Dynamics of Coupled Microbial and Contaminant Transport.

Dynamic microbial attachment/detachment occurs in subsurface systems in response to changing environmental conditions caused by contaminant movement and degradation. This project's objective is to develop the understanding of the environmental conditions and mechanisms by which anaerobic bacteria partition between aqueous and solid phases. In particular this interdisciplinary research project provides fundamental information on the attachment/detachment dynamics of anaerobic bacteria in heterogeneous porous media under growth and growth-limiting conditions. This is a critical requirement for designing and evaluating in situ bioremediation efforts

An Analysis Platform for Multiscale Hydrogeologic Modeling with Emphasis on Hybrid Multiscale Methods

One of the most significant challenges faced by hydrogeologic modelers is the disparity between the spatial and temporal scales at which fundamental flow, transport, and reaction processes can best be understood and quantified (e.g., microscopic to pore scales and seconds to days) and at which practical model predictions are needed (e.g., plume to aquifer scales and years to centuries). While the multiscale nature of hydrogeologic problems is widely recognized, technological limitations in computation and characterization restrict most practical modeling efforts to fairly coarse representations of heterogeneous properties and processes. For some modern problems, the necessary level of simplification is such that model parameters may lose physical meaning and model predictive ability is questionable for any conditions other than those to which the model was calibrated. Recently, there has been broad interest across a wide range of scientific and engineering disciplines in simulation approaches that more rigorously account for the multiscale nature of systems of interest. In this article, we review a number of such approaches and propose a classification scheme for defining different types of multiscale simulation methods and those classes of problems to which they are most applicable. Our classification scheme is presented in terms of a flowchart (Multiscale Analysis Platform), and defines several different motifs of multiscale simulation. Within each motif, the member methods are reviewed and example applications are discussed. We focus attention on hybrid multiscale methods, in which two or more models with different physics described at fundamentally different scales are directly coupled within a single simulation. Very recently these methods have begun to be applied to groundwater flow and transport simulations, and we discuss these applications in the context of our classification scheme. As computational and characterization capabilities continue to improve, we envision that hybrid multiscale modeling will become more common and also a viable alternative to conventional single-scale models in the near future

Influence of Mineral-Bound Humic Substances on the Sorption of Hydrophobic Organic Compounds

The sorption of three hydrophobic organic compounds (HOC) was investigated on hematite and kaolinite that had been coated with natural humic substances over a mass percent carbon range of 0.01-0.5%. Increasing quantities of sorbed humic substances increased the sorption of HOC. Anthracene, the most hydrophobic HOC, showed the greatest sorption enhancement, while the most aromatic coating, peat humic acid, was the strongest sorbent. Depending on the type of humic acid coating and the mineral substrate, the experimental Koc values were either higher or lower than those predicted by the Kow The sorptivity of a given humic acid for HOC was not the same on kaolinite and hematite, suggesting that the orientation and structure of the humic substance on the mineral may affect the surface area of the organic phase and the accessibility of hydrophobic domains that control HOC sorptivity. Sorption isotherms for HOC on the humic-coated mineral substrates were nonlinear, implying that the sorption phenomenon was adsorption onto rather than partitioning into the surface organic phase

Interaction of Hydrophobic Organic Compounds with Mineral-Bound Humic Substances

The sorption of hydrophobic organic compounds (HOC) on mineral-associated peat humic acid (PHA) was evaluated under different pH and electrolyte regimes. Relative size distribution measurements indicated that PHA was “coiled” in solution at high ionic strength (I) and elongated at low I. The sorption of PHA to hematite and kaolinite varied with I and electrolyte cation, suggesting that the configuration of the humic acid in solution influenced its structure on the mineral surface. The sorption maxima for PHA on kaolinite indicated that PHA occupies twice the mineral surface area at low I (0.005) as that observed at high I (0.1). HOC sorption to mineral-bound PHA in Na+ electrolyte was greater at lower I, indicating that humate structure was a plausible determinant of HOC sorption. Freundlich isotherms of dibenzothiophene on the PHA-coated kaolinite did not display unit slope, regardless of pH, I, or cation. Carbazole and anthracene displayed competitive behavior for sorption onto PHA-coated kaolinite. Collectively, the experimental observations indicate that hydrophobic adsorption rather than phase partitioning was the dominant mode of HOC binding

Recharge Data Package for the Immobilized Low-Activity Waste 2001 Performance Assessment

Lockheed Martin Hanford Company (LMHC) is designing and assessing the performance of disposal facilities to receive radioactive wastes that are currently stored in single- and double-shell tanks at the Hanford Site. The preferred method of disposing of the portion that is classified as immobilized low-activity waste (ILAW) is to vitrify the waste and place the product in near-surface, shallow-land burial facilities. The LMHC project to assess the performance of these disposal facilities is known as the Hanford ILAW Performance Assessment (PA) Activity, hereafter called the ILAW PA project. The goal of this project is to provide a reasonable expectation that the disposal of the waste is protective of the general public, groundwater resources, air resources, surface-water resources, and inadvertent intruders. Achieving this goal will require predictions of contaminant migration from the facility. To make such predictions will require estimates of the fluxes of water moving through the sediments within the vadose zone around and beneath the disposal facility. These fluxes, loosely called recharge rates, are the primary mechanism for transporting contaminants to the groundwater. Pacific Northwest National Laboratory (PNNL) assists LMHC in their performance assessment activities. One of the PNNL tasks is to provide estimates of recharge rates for current conditions and long-term scenarios involving the shallow-land disposal of ILAW. Specifically, recharge estimates are needed for a fully functional surface cover, the cover sideslope, and the immediately surrounding terrain. In addition, recharge estimates are needed for degraded cover conditions. The temporal scope of the analysis is 10,000 years, but could be longer if some contaminant peaks occur after 10,000 years. The elements of this report compose the Recharge Data Package, which provides estimates of recharge rates for the scenarios being considered in the 2001 PA. Table S.1 identifies the surface features and time periods evaluated. The most important feature, the surface cover, is expected to be the modified RCRA Subtitle C design. This design uses a 1-m-thick silt loam layer above sand and gravel filter layers to create a capillary break. A 0.15-m-thick asphalt layer underlies the filter layers to function as a backup barrier and to promote lateral drainage. Cover sideslopes are expected to be constructed with 1V:10H slopes using sandy gravel. The recharge estimates for each scenario were derived from lysimeter and tracer data collected by the ILAW PA and other projects and from modeling analyses