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 re sult of water-rock interactions, and they provide site-specific, natural an alog information for assessment of in-situ, long-term migration of radionuc lides in the far held of a nuclear waste disposal site. In this study, a ma ss balance model was used to relate the decay-series radionuclide distribut ions among solution, sorbed and solid phases in an aquifer system to proces ses of water transport, sorption-desorption, dissolution-precipitation, rad ioactive ingrowth-decay, and alpha recoil. Isotopes of U (U-238, U-234), Th (Th-232, Th-230, Th-228, Th-234), Ra (Ra-226, Ra-228, Ra-224), and Rn (Rn- 222) were measured in 23 groundwater samples collected from a basaltic aqui fer 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 U-238/L, with U-234/U-238 ratios of similar to 1.6-3.0. Rn-222 activities range from 20 to 500 dpm/L. Modeling of the observed disequilibr ia places the following constraints on the time scale of radionuclide migra tion 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 tim e for precipitation is days for Th, years for Ra, and centuries for U. (2) Retardation factors due to sorption average >10(6) for Th-232, similar to 1 0(4) for Ra-226, and similar to 10(3) for U-238. (3) Dissolution rates of r ocks are similar to 70 to 800 mg/L/y. (4) Ages of groundwater range from <1 0 to 100 years. Contours of groundwater age, as well as spatial patterns of radionuclide disequilibria, delineate two north-south preferential flow pa thways and two stagnated locales. Relatively high rates of dissolution and precipitation and alpha-recoil of Rn-222 occur near the groundwater recharg ing sites as well as in the major flow pathways. Decay of the sorbed parent radionuclides (e.g., Ra-266 and Ra-228) on micro-fracture surfaces constit utes an important source of their daughter (Rn-222 and Th-228) activities i n groundwater. Copyright (C) 2000 Elsevier Science Ltd.