The effects of barometric pumping on contaminant transport

Variation in the ambient atmospheric pressure results in subsurface motion of air in porous and fractured earth materials. This is known as barometric pumping. We analyze this phenomenon for unfractured media and demonstrate that if hydrodynamic dispersivity is included, barometric pumping can significantly decrease the retention time of volatile subsurface contaminants. Formulae are derived which give analytically the dependence of the effect on the amplitude and frequency of the barometric pump as well as on the material properties. In addition, numerical modeling results using a method which carefully avoids spurious numerical dispersion are presented. Field data will appear to follow a standard diffusive transport model; but in the systems in which barometric pumping is significant, the value for D will need to be greater than that derived from isobaric tests. In addition to enhancing the diffusion, pumping also sweeps out pore gases near the surface and thus, reduces the distance a subsurface contaminant must diffuse before it mixes with the atmosphere. These barometric pumping mechanisms will be enhanced by the presence of fractures, which permit pressure variations to propagate deep into the ground. Because barometric pumping affects the rate of contaminant transport from the subsurface, this process may play an important role in the environmental remediation of volatile organic chemicals in the vadose zone.