Surface to Cave Dye Tracing: Lessons Learned from the Belgian Karst

The vadose zone (unsaturated) of karst aquifers is critical for several reasons. (i) It is the main pathway for karst groundwater recharge from the surface to the phreatic zone (saturated), (ii) it forms the transition zone between surface human activities (potential contamination) and the groundwater resource, and (iii) it gives a support for karst fauna and flora through its hydrogeological dynamic. The hydrogeological functions of the vadose zone are of growing interest for these reasons but, also, because this highly heterogeneous media is still poorly understood by hydrogeologists and karst researchers. In order to more fully apprehend the function of this zone, surface to cave dye tracing was performed above the Rochefort Cave in south Belgium. The goal was to explore the hydrological processes of the 30-meters vadose zone between the surface and the cave by mean of dyes and drip-water monitoring. These methods enable us to discover information about groundwater dynamic. A maximum velocity of around 10 meters/hour was calculated in absence of rainfall and without significant flush during the injection of the dye. A multimodal breakthrough curve was recorded in the percolation water, the tracer being remobilized by every rainfall event during more than 120 days. The dye tracked the reaction of the percolation to rainfall events at the surface. Separation and analysis of the different peaks and their related rainfall events revealed a clear trend. The higher the rainfall intensity, the faster the reaction of the percolation. The effect of the rainfall gives infiltration velocities from 10 to 55 m/h for the higher rainfall intensities. Additionally, the dye tracing allowed us to compare the effect of snowmelt and rainfall on the infiltration efficiency. Snowmelt has a higher remobilization effect on the dye, thus a higher infiltration potential compared to rainfall events. The effect of rainfall intensity on infiltration velocity and the difference between snowmelt and rainfall when considering infiltration efficiency seems of primary importance in a context of global climate changes. These results emphasize the importance of evaluating vadose processes in karst terrain