Coupled thermohaline groundwater flow and single-species reactive solute transport in fractured porous media

A 3D numerical model has been developed to solve coupled fluid flow, heat and single-species reactive mass transport with variable fluid density and viscosity. We focus on a single reaction between quartz and its aqueous form silica. The fluid density and viscosity and the dissolution rate constant, equilibrium constant and activity coefficient are calculated as a function of the concentrations of major ions and temperature. Reaction and flow parameters, such as mineral surface area and permeability, are updated at the end of each time step with explicitly calculated reaction rates. Adaptive time stepping is used to increase or decrease the time step size according to the rate of temporal variation of the solution to prevent physically unrealistic results. The time step size depends on maximum changes in matrix porosity and/or fracture aperture. The model is verified against existing analytical solutions of heat transfer and reactive transport in fractured porous media. The complexity of the model formulation allows studying chemical reactions and variable-density flow in a more realistic way than done previously. The newly developed model has been used to simulate illustrative examples of coupled thermohaline flow and reactive transport in fractured porous media. Simulations indicate that thermohaline (double-diffusive) transport impacts both buoyancy-driven flow and chemical reactions. Hot zones correspond to upwelling and to quartz dissolution while in cooler zones, the plume sinks and silica precipitates. The silica concentration is inversely proportional to salinity in high-salinity regions and proportional to temperature in low-salinity regions. Density contrasts are generally small and fractures do not act like preferential pathways but contribute to transverse dispersion of the plume. Results of a long-term (100 years) simulation indicate that the coexistence of dissolution and precipitation leads to self-sealing of fractures. Salt mass fluxes through fractures decrease significantly due to major fracture aperture reduction in the precipitation zone. The system is the most sensitive to temperature because it impacts both the dissolution kinetics (Arrhenius equation) and the quartz solubility. The system is least sensitive to quartz surface area in the fracture because the volumetric fraction of a fracture is small compared to the volumetric fraction of the porous matrix. (c) 2006 Elsevier Ltd. All rights reserved.