Characterisation of structural and water flow processes in the upper vadose karst zone using a multi-method approach in a cave

The complexity of karst aquifers hampers the assessment of groundwater recharge processes in the upper vadose zone. Consequently processes governing water flow in the soil and epikarst into the vadose zone remain poorly understood. This study aims to explore spatial differences of water recharge, storage, and movement through the upper vadose zone on a field scale at the Hochschwab karst massif (Eastern Alps, Austria).

To achieve this, we combined multiple approaches such as geophysical, hydrological, pedological, and structural geological methods to distinguish spatial variability in infiltration processes. Data were collected at Hirschgruben cave (1896 m above sea level) for dry and wet conditions in winter and summer providing a seasonal comparison of infiltration dynamics in regard to snowmelt and precipitation. Monitoring included cave drip water (discharge, electrical conductivity and temperature) along with soil moisture measurements at depths of 5 to 30 cm, and electrical resistivity tomography (ERT) utilizing 96 electrodes between the cave ceiling and the surface to produce resolved 2D images. A structural geological survey of the fracture density classes and fault characteristics was carried out.

The results show different infiltration processes for snowmelt and precipitation; deep saturation with slow water percolation after snowmelt and rapid transit of water and quick responses at the cave weir after heavy precipitation events. Spatial differences in the ERT images indicate differences in water saturation in the epikarst, bedrock and the frost-weathered cave ceiling. The ERT images show the greatest increase in saturation in the bedrock during snowmelt, while rain events with rapid and heavy water flow show a continuous increase in water saturation in the epikarst and preferential flow paths. The structural geological characterisation of the catchment area enables the interpretation of differences in the spatial distribution of water saturation. These results underline that the integration of multiple sensors and methods is crucial to understand the variability of water fluxes in Alpine karst systems under different meteorological conditions.