Geoenvironmental Factors Affecting Organic Vapor Advection and Diffusion Fluxes from the Unsaturated Zone to the Atmosphere under Natural Conditions

Abstract

This study investigates the importance of various geoenvironmental conditions such as depth to water table, gas permeability, water-table fluctuations, moisture content, and temperature on the advection and diffusion fluxes of volatile organic compounds (VOCs) from the unsaturated zone to the atmosphere under natural conditions. A one-dimensional gas-flow and organic-vapor transport model was used to simulate vapor fluxes under sinusoidal atmospheric pressure changes with an amplitude of 300 N m^-2. The diffusion fluxes were generally greater than the advection fluxes by several orders of magnitude. The thickness of the vadose zone and gas permeability have a relatively significant effect on the advection fluxes at land surface compared to their effect on diffusion fluxes. Increasing the thickness of the vadose zone up to 30 m causes advection fluxes to increase. Increasing the gas permeability of the vadose zone up to 10^-12 m² also increases the advection flux. Further increases of vadose-zone thickness or gas permeability do not cause significant additional increases in advection flux. Water-table fluctuations as small as 0.1 m affect the advection fluxes as significantly as the atmospheric pressure changes. Moisture content effects not only advection flux but also diffusion flux during sinusoidal atmospheric pressure changes. Daily temperature variations also have a significant effect on the fluxes. Heterogeneity in gas permeability and moisture content impacts advection and diffusion fluxes at land surface.