An environmental isotope study of the groundwater regime in large aquifers

Environmental isotope geochemistry has been applied in numerous studies of the groundwater regime in large aquifers. The value of isotopic methods in this work is borne out by three recent studies on the aquifers of the Maestrichtian Sands and the Continental Terminal of Senegal, the aquifer of the Miocene limestones of Cyrenaica and the aquifer of the Jurassic dolomitic limestones of Ain Beni Mathar (Eastern Morocco). Apparent-age groundwater charts drawn up on the basis of 14C data provide a means of determining present and past aquifer charging conditions and the general characteristics of underground flow (direction and velocity). Comparison of these data with piezometric charts serves to confirm or modify the conclusions reached. The contributions of drainage, which can be responsible for the transfer of large volumes of groundwater through the semi-permeable intercalations, can be detected or confirmed by environmental isotopes. In certain cases isotopic geochemistry provides a means of checking the homogeneity or heterogeneity of the aquifer reservoir. Data on the 18O contained in precipitation water and groundwater have been gathered and interpreted, and the results compared with those obtained with other isotopes. (author)