Hydrochemical and isotopic evidence of recharge, apparent age, and flow direction of groundwater in Mayo Tsanaga River Basin, Cameroon: bearings on contamination

摘要

Unplanned exploitation of groundwater constitutes emerging water-related threats to MayoTsanaga River Basin. Shallow groundwater from crystalline and detrital sediment aquifers, together with rain, dams, springs, and rivers were chemically and isotopically investigated to appraise its evolution, recharge source and mechanisms, flow direction, and age which were used to evaluate the groundwater susceptibility to contamination and the basin's stage of salinization. The groundwater which is Ca-Na-HCO_3 type is a chemically evolved equivalent of surface waters and rain water with Ca-Mg-Cl-SO_4 chemistry. The monsoon rain recharged the groundwater preferentially at an average rate of 74 mm/year, while surface waters recharge upon evaporation. Altitude effect of rain and springs show a similar variation of -0.4%o for δ~(18)O/100 m, but the springs which were recharged at 452, 679, and 773 m asl show enrichment of δ~(18)O through evaporation by 0.8‰ corresponding to 3% of water loss during recharge. The groundwater which shows both local and regional flow regimes gets older towards the basins' margin with coeval enrichment in F~- and depletion in NO_3~-. Incidentally, younger groundwaters are susceptible to anthropogenic contamination and older groundwaters are sinks of lithol-ogenic fluoride. The basins salinization is still at an early stage.