Modeling the influence of water content on soil vapor extraction

We present a comprehensive model and simulation results to assess the impact of changing water content on nonaqueous phase liquid (NAPL) mass transfer, vapor phase retardation, and slow desorption during soil vapor extraction (SVE). Contaminant mass transfer and water and energy transport processes for a one-dimensional, noniso-thermal, and single contaminant component system are considered. Literature-derived relationships are used to express the fraction of soil surface area exposed to the vapor phase and the NAPL-gas mass transfer rate expression as a function of water saturation. Simulations are presented for two scenarios: (i) low water saturation, where soil drying is expected and direct vapor sorption to soil may be important, and (ii) high water saturation, where NAPL mass transfer to the gas phase is rate-limited and the gas flow rate is very low. At low water saturation slow desorption controls long tailing in the effluent concentration and the cleanup time. Also, when dry air is purged through the system, water evaporation occurs, the temperature decreases, and hence, desorption rates decrease. The NAPL mass transfer rates were negligibly affected by water evaporation because time scales of the NAPL volatilization were much smaller than time scales of the water evaporation. At high water saturation, NAPL is trapped by water and NAPL mass transfer to the gas phase is limited by diffusion through water films. For this case, NAPL mass transfer is slow and thus controls concentration tailing and cleanup times.