Assessment of Recharge Dynamics Under Extreme Rainfall Using Groundwater Fluctuations and Stable Isotopes Signatures: Insights from an Urban Observatory in Dakar

Rapid urbanization across sub-Saharan Africa intensifies water supply challenges, making groundwater an essential resource for meeting the needs of low-income urban communities. However, the recurrence of extreme rainfall events due to climate change present major challenges for sustainable groundwater management, particularly in urban areas where infrastructures often impedes natural recharge processes and simultaneously amplifies flood risks. Here we investigate the recharge dynamics of the Thiaroye quaternary sand aquifer in Dakar, Senegal, under extreme rainfall conditions, based on high resolution monitoring of groundwater fluctuations and stable isotope analysis. The study combines high-frequency hydrological data from the Dakar suburban groundwater observatory, with isotopic signatures (δ<sup>18</sup>O and δ<sup>2</sup>H) to trace recharge processes and contamination risks. Results indicate that high to extreme rainfall events (>10 mm.d<sup>-1</sup>) are the primary drivers of groundwater level fluctuations, with an identified threshold of approximately 9 mm needed for effective recharge. The isotopic analysis confirms consistent recharge conditions primarily influenced by local rainfall despite substantial variability in precipitation isotopic composition. This research underscores the critical need for improved groundwater management strategies to enhance urban resilience against extreme climatic events such as heavy to extreme rainfall, bridging the knowledge gap regarding groundwater recharge mechanisms in rapidly urbanizing areas like Dakar.