GAS FLUSHING OF EPIKARST CRACKS BY DIURNAL THERMAL GRADIENTS

Cavities within the epikarst that are opened to the atmosphere can experience nightly flushing of gases. Solar heating of the surface will diffuse downward through the rock resulting in an inverted thermal gradient at night where rock at a depth of 20-40 cm is warmer than rock at the surface. The high thermal mass of the rock transfers this same thermal signature to the air trapped within the epikarst cavities. The inverted thermal gradient within the cavities will trigger convective air motion, which in turn triggers invasion of atmospheric air into the cavity system. The depth of penetration of atmospheric air into the cavity system will depend upon cavity aperture and the rate of thermal dissipation of the entrained atmospheric air, whereby under the right conditions the atmospheric plume can reach many meters into the subsurface. By mass balance, it follows that entrainment of air produces flushing of cavity air into the atmosphere. The result of this flushing is a nightly exhalation of vadose zone air, a resetting of the gas composition of the cavity system, and significant drying of cavity walls. Field data will be presented that illustrates the venting system and calculations of the criteria and limitations of the venting system.