Transport of Gas across Interfaces: Cracked Soils and Fractured Rocks

It is well known that the Earth's surface crust is cracked and fractured in many areas. The porous membrane which composes the upper few meters of land surface is full of discontinuities. Recent work, including laboratory scale experiments, in situ field measurements and simulations show that these discontinuities may play an important role in the exchange rate of gases across this membrane. It was found that the temperature differences between the ambient atmospheric air and the air within the fracture at night could enhance gas fluxes through development of free thermal convective cycles.

Although most of the work has been done on fractured rocks, similar conditions prevail in most clayey soils, mainly during the summer when the soil becomes drier and the clay matrix shrinks. For example, in agricultural fields these soil cracks may significantly enhance the flux of Carbon-Dioxide and other soil gas species from the root zone to the atmosphere. Enhanced evaporation from these cracks focuses salt precipitation on crack/fracture walls and subsequent irrigation or rain events may increase the dissolution and transport rate of these salts downward, bypassing the low-permeability region. Simulations revealed that although lateral solute transport (driven by evaporation) would be faster in high permeability soils, it is more important as a bypass mechanism in low permeability soils (and rocks) because the natural (vertical) drainage through these soils is very low.

In summary, we show that cracks and fractures are very important to water and gas cycles not only during flow events but also during dry period. This is especially important in arid environments where natural infiltration events are rare and fractures/cracks are empty most of the time.