Free and forced convection in high permeability soils: impact on gas flux at the soil-atmosphere interface

In low permeability soils, gas flux is mostly governed by diffusion and considered to be a relatively slow and continuous process of soil ventilation and aeration. Recent studies have shown that as the soil permeability increases, gas circulation by convective mechanisms become important. In high permeability matrices, the overall gas flux through the earth-atmosphere interface can be significantly greater than the diffusive gas flux. There are several driving mechanisms which can trigger convective gas exchange at the earth - atmosphere interface, two of those can be of great importance and are being explored in this research. The first one is thermal convection venting (TCV), which develops when there are unstable density gradients. The second mechanism is wind induced convection (WIC), which develops due to surface winds that create pressure differences, thus driving air movement. Development of TCV and WIC are directly affected by soil properties, mostly soil permeability. The objectives of this research are to investigate: (a) the effect of porous media grain size, and the resulting permeability, on TCV and WIC, under homogeneous (one grain size) and simple heterogeneous (two grain sizes) conditions; and (b) the effect of atmospheric conditions on TCV and WIC, and mixed venting on the overall gas flux. The experiments were carried out in a Climate Controlled Laboratory using large columns packed with different ideal spherical particles and under different environmental conditions. Both wind and thermal gradients are imposed and controlled independently in order to isolate the different atmospheric effects. A network of sensors enables continuous monitoring of gas flux and thermal gradient inside the columns. A continuous low flow of CO2 enriched air enables constant CO2 concentration at the bottom of the column. Preliminary results show that in homogenous porous media with high permeability of 6.67 * 10-6 [m^2], using 4-cm diameter spheres, CO2 fluxes were significantly higher under WIC and TCV conditions compared to no-wind, isothermal conditions. Under WIC, surface wind speed of 1.5 [m/s], CO2 flux was 4.2×0.6 [g/m^2h]. Under TCV, with a temperature gradient of 8.8 [°C/m], CO2 flux was 11.15×0.05 [g/m^2h]. Under no-wind isothermal conditions the measured flux was 2.45×0.6 [g/m^2h]. The CO2 flux was the highest when both WIC and TCV conditions were imposed simultaneously (14.6×1.1 [g/m^2h]), suggesting a superposition of the TCV and WIC mechanisms. In the layered heterogeneity experiment, still in process, preliminary results suggest the lower permeability layer is acting as the limiting factor for the TCV and WIC fluxes, (e.g., smaller particles of 1 cm over 4 cm in diameter). Initial results indicate that there is a permeability threshold value, which can be accurately determined and compared to models, above which TCV and WIC will occur and impact fluxes either independently or in conjunction depending on atmospheric conditions.