through the chimney (Thompson, 1999;Smith, 2002). This implies that differences in radionuclide production, transport mechanisms, or collapse phenomena produced significantly different radionuclide distributions at BOBAC and JERBOA compared to HYRAX (Smith, 2002).…”

Site-Scale Saturated Zone Transport

“…Wells were drilled to obtain radionuclide concentrations in both the near field and far field, and a 16-year aquifer pump test was conducted to determine the migration rates through the alluvium from the test cavity to the pumping well under forced-gradient conditions. The pumping well RNM-2S was drilled 91 m south of the CAMBRIC test and screened from 16 m to 41 m below the bottom of the CAMBRIC cavity (Smith 2002 The lack of arrival of less-mobile radionuclides at the pumping well allows a lower bound to be placed on the retardation factor. As an order-of-magnitude estimate, lack of arrival after 16 years when the first arrival of tritium was about two years led to retardation factors of at least 8.…”