Free and forced gas convection in highly permeable, dry porous media

The spatial and temporal distribution of gas species within the vadose zone is determined by biochemical sources and by transport mechanisms. Here, two mechanisms that can transfer gas at high rates across the earth-atmosphere interface are studied. The first is thermal convection venting (TCV), a free convection process that develops under conditions of sufficient temperature and density gradients. The second mechanism is wind-induced convection (WIC), an outcome of atmospheric surface winds that drive air movement within the porous media by a forced-convection process. Both of these advective mechanisms can dominate gas transport in high permeability porous media, and the objective of this study was to determine the permeability values that are relevant for these mechanisms to become significant for gas transport. Experiments were performed using large columns filled with four different single-sized spherical particles of 1 to 4 cm in diameter. The experiments were conducted in a climate-controlled laboratory, where surface winds and temperature gradients were imposed and monitored. A tracer gas of CO₂-enriched air was used to quantify the impact of TCV and WIC on gas exchange between the porous media and the atmosphere. A permeability range of 10⁻⁷ to 10⁻⁶ m² was found to be sufficient for the onset of TCV when imposed temperature gradients were similar to standard nighttime atmospheric conditions, leading to full or partial venting of the column. Surface wind with a velocity of 1.5 m s⁻¹ drove WIC to a depth of 0.3 m in most experimental conditions. The impact of WIC on net gas transport was not observed at the bottom-most sensor (0.9 m), except under conditions of very high permeability (2.4 10⁻⁶ m²) and a large temperature difference (6.5 °C m⁻¹), when both TCV and WIC worked simultaneously. Results confirm that TCV and WIC can significantly contribute to gas transport through porous media with sufficiently high permeability.