Stable Isotopes for Groundwater Assessment of the Quaternary Aquifer of the Western Part of Chad: A Case Study of Bahr El Gazel, Hadjer Lamis, Kanem and Lac Regions

Environmental isotopes were used to decipher sources of groundwater and recharge of the Quaternary aquifer in 4 regions namely, Bahr El Gazel, Hadjer Lamis, Lac and Kanem located in the North and East of the Lake Chad Basin. Four field campaigns were conducted under the IAEA/ RAF/7/011 project for a total of 89 sampling points. Samples were collected during wet and dry seasons from 2013 to 2015. Physico-chemical and isotopes analyses were performed for groundwater and rainfall data. Stable isotopes of rainfall indicated a continental or close-sea moisture origins and evaporation as the main fractionation processes occuring in the study area, as attested by the excess values and seasonal variations. The most enriched values of stable isotopes are observed in the Lac Region in the South-East (SW) and the depleted ones are seen in the Hadjer Lamis Region in the South-East (SE). On one hand, evaporation affects groundwater during rainfalls and the recharge lead to the accumulation heavy isotopes in the unsaturated zone. On another hand, mixing and diffusion processes and conservative ions indicate successive recharge events. The decrease of tritium in precipitation is consistent with the decrease of tritium contents in groundwater, showing recharge events particularly for post and recharge.