Numerical modeling of coupled fluid flows and reactive transport

Subsurface contamination problems of metals and radionuclides are ubiquitous. Metals and radionuclides may exist in the solute phase or bound to soil particles and interstitial portions of the geologic matrix. Accurate tools to reliably predict the migration and transformation of these metals and radionuclides in the subsurface environment enhance the ability of environmental scientists, engineers and decision makers to analyze their impact and to evaluate the efficacy of alternative remediation techniques prior to incurring expense in the field. A mechanistic-based numerical model provides such a tool. This paper communicates the development and verification of a mechanistically coupled fluid flow and reactive chemical transport under both fast and slow reactions in variably saturated media. Theoretical bases, numerical implementations, and modeling experiments of the model are described. Three example problems are presented. The first one is a diffusive reactive transport problem to elucidate the effects of precipitation/dissolution on diffusion. The second problem involves coupled fluid flow and reactive transport in fractured media. Its purposes are to examine the effects of precipitation and dissolution on fluid flow and matrix diffusion. It helps shade some lights on the retardation ability of grain matrices on the migration of metals and radionuclides. The third example focuses on a complicated but realistic advective-dispersive-reactive transport problem. This example exemplifies the need of innovative numerical algorithms to solve problems involving stiff geochemical reactions.