[en] Numerical modeling has become a critical tool to the Department of Energy for evaluating the environmental impact of alternative energy sources and remediation strategies for legacy waste sites. Unfortunately, the physical and chemical complexity of many sites overwhelms the capabilities of even most 'state of the art' groundwater models. Of particular concern are the representation of highly-heterogeneous stratified rock/soil layers in the subsurface and the biological and geochemical interactions of chemical species within multiple fluid phases. Clearly, there is a need for higher-resolution modeling (i.e. more spatial, temporal, and chemical degrees of freedom) and increasingly mechanistic descriptions of subsurface physicochemical processes. We present research being performed in the development of PFLOTRAN, a parallel multiphase flow and multicomponent reactive transport model. Written in Fortran90, PFLOTRAN is founded upon PETSc data structures and solvers and has exhibited impressive strong scalability on up to 4000 processors on the ORNL Cray XT3. We are employing PFLOTRAN in the simulation of uranium transport at the Hanford 300 Area, a contaminated site of major concern to the Department of Energy, the State of Washington, and other government agencies where overly-simplistic historical modeling erroneously predicted decade removal times for uranium by ambient groundwater flow. By leveraging the billions of degrees of freedom available through high-performance computation using tens of thousands of processors, we can better characterize the release of uranium into groundwater and its subsequent transport to the Columbia River, and thereby better understand and evaluate the effectiveness of various proposed remediation strategies

[en] We have completed the studies on reactions of minerals with caustic Hanford tank waste solutions. Systematic studies on the effects of different anions, cations, and the radionuclide Cs-137 were completed and technical manuscripts on these experiments were submitted for publication. The concentration of NaOH and the type of anion played the dominant roles in determining minerals formed. Increasing NaOH concentration and temperature enhanced the formation of feldspathoids; when NaOH concentration was high (e.g.,16 M), stable cancrinite and sodalite formed rapidly. Cancrinite formed in the presence of nitrate or sulfate; sodalite formed in the presence of chloride, carbonate or without added anions. Low concentration of Cs (< 100 mM) did not affect the formation of lepispheric cancrinite and sodalite, whereas only highly crystalline cancrinite formed when Cs concentration was >250mM. The presence of K did not alter but slowed down the formation of cancrinite and sodalite. The presence of divalent cations led to the formation of intermediate or stable silicates, aluminates, hydroxides or even aluminosilicates. We investigated the incorporation of Cs and the stability of the incorporated Cs in feldspathoids, zeolites, and allophane that may form in the sediments under conditions mimicking Hanford tank leaks. The incorporated Cs was quantified by atomic absorption spectroscopy after digestion in 1 M HCl. Cancrinite, sodalite, LTA zeolite, the 3-D cross-shaped zeolite, and allophane were capable to preferentially incorporate Cs when they form in the alkaline simulants

[en] Migration of radionuclides under the SX-tank farm at the Hanford nuclear waste complex involves interaction of variably water saturated sediments with concentrated NaOH-NaNO₃-NaNO₂ solutions that have leaked from the tanks. Constant K_d models for describing radionuclide retardation are not valid under these conditions because of strong competition for sorption sites by abundant Na⁺ ions, and because of dramatically changing solution compositions with time as the highly concentrated tank fluid becomes diluted as it mixes with infiltrating rainwater. A mechanistic multicomponent sorption model is required that can account for effects of competition and spatially and temporally variable solution compositions. To investigate the influence of the high ionic strength tank fluids on Cs⁺ migration, numerical calculations are performed using the multiphase-multicomponent reactive transport code FLOTRAN. The computer model describes reactive transport in nonisothermal, variably saturated porous media including both liquid and gas phases. Pitzer activity coefficient corrections are used to describe the high ionic strength solutions. The calculations take into account multicomponent cation exchange based on measured selectivity coefficients specific to the Hanford sediments. Solution composition data obtained from Well 299-W23-19, documenting a moderately concentrated leak from the SX-115 tank, are used to calibrate the model. In addition to exchange of cations Na⁺, K⁺, Ca²⁺, and Cs⁺, aqueous complexing and a kinetic description of precipitation and dissolution of calcite are also included in the calculations. The fitted infiltration rate of 0.08 m yr^-1, and fitted cation exchange capacity of 0.05 mol kg^-1 are consistent with measured values for the Hanford sediments. A sensitivity analysis is performed for Na⁺ concentrations ranging from 5 to 20 m to investigate the mobility of Cs⁺ interacting with a highly concentrated background electrolyte solution believed to have been released from the SX-108/SX-109 tanks. The calculations indicate that during the initial period of the tank leak when Cs⁺ is associated with high Na⁺ concentrations, there is little retardation of the Cs⁺ plume. However, as time increases the Na⁺ and Cs⁺ profiles become chromatographically separated due to differences in their selectivity coefficients and dilution of the tank leak plume with infiltrating rainwater. Eventually the two species become separated spatially, and Cs⁺ becomes highly retarded and remains essentially fixed in the sediments by cation exchange. For the 20 m Na⁺ simulated tank leak, the sorbed Cs⁺ profile is in close agreement with data obtained from the slant borehole and consistent with the estimated tank supernatant concentration. The simulations suggest that natural attenuation processes should result in strong fixation of Cs⁺ in the vadose zone in spite of the release of high Na⁺ concentrations during a tank leak event

