Anderson L. Ward

Variably saturated flow and multicomponent biogeochemical reactive transport modeling of a uranium bioremediation field experiment.

Three-dimensional, coupled variably saturated flow and biogeochemical reactive transport modeling of a 2008 in situ uranium bioremediation field experiment is used to better understand the interplay of transport and biogeochemical reactions controlling uranium behavior under pulsed acetate amendment, seasonal water table variation, spatially variable physical (hydraulic conductivity, porosity) and geochemical (reactive surface area) material properties. While the simulation of the 2008 Big Rusty acetate biostimulation field experiment in Rifle, Colorado was generally consistent with behaviors identified in previous field experiments at the Rifle IFRC site, the additional process and property detail provided several new insights. A principal conclusion from this work is that uranium bioreduction is most effective when acetate, in excess of the sulfate-reducing bacteria demand, is available to the metal-reducing bacteria. The inclusion of an initially small population of slow growing sulfate-reducing bacteria identified in proteomic analyses led to an additional source of Fe(II) from the dissolution of Fe(III) minerals promoted by biogenic sulfide. The falling water table during the experiment significantly reduced the saturated thickness of the aquifer and resulted in reactants and products, as well as unmitigated uranium, in the newly unsaturated vadose zone. High permeability sandy gravel structures resulted in locally high flow rates in the vicinity of injection wells that increased acetate dilution. In downgradient locations, these structures created preferential flow paths for acetate delivery that enhanced local zones of TEAP reactivity and subsidiary reactions. Conversely, smaller transport rates associated with the lower permeability lithofacies (e.g., fine) and vadose zone were shown to limit acetate access and reaction. Once accessed by acetate, however, these same zones limited subsequent acetate dilution and provided longer residence times that resulted in higher concentrations of TEAP reaction products when terminal electron donors and acceptors were not limiting. Finally, facies-based porosity and reactive surface area variations were shown to affect aqueous uranium concentration distributions with localized effects of the fine lithofacies having the largest impact on U(VI) surface complexation. The ability to model the comprehensive biogeochemical reaction network, and spatially and temporally variable processes, properties, and conditions controlling uranium behavior during engineered bioremediation in the naturally complex Rifle IFRC subsurface system required a subsurface simulator that could use the large memory and computational performance of a massively parallel computer. In this case, the eSTOMP simulator, operating on 128 processor cores for 12h, was used to simulate the 110-day field experiment and 50 days of post-biostimulation behavior.

Pore-scale simulations of drainage of heterogeneous and anisotropic porous media

A numerical model, based on smoothed particle hydrodynamics, was used to simulate pore-scale liquid and gas flow in synthetic two-dimensional porous media consisting of nonoverlapping grains. The model was used to study the effects of pore-scale heterogeneity and anisotropy on the relationship between the average saturation and the Bond number (strength of the gravitational field acting on fluid density differences relative to capillary forces). Pore-scale anisotropy was created by using co-oriented nonoverlapping elliptical grains, and heterogeneity was created by inserting a microfracture in the middle of the porous domain consisting of nonoverlapping circular grains. The effect of the wetting fluid properties on drainage was also investigated. It is shown that pore-scale heterogeneity and anisotropy can give rise to saturation/Bond number relationships and entry (bubbling) pressures that depend on the flow direction, suggesting that these properties should be described by tensor rather than scalar quan...

