Assessment of hydrogeochemical evolution of groundwater from the basement aquifer in the upper part of transboundary Mono River Basin, Togo

Abstract

Groundwater acts as a vital resource in upstream of the transboundary Mono River basin (MRB) to meet domestic, drinking, and industrial needs. Groundwater quality and hydrogeochemistry processes have been investigated in the MRB using classical hydrogeochemical and multivariate statistical methods. Groundwater samples (187) were collected for physicochemical parameters analysis. The results show that groundwater is predominantly fresh with circumneutral pH. All parameters were within the permissible limit except 21.58 % and 6.32 % of samples, respectively, for nitrates and pH that exceed the WHO drinking water guidelines. Cations and anions are in the order of Ca2+>Mg2+>Na+ >K+ and HCO-3> C1-> NO3->SO24 where Ca-Mg-HCO3 are dominant water type. The PCA revealed three main factors: silicate minerals weathering, nitrification, and hydrolysis of S-compounds, accounting for 75.5 % of the variance. Groundwater chemistry showed significant spatial heterogeneity. Nitrate sources likely include agrochemicals, human and animal waste, deforestation, and domestic waste. The chemical evolution of groundwater is influenced by equilibrium with secondary minerals such as Ca-montmorillonite, Mg-montmorillonite, K-feldspar, and kaolinite. This process reflects the ongoing incongruent dissolution of primary silicates, leading to secondary minerals through mass transfer and ion exchange. The presence of montmorillonite suggests alteration of amphiboles and plagioclases, indicating a balanced geochemical environment. These findings enhance the understanding of groundwater chemistry evolution and support water resource management in the MRB.