Reactive Transport Processes from Outcrop Scale to Nano Scale

Abstract

 Water–rock interactions proceed as a result of reaction and transport occurring at an outcrop scale and in pores at a millimeter to nanometer scale. To quantitatively evaluate these processes, various parameters including porosity, dissolution rate, fluid flow rate, effective diffusion coefficient, and surface area are required. Focusing on rock weathering, some examples are presented of how these parameters are determined, and a brief review is provided of the nature of the parameters. Hydraulic conductivities, a measure of fluid flow rate, ranging from < 10⁻¹³ to 1 m s⁻¹, largely depend on pore radius or fracture opening size and porosity. Effective diffusion coefficients, ranging from ∼10⁻¹⁵ to 10⁻⁹ m² s⁻¹, are known to be a power law function of porosity. Dissolution rate constants of silicate minerals, ranging from ∼10⁻¹⁴ to 10⁻⁹ mol m⁻² s⁻¹ under ambient conditions, are affected by the connectedness of the SiO₄ tetrahedra. Using those values, some simple examples are presented of calculations of the profiles of concentration and reaction rate at an outcrop scale and at a pore scale, as well as some recent advances in pore scale geochemical processes.