In-situ radionuclide transport and preferential groundwater flows at INEEL (Idaho): Decay-series disequilibrium studies
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 {alpha} recoil. Isotopes of U and Rn 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. 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{sup 6} for {sup 232}Th, {approximately}10{sup 4} for {sup 226}Ra, and {approximately}10{sup 3} for {sup 238}U. (3) Dissolution rates of rocks are {approximately}70 to 800 mg/L/y. (4) Ages of groundwater range from <10 to 100 years. Contours of groundwater age, as well as spatial patterns of radionuclide disequilibria, delineate two north-south preferential flow pathways and two stagnated locales. Relatively high rates of dissolution and precipitation and {alpha}-recoil of {sup 222}Rn occur near the groundwater recharging sites as well as in the major flow pathways. Decay of the sorbed parent radionuclides (e.g., {sup 226}Ra and {sup 228}Ra) on micro-fracture surfaces constitutes an important source of their daughter ({sup 222}Rn and {sup 228}Th) activities in groundwater.
- Research Organization:
- Univ. of Southern California, Los Angeles, CA (US)
- OSTI ID:
- 20020872
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 64, Issue 5; Other Information: PBD: Mar 2000; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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