Two-species diffusion and reaction in a porous medium: Application to chemical oxidation of DNAPLs in fractured rock

Chemical oxidation of DNAPL contaminated bedrock is an emerging technology in environmental remediation. Little is understood about the behavior of simultaneous reaction and diffusion of DNAPL and chemical oxidant in porous bedrock. This system was modeled using 1-D homogeneous and 2-D numerical models with heterogeneous porosity. Under fast reaction conditions, a stable, sharp, moving reaction front was observed. A semi-analytical solution was developed which simulates the propagation of this reaction front. The reaction front was shown to propagate as the square-root of time, indicating that it is a diffusive process. A reaction front diffusivity was defined to quantify this propagation. Estimates of the reaction front diffusivity were on the order of 10 -6 to 10-5 cm2/s. An early time regime was discovered during which a reaction front has not yet formed. The time required to for a reaction front to form was shown to scale as the inverse of the Damkohler number. A late time regime was also analyzed during which the effects of no-flux boundary conditions impacted propagation of the reaction front. In 2-D heterogeneous simulations, the reaction front was shown to be stable against perturbations introduced by heterogeneous porosity and perturbations in boundary conditions.