Advances in Tensiometry for Long-term Monitoring of Soil Water Pressures

Soil water pressures, measured over space and time, are needed to predict the direction of water flow and chemical transport in the vadose zone. Advanced tensiometers (ATs), which utilize a water-filled porous cup connected directly to a pressure transducer, can be installed at almost any location and depth using standard drilling techniques such as auger drilling, but these methods can significantly disturb the site. For sites where minimal disturbance is desired, alternate approaches for tensiometer placement have been sought. To test installation techniques and performance longevity, advanced tensiometers were placed into the ground at a test site near Richland, WA using two different installation methods, auger drilling and a drive-cone push technique. The tensiometers were subsequently monitored for nearly 2 yr without refilling or recalibration. The data indicated that tensiometers placed by the auger technique took several months to equilibrate, while the cone push units came to equilibrium within 24 h following their installation. Soil water pressures always remained above -90 cm pressure head (-90 mbar) at depths >90 cm. At the greatest depth (730 cm), positive then negative pressures were observed as the water table was lowered and the soil drained. The results suggest that for our test conditions (coarse sandy soil, no vegetation), soil water pressures stay well within the tensiometer range and unit gradient conditions persist, indicating a draining profile. Advanced tensiometers, placed either by auger or cone penetrometer, provide a robust and reliable method for long-term monitoring of soil water pressure profiles.

Laboratory and Modeling Evaluations in Support of Field Testing for Desiccation at the Hanford Site

The Deep Vadose Zone Treatability Test Plan for the Hanford Central Plateau includes testing of the desiccation technology as a potential technology to be used in conjunction with surface infiltration control to limit the flux of technetium and other contaminants in the vadose zone to the groundwater. Laboratory and modeling efforts were conducted to investigate technical uncertainties related to the desiccation process and its impact on contaminant transport. This information is intended to support planning, operation, and interpretation of a field test for desiccation in the Hanford Central Plateau.

Vadose Zone Transport Field Study: Status Report

Studies were initiated at the Hanford Site to evaluate the process controlling the transport of fluids in the vadose zone and to develop a reliable database upon which vadose-zone transport models can be calibrated. These models are needed to evaluate contaminant migration through the vadose zone to underlying groundwaters at Hanford. A study site that had previously been extensively characterized using geophysical monitoring techniques was selected in the 200 E Area. Techniques used previously included neutron probe for water content, spectral gamma logging for radionuclide tracers, and gamma scattering for wet bulk density. Building on the characterization efforts of the past 20 years, the site was instrumented to facilitate the comparison of nine vadose-zone characterization methods: advanced tensiometers, neutron probe, electrical resistance tomography (ERT), high-resolution resistivity (HRR), electromagnetic induction imaging (EMI), cross-borehole radar, and cross-borehole seismic.

Electrical Resistivity Correlation to Vadose Zone Sediment and Pore-Water Composition for the BC Cribs and Trenches Area

This technical report documents the results of geochemical and soil resistivity characterization of sediment obtained from four boreholes drilled in the BC Cribs and Trench area. Vadose zone sediment samples were obtained at a frequency of about every 2.5 ft from approximately 5 ft bgs to borehole total depth. In total, 505 grab samples and 39 six-inch long cores were obtained for characterization. The pore-water chemical composition data, laboratory-scale soil resistivity and other ancillary physical and hydrologic measurements and analyses described in this report are designed to provide a crucial link between direct measurements on sediments and the surface-based electrical-resistivity information obtained via field surveys. A second goal of the sediment characterization was to measure the total and water-leachable concentrations of key contaminants of concern as a function of depth and distance from the footprints of inactive disposal facilities. The total and water-leachable concentrations of key contaminants will be used to update contaminant distribution conceptual models and to provide more data for improving base-line risk predictions and remedial alternative selections. The ERC “ground truthing” exercise for the individual boreholes showed mixed results. In general, the high concentrations of dissolved salts in the pore waters of sediments from C5923, C5924 and C4191 produced a low resistivity “target” in the processed resistivity field surveys, and variability could be seen in the resistivity data that could relate to the variability in pore- water concentrations but the correlations (regression R2 were mediocre ranging from 0.2 to 0.7 at best; where perfect correlation is 1.0). The field-based geophysical data also seemed to suffer from a sort of vertigo, where looking down from the ground surface, the target (e.g., maximum pore-water salt concentration) depth was difficult to resolve. The best correlations between the field electrical resistivity surveys and borehole pore water data sets were obtained when focusing on areal extent of the salt plume. Lateral resolution of the geophysical field data is best conducted by comparing an aggregated set of geophysical data on all boreholes together. When assembling the pore-water data for all four boreholes in an aerial view, the field ERC data produce a reasonable aerial picture of where high salt plumes exist below the BC Cribs and Trenches area. Future work that relies on more laboratory soil resistivity and incorporation of other field data (spectral gamma, neutron moisture and soil density logs) and physical and hydraulic measurements on samples obtained from the boreholes will used develop a more detailed petrophysical model of the sediments below BC Cribs and Trenches. This more detailed model can be used as a more realistic “earth model” in the inversion process to better manipulate the raw field survey data. It is also recommended that one more borehole be drilled after a thorough vetting of the current data with geophysics experts and other Hanford stakeholder to optimize where to place the borehole, what electrical and other geophysical surveys should be conducted , where to take sediment samples and what parameters should be measured on the sediments to attempt one more “ground truthing” exercise.

