Analytical Model for Coupled Water Flow and Bubble-facilitated VOC Transport From the Saturated Zone to the Atmosphere

An analytical model is presented for assessing the coupled processes that govern water flow and volatile organic compound (VOC) transport from the saturated zone through the vadose region and into the atmosphere. The model is verified by a finite element solution. The sensitivity analyses are performed to evaluate the influence of key parameters, such as bubble upward velocity, water flow, atmospheric conditions on VOC transport and emissions from subsoil environments. VOC transport is sensitive to soil texture, which significantly impacts the capillary fringe in the vadose zone. Higher VOC concentrations are observed in sandy soils compared to silt loam, as the larger volumetric water content observed in the silt loam reduces effective VOC diffusivity. Traditional diffusion-limited models show a sharp concentration decrease in the saturated zone due to low diffusion coefficients of VOC in water, while bubble-facilitated transport maintains higher VOC concentrations in the saturated zone. The relative VOC concentration for diffusion-limited models can be around four orders magnitude lower than the calculated value for bubble-facilitated VOC transport model. Increased bubble transport velocity or reduced saturated zone thickness enhances the VOC concentration gradient, resulting in significantly higher emission fluxes. The atmospheric boundary layer also significantly impacts VOC concentrations and emissions. Ignoring the effects of the atmospheric boundary layer can lead to underestimations of VOC emission flux by a factor of 1.2. These findings highlight the significance of coupled bubble and water flow for the transport of VOCs in the saturated-unsaturated-atmospheric system.