[en] Migration of radionuclides under the SX-tank farm at the Hanford nuclear waste complex involves interaction of variably water saturated sediments with concentrated NaOH-NaNO3-NaNO2 solutions that have leaked from the tanks. Constant Kd models for describing radionuclide retardation are not valid under these conditions because of strong competition for sorption sites by abundant Na+ ions, and because of dramatically changing solution compositions with time as the highly concentrated tank fluid becomes diluted as it mixes with infiltrating rainwater. A mechanistic multicomponent sorption model is required that can account for effects of competition and spatially and temporally variable solution compositions. To investigate the influence of the high ionic strength tank fluids on Cs+ migration, numerical calculations are performed using the multiphase-multicomponent reactive transport code FLOTRAN. The computer model describes reactive transport in nonisothermal, variably saturated porous media including both liquid and gas phases. Pitzer activity coefficient corrections are used to describe the high ionic strength solutions. The calculations take into account multicomponent cation exchange based on measured selectivity coefficients specific to the Hanford sediments. Solution composition data obtained from Well 299-W23-19, documenting a moderately concentrated leak from the SX-115 tank, are used to calibrate the model. In addition to exchange of cations Na+, K+, Ca2+, and Cs+, aqueous complexing and a kinetic description of precipitation and dissolution of calcite are also included in the calculations. The fitted infiltration rate of 0.08 m yr-1, and fitted cation exchange capacity of 0.05 mol kg-1 are consistent with measured values for the Hanford sediments. A sensitivity analysis is performed for Na+ concentrations ranging from 5 to 20 m to investigate the mobility of Cs+ interacting with a highly concentrated background electrolyte solution believed to have been released from the SX-108/SX-109 tanks. The calculations indicate that during the initial period of the tank leak when Cs+ is associated with high Na+ concentrations, there is little retardation of the Cs+ plume. However, as time increases the Na+ and Cs+ profiles become chromatographically separated due to differences in their selectivity coefficients and dilution of the tank leak plume with infiltrating rainwater. Eventually the two species become separated spatially, and Cs+ becomes highly retarded and remains essentially fixed in the sediments by cation exchange. For the 20 m Na+ simulated tank leak, the sorbed Cs+ profile is in close agreement with data obtained from the slant borehole and consistent with the estimated tank supernatant concentration. The simulations suggest that natural attenuation processes should result in strong fixation of Cs+ in the vadose zone in spite of the release of high Na+ concentrations during a tank leak event

