Characterization and Modeling of Spatial Variability in a Complex Alluvial Aquifer: Implications on Solute Transport

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Field investigations of stratified alluvial deposits suggest that they can give rise to a hierarchy of permeability modes across scales, corresponding to a hierarchy of sedimentary unit types and thus may lead to enhanced plume spread in such media. In this work, we model the sedimentary architecture of the alluvium deposits in Fortymile Wash, Nevada, using a hierarchical transition probability geostatistical approach. The alluvial aquifer comprises a segment of the groundwater flow pathway from the potential high-level nuclear waste repository at Yucca Mountain, Nevada to the downstream accessible environment and may be a natural barrier to radionuclide migration. Thus our main goal is to quantify the impact of spatial variability in the alluvium on solute transport. The alluvial aquifer is a gravel-dominated braid-belt deposit, having lower-permeability paleosols interstratified with higher-permeability gravel-bar deposits. A three-dimensional hierarchical hydrofacies model is developed through fusion of multiple geologic data types and sources. Markov chain models of transition probabilities are employed to represent complex patterns of spatial variability at each hierarchical level in a geostatistical fashion and to impose realistic constraints to such variations through conditioning on existing data. The link between the alluvium spatial variability and solute dispersion at different spatiotemporal scales is demonstrated using the stochastic-Lagrangian transport theory. We show that the longitudinal macrodispersivity can be on the order of hundreds to thousands of meters, and it may not reach its asymptotic value until after 1,000 years of traveltime.