Fluid flows and reactive chemical transport in variably saturated subsurface media

The couplings among fluid flow, advective and dispersive/diffusive transport, and chemical reactions are important for both waste disposal and remediation of contaminated sites in fractured media or soils. The feedback of chemical reactions on fluid flow and reactive chemical transport is particularly important because changes in hydraulic properties due to precipitation and dissolution along fractures and rock matrix might be pronouncing. This chapter presents the fundamental physical, chemical, and biological processes that control the movement of fluids, the migration of chemicals, and their interactions with the media. It describes the development and verification of a mechanistic-based numerical model for simulation of coupled fluid flow and reactive chemical transport, including both fast and slow reactions, in variably saturated media. Mathematical formulations, numerical and computer implementations, and numerical experiments are described. Six example problems are employed to illustrate the applications of the model. The first three problems are used to demonstrate the versatility and flexibility in modeling various combinations of fluid flows and reactive chemical transport. The fourth and fifth problems are used to illustrate the interplay and effects of advective/dispersive transport, chemical reactions, and fluid flows. The final problem is used to show that species switching algorithms are necessary to deal with highly nonlinear, stiff geochemistry problems.