Monitoring karst aquifers hydrodynamics in metamorphic carbonates: the example of the Apuan Alps, Italy

Abstract

The hydrodynamic behavior of karst aquifers is usually complex due to the high degree of heterogeneity and anisotropy of their geologic and hydrogeologic properties. Among karstified rocks, metamorphic carbonate rocks (e.g marble and meta-dolostone) are characterized by very low porosity and permeability, but their hydraulic properties can be altered by geologic processes, and significant aquifers can develop into these rocks eventually. However, the functioning of metamorphic karst aquifers is still poorly known. This contribution presents the hydrological monitoring of three of the main karst springs of the Apuan Alps (Italy). This mountain chain is characterized by extensive metamorphosed carbonate outcrops in which karst aquifers have developed. Pollaccia, Renara and Equi springs were equipped with multi parametric sensors for water level and discharge (L, Q), temperature (T), and electrical conductivity (EC). Their hydrographs and chemographs were compared with precipitation time series collected by the meteorological stations to investigate the response of the karst systems to infiltration and to infer their structure. The mean discharge for Pollaccia, Renara and Equi springs were respectively about 900, 200 and 800 l/s, their EC varied between 130 and 300 µS/cm, whereas T ranged between 7.8 and 10.6 °C. Water discharge, T, and EC response to infiltration was quick and impulsive with lag times of few hours. EC and T trends were complex for all the three springs, but dilution was the prevalent hydrodynamic response to precipitation observed during the monitoring. Piston-flow phenomena were scarce and limited to particular conditions. No delay between discharge increments and EC and T responses occurred. These observations suggest that the springs are fed by dominant conduit systems in which water flows only in few, enlarged conduits, with limited fracture storage. Piston-flow phenomena could be related to the presence of secondary flow networks that are connected with the main flow pathways only in specific conditions and/or to the flushing of limited volumes of water stored in the aquifers. Monitoring results suggest that the functioning of meta-carbonate aquifers in the Apuan Alps is controlled by several factors such as the low degree of fracturing of marbles, the local structural arrangement, and the primary porosity of the karstified rocks. As a consequence, storage is limited and flow velocity is high, preventing sustained water-rock interactions.