Modeling of multicomponent organic chemical transport in three phase porous media
A two-dimensional finite-element model was developed to predict coupled transient flow and multicomponent transport of organic chemicals which can partition between nonaqueous phase liquid, water, gas and solid phases in porous media under the assumption of local chemical equilibrium. Gas-phase pressure gradients are assumed negligible and liquid flow equations are solved simultaneously using an upstream weighted solution method with time-lagged interphase mass-transfer terms and phase densities. Phase-summed component transport equations are solved serially after computation of the velocity field also by an upstream weighted finite-element method. Mass-transfer rates are evaluated from individual phase transport equations by back-substitution and corrected for mass-balance errors. A number of hypothetical one- and two-dimensional simulations were performed to evaluate the applicability of the model to predict the transport of slightly soluble and volatile organics in three-fluid-phase porous media. Results indicate that mass-transfer rate and fluid density updating have negligible effects during periods of highly transient nonaqueous liquid phase migration but become important for long-term simulations as cumulative dissolution to the water phase and volatilization to the gas phase account for longer proportions of the total mass. Due to low solubilities of environmentally important organic liquids, the efficiency of organic removal by aqueous-phase dissolution and transport can be very slow. Gas-phase diffusion can have a significant influence on the mass transport of organics with large Henry's constants.