Flow Zones In Unsaturated Soil Due To Barometric Pumping


The study of gas flow in unsaturated soil is important for better modeling ofvolatile organic compounds (VOCs) transport. A gas flow in unsaturated soil can beinduced naturally by the atmospheric pressure fluctuation. Oscillations in barometricpressure are both diurnal, corresponding to daily heating and cooling of theatmosphere, and of longer time periods, resulting from the passage of weather fronts.Daily variations will average about 4 to 5 mbar while those due to weather frontpassage can be 25 mbar or more.A one-dimensional conceptual model was used to investigate the advectivegas flow zones in the subsurface induced by the natural atmospheric pressurefluctuation. From analytical solution, it is clear that the gas phase inside unsaturatedzone moves with sinusoidal velocity whose amplitude decrease with depth. Two zonescan be distinguished. First in which the gas phase can reach the soil surface andcontinuously mixed with clean air. The depth "Penetration Depth" of this zone mayrange from 0.05m to 0.8m. Second is "Oscillation Zone" in which the air oscillatesaround its original position but still remains in the soil. Maximum air displacementtoward the upper boundary may reach 0.24m when the depth of the lowerimpermeable boundary is 10m. This displacement is more as the lower impermeableboundary is deeper. The mixing of air above penetration depth with clean air abovesoil surface and the oscillation of air below penetration depth may have a significanteffect on natural VOCs transport and fate in the soil region within these depths.