Assessing Vadose Zone Biodegradation by a Multicomponent Gas Transport Model

Abstract

This study calibrated a gas transport model that uses multicomponent approaches, i.e., the dusty gas model (DGM) and Stefan–Maxwell (SM) equations, to soil gas concentration depth profiles to determine aerobic biodegradation rates of hydrocarbons in unsaturated soils. I found that viscous advection induced by multicomponent gas transport in porous systems and Knudsen diffusion may be neglected for soil systems with intrinsic permeabilities >10−13 m2, and intrinsic permeability of low‐permeability layers, such as cemented limestone or concrete pavement on the ground surface, calibrated by the DGM model ranged from 10−15 to 10−18 m2. The SM model may be applicable for gas transport in soil systems with low‐permeability layers if low obstruction factor values are assigned for these layers. The obstruction factor defined in the DGM and SM equations accounts for impedance to gas diffusion in unsaturated soils and is similar to tortuosity for Fick's law of diffusion. Results further showed that flux rates of hydrocarbon vapors from sources in unsaturated soils need to be measured or assessed before models are fitted to hydrocarbon vapor depth profiles. If the flux rates are not assessed, and if the obstruction factor and intrinsic permeability in the low‐permeability layers are of uncertain magnitudes, models may produce nonunique results for biodegradation rates of hydrocarbons. Thus, I propose a two‐step procedure for determining aerobic biodegradation rates of hydrocarbons.