Source of groundwater salinity in coastline aquifers based on environmental isotopes (Portugal): Natural vs. human interference. A review and reinterpretation

• Evaluation of natural vs. human groundwater deterioration.
• Update the assessment of salt origin in groundwaters from coastal areas.
• Interpretation on water’s age and water–salt diapir/brines/seawater interaction.
• 14C content to redefine the origin of groundwater mineralization.
• Results are vital to local authorities to reach sustainable water management.

Environmental stable (δ18O, δ2H, δ13C) and radioactive (3H and 14C) isotopes, together with geochemical data were used to identify the origin of salinization in different environments. Three case studies from sedimentary basins of continental Portugal are presented: (i) two at the Meso-Cenozoic Portuguese southern border (Algarve basin) and (ii) one at the Lower Tagus–Lower Sado basin (central Portugal), with a new data interpretation. Groundwater salinization occurs in all three cases, and may reach values of several grams of Total Dissolved Solids per liter; above accepted limits for drinking water. The source of this high mineralization could be: (a) seawater intrusion (ancient or modern); (b) dissolution from diapiric structures intruding on the aquifer systems; (c) brine dissolution at depth; and (d) evaporation of irrigation water. The results obtained have provided an effective label for seawater and freshwater, to enable seawater intrusion to be traced, as well as the identification of other processes that might be responsible for groundwater salinization, such as salt minerals dissolution and ion exchange.