Advective and diffusive contributions to reactive gas transport during pyrite oxidation in the unsaturated zone

Abstract

Pyrite oxidation in unsaturated mine waste rock dumps and soils is limited by the supply of oxygen from the atmosphere. In models, oxygen transport through the subsurface is often assumed to be driven by diffusion. However, oxygen comprises 23.2% by mass of dry air, and when oxygen is consumed at depth in the unsaturated zone, a pressure gradient is created between the reactive zone and the ground surface, causing a substantial advective air flow into the subsurface. To determine the balance between advective and diffusive transport, a one‐dimensional multicomponent unsaturated zone gas transport model is developed. Both advection‐diffusion and Maxwell‐Stefan model formulations are presented. A steady state analytical solution is derived that provides insight into solution behavior and which can be used to test numerical models. A numerical solution is obtained for both the steady and transient cases. At steady state, advection comprises approximately 23% of the total oxygen flux, with the contribution of advection being almost entirely determined by the composition of the atmosphere. Other parameters, such as the permeability, have a negligible effect on the proportion of advective flows at steady state. However, greater pressure gradients are found in low‐permeability soils. In transient cases, advective fluxes depend on the initial conditions and can be far greater than diffusive fluxes. In contrast to steady state conditions the transient case is sensitive to other model parameters; for example, the time to approach steady state depends exponentially on the distance between the soil surface and the subsurface reactive zone.