IMPACTS AND LIMITS OF THE USE OF ISOTOPE TECHNIQUES IN THE DETERMINATION OF THE ORIGIN OF GROUNDWATER SALINITY IN THE SOUTHERN CRYSTALLINE BASEMENT OF MADAGASCAR

摘要

The origin of groundwater salinity in the crystalline basement of southern Madagascar is examined by means of chemical and isotope signatures. These include the isotopes (~(18)O, ~2H, ~3H) of water and (~987)Sr/~(86)Sr, ~(13)C, ~(14)C) of solutes. Chemical signatures are characterized by trace elements (Br, Sr, F) and the major ion composition. The results suggest two possible mechanisms of mineralization in the Graphite and Androy formations. In the former, palaeomarine intrusion might have contributed to the salinity, whereas water-rock interaction seems to be important in the latter. In both cases, marine aerosols seem to contribute to groundwater salinity. Environmental isotopes have contributed to assessing the origin of the groundwater salinity, although the overall mechanism is not yet clear.