Impact of wind speed, air temperature, and soil permeability on gas transport in the upper vadose zone

In high permeability soils, the overall gas flux through the Earth-atmosphere interface can be significantly greater than the diffusive gas flux. Here we focused on the role of atmospheric conditions and porous media properties on convective gas transport mechanisms. Three different experiments quantitatively explored the correlation between media particle size and the development of three gastran sport mechanisms: enhanced diffusion, thermal-induced convection, and wind-induced convection. These mechanisms, resulting from the development of thermal gradient and surface wind, were analyzed both independently and in combination. Two types of experiments were carried out: (1) under field conditions, and (2) under highly controlled laboratory conditions using highly-permeable porous media composed of well-defined single-sized spherical particles. Porous media permeability ranged from 3.87×10-10 m2 (sand) up to2.67×10-6 m2 (large aggregates collected from an agricultural field). In all studies, temperature and wind conditions across the media and at the media-atmosphere interface were monitored. Results show that the magnitudes of thermal- and wind-induced convection were directly related to porous media particle size (and, subsequently, media permeability) given favoring ambient conditions at the media-atmosphere interface. Such ambient conditions were either high diurnal temperature amplitude (~± 10 ᵒC) or high surface wind (~2 m/s measured 10 m above ground). In addition, specific results from the field experiment were used to establish an empirical model that predicts gas transport magnitude as a function of wind speed and media permeability.