Multivariate assessment of groundwater contamination levels associated with saline intrusion processes in Mediterranean coastal aquifers

Abstract

Groundwater in Mediterranean coastal aquifers supplies a large part of the demand for freshwater but is increasingly threatened by seawater intrusion and anthropogenic pollution. During the springs of 2023 and 2024, six coastal carbonate aquifers in Mallorca were sampled to assess the present-day spatial and vertical variability of salinization and pollution. Hydrochemistry showed elevated Cl − (2140–18,800 mg/L), Na + (1317–10,983 mg/L) and SO₄ 2− (440–2890 mg/L), with high electrical conductivity (7480–53,850 μS/cm in 2023 and 16,200–42,000 μS/cm in 2024). Nutrients (NO₃ − , NH₄ + , PO₄ 3− , NO₂ − ) and fecal indicators ( Escherichia coli , Enterococci ) were detected. Vertical profiles showed salinity increases with depth (except at Drac de Santanyí). The ƒ sea index indicated marine intrusion in >96% of samples. Modified Piper indices (GQI Piper(mix) , GQI Piper(dom) ) and GQI SWI values (28–56 in 2023; 32–51 in 2024) pointed to dominant to mixed Na Cl facies. PCA and hierarchical clustering revealed marked hydrochemical heterogeneity among sites, with differences between 2023 and 2024 and site-specific anomalies associated with freshwater inputs and anthropogenic pressure. Overall, the results document widespread brackish to saline groundwater conditions and the co-occurrence of microbiological contamination in Mallorca's coastal aquifers, highlighting their high vulnerability to salinization and water-quality degradation. This work presents a data-driven, site-specific conceptual model of the marine intrusion system in Mediterranean coastal aquifers, characterized by a laterally extensive brackish zone overlying saline groundwater and a limited or locally absent freshwater lens near the coast. These findings underscore the need for integrated groundwater management, including salinity monitoring, regulation of abstraction, and improved wastewater treatment, to mitigate ecological and public-health risks in Mediterranean coastal aquifer systems. • Integrated hydrochemical, microbiological, and multivariate analyses resolve depth-dependent salinity architecture in Mallorca's coastal karst aquifers. • Hydrochemical indices reveal widespread mixed Na Cl waters and strong vertical salinity stratification across sites (2023–2024). • Microbiological indicators co-occur with salinization patterns, highlighting vulnerable coastal mixing zones. • PCA and clustering delineate the spatial and vertical structure of saline–freshwater mixing and support a site-specific conceptual synthesis for coastal aquifer management.

Topics & Concepts