Author : A.Abdul-Majeed, Muhammad
Engineering and Technology Journal,
2010, Volume 28, Issue 10, Pages 1900-1909
The study of gas flow in unsaturated soil is important for better modeling of
volatile organic compounds (VOCs) transport. A gas flow in unsaturated soil can be
induced naturally by the atmospheric pressure fluctuation. Oscillations in barometric
pressure are both diurnal, corresponding to daily heating and cooling of the
atmosphere, 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 front
passage can be 25 mbar or more.
A one-dimensional conceptual model was used to investigate the advective
gas flow zones in the subsurface induced by the natural atmospheric pressure
fluctuation. From analytical solution, it is clear that the gas phase inside unsaturated
zone moves with sinusoidal velocity whose amplitude decrease with depth. Two zones
can be distinguished. First in which the gas phase can reach the soil surface and
continuously mixed with clean air. The depth "Penetration Depth" of this zone may
range from 0.05m to 0.8m. Second is "Oscillation Zone" in which the air oscillates
around its original position but still remains in the soil. Maximum air displacement
toward the upper boundary may reach 0.24m when the depth of the lower
impermeable boundary is 10m. This displacement is more as the lower impermeable
boundary is deeper. The mixing of air above penetration depth with clean air above
soil surface and the oscillation of air below penetration depth may have a significant
effect on natural VOCs transport and fate in the soil region within these depths.