Résumé
Recharge of the Quaternary water table in the northwestern Lake Chad basin (southeastern Niger) estimated from isotopes. The Quaternary water table is present over almost the whole Lake Chad basin. In its Niger part, where annual rainfall varies from 0 to 350 mm, radioactive isotopes ( 3 H and 14 C) are interpreted in terms of renewal rate; medians are 0.10 and 0.05 %, respectively, which means a recharge of about 2 or 3 mm⋅yr -1 . This very weak infiltration is compatible with the stable isotope contents ( 2 H and 18 O) in groundwater, which show a mixing of old and recent waters, infiltrated during the last humid period and the present drier times.