Hydrogeochemistry and stable isotopes of groundwater in the Sudano-Sahelian zone: A case of the Mayo Bocki watershed in North Cameroon

Abstract

Groundwater is the main source of water supply for domestic, agricultural and industrial uses, especially in the sahelian regions. The present study is carried out in the basement aquifers of the Mayo Bocki catchment, Cameroon. It aims to determine the controlling factors of groundwater mineralization and to assess the relative mobility of chemical elements in the aquifers. The methodology adopted was based on conventional geochemical classifications and multivariate statistical analyses. The results revealed that these waters are characterised by low to medium mineralization in both the altered and fractured aquifers. The average concentration of cations followed the trend Ca2+ > Na+ > Mg2+ > K+ while that of anions was HCO3− > NO3− > Cl− > SO42−. The predominant hydrochemical facies was Ca Mg- HCO3 (77%). The acquisition of mineralization by groundwater in this basin involves several natural geochemical processes, including hydrolysis of silicates, dissolution of carbonate minerals, base exchange and leaching. In addition, the chemical inputs used in agriculture make a significant contribution to the chemical signature of these waters. Stable isotopes (18O, 2H) have shown that the water masses in the reservoirs are derived from rainwater and that their chemistry is not influenced by evaporation process. The predominant chemical alteration type in the area under study is monosiallitization. During this weathering process, the behaviour of chemical elements varies from one rock to another. In general, altered rock forms (granite, basalt, trachyte, schist and clay) are enriched in K2O, Na2O, P2O5, Al2O3, SiO2 and depleted in CaO, MgO, MnO, F2O3, SO3, Cl. The relative mobility of the constituents in the aqueous phase revealed that Mn, Zn, Sr, Ca, Na and Mo are the most mobile elements while Al, Th, Cs, Fe are the least mobile elements.