11th Applied Isotope Geochemistry Conference, AIG-11 BRGM Investigation of 36 Cl distribution: towards a new estimation of groundwater residence times in the confined aquifer of the LCB?

Cl measurements in groundwaters of the deep confined aquifer of the Lake Chad Basin (LCB) were performed in order to constrain groundwater geochemical ages and residence times. Twenty-seven wells were sampled in Nigeria, Niger and Chad in the southern parts of the large (700 000 km 2 ) multilayered aquifer of the LCB. 36 Cl/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 Continental Terminal aquifer, while the lowest ones are found in wells located near the southern fringe of the present-day lake Chad. Chloride concentrations are low (below 100 mg/l) and not correlated to the 36 Cl/Cl values, indicating negligible dissolution of evaporitesin most samples. Reliable 36 Cl ages can be calculated along the different flow paths investigated, suggesting residence timesof the deep groundwaters larger than 300 000 years. These results are consistent with new AMS- 14 C data below the detection limit but are in contradiction with previous 14 C data obtained in the area.