Chlorine-36 dating of the deep confined aquifer groundwaters of lake Chad Basin

International audience36Cl measurements in groundwaters of the deep confined aquifer of the Lake Chad Basin were performedin order to constrain groundwater apparent ages and residence times. Twenty-seven wells were sampled inNigeria, Niger and Chad in the southern parts of this large (700 000 km2) multilayered aquifer. 36Cl/Cl valuesrange between 11±1.10-15 to 148±8.10-15 at/at. The highest ratios are observed near the recharge zone of theNigerian part of the aquifer, while the lowest ones are found in wells located near the southern fringe of thepresent-day lake Chad. Chlorine concentrations are low (below 100mg/l) and not correlated to the 36Cl/Clvalues, indicating negligible dissolution of evaporites in most samples. Reliable 36Cl ages can be calculatedalong the different flow paths investigated, suggesting residence times of the deep groundwaters larger than300 000 years. These results are consistent with new AMS-14C data below the detection limit but are incontradiction with previous 14C data