MODELING MINERAL DISSOLUTION AND PRECIPITATION IN DUAL-POROSITY FRACTURE MATRIX SYSTEMS

A dual-porosity chemical transport model is applied to saturated fractured porous media. In this model, transport through fractures is advection-dominated while transport within the matrix and between the matrix and the fractures is diffusion-dominated. Recent studies in the literature indicate that conservative tracers and radionuclides in these systems will be retarded by matrix diffusion even in the absence of chemical reactions. This study examines systems that include chemical reactions (e.g., dissolution/precipitation and aqueous complexation) and examines changes in retardation caused by precipitation. The two-dimensional simulation domain consists of a single fracture and the adjacent permeable matrix. The system is modeled using finite differences and a chemical equilibrium simulator based cn the Villars-Cruise-Smith algorithm. A constant flow rate is assumed in the fracture, and diffusion rates are functions of the local matrix porosity. Results from simulations with idealized chemistry are presented and discussed to illustrate basic characteristics of this system. These characteristics include both advection-induced and diffusion-induced sequential dissolution/precipitation waves. Results are contrasted with limiting single porosity cases. An additional simulation using the Pitzer activity coefficient model illustrates the chemical retardation of a contaminant due to contaminant precipitation.