The Karst Vadose Zone as an Important Water Storage System

Recent field and modelling investigations have shown the karst vadose zone to act as an important factor in the assessment of available water resources, particularly in regions, characterised by thick unsaturated zones. In particular, in semi-arid regions of the Eastern Mediterranean, very wet years have shown to have prolonged effects of elevated groundwater discharge as well as elevated groundwater levels, compared to long-term average hydraulic conditions.

The above prolonged storage effects can generally be attributed either to delayed groundwater discharge or the sustained infiltration processes in the matrix of the vadose zone.

The research focussed on the analysis of the geohydrological processes in the field, i.e. the analysis of spring discharge and groundwater hydrograph records, both for humid-temperate as well as semi-arid conditions, the analysis of water tracers (Krypton an T/He; trace organics) as well as the coupled modelling of saturated / unsaturated flow, employing a double-continuum approach (HydroGeoSphere).

Our findings show that in less maturely karstified aquifer systems, the contribution of delayed seepage from the vadose zone can reach up to 40% of total spring discharge which is of particular importance for regions with prolonged drought periods, expected for semi-arid environments. The analysis of the tracer information allowed the discrimination of the source of the delayed discharge, in particular Krypton tracer analysis demonstrated the extended residence time of infiltrating water in the vadose zone. The quantification of the partitioning between rapid recharge and slow vadose seepage proved to be a challenge.