Dynamic monitoring of unconfined and semi-confined aquifers in the Quaternary system of N’Djamena, Chad

This study investigates the spatiotemporal variability of unconfined and semi-confined aquifers in N’Djamena, Chad, based on dynamic monitoring conducted from 2020 to 2024. A total of 40 boreholes—18 in unconfined aquifers and 22 in semi-confined systems—were monitored using piezometers, probes, and GPS instruments during two key periods each year: May (end of the dry season) and November (post-rainy season). Groundwater depths exhibited marked seasonal fluctuations, ranging from 13.5 m to 4.1 m in unconfined aquifers and from 29.1 m to 6.3 m in semi-confined aquifers. Pearson correlation analysis revealed only a moderate relationship with temperature (r = 0.582) and a weak correlation with precipitation (r = 0.390), suggesting that groundwater level variations are not solely or linearly governed by climatic parameters. Instead, they likely result from a combination of thermal influences, delayed infiltration, and lateral recharge through semi-permeable layers. A comparative analysis with the Lake Chad Basin highlighted distinct recharge mechanisms shaped by geological and hydrological contrasts. These findings support differentiated aquifer management strategies—such as using unconfined aquifers for emergency supply and preserving semi-confined aquifers for long-term needs—as well as urban planning interventions, including improved drainage in flood-prone areas such as Sabangali and Gassi. This study contributes to the development of adaptive groundwater governance frameworks in water-stressed and rapidly urbanizing Sahelian environments.