Tracing salinization processes in coastal aquifers using an isotopic and geochemical approach: comparative studies in western Morocco and southwest Portugal

Abstract

Environmental stable and radioactive isotopes (δ2H, δ13C, δ18O; 3H and 14C), together with physical and geochemical data, were used in the determination of the origins of groundwater salinization and geochemical evolution processes in coastal regions. Two case studies on the Atlantic Coast are discussed, one located in the Essaouira sedimentary basin, western Morocco, and the second, in the Lower Tagus–Sado sedimentary basin, southwest Portugal. In both regions, groundwater degradation occurs by salinization increase to different concentrations and in relation to different origins. The main quality issues for the groundwater resources are related to seawater intrusion, dissolution of diapiric structures intruding the aquifer layers, brine dissolution at depth, and/or evaporation of irrigation water. Anthropogenic pollution ascribed to agricultural activities is another source for groundwater degradation, affecting mainly the shallow aquifers. The apparent 14C age of the analysed samples ranges from 2.9 ± 0.3 up to 45.6 ± 0.6 pmC in the Miocene groundwater samples from the basin in Portugal; at the Essaouira basin in Morocco, the 14C content varies from 60 to 86 pmC. In most of the water samples, the 3H concentration is below the detection limit. In both basins, the isotopic results together with the geochemical data provided an effective label for tracing the mineralization origin and groundwater degradation processes. Further, the isotopic signatures were used in the identification of a paleoclimate (colder period), recorded in the stable isotopic composition and corroborated with the 14C data.