Seawater intrusion in carbonate aquifers: the case of the spring group of Castellammare di Stabia (Naples, southern Italy)

Abstract

Understanding salt water intrusion in coastal aquifers is very important for appropriately managing water resources, especially in fractured and karst aquifers. This study analyses the salt water intrusion phenomenon in the carbonate aquifer of the Mount Faito near the spring area of Castellammare di Stabia. In this area, groundwater outflow near the coastline reaches a total of about 0.5 m3/s, from various springs that differ from one another in discharge rate and water chemistry.Results of the previous work were considered, re-examining in particular the chemical data available since 1979. New investigations were carried out in the springs area, including chemical and isotopic analyses of the main representative springs and measurements of flow rate. Furthermore, an experimental borehole was drilled near the springs.According to previous studies, the different springs located near the coastline are the results of different percentages of mixing between two end-members. A first end-member represents the groundwater circulation typically occurring in the carbonate aquifer and outflowing from the less mineralised springs. A second end-member is constituted by the salt water intrusion in the carbonate aquifer. The sea water wedge and particularly the zone of dispersion related to the interface between fresh water and salt water determine different percentages of mixing between two end-members that affect the water chemistry of the brackish and saline springs. Based on the results of the experimental borehole, on the percentage of mixing that explains the difference in hydrochemical facies of the different spring groups and on the isotopic analyses, it was reconstructed a conceptual hydrogeological model of the peculiar spring area, where the phenomenon of the saltwater intrusion is complicated by the presence of fractured and sometimes karstified aquifer, of the interface upconing near the springs and of the localised rise of deep gases.