HR: 1340h
AN: H23B-1319    [Abstracts]
TI: Stochastic Simulation of the Effects of Physical and Chemical Heterogeneities on Solute Transport in an Alluvial Aquifer
AU: * Sun, A Y
EM: asun@swri.org
AF: CNWRA, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78254, United States
AU: Ritzi, R W
EM: robert.ritzi@wright.edu
AF: Wright State University, 260 Brehm Lab 3640 Colonel Glenn Highway, Dayton, OH 45435, United States
AU: Bertetti, F P
EM: paul.bertetti@swri.org
AF: CNWRA, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78254, United States
AB: The valley-fill alluvial aquifer of Fortymile Wash is conceptualized as a natural barrier to radionuclide transport from a potential high-level nuclear waste repository in Yucca Mountain, Nevada. The valley-fill alluvial aquifer is being represented as a hydrogeologically homogeneous unit in site-scale saturated zone models. Geological and geophysical surveys conducted in the area indicate strong spatial variability of facies distributions, leading to uncertainty about flow and transport processes within the alluvium, and particularly the impact of preferential flow paths on site-scale radionuclide transport. The main purpose of this work is to evaluate the effectiveness of the alluvium in isolating radioactive waste from the accessible environment, by considering the spatial variability of the alluvium structure. A facies-driven approach is adopted to simulate both physical and chemical heterogeneities in the alluvium. Based on borehole cutting log analyses, outcrop studies, and sedimentological scaling relationships, a multiscale, hierarchical hydrofacies model is developed where the main hydrofacies are classified corresponding to different scales of bedforms and deposits, and to the contrasts in permeability. A transition probability/Markov chain geostatistical approach is used to generate realistic replicas of subsurface facies distributions, which in turn, are translated into permeability distributions using site-specific values. A set of normalized Neptunium (Np) sorption parameters pertaining to the Fortymile Wash alluvial aquifer are analyzed using geostatistical methods, and the results are combined with surface area distributions to enable simulation of Np parameter distributions. Monte Carlo simulations of non-reactive and reactive transport are performed using fine-resolution models that have dimensions on the order of a block in the site-scale models. Flow-based upscaling indicates that the magnitudes of mean block hydraulic conductivities are close to the hydraulic conductivity values assigned to the valley-fill alluvial unit in site-scale models. The results of reactive transport modeling indicate at most a two-fold increase in solute spread in the longitudinal direction. Additional numerical tests are conducted to quantify the moments of breakthrough curves. This abstract is an independent product of the CNWRA and does not necessarily reflect the view or regulatory position of NRC.
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1869 Stochastic hydrology
DE: 1873 Uncertainty assessment (3275)
SC: Hydrology [H]
MN: 2007 Fall Meeting