Groundwater hydrochemistry and identification of nitrate pollution sources in the Ouémé Delta (Southern-Benin) using dual isotopes (15N–NO3 and 18O–NO3) and a Bayesian isotope mixing model

The watershed of the Delta of Ouémé is part of the southern coastal sedimentary basin of Benin. Economic development in this region is essentially based on farming, fishing, and livestock breeding, which are activities that may lead to environmental pollution, including both surface and groundwater pollution. The major aim of the study is to examine the quality of groundwater and to find out the likely sources of pollution in this watershed, using an isotope mixing model (MixSIAR). The methodology applied calls on classical hydrochemical approaches, Self-Organizing Map (SOM), and stable nitrate isotopes (15N;18O) (allowing more accurate discrimination of contamination sources compared to hydrochemical methods). Results demonstrated that the origins of nitrate in groundwater are mainly from manure and sewage and soil nitrogen nitrification. The Bayesian model estimated the contribution of each source and the results have shown that manure and sewage represent the dominant contributors to groundwater nitrate (75.2 % and 87.3 % in the Mio-Pliocene and the Quaternary aquifers, respectively). Then, groundwater pollution during both the dry and rainy periods was mainly due to anthropogenic activities. These results highlight some major implications: (i) groundwater pollution is mainly of anthropogenic origin, calling for better control of agricultural and domestic practices; (ii) the isotopic approach coupled with advanced statistical methods is an effective tool for distinguishing and quantififing pollution sources; and (iii) the study provides a scientific basis for strengthening aquifer protection policies, limiting nitrate-related health risks and promoting sustainable management of water resources.