Non-Passive Transport of Volatile Organic Compounds in the Unsaturated Zone

1 2 A detailed model was formulated to describe the non-passive transport of water-soluble 3 chemicals in the unsaturated zone and used to illustrate one-dimensional infiltration and 4 redistribution of alcohol-water mixtures. The model includes the dependence of density, 5 viscosity, surface tension, molecular diffusion coefficient in the liquid phase, and gas-liquid 6 partition coefficient on the aqueous mixture composition. It also takes into account the 7 decrease in the gas-liquid partition coefficient at high capillary pressures, in accordance with 8 Kelvin's equation for multi-component mixtures. Simulation of butanol-water mixtures 9 infiltration in sand was in agreement with the experimental data and simulations reported in 10 the literature. Simulation of methanol infiltration and redistribution in two different soils 11 showed that methanol concentration significantly affects volumetric liquid content and 12 concentration profiles, as well as the normalized volatilization and evaporation fluxes. 13 Dispersion in the liquid-phase was the predominant mechanism in the transport of methanol 14 when dispersivity at saturation was set to 7.8 cm. Liquid flow was mainly due to capillary 15 pressure gradients induced by changes in volumetric liquid content. However, for dispersivity 16 at saturation set to 0.2 cm, changes in surface tension due to variation in composition induced 17 important liquid flow and convection in the liquid-phase was the most active transport 18 mechanism. When the Kelvin effect was ignored within the soil, the gas-phase diffusion was 19 significantly lower, leading to lower evaporation flux of water and higher volumetric liquid 20 contents near the soil surface.