Stable isotopes (2H, 18O and 87/86Sr) and hydrochemistry monitoring for groundwater hydrodynamics analysis in a karst aquifer (Gran Sasso, Central Italy).

This paper deals with chemical and isotope analyses of 21 springs, which were monitored 3 times in the course of 2001; the monitoring program was focused on the groundwater of the Gran Sasso carbonate karst aquifer (Central Italy), typical of the mountainous Mediterranean area. Based on the hydrogeological setting of the study area, 6 groups of springs with different groundwater circulation patterns were distinguished. The hydrogeochemistry of their main components provided additional information about groundwater flowpaths, confirming the proposed classification. The spatial distribution of their ion concentrations validated the assumptions underlying the hydrogeological conceptual model, showing diverging groundwater flowpaths from the core to the boundaries of the aquifer. Geochemical modelling and saturation index computation elucidated water-carbonate rock interaction, contribution by alluvial aquifers at the karst aquifer boundaries, as well as impacts of human activities. The analysis of 18O/ 16O and 2H/H values and their spatial distribution in the aquifer substantiated the hydrogeology-based classification of 6 groups of springs, making it possible to trace back groundwater recharge areas based on mean isotope elevations; the latter were calculated by using two rain monitoring stations. 87Sr/ 86Sr analyses showed seasonal changes in many springs: in winter-spring, the changes are due to inflow of new recharge water, infiltrating into younger rocks and thus increasing 87Sr/ 86Sr values; in summer-autumn, when there is no recharge and spring discharge declines, changes are due to base flow groundwater circulating in more ancient rocks, with a subsequent drop in 87Sr/ 86Sr values. The results of this study stress the contribution that spatio-temporal isotope monitoring can give to the definition of groundwater flowpaths and hydrodynamics in fissured and karst aquifers, taking into account their hydrogeological and hydrogeochemical setting