Coastal aquifers serve as a lifeline for agriculture, drinking water and ecosystems. However, they are increasingly under threat worldwide from seawater intrusion, primarily driven by excessive groundwater extraction, intensified land use, population growth, and climate change, including sea-level rise. This study applies an integrated, multi-seasonal hydrochemical and isotopic approach to assess groundwater and surface water quality in the Metaponto coastal plain (southern Italy). Groundwater and surface water were sampled seasonally over one year and analysed for major ions, trace metals, stable isotopes (δ 18 O, δ 2 H), and radioactive isotopes ( 3 H, 14 C), supported by hydrochemical diagrams, ionic deviation analysis, seawater fraction (f sea ), and multivariate statistics. Electrical conductivity in groundwater ranged from freshwater values (10,000 μS/cm in coastal and deep wells, with maximum seawater fractions reaching ∼0.41. Hydrochemical facies evolve from Ca–HCO 3 inland to Na–Cl dominance near the coast, reflecting salinization driven by seawater mixing and cation exchange. Stable isotopes indicate predominantly meteoric recharge, while enriched δ 18 O and δ 2 H values (up to −2.5‰ and −15‰, respectively) in coastal and deep groundwater reflect evaporation and saline mixing. Tritium (≤0.5–4 TU) and radiocarbon ages (10,000 years BP) reveal a vertically stratified aquifer, with modern shallow groundwater overlying confined paleo-groundwater. Despite elevated salinity, Heavy Metal Pollution Index (HPI) values remained low (≈10–15), indicating negligible metal contamination. These results demonstrate that groundwater quality degradation in the Metaponto coastal aquifer is primarily controlled by salinization rather than metal pollution and highlight the value of integrated hydrochemical–isotopic tools for identifying aquifer vulnerability and supporting sustainable groundwater management in coastal plains. • Integrate methods to identify processes causing groundwater quality degradation. • Reveal inland-to-coast salinity shifts with seasonal and depth-specific variations. • Differentiates mixing and reactive processes using f sea and ionic deviation analyses. • Apply δ 18 O, δ 2 H, 3 H, and 14 C to assess recharge sources and aquifer stratification. • Reveals isolated paleo-groundwater (>10,000 yrs) beneath a vulnerable coastal aquifer.
Singh et al. (Sun,) studied this question.