A ground based gravity network for monitoring water mass movements in the Classical Karst region

The Classical Karst is a limestone plateau stretching between Italy and Slovenia over an area of about 600km2.The aquifer of Classical Karst contains a complex network of conduits, shafts and large voids that are fed by theautogenic recharge and allogenic input of the Reka River. The Reka River sinks underground in the Škocjan Cavesand continues its underground flow for almost 40km until it reaches the Adriatic Sea at the Timavo Springs. Theriver shows high discharge variations; with minimal discharge below 0.3 m3/s and maximal discharge reachingover 350 m3/s. As the conduit system cannot efficiently drain large discharge, huge water masses are stored inthe epiphreatic voids of the aquifer during flood events. Škocjan Caves present such storage, where a vast amountof water is temporary stored during intense rain. Further evidences of the impressive water movements in thissystem could be found in the geodetic time-series recorded by the Grotta Gigante horizontal pendulums, whichshow deformation transients during Reka flood events. Gravimetry could be a useful tool to obtain local mass balances of such complex system, contributing together withthe other classical hydrologic prospections in depicting the water dynamics in this karstic environment. In additionto this gravimetry represents the ideal completion to the already set up geodetic instrumentation in the ClassicalKarst region. In the last year, we installed two continuous recording gravity stations, near the Škocjan caves and inside the GrottaGigante cave. The Škocjan caves serve as a test site because the cave geometry and the hydraulic system here arewell known. The Grotta Gigante site offers a quiet place and long term geodetic time-series but on the other hand,the hydrodynamics here are less clear. In this contribution, we present the simulations in support to the placement of the instruments and the first analysisconducted on the observed gravity time-series. For the Škocjan caves our simulations estimate that this mass accu-mulation could generate gravity signals up to 30 microGal for extreme events with peak discharge over 250m3/slasting for 1-1.5 days, accumulating over 35 106m3of water. The recorded data in Škocjan supports our simula-tions: a prominent peak up to 5 microGal of amplitude was recorded during a 14 106m3flood event on October2018. We believe that the Classical Karst represents an interesting study case for both the geodetic and hydrologic com-munities; the Škocjan cave offers a natural laboratory to optimally assess the contribution of gravimetry as a toolfor monitoring underground fluid mass movements.