Résumé
Over the last decades, a continuous increase in groundwater reserves has been observed in southwest Niger due to a rapid clearing of the native vegetation. In this rural region, high levels of nitrate occur in groundwater, which are difficult to explain given the lack of any usual source of pollution. Groundwater surveys show that important increases in nitrate content occur during exceptional recharge events near infiltrating ponds, indicating a current nitrogen flux to the aquifer. Groundwater samples with nitrate contents of between 1 and 100 mg l (super -1) have delta (super 15) N values between +3 to +8 per mil, a range fully consistent with natural derivation from the soil. These results strongly suggest that a break in the nitrogen cycle following land clearance is the main source of dissolved nitrate in this unconfined aquifer.