Présentations Colloques

Oral Presentation
2.01
Session 2.01: Groundwater in semiarid regions – a long-term view on changes in aquifer balances. Hydrogeology in developing countries.
Kadjangaba Edith
Groundwater recharge and vulnerability of shallow aquifers under a Sahelian metropole- N'Djamena (Chad Republic)
In the Lake Chad basin, N’Djamena, the capital of Chad, lies in the north western part of the Chari Baguirmi region, on the Chari floodplain and the right bank of the Chari River. In detrital Quaternary sediments, the groundwater system globally consists of to main layers between 6 and 13 m, and 30 and 46 m deep respectively. These two aquifer layers might locally be hydraulically connected. Only about 23% of the urban population, which is steadily increasing, is connected to a network of drinking water supply originating from the deeper layer. The city has no functional sewage system. An open canal, which collects domestic sewage, crosses the city from the north to the south. Moreover, many old mines in the outskirts of the city form depression where water remains stagnant almost permanently. **As in most African cities, groundwater pollution occurs firstly from direct infiltration of rainfall after leaching the soil, and in a second time when the water table reaches the bottom of the latrines. In such a context deciphering recharge processes is of major concern.** The data obtained from different sampling campaigns and analyses (major ions, traces, 18O, 2H, 13C, 3H, 14C), conducted between 2002 and 2003 (Kadjangaba thesis), and 2010 (IAEA TC project) underlined the major role of the Chari River, and allowed to characterize and localize the recharge of urban runoff. **The results also reveal that the deeper part of the Quaternary water body is still relatively well preserved despite anthropogenic pollution on the surface. Nevertheless, this aquifer layer stays vulnerable in case of over exploitation that might drain polluted surface water to deeper levels.**
Chad