ABSTRACT
Multi-scale processes involved in subsurface reactive transport may play an essential role that has not been fully understood and appreciated. Such processes are important in the Hanford 300 Area where it has been noted that U(VI) is leached extremely slowly from contaminated sediments. The primary reason for this slow leaching process has been identified as dissolution and/or desorption of U(VI) from grain interiors combined with diffusional mass transfer to the bulk fluid. Multi-scale processes may also be partially responsible for the large discrepancy observed between laboratory and field- derived kinetic rate constants through distinct diffusion-limited transport domains coupled to preferential pathways that reduce the effective reactive surface area.

In general, modeling multi-scale processes in porous media requires a multiple interacting continuum approach. Starting at the pore scale, it is demonstrated that a multi-scale continuum model is required to fit upscaled pore-scale simulations involving reactive and non- reactive transport using Lattice Boltzmann techniques. Although in some cases a single continuum model may suffice using effective parameters (e.g. effective mineral surface area), the single continuum model cannot be predictive since it is based on upscaled parameters that are not directly accessible through measurement. This is demonstrated with a simple single component system involving linear kinetics for which an analytical expression can be derived for the upscaled effective surface area. In the remainder of the presentation U(VI) transport in sediment from the Hanford 300 Area is considered, and an efficient parallel implementation of the multiscale continuum formulation is discussed in which primary and sub-grid scale continua are rigorously decoupled

BIOGRAPHICAL SKETCH
Dr. Lichtner's primary interests are in numerical modeling of fluid flow and transport combined with chemical reactions of minerals, aqueous species and gases in porous and fractured media. His research has ranged from theoretical investigations to modeling field and laboratory studies. He has co-edited a book on reactive transport entitled "Reactive Transport in Porous Media," published by the Mineralogical Society of America. Dr. Lichtner’s current research interests include modeling multi-scale reactive transport processes in heterogeneous and fractured media, geologic sequestration of carbon dioxide, and upscaling micro- and macro-scale descriptions of coupled flow, transport and chemical reactions. His interests also include developing massively parallel computing techniques for the next-generation reactive transport model and he is a recent receipient of a DOE SciDAC II (Scientific Discovery through Advanced Computing) award.