Aeration Zone in Karst – Properties and Investigations

Abstract

Aeration, vadose or unsaturated zone in karst aquifer is the zone above the water table (the zone between land surface and piezometric level measured in stand pipe piezometers). This zone can be divided into two sections: an upper section permanently water free and a deeper section temporarily saturated by water. During dry periods, the deeper section is temporarily an aeration zone; however, during the rainy season it is the most dynamic part of the karst aquifer. Within this section, there exist karst channels, caverns, underground flows with free surface (underground rivers), and underground siphonal lakes. An aeration zone plays an important role during the construction of dams, reservoirs and tunnels in karst and can cause various kinds of problems. A particular problem appears if the dam sites and reservoirs are located above the aeration zone or when tunnels cross huge empty caverns or temporarily saturated aeration zones. Due to an abrupt rising of the water table the pressure of air trapped in the karst channels and siphons significantly increase. As consequence, the water tightness of the reservoir bottom can be endangered. The induced seismicity, also, has been documented at the vicinity of some large temporary springs. Some common investigation methods in karst are not successful in the case of aeration zone. The most effective methods in karst as groundwater level monitoring and tracer tests are not useful. Speleology is the most common method but only if karst channels are accessible. TV logging was successfully applied at different locations. Experiments with smoke and ear as tracers also provided good results. Some of those methods are presented in this article.