Stalactite drip-water monitoring and tracer tests approach to assess hydrogeologic behavior of karst vadose zone: case study of Han-sur-Lesse (Belgium)

This paper focuses on the unsaturated zone and epikarst of karst aquifers in order to identify the hydrogeological behavior of these important but poorly understood parts of karst systems. A high-resolution monitoring of stalactite drips and artificial tracer test experiment in the vadose zone allows to draw a conceptual model for the local recharge processes through the unsaturated zone of karst aquifers. Those aquifers represent major groundwater resources in Belgium. Seven years of high-resolution stalactite dripping were recorded on 3 sites of the Han-sur-Lesse cave system (South of Belgium). The three sites show very different discharge variations but a similar behavior from one year to another. Analysis and comparison of the dripping cycles over 7 years highlight the vadose zone behavior and its relationship with surface infiltration (water excess). For each site, a water excess threshold at the beginning of the winter has been identified. This threshold is needed to refill the vadose zone/epikarst capacity before the dripping. The value of the threshold is relatively constant for each site during the 7 years, indicating the regularity of the capacitive system of the vadose zone. Unknown features have also been observed with discharge decrease during high water excess periods, linked to the drainage processes of epikarst. Finally, artificial tracer test reveals the duality of the unsaturated zone behavior, with fast transmissive pathways (velocity of >6 m/h) related to a highly capacitive system (>150 days of tracer restitution).