Isotope hydrology and geochemical modeling: new insights into the recharge processes and water–rock interactions of a fissured carbonate aquifer (Gran Sasso, central Italy)

Abstract

The goal of this paper was to characterize the recharge process and water–rock interactions in a regional homogeneous fractured carbonate aquifer in the Mediterranean environment (Gran Sasso, central Italy) through isotope data (δ2H, δ18O and δ13C-DIC) collected between 2006 and 2010. Samples were collected from the springs of the Gran Sasso aquifer and from the Underground Nuclear Physics Laboratories, located within the aquifer. Additionally, the hydrochemical data and the reference hydrogeological frameworks of previous studies have been used as a starting point for the geochemical modeling. The Gran Sasso aquifer, which is bounded by terrigenous and clastic units acting as aquitards, accommodates a uniquely broad regional groundwater, which feeds springs mainly at its border with high, steady discharge. In total, these springs discharge of more than 18 m3/s. At a local scale for the aquifer core and at a regional scale for the overall aquifer, δ 2H, δ18O and δ13C-DIC isotope data, the geochemical inverse modeling through PHREEQC and the δ13C-DIC fractionation modeling through NETPATH 2.0 show the following. (1) Clear processes of evaporation and related isotope enrichment may be ruled out. (2) Groundwater flow is active and extends to the overall aquifer without clear signs of layering and partitioning. (3) Groundwater flowpaths radiate from the core toward the periphery. (4) In L’Aquila Plain, Gran Sasso groundwater mixes with shallow Quaternary water at a ratio of one half. (5) The main geochemical processes are the dissolution of calcite and dolomite, and in some cases, ion exchange occurs (Mg2+ and SO42− release, Ca2+ adsorption). (6) In the less mineralized recharge groundwater, the δ13C-DIC value is influenced by the δ13C-DIC value of rainfall, while in more evolved groundwater, the final δ13C-DIC value is reached by fractionation during the flowpaths, indicating a lengthy interaction with limestone.