Assessment of nonequilibrium in gas–water mass transfer during advective gas-phase transport in soils

In soils advective gas flow may be due to natural processes, e.g., water-table fluctuations, or may be a result of remediation techniques, such as soil vapor extraction (SVE), which are used to remove volatile organic compounds (VOCs) from the vadose zone. Recently, multicomponent experiments as well as numerical simulations provided evidence that the efficiency of SVE operations can be limited by slow diffusion of VOCs from interparticle water into the gas phase. Using a first-order kinetics approach, we show that the degree of local nonequilibrium between the two phases and, thus, of the validity of the local equilibrium assumption (LEA) can be characterized by the prefix denominator, PD, for gas–water mass transfer which is derived by the SKIT (Separation of the Kinetically Influenced Term) procedure of Bahr and Rubin [Bahr, J.M., Rubin, J., 1987. Direct comparison of kinetic and local equilibrium formulations for solute transport affected by surface reactions. Water Resour. Res. 23(3), 438–452]. Values of the prefix denominator smaller than 1 indicated nonequilibrium between the two phases while PD-values of 10 or larger indicated local equilibrium between aqueous and gaseous VOC concentrations. These findings can be used to optimize field applications of soil vapor extraction (SVE).