La zone d\u27infiltration des aquifères karstiques. Méthodes d\u27étude. Structure et fonctionnement

Karst aquifers differ from other aquifers because of the spectacular and rapid change of their initial hydraulic properties, by solution enlargement of certain discontinuities. As a consequence, underground flow paths concentrate in a hierarchised network from surface to a unique spring, while groundwater storage develops in connection to drainage. Because of its very active role in karst landform evolution, the infiltration zone shows typical characters and properties. The nature of flows (hydraulic behavior), the karstic void distribution (structure), and the processes obviously distinguish it from the unsaturated zone of porous and fissure aquifers. Among these properties, the fact that man is able to enter it through caves and can directly observe some infiltration seepage, has been essential in karst concepts and its approach. Investigation methods are reviewed. They are related either to a global approach of karst system, by studying its hydraulic behavior, or to a local approach, devoted to a natural (cave) or artificial (borehole) observation site. The most common methods are based on hydrodynamics (recession analysis, discharge or water level time series analysis) and on water geochemistry. But dye tracing tests, slug tests and neutron probe logging in boreholes, and morphostructural analysis and geophysics are used in some specific works. Therefore the most original investigations certainly are those based on air, water, CO2 and heat flows in caves. The peculiarities of karst infiltration induce some practical consequences. Some of them are related to groundwater resources (recharge conditions, travel time) and quality (high variability of chemical contents). Those peculiarities must be taken into account in land management projects. In short infiltration characters and related mechanisms are responsible in a large part for karst aquifer vulnerability