Geochemistry, origin and recharge mechanisms of groundwaters from the Garoua Sandstone aquifer, northen Cameroon

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This paper presents results of the geochemical and isotopic studies on groundwater samples from the Garoua Sandstone Basin, in northern Cameroon. Chemical and environmental isotope data are presented and discussed in terms of the origin of dissolved species and of groundwater. All of the investigated groundwaters are categorized into two chemical types: low mineralized and acidic waters of CaNaHCO 3 type, and relatively high mineralized waters of NaHCO 3 type, with relatively high pH, between 7.40 and 8.22. Interpretation of chemical data, based upon both thermodynamic calculations and stability diagrams, suggests that the chemical evolution of groundwater is primarily controlled by water-rock interactions, involving (1) acidic weathering of aluminosilicates, (2) dissolution of secondary carbonate minerals, and (3) cation exchange of Na + for Ca 2+. However, the original composition of groundwater may have been modified by further secondary processes such as mixing of chemically different water masses, and anthropogenic NO − 3 pollution. Tritium contents in the considered groundwaters are very heterogeneous, suggesting differences in the water turnover time or in the recharge conditions, or both. On the basis of the geographical distribution of 3H contents, a hydrogeological relation between river and the groundwater systems has been proposed for the study site. Interpretation of both 18O and 2H, as well as 3H, suggests that the recharge of the investigated groundwaters may result from three major mechanisms: (1) direct infiltration of local precipitation; (2) lateral inflow of river waters; (3) probably upward leakage of old groundwaters.