In-situ radionuclide transport and preferential groundwater flows at INEEL (Idaho): decay-series disequilibrium studies

• Published in: Geochimica et cosmochimica acta 2000-03, Vol.64 (5), p.867-881
• Main Authors: Luo, Shangde, Ku, Teh-Lung, Roback, Robert, Murrell, Michael, McLing, Travis L.
• Format: Article
• Language: English
• Subjects: AQUIFERS; GROUND WATER; HYDROLOGY; IDAHO NATIONAL ENGINEERING LABORATORY; MANAGEMENT OF RADIOACTIVE WASTES, AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES; NUCLEAR FUEL CYCLE AND FUEL MATERIALS; RADIOACTIVE WASTE DISPOSAL; RADIONUCLIDE MIGRATION; ROCK-FLUID INTERACTIONS; SITE CHARACTERIZATION; USA, Idaho
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/S0016-7037(99)00373-7

Uranium and thorium-decay series disequilibria in groundwater occur as a result of water-rock interactions, and they provide site-specific, natural analog information for assessment of in-situ, long-term migration of radionuclides in the far field of a nuclear waste disposal site. In this study, a mass balance model was used to relate the decay-series radionuclide distributions among solution, sorbed and solid phases in an aquifer system to processes of water transport, sorption-desorption, dissolution-precipitation, radioactive ingrowth-decay, and α recoil. Isotopes of U ( 238U, 234U), Th ( 232Th, 230Th, 228Th, 234Th), Ra ( 226Ra, 228Ra, 224Ra), and Rn ( 222Rn) were measured in 23 groundwater samples collected from a basaltic aquifer at the Idaho National Engineering and Environmental Laboratory (INEEL), Idaho. The results show that groundwater activities of Th and Ra isotopes are 2–4 orders lower than those of their U progenitors which average 1.35 ± 0.40 dpm 238U/L, with 234U/ 238U ratios of ∼1.6–3.0. 222Rn activities range from 20 to 500 dpm/L. Modeling of the observed disequilibria places the following constraints on the time scale of radionuclide migration and water-rock interaction at INEEL: (1) Time for sorption is minutes for Ra and Th; time for desorption is days for Ra and years for Th; and time for precipitation is days for Th, years for Ra, and centuries for U. (2) Retardation factors due to sorption average >10 6 for 232Th, ∼10 4 for 226Ra, and ∼10 3 for 238U. (3) Dissolution rates of rocks are ∼70 to 800 mg/L/y. (4) Ages of groundwater range from