Nuclear wastes from Hanford’s processing for separation of plutonium are stored in massive, buried, single-shell tanks in 18 tank farms. These so-called tank wastes were initially thermally hot because of radioactive decay, and many exhibited extreme chemical character in terms of pH, salinity, and radionuclide concentration. At present, 67 of the 149 single shell tanks are suspected to have released over 1.9 million L of tank waste to the vadose zone, with most leak events occurring between 1950 and 1975. Boreholes have been placed through the largest vadose zone plumes to defi ne the extent of contaminant migration and to develop conceptual models of processes governing the transformation, retardation, and overall transport of tank waste residuals. Laboratory studies with sediments so collected have shown that ion exchange, precipitation and dissolution, and surface complexation reactions have occurred between the tank wastes and subsurface sediments, moderating their chemical character and retarding the migration of select contaminants. Processes suspected to facilitate the far-fi eld migration of immobile radionuclides including stable aqueous complex formation and mobile colloids were found to be potentially operative but unlikely to occur in the fi eld, with the exception of cyanide-facilitated migration of 60 Co. Certain fi ssion product oxyanions (Mo, Ru, Se, Tc) and nitrates are the most mobile of tank waste constituents because their adsorption is suppressed by large concentrations of waste anions, the vadose zone clay fraction is negative in surface charge, and, unlike Cr, their reduced forms are unstable in oxidizing environments. Reaction/process-based transport modeling is beginning to be used for predictions of future contaminant mobility and plume evolution.

/pdf/geochemical-processes-controlling-migration-of-tank-wastes-28lv5ukngw.pdf

Geochemical Processes Controlling Migration of Tank Wastes in Hanford's Vadose Zone

[1] While pedotransfer functions (PTFs) have long been applied to estimate soil hydraulic parameters for unsaturated flow and solute transport modeling, the uncertainty associated with the estimates is often ignored. The objective of this study is to evaluate uncertainty of the PTF-estimated soil hydraulic parameters and its effect on numerical simulation of moisture flow. Contributing to the parameter estimation uncertainty are (1) the PTF intrinsic uncertainty caused by limited data used for PTF training and (2) the PTF input uncertainty in pedotransfer variables (i.e., PTF inputs). The PTF intrinsic uncertainty is assessed using the bootstrap method by generating multiple bootstrap realizations of the soil hydraulic parameters; the realizations follow normal or lognormal distributions. The PTF input variables (i.e., bulk density and soil texture) are obtained using the cokriging technique. The PTF input uncertainty is quantified by assuming that the cokriging estimates follow a normal distribution. Our results show that the PTF input uncertainty dominates over the PTF intrinsic uncertainty and determines the spatial distribution of the PTF parameter estimation uncertainty. When the parameter estimation uncertainty is included, the spatial variability of the measured soil hydraulic parameters is better captured. This is also the case for the observed moisture contents, whose spatial variability is well bracketed by the prediction intervals. However, this is only possible after the PTF input uncertainty is considered. These results suggest that additional sample acquisition for the PTF input variables would have a more favorable impact on reduction of the parameter estimation uncertainty than collecting additional soil hydraulic parameter measurements for PTF development.

/pdf/quantification-of-uncertainty-in-pedotransfer-function-based-15m7cxq7wk.pdf

Quantification of uncertainty in pedotransfer function-based parameter estimation for unsaturated flow modeling

[1] By leveraging use of ‘‘soft’’ data (e.g., initial moisture content, qi), this study applies the transition probability (TP) based Markov chain (MC) model to sediment textural classes for characterizing the medium heterogeneity and sediment layering structure. The TP/MC method is evaluated by simulating the vadose zone moisture movement at a field site, where the stratigraphy consists of imperfectly stratified soil layers. Soil heterogeneity is characterized via spatial variability of the geometry of soil textural classes. When the qi measurements, which carry signature about medium heterogeneity and stratigraphy, are not included in the TP/MC model, it is not possible to identify the horizontal TP. The qi measurements, when transformed into soil classes, are necessary in mapping the soil layering structure prevalent at the site. The soil hydraulic parameters for each soil class are treated deterministically and are estimated on the basis of core samples. To evaluate uncertainty in characterizing geometry of the soil classes, multiple conditional realizations of the soil classes are generated. A Monte Carlo simulation shows that the simulated mean moisture contents agree well with corresponding field observations. The observed splitting of the moisture plume in a coarse sand layer that is sandwiched between two fine-textured layers, the southeastward movement of the plume during the redistribution period, and the near-zero fluid flux below the bottom fine layer are adequately simulated. Spatial variability of the field-measured moisture content is sufficiently captured by the 95% confidence intervals calculated from the Monte Carlo simulations. Investigating the effect of data conditioning on the simulated results shows that a reduction of conditioning data does not necessarily deteriorate simulation results if other conditioning data exist within the mean length of the soil classes. The TP/MC method is flexible so that other types of site characterization data (e.g., geophysical data) can be incorporated as they become available.

