HR: 08:45h
AN: GS22A-03    [Abstracts]
TI: Increased Concentrations of Short-Lived Decay-Series Radionuclides in Groundwaters Underneath the Nopal I Uranium Deposit at Pena Blanca, Mexico
AU: * Luo, S
EM: sluo@usc.edu
AF: National Cheng-Kung University, Department of Earth Sciences, Tainan, 701, Taiwan
AU: * Luo, S
EM: sluo@usc.edu
AF: University of Southern California, Department of Earth Sciences, Los Angeles, CA 90089- 0740, United States
AU: Ku, T
EM: rku@usc.edu
AF: University of Southern California, Department of Earth Sciences, Los Angeles, CA 90089- 0740, United States
AU: Todd, V
EM: vtodd@usc.edu
AF: University of Southern California, Department of Earth Sciences, Los Angeles, CA 90089- 0740, United States
AU: Murrell, M T
EM: mmurrell@lanl.gov
AF: Los Alamos National Laboratory, CST-11, Los Alamos, NM 87545, United States
AU: Dinsmoor, J C
EM: john_dinsmoor@lanl.gov
AF: Los Alamos National Laboratory, CST-11, Los Alamos, NM 87545, United States
AB: The Nopal I uranium ore deposit at Pena Blanca, Mexico, located at > 200 meters above the groundwater table, provides an ideal natural analog for quantifying the effectiveness of geological barrier for isolation of radioactive waste nuclides from reaching the human environments through ground water transport. To fulfill such natural analog studies, three wells (PB1, PB2, and PB3 respectively) were drilled at the site from the land surface down to the saturated groundwater zone and ground waters were collected from each of these wells through large- volume sampling/in-situ Mn-filter filtration for analyses of short-lived uranium/thorium-series radionuclides. Our measurements from PB1 show that the groundwater standing in the hole has much lower 222Rn activity than the freshly pumped groundwater. From this change in 222Rn activity, we estimate the residence time of groundwater in PB1 to be about 20 days. Our measurements also show that the activities of short-lived radioisotopes of Th (234Th), Ra (228Ra, 224Ra, 223Ra), Rn (222Rn), Pb (210Pb), and Po (210Po) in PB1, PB2, and PB3 are all significantly higher than those from the other wells near the Nopal I site. These high activities provide evidence for the enrichment of long-lived U and Ra isotopes in the groundwater as well as in the associated adsorbed phases on the fractured aquifer rocks underneath the ore deposit. Such enrichment suggests a rapid dissolution of U and Ra isotopes from the uranium ore deposit in the vadose zone and the subsequent migration to the groundwater underneath. A reactive transport model can be established to characterize the in-situ transport of radionuclides at the site. The observed change of 222Rn activity at PB1 also suggests that the measured high radioactivityies in ground waters from the site isare not an artifact of drilling operations. However, further studies are needed to assess if or to what extent the radionuclide migration is affected by the previous mining activities at the site.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 1806 Chemistry of fresh water
DE: 1832 Groundwater transport
SC: Geochemical Society [GS]
MN: 2007 Joint Assembly