Integrating Sr isotopes into coastal aquifer management: evidence for early marine intrusion in Tenerife

The island of Tenerife (Canary Islands, Spain) relies on basalt-hosted aquifers to supply approximately 90% of its water for agriculture and human consumption. The hydrogeological system is highly compartmentalised, consisting of a low-permeability volcanic core overlain by permeable units and dissected by impermeable dykes that form discrete groundwater “pockets”. Recharge is spatially variable and frequently intercepted by wells and horizontal galleries, while water demand is highest in coastal areas, where intensive extraction has historically led to marine intrusion, conventionally identified using chloride concentrations and electrical conductivity. In volcanic ocean island settings, salinity-based indicators can be ambiguous due to evaporative concentration, marine aerosol input, and diffuse volcanic degassing. To improve detection of marine influence, strontium isotopes (87Sr/86Sr) were analysed together with Sr concentrations and major ions in 43 coastal groundwater extraction sites across Tenerife. Strontium isotopes provide a conservative tracer of fluid source, unaffected by biological or physical fractionation, with distinct end-members for basalt-derived groundwater and seawater. Groundwaters display unradiogenic 87Sr/86Sr ratios (0.7032–0.7039), consistent with interaction with basaltic lithologies and defining a well-constrained freshwater end-member. Seawater samples show homogeneous 87Sr/86Sr values (~0.70917) and high Sr and chloride concentrations, providing a clear marine reference. Several groundwaters exhibit isotopic enrichment toward the marine signature (up to 87Sr/86Sr = 0.7055) at moderate chloride concentrations (