Search

At the Hanford Site in Washington, an extensive in situ redox manipulation (ISRM) permeable reactive barrier was installed to prevent chromate from reaching the Columbia River. However, chromium has been detected in several wells, indicating a premature loss of the reductive capacity in the aquifer. One possible cause for premature chromate breakthrough is associated with the presence of high-permeability zones in the aquifer. The potential emplacement of zero-valent iron (Fe0) into high-permeability Hanford sediments to enhance the barrier’s reductive capacity using shear-thinning fluids containing polymers was investigated in three-dimensional wedge-shaped aquifer models. Porous media were packed in the wedge-shaped flow cell to create either a heterogeneous layered system with a high-permeability zone between two low-permeability zones or a high-permeability channel surrounded by low-permeability materials. The injection flow rate, polymer type, polymer concentration, and injected pore volumes were determined based on preliminary short- and long-column experiments. The flow cell experiments indicated that iron concentration enhancements of at least 0.6% w/w could be obtained using moderate flow rates and injection of 30 pore volumes. The aqueous pressure increased by a maximum of 25 kPa during infiltration, but a decrease in permeability was not observed. Under optimal conditions, the 0.6% amended Fe0 concentration would provide approximately 20 times the average reductive capacity that is provided by the dithionite-reduced Fe(II) in the ISRM barrier. [ABSTRACT FROM AUTHOR]

A testing methodology is presented that supports the development of a field design for in situ gaseous treatment of sediments with diluted hydrogen sulfide. This approach involves the collection of column breakthrough test results at various flow rates, allowing a relationship to be developed between pore velocity of the carrier gas and velocity of the hydrogen sulfide reaction front that permits sizing to the field scale. A regression fit of a set of laboratory column breakthrough test data collected in this study is utilized to illustrate the development of a field design based on a two-dimensional radial flow analytical model. Information regarding treatment time and hydrogen sulfide consumption characteristics associated with in situ gaseous treatment can then be obtained from this model and used as a basis for estimation of treatment schedule and costs. The regression relationship can also be utilized in numerical models in more complex geometries to support the field design of in situ gaseous treatment operations. [ABSTRACT FROM AUTHOR]

Numerical simulation has been applied in support of the U.S. Department of Energy's efforts to characterize the nature and distribution of CCl4 contamination in the deep vadose zone at the Hanford site, near Richland, WA. Three-dimensional simulations were executed using layered and heterogeneous distributions of soil properties to investigate the vertical and lateral distribution of CCl4 beneath its release point (216-Z-9 trench) and the effects of soil vapor extraction. The complexity of the modeled physical processes--namely, the nonlinearities associated with multifluid subsurface flow, including phase transitions and hysteresis in the relative permeability--saturation--capillary pressure functions--limits the grid resolution when executed using single-processor computers. To achieve higher grid resolutions and acceptable detail in the simulation results for subsurface distribution and remediation of CCl4, execution on multiple processors was required. We have developed a scalable implementation of a multifluid subsurface flow and transport simulator, STOMP, with capabilities for volatile organic compounds. Developing scientific software for execution on parallel computers has unique challenges. The guiding objectives for developing this scalable code were to keep the source coding readable and modifiable by subsurface scientists, to allow for both sequential and scalable processing, and to depend on domain scientists for code parallelization and scalable linear system solvers. [ABSTRACT FROM AUTHOR]

Large quantities of radioactive and chemical wastes resulting from Pu production for nuclear weapons are located in the vadose zone at the USDOE's Hanford Site, north of Richland, WA. The vadose zone here is characterized by often highly stratified glacial-fluvial sediments that give rise to complex subsurface-flow paths that contribute to uncertainty of contaminant fate and transport. Research efforts have focused on answering questions of contaminant transport from the viewpoint of geologic, biologic, geochemical, and hydrologic controls. This special section highlights key research topics concerning vadose zone problems at the Hanford Site. Research indicates that some of the contaminant species (137Cs, 60Co, 90Sr) are retained by Hanford sediments as a result of geochemical reactions, rendering them effectively immobile except under extremely saline or acidic conditions, while other species (99Tc, 129I, ³H) are typically mobile and have moved deep into the vadose zone and subsequently into groundwater. In addition, large quantities of organics, including carbon tetrachloride, have moved in complex ways as both vapor and liquid in the subsurface. Observed transport of mobile species is linked to liquid discharges and to elevated recharge rates that occur primarily at waste sites where land surfaces are void of vegetation and where winter rains have subsequently penetrated the subsurface wastes. A series of papers in this issue documents progress to date in understanding transport rates at Hanford, why anisotropy strongly affects the distribution of subsurface contaminants, why organic contaminants are difficult to find in the deep vadose zone, and what the impacts of hypersaline fluids are on waste form degradation and subsequent transport. [ABSTRACT FROM AUTHOR]

The feasibility of using in situ gaseous reduction to establish a vadose zone permeable reactive barrier was evaluated through a combination of laboratory testing and consideration of fundamental vadose zone transport concepts. For the experimental evaluation, a series of laboratory column tests were conducted in which Hanford formation sediment from the USDOE Hanford Site in Richland, WA, was first treated with a diluted hydrogen sulfide gas mixture to reduce sediment iron oxide to ferrous sulfide. Water containing dissolved oxygen was then pumped through the columns at different flow rates to determine the reoxidation rate and the reductive capacity of the treated sediment. The results indicated that the treated sediment has a significant reductive capacity consistent with the basic reactions associated with the treatment and reoxidation processes. The observed reductive capacity was found to be dependent on the flow rate of water during the reoxidation phase of the tests. The reductive capacity approached the maximum value predicted on the basis of the treatment reaction as the flow rate was decreased. Thus, laboratory treatment tests provide a means for predicting the reductive capacity of the barrier under field conditions. In the theoretical assessment, oxygen diffusion was identified as the dominant mechanism leading to reoxidation of the barrier. Depending on vadose zone characterisitics, the predicted barrier lifetime varies from several years to more than 100 years. [ABSTRACT FROM AUTHOR]