Hydrogeochemical characterization and isotopic composition as tracers of groundwater in southeastern Douala, Cameroon

Groundwater is a critical resource for sustaining human and ecological systems, yet anthropogenic activities and natural processes increasingly threaten its quality and recharge mechanisms. This study investigates the hydrogeochemical characteristics, trace element contamination, and groundwater recharge mechanisms in south-eastern Douala, Cameroon. Groundwater samples were collected from 12 boreholes, 7 open wells, and springs, with in-situ measurements of physicochemical parameters. Laboratory analyses included major ions, trace elements, and stable isotopes (δD and δ1⁸O). Results revealed a pH range of 4.1–7.9, with an average of 5.45, indicating an acidic aquifer system. TDS concentrations (< 352 mg/L) confirmed that the groundwater is fresh. Major ions followed the order Na⁺ > Ca2⁺ > K⁺ > Mg2⁺ for cations and NO₃⁻ > Cl⁻ > SO₄2⁻ > HCO₃⁻ for anions, while trace elements were present as B > Zn > Al > Cu > Ba > Fe > Mn > Ti > Li. Hydrochemical facies identified Na–K-Cl (76%) as the dominant water type, followed by Ca–Mg–Cl (14%), Ca–Cl (5%), and Na-HCO₃ (5%). Silicate weathering and cation exchange were identified as key processes controlling groundwater chemistry. Isotopic analysis (δD = 6.8378 × δ1⁸O + 10.03) aligned with the Douala Meteoric Water Line, indicating modern recharge primarily through direct precipitation. However, nitrate contamination at shallow depths (up to 35 m) highlights the vulnerability of the aquifer to anthropogenic pollution. While the groundwater is suitable for irrigation, elevated levels of Al, B, Fe, Zn, and Si in some locations pose potential health risks for drinking purposes.