[en] At the Hanford site, highly radioactive and chemically aggressive waste fluids have leaked from underground storage tanks into the vadose zone. This paper addresses hydrogeological issues at the 241-SX tank farm, especially focusing on tank SX-108 which is one of the highest heat load, supernate density and ionic strength tanks at Hanford and a known leaker. The behavior of contaminants in the unsaturated zone near SX-108 is determined by an interplay of multiphase fluid flow and heat transfer processes with reactive chemical transport in a complex geological setting. Numerical simulation studies were performed to obtain a better understanding of mass and energy transport in the unique hydrogeologic system created by the SX tank farm. Problem parameters are patterned after conditions at tank SX-108, and measured data were used whenever possible. Borrowing from techniques developed in geothermal and petroleum reservoir engineering, our simulations feature a comprehensive description of multiphase processes, including boiling and condensation phenomena, and precipitation and dissolution of solids. We find that the thermal perturbation from the tank causes large-scale redistribution of moisture and alters water seepage patterns. During periods of high heat load, fluid and heat flow near the tank is dominated by vapor-liquid counterflow (heat pipe), which provides a much more efficient mechanism than heat conduction for dissipating tank heat. The heat pipe mechanism is also very effective in concentrating dissolved solids near the heat source, where salts may precipitate even if they were only present in small concentrations in ambient fluids. Tank leaks that released aqueous fluids of high ionic strength into the vadose zone were also modeled. The heat load causes formation dryout beneath the tank, which is accompanied by precipitation of solutes

[en] Modeling uranium transport at the Hanford 300 Area presents new challenges for high performance computing. A field-scale three-dimensional domain with an hourly fluctuating Columbia river stage coupled to flow in highly permeable sediments results in fast groundwater flow rates requiring small time steps. In this work, high-performance computing has been applied to simulate variably saturated groundwater flow and tracer transport at the 300 Area using PFLOTRAN. Simulation results are presented for discretizations up to 10.8 million degrees of freedom, while PFLOTRAN performance was assessed on up to one billion degrees of freedom and 12,000 processor cores on Jaguar, the Cray XT4 supercomputer at ORNL

[en] This project seeks to improve the basic understanding of the role of colloids in facilitating the transport of contaminants in the vadose zone. We focus on three major thrusts: (1) thermodynamic stability and mobility of colloids formed by reactions of sediments with highly alkaline tank waste solutions, (2) colloid-contaminant interactions, and (3) in-situ colloid mobilization and colloid facilitated contaminant transport occurring in both contaminated and uncontaminated Hanford sediments

[en] A sizable groundwater U plume exists in Hanford's 300 A resulting from the disposal of fuel rod dissolution wastes containing Al, Cu, and U to the vadose zone. This project is studying U-contaminated samples collected along a flow path from the waste source to the Columbia River. Three primary objectives are being pursued: (1) To develop microscopic models for U desorption/adsorption in sediments along the flow path including both geochemical reaction and diffusive mass transport processes. (2) To parameterize the microscopic models with appropriate laboratory measurements and data within context of a dual continuum, reactive transport model (DCM). (3) To apply the parameterized DCM to laboratory columns of different size and sediment texture for testing of scaling hypotheses

[en] Wellbore integrity is essential to ensuring long-term isolation of buoyant supercritical CO₂ during geologic sequestration of CO₂. In this report, we summarize recent progress in numerical simulations of cement-brine-CO₂ interactions with respect to migration of CO₂ outside of casing. Using typical values for the hydrologic properties of cement, caprock (shale) and reservoir materials, we show that the capillary properties of good quality cement will prevent flow of CO₂ into and through cement. Rather, CO₂, if present, is likely to be confined to the casing-cement or cement-formation interfaces. CO₂ does react with the cement by diffusion from the interface into the cement, in which case it produces distinct carbonation fronts within the cement. This is consistent with observations of cement performance at the CO₂-enhanced oil recovery SACROC Unit in West Texas (Carey et al. 2007). For poor quality cement, flow through cement may occur and would produce a pattern of uniform carbonation without reaction fronts. We also consider an alternative explanation for cement carbonation reactions as due to CO₂ derived from caprock. We show that carbonation reactions in cement are limited to surficial reactions when CO₂ pressure is low (< 10 bars) as might be expected in many caprock environments. For the case of caprock overlying natural CO₂ reservoirs for millions of years, we consider Scherer and Huet's (2009) hypothesis of diffusive steady-state between CO₂ in the reservoir and in the caprock. We find that in this case, the aqueous CO₂ concentration would differ little from the reservoir and would be expected to produce carbonation reaction fronts in cements that are relatively uniform as a function of depth.

[en] Estimation of Hanford SX tank waste compositions from historically derived inventories