Integrated geophysics and water geochemistry characterization of groundwater salinization in the La Misión coastal aquifer, Baja California, Mexico

Coastal aquifers in arid regions are increasingly threatened by salinization due to groundwater overexploitation and climate variability.The La Misión coastal aquifer, located in Ensenada, Baja California, Mexico, is a strategic freshwater resource currently affected by seawater intrusion and related hydrogeochemical processes. This study presents the first integrated geophysical (Transient Electromagnetic Method, TEM), hydrogeochemical, and isotopic investigation of groundwater salinization in the region. TEM soundings revealed four geoelectrical units that define the hydrogeological structure of the aquifer and control the spatial distribution of saline groundwater near the La Misión estuary. Hydrochemical analyses identified Ca–Mg–Cl, Ca–Cl, and Na–Cl facies controlled by water–rock interaction, ion exchange processes, and seawater mixing. Multivariate statistical analysis confirmed that the dominant hydrochemical gradient is controlled by salinity-related variables (Cl - , Na + , EC, and TDS). Stable isotopes (δ 18 O and δ 2 H) indicate that groundwater recharge is predominantly meteoric, while isotopic enrichment observed in coastal wells reflects evaporation and partial seawater mixing. The results indicate that groundwater salinization in the basin is governed by three coupled processes: (i) seawater intrusion along the coastal margin, (ii) water–rock interaction within alluvial sediments, and (iii) evaporative concentration in shallow groundwater systems. The integrated interpretation provides a conceptual framework for understanding groundwater evolution and offers scientific support for groundwater monitoring and sustainable management strategies in semi-arid coastal aquifers of northwestern Mexico.