Robert C. Roback

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 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 >106 for 232Th, ∼104 for 226Ra, and ∼103 for 238U. (3) Dissolution rates of rocks are ∼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 α-recoil of 222Rn occur near the groundwater recharging sites as well as in the major flow pathways. Decay of the sorbed parent radionuclides (e.g., 226Ra and 228Ra) on micro-fracture surfaces constitutes an important source of their daughter (222Rn and 228Th) activities in groundwater.

Groundwater “fast paths” in the Snake River Plain aquifer: Radiogenic isotope ratios as natural groundwater tracers

Preferential flow paths are expected in many groundwater systems and must be located because they can greatly affect contaminant transport. The fundamental characteristics of radiogenic isotope ratios in chemically evolving waters make them highly effective as preferential flow path indicators. These ratios tend to be more easily interpreted than solute-concentration data because their response to water-rock interaction is less complex. We demonstrate this approach with groundwater {sup 87}Sr/{sup 86}Sr ratios in the Snake River Plain aquifer within and near the Idaho National Engineering and Environmental Laboratory. These data reveal slow-flow zones as lower {sup 87}Sr/{sup 86}Sr areas created by prolonged interaction with the host basalts and a relatively fast flowing zone as a high {sup 87}Sr/{sup 86}Sr area.

Uranium isotopic evidence for groundwater chemical evolution and flow patterns in the eastern Snake River Plain aquifer, Idaho

The isotopic composition and concentration of uranium and strontium in groundwater, combined with solute concentration data, provide important details regarding groundwater geochemical evolution and flow-pathways in the eastern Snake River Plain aquifer. The study was conducted in the vicinity of the Idaho National Engineering and Environmental Laboratory (INEEL), Idaho, which has a long history of storing and disposing of radioactive waste, some of which has entered the aquifer. Uranium concentrations in INEEL groundwater range from 0.3 to 3.6 ppb, and 234 U/ 238 U atomic ratios range between 0.000085 and 0.000168 (activity ratios of 1.5 to 3.1). All of the samples have natural 235 U/ 238 U ratios, and 236 U was not detected; thus, the trends delineated by the 234 U/ 238 U ratios reflect natural variations in the aquifer. Groundwater nearest the valleys that provides focused recharge to the Snake River Plain aquifer from the northwest has high 234 U/ 238 U ratios when compared to values of regional groundwater flowing southwestward in the aquifer. Mixing of these water masses can account for the intermediate uranium isotope ratios of some of the samples; however, water-rock interaction must also be invoked to account for the observed trends in isotopic data. Uranium and 87 Sr/ 86 Sr isotope ratios are positively correlated and define a trend toward isotope ratios of the aquifer host rock. These relations indicate that dissolution and/or ion exchange are more important than alpha recoil or selective leaching in controlling 234 U/ 238 U ratios. As a result, 234 U/ 238 U ratios decrease along flow pathways toward the secular equilibrium values of the aquifer host rock. Uranium and strontium isotopic modification can be explained by incongruent dissolution of the host basalt. Lateral distributions of 234 U/ 238 U ratios indicate elongate zones of high 234 U/ 238 U ratios extending southward from the mouths of Birch Creek and the Little Lost River. These elongate zones are interpreted as preferential flow paths. Two isolated pockets of groundwater located in the central and western parts of the study area have lower 234 U/ 238 U ratios than the adjacent aquifer water. Both of these zones are interpreted to contain stagnant waters that are relatively isolated from flow in the regional aquifer due to lower permeability. Physical and chemical evidence strongly suggests that the stagnant zones are dominated by water from the Big Lost River that infiltrated via flood control ponds (spreading areas), playas, and the riverbed.

A research park for studying processes in unsaturated fractured media

A field research site has been developed to explore the combined use of physical experiments and mathematical modeling to analyze large-scale infiltration and chemical transport through the unsaturated media overlying the Snake River Plain Aquifer in southeastern Idaho. This site offers opportunities to observe water and contaminant migration influenced by fluid dynamics and microbial activity through heterogeneous-porous and fractured media. At many waste disposal facilities, the presence of toxic or radioactive wastes between the land surface and underlying aquifers poses a serious and ongoing threat to public health and safety.To reduce the risk associated with these industrial and Cold War by-products, a combination of remediation and long-term monitoring will be required.

Characterization of Contaminant Transport Using Naturally-Occurring U Series Disequilibria: In-Situ Radionuclide Transport and Preferential Groundwater flows at INEEL (Idaho)

The goal of the research is to study the migration of nuclear waste contaminants in subsurface fractured systems using naturally occurring uranium- and thorium-series radionuclides as tracers under in-situ physico-chemical and hydrogeologic conditions. We develop a model of contaminant migration in the Snake River Plain Aquifer beneath the INEEL by evaluating the retardation processes involved in the rock/water interaction. The major tasks are to determine: (1) the distribution of U, Th, Pa, Ra, Rn, Po and Po isotopes in groundwater as well as in rock minerals and sorbed phases, (2) through modeling the extent of disequilibria the in-situ retardation factors of radionuclides and rock/water interaction time scales, and (3) the water residence time in the aquifer and the preferential flow paths. The study provides an improved characterization of preferential flow and contaminant transport in fractured rocks - information that pertains to risk and performance assessment and remediation action at INEEL and other contaminated DOE sites.

Decay-series disequilibrium study of in-situ, long-term radionuclide transport in water-rock systems

Uranium and thorium-series disequilibrium in nature permits the determination of many insitu physico-chemical, geologic and hydrologic variables that control the long-term migration of radionuclides in geologic systems. It also provides site-specific, natural analog information valuable to the assessment of geologic disposal of nuclear wastes. In this study, a model that relates the decay-series radioisotope distributions among solution, sorbed and solid phases in water-rock systems to processes of water transport, sorption-desorption, dissolutionprecipitation, radioactive ingrowth-decay, and α recoil is discussed and applied to a basaltic aquifer at the Idaho National Engineering and Environmental Laboratory (INEEL), Idaho.