HR: 08:30h
AN: GS22A-02    [Abstracts]
TI: Modeling of U-series Radionuclide Transport Through Soil at Pena Blanca, Chihuahua, Mexico
AU: * Pekar, K E
EM: pekar@geo.utep.edu
AF: Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 79968, United States
AU: Goodell, P C
EM: goodell@geo.utep.edu
AF: Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 79968, United States
AU: Walton, J C
EM: walton@utep.edu
AF: Department of Civil Engineering, University of Texas at El Paso, El Paso, TX 79968, United States
AU: Anthony, E Y
EM: eanthony@geo.utep.edu
AF: Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 79968, United States
AU: Ren, M
EM: ren@geo.utep.edu
AF: Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 79968, United States
AB: The Nopal I uranium deposit is located at Pena Blanca in Chihuahua, Mexico. Mining of high-grade uranium ore occurred in the early 1980s, with the ore stockpiled nearby. The stockpile was mostly cleared in the 1990s; however, some of the high-grade boulders have remained there, creating localized sources of radioactivity for a period of 25-30 years. This provides a unique opportunity to study radionuclide transport, because the study area did not have any uranium contamination predating the stockpile in the 1980s. One high-grade boulder was selected for study based upon its shape, location, and high activity. The presumed drip-line off of the boulder was marked, samples from the boulder surface were taken, and then the boulder was moved several feet away. Soil samples were taken from directly beneath the boulder, around the drip-line, and down slope. Eight of these samples were collected in a vertical profile directly beneath the boulder. Visible flakes of boulder material were removed from the surficial soil samples, because they would have higher concentrations of U-series radionuclides and cause the activities in the soil samples to be excessively high. The vertical sampling profile used 2-inch thicknesses for each sample. The soil samples were packaged into thin plastic containers to minimize the attenuation and to standardize sample geometry, and then they were analyzed by gamma-ray spectroscopy with a Ge(Li) detector for Th-234, Pa-234, U-234, Th-230, Ra-226, Pb-214, Bi-214, and Pb-210. The raw counts were corrected for self-attenuation and normalized using BL-5, a uranium standard from Beaverlodge, Saskatchewan. BL-5 allowed the counts obtained on the Ge(Li) to be referenced to a known concentration or activity, which was then applied to the soil unknowns for a reliable calculation of their concentrations. Gamma ray spectra of five soil samples from the vertical profile exhibit decreasing activities with increasing depth for the selected radionuclides. Independent multi-element analyses of three samples by ICP-MS show decreasing uranium concentration with depth as well. The transport of the radionuclides is evaluated using STANMOD, a Windows-based software package for evaluating solute transport in porous media using analytical solutions of the advection-dispersion solute transport equation. The package allows various one-dimensional, advection-dispersion parameters to be determined by fitting mathematical solutions of theoretical transport models to observed data. The results are promising for future work on the release rate of radionuclides from the boulder, the dominant mode of transport (e.g., particulate or dissolution), and the movement of radionuclides through porous media. The measured subsurface transport rates provide modelers with a model validation dataset.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1040 Radiogenic isotope geochemistry
SC: Geochemical Society [GS]
MN: 2007 Joint Assembly