Vadose Zone Transport Field Study: Soil Water Content Distributions by Neutron Moderation

Contaminant transport through the vadose zone is a complex process controlled largely by interactions between subsurface lithologic features, water flow, and fluid properties. Understanding the processes controlling transport is an important prerequisite to the development and implementation of effective soil and ground water remediation programs. However, difficulties in directly observing and sampling the subsurface can complicate attempts to better describe subsurface transport processes and is mostly responsible for the large amount of uncertainty associated with vadose zone processes. The reduction of the uncertainty has been identified as a site need at Hanford by the STCG and the National Research Council (2000a) and is a key aspect of the site?s science and technology effort.

Vadose Zone Transport Field Study: FY 2002 Status Report

This work reported here is part of the U. S. Department of Energy’s Science and Technology Initiative to develop improved conceptual models of flow and transport in the vadose zone, particularly for the Hanford Site, Washington. The National Academy of Sciences has identified significant knowledge gaps in conceptual model development as one reason for discovery of subsurface contamination in unexpected places. Inadequate conceptualizations limits, not only the understanding of long-term fate and transport, but also the selection and design of remediation technologies. Current conceptual models are limited partly because they do not account for the random heterogeneity that occurs under the extremes of very nonlinear flow behavior typical of the Hanford vadose zone. A major improvement in conceptual modeling of the Hanford vadose zone includes a better understanding and description of soil anisotropy, a property that appears to control much of the subsurface flow and transport in layered sediments at the Hanford Site.

Hydrologic Characterization Using Vadose Zone Monitoring Tools: Status Report

Hydrologic characterization of the vadose zone (from soil surface to the underlying water table) is needed to assess contaminant migration from buried wastes. The Pacific Northwest National Laboratory, under contract with the U. S. Department of Energys EM-50 (Subsurface Contamination Focus Area), and in collaboration with CH2MHILL Hanford Group, the Idaho National Engineering and Environmental Laboratory (INEEL), and Duratek Federal Services (DFS), deployed a suite of vadose-zone instruments at the Hanford Site near Richland, Washington. Several new instruments were tested.

Vadose Zone Transport Field Study: FY 2001 Test Plan

The primary objective of the Vadose Zone Transport Field Study is to obtain hydrologic, geophysical, and geochemical data from controlled field studies to reduce the uncertainty in vadose-zone conceptual models and to facilitate the calibration of numerical models for water flow and contaminant transport through Hanfords heterogeneous vadose zone. A secondary objective is to evaluate advanced, cost-effective characterization methods with the potential to assess changing conditions in the vadose zone, particularly as surrogates of currently undetectable high-risk contaminants. The study is designed to assure the measurement of flow-and-transport properties in the same soil volume, a pre-requisite for developing techniques for extrapolating parameters derived from investigations at clean representative sites to contaminated sites with minimal characterization.