HR: 1340h
AN: H23B-1317    [Abstracts]
TI: A Markov Chain Model for Characterizing Vadose Zone Soil Heterogeneity and Layering Structure
AU: * Ye, M
EM: mingye@scs.fsu.edu
AF: School of Computational Science and Department of Geological Sciences, 441 Dirac Science Library Florida State University, Tallahassee, FL 32306, United States
AU: Khaleel, R
EM: Raziuddin_Khaleel@RL.gov
AF: Fluor Government Group, P. O. Box 1050, Richland, WA 99352, United States
AB: Characterization of media heterogeneity in unsaturated media is always hampered by lack of site-specific soil samples. In particular, geologic heterogeneity, which controls unsaturated flow and solute transport, cannot be identified based on samples from one or multiple boreholes. This study develops a geologically based Markov chain model to characterize vadose zone soil heterogeneity and layering structure. Soil textural classes (categorical variables) are first categorized, and spatial variability of the soil classes is measured by transition probability from one class to another in space. The Markov chain model is developed based on the transition probability. Although the model in the vertical direction can be developed based on borehole samples, developing the model at the horizontal direction always needs geological information (e.g., mean length and juxtaposition tendencies of the soil classes) obtained from soil (or outcrop) surveys. The geological information is critical to characterization of heterogeneity and soil layering structure. The method is applied to a field injection experiment at the Sisson and Lu injection site at the U.S. Department of Energy Hanford Site, WA. Four soil classes (coarse sand, sand, loamy sand, and sandy loam) are categorized based on particle size distribution of 93 samples from 6 boreholes at the site. Among the 93 samples, 60 samples are collected from 3 boreholes closely spaced within a maximum distance of 2.17 m. The vertical Markov chain model is developed based on the 93 samples. The horizontal model, however, cannot be developed due to a close spacing of the 3 boreholes providing majority of the samples. Initial moisture content measurements at 1334 locations (from 32 boreholes and at 1-foot vertical interval) are used to develop the horizontal Markov chain model. The measurements at the site are transformed into soil textural classes based on the field observation that higher and lower measurement values correspond to fine- and coarse-textured media, respectively. Volumetric portions of the soil classes are used as thresholds of the transform. The Markov chain models developed are then used to represent the spatial structure of the soil classes to characterize soil heterogeneity. Multiple realizations are generated to represent uncertainty of the spatial structure. Soil layering structure is well represented because of the conditioning of the soil classes, as transformed from the initial moisture contents. Soil hydraulic parameters for each soil class are determined from core samples of the soil class. Next, the field injection experiment is simulated; the simulated moisture contents compare well with the field observations for both infiltration and redistribution periods. In particular, in comparison with our previous modeling at the site, the vertical movement of the injected water is improved because of a more accurate characterization of layering structure at the site. Effects of uncertainty of spatial structure of the soil classes on prediction of unsaturated flow at the site are also investigated. Uncertainty of the soil hydraulic parameters within each soil class will be explored in a future study.
DE: 1829 Groundwater hydrology
DE: 1847 Modeling
DE: 1866 Soil moisture
DE: 1873 Uncertainty assessment (3275)
DE: 1875 Vadose zone
SC: Hydrology [H]
MN: 2007 Fall Meeting