Stable isotopes of water as a tool to study groundwater–seawater interactions in coastal south-eastern Sicily

Stable isotopes of hydrogen (δD) and oxygen (δ18O) in groundwater, submarine springs and seawater collected in coastal south-eastern Sicily were investigated. While the spatial distribution of δ18O in groundwater showed that most of the heavy-isotope-depleted samples (from −5.7‰ to −6.9‰) were located in the higher portion of the Hyblean Mountains, more positive values (from −3.4‰ to −5.6‰) were found in the marginal areas. The isotope composition of submarine springs was characterized by significant variability (δ18O between −5.6‰ and +1.0‰ and δD between −30‰ and +5.0‰) reflecting seasonal variations and different seawater contributions. The isotopic and chemical composition of coastal groundwater showed the presence of a surficial aquifer, while the composition of inland groundwater and beach springs characterizes a deeper limestone aquifer. The amount of NO3 entering the sea from land-based sources was estimated to be >60 kg/day, which could be of potential environmental concern. Since the submarine groundwater manifestations are fed by coastal contaminated groundwater and re-circulated seawater, environmental implication on the freshwater resources management in the region is of particular interest, especially their protection against contaminants from land-based sources represented by growing agricultural activities in the region.