Predicting soil-water and soil-air transport properties and their effects on soil-vapor extraction efficiency

Accurate prediction Of water and air transport parameters in variably saturated soil is necessary for modeling of soil-vapor extraction (SVE) at soil sites contaminated with volatile organic chemicals (VOCs). An expression for predicting saturated water permeability (k(l,s)) in undisturbed soils from the soil total porosity and;the field capacity soil-water content was developed by fitting a tortuous-tube fluid flow model to measured water permeability and gas diffusivity data: The new k(l,s) expression gave accurate-predictions when tested against independent k(l,s) data. The k(l,s) expression was implemented in the Campbell relative Water permeability model to yield a predictive model for water permeability in variably saturated, undisturbed soil. The water permeability model, together with recently developed predictive equations for gas permeability and gas diffusivity, was-used in a two-dimensional numerical SVE model that also included non-equilibrium mass transfer of VOC from a separate phase (nonaqueous phase liquid [NAPL]) to the air phase. SVE calculations showed:that gas,permeability is likely the most important factor controlling VOC migration and vapor extraction efficiency. Water:permeability and gas diffusivity effects became significant at water contents near and above field capacity. The NAPL-air mass transfer coefficient also had large:impacts On simulated vapor extraction efficiency. The calculations suggest that realistic VE models need to include predictive expressions:for both convective, diffusive, and phase-partitioning processes in natural, undisturbed soils.