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

36Cl measurements in groundwaters of the deep confined aquifer of the Lake Chad Basin were performed in order to constrain groundwater apparent ages and residence times. Twenty-seven wells were sampled in Nigeria, Niger and Chad in the southern parts of this large (700 000 km2) multilayered aquifer. 36Cl/Cl values range between 11±1.10-15 to 148±8.10-15 at/at. The highest ratios are observed near the recharge zone of the Nigerian part of the aquifer, while the lowest ones are found in wells located near the southern fringe of the present-day lake Chad. Chlorine concentrations are low (below 100mg/l) and not correlated to the 36Cl/Cl values, indicating negligible dissolution of evaporites in most samples. Reliable 36Cl ages can be calculated along the different flow paths investigated, suggesting residence times of the deep groundwaters larger than 300 000 years. These results are consistent with new AMS-14C data below the detection limit but are in contradiction with previous 14C data.