Hydrogeochemical and isotopic analysis of groundwater and surface water interactions in the Southern Nyong river basin, Mbalmayo–Cameroon

Abstract Erratic water supply in Mbalmayo compels residents to rely on groundwater—accessed through boreholes and hand-dug wells, which are vulnerable to point-source pollution from pit latrines. This study investigated groundwater geochemistry in parts of the Nyong River Basin using field measurements and random sampling. Major cation concentrations were analyzed using Microwave Plasma–Atomic Emission Spectroscopy (MP-AES), Major anions were measured via Ion Chromatography using a DIONEX ICS-900 system, and stable isotopes (δ¹⁸O, δ²H) were analyzed using the Cavity Ring-Down Spectrometer (Picarro L2120-i) and used to evaluate water quality, recharge sources, and contamination pathways. The pH ranged from 5.6 to 8.8, and 33% of the samples indicated an acidification risk. The dominant hydrochemical facies was Na-K-Cl (60%), and the NO3 − concentration in 24% of the samples exceeded the WHO (2017) guideline of 50 mg/L for drinking water. Data analytical techniques included Gibbs plot, Na/Cl molar ratios, and mineral saturation indices, which revealed water-rock interaction, silicate weathering, and kaolinite equilibrium as main geochemical processes. Cl−/Br mass ratios aligned with septic tank leachates and sewage effluents, indicating pit latrine seepage. Groundwater chemistry in Mbalmayo reflects both geogenic mineralization and anthropogenic nutrient inputs, as shown by the agreement between PCA and HCA. Stable isotope values (δ¹⁸O: ranged from − 5.83‰ to − 2.44‰, and δ²H: –from 25‰ to 15.9‰) are broadly consistent with Central African rainfall, but exhibit local enrichment and depletion patterns shaped by hydroclimatic variability.