/pdf/a-markov-chain-model-for-characterizing-medium-heterogeneity-1oejtpf7li.pdf

A Markov chain model for characterizing medium heterogeneity and sediment layering structure

Researchers at Pacific Northwest National Laboratory developed field-scale groundwater flow and transport simulations of the 300 Area to support the 300-FF-5 Operable Unit Phase III Feasibility Study. The 300 Area is located in the southeast portion of the U.S. Department of Energy’s Hanford Site in Washington State. Historical operations involving uranium fuel fabrication and research activities at the 300 Area have contaminated engineered liquid-waste disposal facilities, the underlying vadose zone, and the uppermost aquifer with uranium. The main objectives of this research were to develop numerical groundwater flow and transport models to help refine the site conceptual model, and to assist assessment of proposed alternative remediation technologies focused on the 300 Area uranium plume.

/pdf/three-dimensional-groundwater-models-of-the-300-area-at-the-cqy8du8nrb.pdf

Three-Dimensional Groundwater Models of the 300 Area at the Hanford Site, Washington State

This data package discusses the geochemistry of vadose zone sediments beneath the single-shell tank farms at the U.S. Department of Energy’s (DOE’s) Hanford Site. The purpose of the report is to provide a review of the most recent and relevant geochemical process information available for the vadose zone beneath the single-shell tank farms and the Integrated Disposal Facility. Two companion reports to this one were recently published which discuss the geology of the farms (Reidel and Chamness 2007) and groundwater flow and contamination beneath the farms (Horton 2007).

/pdf/geochemical-processes-data-package-for-the-vadose-zone-in-458y0gn1nj.pdf

Geochemical Processes Data Package for the Vadose Zone in the Single-Shell Tank Waste Management Areas at the Hanford Site

From FY 2000 through FY 2003, a series of vadose zone transport field experiments were conducted as part of the U.S. Department of Energy’s Groundwater/Vadose Zone Integration Project Science and Technology Project, now known as the Remediation and Closure Science Project, and managed by the Pacific Northwest National Laboratory (PNNL). The series of experiments included two major field campaigns, one at a 299-E24-11 injection test site near PUREX and a second at a clastic dike site off Army Loop Road. The goals of these experiments were to improve our understanding of vadose zone transport processes; to develop data sets to validate and calibrate vadose zone flow and transport models; and to identify advanced monitoring techniques useful for evaluating flow-and-transport mechanisms and delineating contaminant plumes in the vadose zone at the Hanford Site. This report summarizes the key findings from the field studies and demonstrates how data collected from these studies are being used to improve conceptual models and develop numerical models of flow and transport in Hanford’s vadose zone. Results of these tests have led to a better understanding of the vadose zone. Fine-scale geologic heterogeneities, including grain fabric and lamination, were observed to have a strong effect on the large-scale behavior of contaminant plumes, primarily through increased lateral spreading resulting from anisotropy. Conceptual models have been updated to include lateral spreading and numerical models of unsaturated flow and transport have revised accordingly. A new robust model based on the concept of a connectivity tensor was developed to describe saturation-dependent anisotropy in strongly heterogeneous soils and has been incorporated into PNNL’s Subsurface Transport Over Multiple Phases (STOMP) simulator. Application to field-scale transport problems have led to a better understanding plume behavior at a number of sites where lateral spreading may have dominated waste migration (e.g. BC Cribs and Trenches). The improved models have been also coupled with inverse models and newly-developed parameter scaling techniques to allow estimation of field-scale and effective transport parameters for the vadose zone. The development and utility of pedotransfer functions for describing fine-scale hydrogeochemical heterogeneity and for incorporating this heterogeneity into reactive transport models was explored. An approach based on grain-size statistics appears feasible and has been used to describe heterogeneity in hydraulic properties and sorption properties, such as the cation exchange capacity and the specific surface area of Hanford sediments. This work has also led to the development of inverse modeling capabilities for time-dependent, subsurface, reactive transport with transient flow fields using an automated optimization algorithm. In addition, a number of geophysical techniques investigated for their potential to provide detailed information on the subtle changes in lithology and bedding surfaces; plume delineation, leak detection. High-resolution resistivity is now being used for detecting saline plumes at several waste sites at Hanford, including tank farms. Results from the field studies and associated analysis have appeared in more than 46 publications generated over the past 4 years. These publications include test plans and status reports, in addition to numerous technical notes and peer reviewed papers.

/pdf/vadose-zone-transport-field-study-summary-report-1nk50kz7ll.pdf

Vadose Zone Transport Field Study: Summary Report

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Vadose Zone Transport Field Study: Summary Report