USING MULTIPLE FIELD FLUOROMETERS TO HIGHLIGHT DYE BEHAVIOR IN KARST CONDUITS

Dye tracing is a powerful tool in karst hydrogeology to specify groundwater flow characteristics. The use of in situ monitoring devices enables improved investigation quality. The development of compact and autonomous fluorometers (100% submersible) opens new perspectives for groundwater behavior investigation by mean of original experimentations.

We evaluated the relevance of single point monitoring when performing dye tracing in karst conduits. Ten fluorometers (Fluo-Green) were used in the river of the Han-sur-Lesse cave (Belgium). This network was placed along a 2D profile, covering the whole section of the river. We choose a portion of the river with homogeneous flow velocity and nearly laminar flow.

The results in this first section show small but significant differences in the dye arrival given the place of the monitoring across the profile. The faster arrivals were recorded in the center of the river, while the borders with lower velocity show slight shifts in the dye arrival. All breakthrough curves show similar characteristics and shapes, resulting in similar interpretations if used for advanced analysis.

This highlights the importance of monitoring location when measuring in a karst conduit. Local particularities driven by the shape of the conduit or flow velocity may influence the probes record. Here, only small differences in the dye arrival where recorded in a homogeneous karst conduit with laminar flow. Future experiments will be performed on various flow environments to inform about dye behavior in karst conduit. It is expected that flow velocity variability, backwater effects or heterogeneous conduit section will influence the dye restitution and the interpretation based on breakthrough curves. If the attempt is to perform statistical analysis or flow modelling, the accuracy and possible variability at local scale might be useful to investigate. At least, consideration about probe installation before dye tracing or long-term monitoring might be useful to avoid misinterpretation or local conditions influence.