THE EVOLUTION OF ALKALINE GROUNDWATERS IN THE CONTINENTAL INTERCALAIRE AQUIFER OF THE IRHAZER-PLAIN, NIGER

This paper discusses the hydrochemical evolution of groundwaters in the multilayer artesian aquifer of the Continental Intercalaire (the mainly Jurassic Agadez-Dabla sandstones) aquifer of northern Niger. Isotopic and chemical changes in relation to the geological setting suggest that the increasing HCO3- alkalinity of the groundwaters is caused by interaction of deep CO2 with the aquifer matrix. Alteration of feldspars by CO2 with a delta(13)C value of -3.0 parts per thousand accounts for the observed alkalinity and isotopic and trends. Carbon 14 ages have been estimated for recent and Holocene groundwaters which have a distinct stable isotopic (H-2 and O-18) signature compared with C-14 dead groundwaters from further west on the Irhazer Plain. Recharge temperatures, estimated by analysis of noble gas contents, show that the C-14 dead waters were recharged under conditions cooler than present averages in the region, possibly at the beginning of deglaciation (approximate to 16 ka B.P.) or during the period 23-30 ka B.P. Trends in the hydrochemistry of U are related to the deep CO2 alteration process. Radiogenic He contents increase along the flow direction and extremely high values occur in the proximity of U ores. Denitrification in the paleowaters of the Irhazer was assessed by measurements of nitrogen/argon ratios.