Geological, hydrogeological and geochemical characterization of aquifers in coastal areas (Ragusa-Donnalucata Area, Sicily, Italy)

Carbonate groundwater systems hosted in coastal areas are of particular concern, for multi-fold reasons. First, in Sicily - as in many other islands – coastal areas are the most densely populated: this increase of anthropogenic pressure considerably raises the potential risks of groundwater contamination. This is even more dangerous considering the fact that groundwater is often the main source of potable water for coastal communities, especially since rivers and ponds near the shore are brackish. Secondly, coastal areas are the sites of potential interactions between the groundwater systems and seawater, the morphology and the location of this interface being a complex function, of groundwaters discharge rate, permeability of the host rocks and hydrodynamical features. As the groundwater/seawater interface results from a complex dynamic equilibrium of several contemporaneously acting forces, it turns out that the diffuse and uncontrolled overexploitation of coastal aquifers, along with structural and climatic circumstances may increase the possibility of seawater intrusion, as encountered all over the world. The outcropping successions consist of thick carbonatic formations that derive from northern sector of African foreland. The upper part of these carbonatic succession are the Fm Ragusa deposits (glauconitic calcarenites) strongly carsified and yointed and the Fm Tellaro (marls and marly limestones) of meddle-upper Miocene) The plio-pleistocenic tectonic phase produced NE-SO and NO-SE faults that represent preferential drainage for groundwaters and surface waters. The mineralogical and chemical composition or carbonate formations making up the aquifer play a decisive role, as demonstrated by prevailing Ca-HCO3 groundwater compositions. This finding is also consistent with thermodynamic model, showing that the studied are at thermodynamic equilibrium with calcite at the prevailing pH and pCO2 conditions, such equilibrium conditions being a result of prolonged interaction with fast-congruently-dissolving carbonate minerals. Mixing with seawater component is a second main source of groundwater mineralization. The importance of this geochemical process is made clear by the widespread occurrence of Na-Cl-rich saline groundwaters which –also according to 18O and D enrichment – claim for non-negligible seawater contribution (exceptionally up to 50%). This is not god news, indeed: it reveals that the indiscriminate proliferation of private and public wells during the last decades is already altering the delicate balance of water budgets typical of coastal aquifers.