Travis L. McLing

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.

Cr Stable Isotopes in Snake River Plain Aquifer Groundwater: Evidence for Natural Reduction of Dissolved Cr(VI)

At Idaho National Laboratory, Cr(VI) concentrations in a groundwater plume once exceeded regulatory limits in some monitoring wells but have generally decreased over time. This study used Cr stable isotope measurements to determine if part of this decrease resulted from removal of Cr(VI) via reduction to insoluble Cr(III). Although waters in the study area contain dissolved oxygen, the basalt host rock contains abundant Fe(II) and may contain reducing microenvironments or aerobic microbes that reduce Cr(VI). In some contaminated locations, (53)Cr/(52)Cr ratios are close to that of the contaminant source, indicating a lack of Cr(VI) reduction. In other locations, ratios are elevated. Part of this shift may be caused by mixing with natural background Cr(VI), which is present at low concentrations but in some locations has elevated (53)Cr/(52)Cr. Some contaminated wells have (53)Cr/(52)Cr ratios greater than the maximum attainable by mixing between the inferred contaminant and the range of natural background observed in several uncontaminated wells, suggesting that Cr(VI) reduction has occurred. Definitive proof of reduction would require additional evidence. Depth profiles of (53)Cr/(52)Cr suggest that reduction occurs immediately below the water table, where basalts are likely least weathered and most reactive, and is weak or nonexistent at greater depth.

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.

Cs+ speciation on soil particles by TOF-SIMS imaging

Soil particles exposed to CsI solutions were analyzed by imaging time-of-flight secondary ion mass spectrometry and also by scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS). The results showed that Cs(+) could be detected and imaged on the surface of the soil particles readily at concentrations down to 160 ppm, which corresponds to 0.04 monolayer. Imaging revealed that most of the soil surface consisted of aluminosilicate material. However, some of the surface was more quartzic in composition, primarily SiO(2) with little Al. It was observed that adsorbed Cs(+) was associated with the presence of Al on the surface of the soil particles. In contrast, in high SiO(2) areas of the soil particle where little Al was observed, little adsorbed Cs(+) was observed on the surface of the soil particle. Using EDS, Cs(+) was observed only in the most concentrated Cs(+)-soil system, and Cs(+) was clearly correlated with the presence of Al and I. These results are interpreted in terms of multiple layers of CsI forming over areas of the soil surface that contain substantial Al. These observations are consistent with the hypothesis that the insertion of Al into the SiO(2) lattice results in the formation of anionic sites, which are then capable of binding cations.

Chapter 7 – Uranium Sorption onto Natural Sands as a Function of Sediment Characteristics and Solution pH

Uranium sorption studies were conducted on twenty-five sandy sediments obtained from Virginia`s Easter Shore Peninsula using batch contact methods. Distribution coefficient (Kd) and sorption isotherms have been determined as function of solution pH. All sediment samples strongly sorbed dissolved uranium species at pH values above 5. Sediments characterized by high iron, aluminum, and surface area, possessed the highest sorption characteristics. Both, Freundlich and Dubinin Radushkevich equations were able to describe the sorption behavior. From the Dubinin-Radushkevich equation, the solution-component-solid surface bounding energy and the sorption capacities, were estimated. Least square regression utilizing sediment characteristics provided an effective statistical method for Kd prediction. Predictions with relative errors of about 30% were obtained using only two sediment variables, such as, iron or aluminum content and surface area. In conclusion, the results support that ion exchange and surface complexation reactions with the ferric and aluminum oxide/oxyhydroxides groups are the predominant sorption mechanisms.

Physical constraints on geologic CO2 sequestration in low-volume basalt formations

Deep basalt formations within large igneous provinces have been proposed as target reservoirs for carbon capture and sequestration on the basis of favorable CO 2 -water-rock reaction kinetics that suggest carbonate mineralization rates on the order of 10 2 –10 3 d. Although these results are encouraging, there exists much uncertainty surrounding the influence of fracture-controlled reservoir heterogeneity on commercial-scale CO 2 injections in basalt formations. This work investigates the physical response of a low-volume basalt reservoir to commercial-scale CO 2 injections using a Monte Carlo numerical modeling experiment such that model variability is solely a function of spatially distributed reservoir heterogeneity. Fifty equally probable reservoirs are simulated using properties inferred from the deep eastern Snake River Plain aquifer in southeast Idaho, and CO 2 injections are modeled within each reservoir for 20 yr at a constant mass rate of 21.6 kg s –1 . Results from this work suggest that (1) formation injectivity is generally favorable, although injection pressures in excess of the fracture gradient were observed in 4% of the simulations; (2) for an extensional stress regime (as exists within the eastern Snake River Plain), shear failure is theoretically possible for optimally oriented fractures if S h ≤ 0.70S V ; and (3) low-volume basalt reservoirs exhibit sufficient CO 2 confinement potential over a 20 yr injection program to accommodate mineral trapping rates suggested in the literature.

CO2 sequestration in basalt: Carbonate mineralization and fluid substitution

Geological sequestration of carbon dioxide in deep reservoirs may provide a large-scale option for reducing the emissions of this gas into the atmosphere. The effectiveness of sequestration depends on the storage capacity and stability of the reservoir and risk of leakage into the overburden. Reservoir rocks can react with a CO2-water mixture, potentially resulting in the precipitation of minerals in the available matrix pore space and within pre-existing fractures. This induced mineralization may form internal seals that could help mitigate the leakage of CO2 into the overburden. For basaltic host rocks, carbonic acid partially dissolves minerals in the host rock, such as the calcium plagioclase mineral, freeing various cations (e.g., Ca2+ and Mg2+) for later precipitation as carbonate cements (Gislason et al., 2010).

A field sampling strategy for semivariogram inference of fractures in rock outcrops

The stochastic continuum (SC) representation is one common approach for simulating the effects of fracture heterogeneity in groundwater flow and transport models. These SC reservoir models are generally developed using geostatistical methods (e.g., kriging or sequential simulation) that rely on the model semivariogram to describe the spatial variability of each continuum. Although a number of strategies for sampling spatial distributions have been published in the literature, little attention has been paid to the optimization of sampling in resource- or access-limited environments. Here we present a strategy for estimating the minimum sample spacing needed to define the spatial distribution of fractures on a vertical outcrop of basalt, located in the Box Canyon, east Snake River Plain, Idaho. We used fracture maps of similar basalts from the published literature to test experimentally the effects of different sample spacings on the resulting semivariogram model. Our final field sampling strategy was based on the lowest sample density that reproduced the semivariogram of the exhaustively sampled fracture map. Application of the derived sampling strategy to an outcrop in our field area gave excellent results, and illustrates the utility of this type of sample optimization. The method will work for developing a sampling plan for any intensive property, provided prior information for a similar domain is available; for example, fracture maps or ortho-rectified photographs from analogous rock types could be used to plan for sampling of a fractured rock outcrop.

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.