Principles and Practice of Reactive Transport Modeling
- PDF / 895,787 Bytes
- 14 Pages / 414.72 x 648 pts Page_size
- 13 Downloads / 229 Views
PRINCIPLES AND PRACTICE OF REACTIVE TRANSPORT MODELING PETER C. LICHTNER Center for Nuclear Waste Regulatory Analyses Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas, 78238-5166
ABSTRACT This paper presents a review of the principles underlying a continuum formulation of reactive transport in a porous medium. Partial differential equations representing conservation of mass are derived for transport by advection, diffusion, and electrochemical migration combined with chemical reaction of aqueous species and solids. Several examples are presented to illustrate the general theory. These include weathering along a narrow crevice, electrochemical migration in a dilute NaCl solution, secondary pyrite formation mediated through intermediate sulfur oxidation states, and a description of a uranium roll-front deposit. Numerical techniques which take advantage of the quasi-stationary state approximation, based on the much longer time scale involved in mineral alteration compared to that characterizing changes in the aqueous phase, permit solving the reactive transport equations over geologic time scales. INTRODUCTION Application of reactive transport modeling to the Earth sciences is receiving increased attention as understanding of fluid/rock interaction improves concurrently with the rapid advance in computer technology. Models describing physicochemical phenomena are applicable to basic geochemical processes involving solute transport, as well as to migration of radionuclides and other toxic substances in groundwater. This paper reviews the principles underlying time-space continuum models describing fluid/rock interaction combined with advective, diffusive and dispersive transport, and electrochemical migration of solute species. Such models provide, in
principle, a comprehensive spatial and temporal description of fluid flow, transport, and rock alteration. The mathematical formulation presented here includes electrochemical processes as well as both reversible and irreversible homogeneous reactions in the aqueous phase, essential to provide for disequilibrium of redox couples, a common occurrence in natural systems. Electrochemical processes occur in natural systems as well as in industrial processes, including treatment of acid mine drainage, oxidation of organic pollutants, nuclear waste disposal, and many others. Electrochemical corrosion processes involving transport and reaction have been considered by a number of authors (e.g. [2]). The purpose of this contribution is in part to present a unified description of electrochemical and non-electrochemical reactions combined with fluid flow and transport in multicomponent systems. Reactive transport processes have been incorporated into the computer code GEM [3], which includes electrochemical migration effects and takes into account species-dependent diffusion. Reactions considered include mineral precipitation/dissolution described by kinetic rate laws, adsorption and ion-exchange reactions, redox reactions, and reversible and irreversible homoge
Data Loading...
