A coupled hydrogeological and multi-isotopic approach to investigate saltwater intrusion in a coastal groundwater system (Sardinia, Italy)

Many coastal areas around the world, especially low-lying delta areas, have a high density population and host important economic activities. In such context groundwater abstraction for public water, irrigation and private water supply can lead to over-exploitation and seawater intrusion phenomena. Saltwater intrusion is a critical socio-economic and environmental issue in the coastal plain of Muravera, south-eastern Sardinia (Italy). Since the early fifties the natural hydrodynamic equilibrium between groundwater, surface-water and seawater has been deeply modified by human interventions mainly related to the development of agriculture and tourism activities. The aim of this work is to deepen the knowledge about groundwater recharge areas, salinization mechanisms and water chemistry evolution through a combined hydrogeological and multi-isotopic approach. In this frame, a monthly piezometric and electrical conductivity monitoring survey was carried out for one year, integrated with chemical and isotope analyses of δ¹⁸O_H2O e δ²H_H2O, δ¹¹B, δ¹⁸O_SO4, δ³⁴S_SO4, ⁸⁷Sr/⁸⁶Sr. Isotope analyses of δ¹⁸O_H2O e δ²H_H2O from two precipitation samples are also performed to provide a reference for local meteoric composition.

Results from hydrochemistry analysis show the occurrence of seawater-freshwater mixing, extending up to 4 km inland. δ¹⁸O_H2O & δ²H_H2O, δ¹¹B, δ¹⁸O_SO4 & δ³⁴S_SO4 isotopes analysis confirms the mixing processes and indicates the meteoric origin of recharge waters for both shallow and semi-confined aquifers. Moreover, a clear correlation between precipitation and seawater H₂O isotopic composition is observed. Strontium isotopes ratio has allowed the identification of four main groundwater flow paths, including lateral recharge from bedrock, surface water infiltration from the Flumendosa river and Rio Flumini Uri, and the occurrence of young mixing processes between fresh and sea waters. Outcomes from the combined investigation approach are crucial in the implementation of an integrated and sustainable management system which aims, on the one hand, at slowing the process of saltwater intrusion, and on the other hand to meet socio-economic needs for local communities’ development.