INFLUENCE OF THE UNSATURATED ZONE ON TRANSIT TIME DISTRIBUTION OF WATER IN KARST SYSTEMS

In this work results from spring response monitoring, artificial tracer experiments, and sampling campaigns of several environmental tracers at the Gallusquelle spring (Swabian Alb, Germany) are compared and attributed to the transit time distribution of water in the karst system. It is shown that the karst system responds highly dynamic to recharge events and that flow velocities in the subsurface can exceed one hundred meters per day. On the contrary, interpretation of a long term record of tritium in spring water reveals a mean transit time of several decades. The difference can be explained by the well known duality concept of diffuse and concentrated flow in karst aquifers. Furthermore it was found that the estimated tritium age differs up to one order of magnitude from ages obtained with other environmental tracers (krypton-85, helium/tritium) which have also a diffuse tracer input function. The result reveals the importance of slow percolation of water through the thick unsaturated zone of the karst system. Tritium is part of the water molecule and enters a karst system via precipitation, i.e. the estimated mean tritium age is a measure of water flow through the whole system, including the saturated and unsaturated zone. In contrast, the mean krypton-85 age and helium/tritium age are a measure of time since the water sample was recharged at the water table. Therefore, our finding indicates a long transit time of water through the unsaturated zone of the karst system. This interpretation is supported by a two-dimensional numerical simulation of flow and transport in a fissured matrix block that contains a thick unsaturated zone and is drained by a conduit. The simulation provides the transit time distribution of water recharged on the top of the karst system and discharged at the karst spring. It shows that depending on boundary conditions, the unsaturated zone of a karst system may provide a large water storage since the porous matrix is nearly saturated and the volume fraction of fissures and conduits is small.