Application of Stable Isotopes to Investigate Hydrological Processes in Rivers in the Tropical Zones, South-Center, Cameroon

Aim: Surface water remains the primary source of drinking and agricultural supply in the tropical rainforests of South-Centre Cameroon, yet seasonal hydrological processes that govern its availability remain limited. The study examines river recharge mechanisms and water residence time. Methods: The study used stable isotopes, oxygen-18 (δ¹⁸O) and deuterium (δ²H), from 56 river systems sampled in February 2024, the dry season and July 2024, the wet season. Sampling was done along a latitudinal transect from Yaoundé to Manjo and Manjo to the Littoral, covering diverse elevations and hydrological settings. Isotope Ratio Mass Spectrometry (IRMS) was used to measure δ¹⁸O and δ²H, alongside hydrochemical indicators such as total dissolved solids (TDS), chloride, and deuterium excess (d-excess). Results: The study found a strong seasonal contrast: wet season δ¹⁸O and δ²H values ranged from -5.22‰ to -2.77‰ and -25.74‰ to -8.68‰, respectively, suggesting direct rainfall recharge. In the dry season, δ¹⁸O values ranged from -4.16‰ to -0.04‰ and δ²H from -20.82‰ to 0.19‰, with elevated TDS (up to 10,020 mg/L) and lower d-excess, indicating increased evaporation and groundwater input. LMWL-GMWL comparisons confirmed rapid recharge during the wet season and stronger evaporative enrichment in the dry season. Conclusion: These findings point to shorter residence times in the wet season and longer retention during dry periods, highlighting climate sensitivity in river recharge. Recommendations: These findings support the need for integrated surface water management, emphasizing protection of recharge areas and implementation of long-term isotopic monitoring. The study offers valuable baseline data for safeguarding surface waters and informs strategies for other vulnerable sources in other regions.