Hydrodynamics of karst aquifers in metamorphic carbonate rocks: results from spring monitoring in the Apuan Alps (Tuscany, Italy)

During the last 40 years, extensive research has characterized the hydrogeology of many karst aquifers, as they are important water resources. Despite that, a systematic investigation on metamorphic karst aquifers is still lacking. The present study investigates the functioning of marble karst aquifers by means of spring hydrological monitoring, coupled with storm-hydrograph, thermograph, and chemograph (HTC) analysis and lag time analysis. Renara and Equi springs (Apuan Alps, Italy) were selected for this investigation. These springs drain catchments that have different degrees of structural complexity. Piston flow is the common hydrodynamic response of Renara spring to infiltration. Strong dilution effects were observed only during the heaviest rainfall events. Prolonged dry conditions after a sustained recharge phase showed the delayed arrival of infiltration water about a month later. Equi spring has a more complex behaviour due to its wider and more hydrologically heterogeneous catchment but the comparison of HTC graphs during the winter dry phase helped to recognize the differential contributions of proximal and distal sectors. Both springs show a rapid discharge increase in response to impulse infiltrative events. Conversely, water temperature and specific electrical conductivity increase only slightly during floods, indicating limited chemical and thermal exchanges between the rock and the water stored in these aquifers. The hydrodynamic behaviour of these karst springs suggests that the Apuan metamorphic aquifers are characterized by the predominance of conduit porosity over fissure and matrix porosities. This is explained by a reduced interstitial porosity and fracturing of the metamorphic carbonate rocks.