Water mass movements in Classical Karst depicted by continuous gravity measurements

Depicting the water dynamics in a Karst environment frequently represents a challenge due to the more complex drainage system compared to porous media and to logistic difficulties in deploying an efficient monitoring network. The observations of key physical parameters such as discharge are usually carried out sparsely and only if a direct access to the drainage channels is possible. For this reason, indirect geophysical methods, such as gravimetry, could be a valid complement to hydrological prospections to study the hydrodynamics of such systems. A typical karst environment is the “Carso/Kars” region, which is a 600km2 limestone aquifer, shared between Italy and Slovenia. The aquifer is constituted by an extended network of caves, shafts, and conduits fed by autogenic water and the allogenic contribution of the Reka river. The Reka river sinks in the Škocjan caves and continues its underground flow for over 30 km, finally outflowing in the Adriatic sea at the Timavo Springs. The river shows high discharge variability between dry and wet periods (0.3- 350 m3/s) and as the karst conduit system cannot efficiently drain large discharge, huge water masses are temporarily stored in the epiphreatic voids during flood events. In July 2018 we installed a gravimeter near the Škocjan caves with the aim to monitor continuously the water variations. In this contribution, we present the gravity data and the processing in order to remove tidal and other non-hydrologic components and, thus, to decipher the gravity variations linked to the Karst water circulation. Some challenges in the processing arise since the study area is close to the Adriatic sea, where marine tidal and non-tidal contributions are unneglectable. Our gravity measurements revealed many effects that are linked to the local hydrology: one important event flooded the caves in February 2019 causing a gravity change of 40 microGal. The event is compatible with the gravity estimate derived from a hydraulic model of the cave which reported an accumulation of about 10 106 m3 of water in the whole cave system in 1.5 days. The case discussed in this contribution represents a first promising application of gravimetry for tracking the water paths in the Classical Karst, easily applicable to other sectors of the Karst for which only little information about the hydrological system exist.