Dual-flow in karst aquifers toward a steady discharge spring (Presciano, Central Italy): influences on a subsurface groundwater dependent ecosystem and on changes related to post-earthquake hydrodynamics

The Presciano spring system (Central Italy) is fed by a fractured carbonate regional aquifer and shows peculiar hydrogeological, physicochemical, hydrogeochemical and biological features that led to hypothesis about local superimposition of a dual groundwater flow system. The components of this system are fast-flow and baseflow groundwater paths. Hydrogeochemical and isotopic (δ¹⁸O and δD for H₂O, and δ³⁴ \( {\text{S}}_{{{\text{SO}}_{ 4} }} \) and δ¹⁸ \( {\text{O}}_{{{\text{SO}}_{ 4} }} \)) data were used to test this conceptual model. Differences in physico-chemistry and ion concentrations were recorded for two main spring sectors, attributable to different groundwater flowpaths. Environmental isotope analyses allowed distinguishing different recharge areas for the two groups: more depleted values (−10.4 ‰ δ¹⁸O and −71 ‰ δD) indicated the existence of a flowpath influenced by a higher elevation recharge area, whereas enriched isotope signals (−9.9 ‰ δ¹⁸O and −66 ‰ δD) supported a regional recharge area of local water infiltration. The existence of a dual flow system is also reflected in the relationship between SO₄ ²⁻ concentration and SO₄ ²⁻ isotope data. The isotopic pattern of SO₄ ²⁻ reflects the seasonal contributions by the two flow paths to the springs: after snowmelt, the isotopic sulphate value in the spring outlet is very similar to the spring sector fed by the fast flow component, while, after the spring exhaustion period, its isotopic composition is very similar to the base flow spring sectors fed by the fracture network. The influence of the different flowpaths has also been highlighted by changes in water isotope composition after the 2009 L’Aquila earthquake in this area: the strong homogeneous response of the spring system can be attributed to a cleaning of the fracture system, which affected the aquifer at regional scale. Additional investigations adopting the geoelectrical tomography technique highlighted the possibility of the existence of a buried karst channel or at least high-permeability shear zone at the contact between the carbonate aquifer and the fluvio-lacustrine quaternary deposits, where the spring system is located. Biological indicators, such as groundwater copepods also showed a response to the dual flowpath regime. The results of this study have great relevance for a better management of the Presciano spring habitat, which is increasingly threatened by anthropogenic disturbance.