Groundwater–rock interactions and mixing in fault–controlled karstic aquifers: A structural, hydrogeochemical and multi-isotopic review of the Pontina Plain (Central Italy)

Karstic aquifers represent crucial water resources and are categorized as either stratigraphically or fault-controlled. This study investigates groundwater-rock interactions and mixing processes within one of the largest fault-controlled karstic aquifers in Central Italy, adjacent to the Pontina plain, which is a highly populated area where agricultural activities and climate change challenge the groundwater assessment of a complex aquifer. We conducted structural, hydrogeochemical, and multi-isotopic screening of ten selected springs with different degrees of mineralization (ranging from Ca-HCO3 to Na-Cl hydrofacies), incorporating new analyses and modeling of delta S-34(SO4), delta O-18(SO4), Sr-87/Sr-86, and delta B-11. Additionally, the reinterpretation of a seismic section provides a more detailed framework extending to depths of approximately 5-7 km that allows the identification of the geometry of normal faults, which act as pathways for upwelling fluids. Our findings reveal that hydrogeochemical compositions result from multiple interactions between karstic water and deeper fluids that have interacted with different rocks. Concentration (Na/Li) and isotope (SO4-H2O) geothermometers, coupled with geochemical modeling and trace element analysis, enabled the estimation of a water temperature equilibrium of approximately 95.5 degrees C, with Triassic evaporites generally corresponding to a depth of approximately 3 km and a temperature of 40 degrees C with magmatic rocks at approximately 1 km depth, which is likely associated with ongoing tectonics and the Quaternary tectonically controlled Volsci Volcanic Field. To obtain the latter estimate, we used a new geothermometer activity based on the equilibrium between analcime and pollucite. Furthermore, this multidisciplinary approach enhances the understanding of groundwater behavior in fault-controlled karstic aquifers, where mantle-derived CO2 dissolved in groundwater is the driving force behind water-rock interactions. Given the potential for further variations in mixing, which may worsen water quality and increase aquifer vulnerability, periodic monitoring of these processes is essential in a human-impacted environment amidst ongoing climate change.