Hydrogeochemical and isotopic characteristics of groundwater in Mbanga, Njombe and Penja (Banana Plain) – Cameroon

Hydrochemical and environmental isotopic (2H, 18O and 3H) data were used to investigate the hydrogeochemical and isotopic characteristics of groundwater within Mbanga, Njombe and Penja (Banana Plain). Hydrogeochemically, the groundwaters are mainly Ca–HCO3, Ca–Mg–HCO3, Ca–Na–HCO3, and Ca–Na–NO3–Cl–HCO3 water types. The groundwater chemistry may be adequately explained by the incongruent dissolution of silicate and alumino-silicate minerals, impact of anthropogenic activities and cation exchange between the groundwaters and clay minerals. The isotopic contents of groundwaters ranged from −4.2‰ to −2.1‰ for δ18O, from −23.4‰ to −10.6‰ for δD and from 0.6 to 1.4 TU for tritium. In the conventional δD–δ18O diagram, the distribution of data points indicates that the groundwaters are of meteoric origin and have not been affected by evaporation. Environmental isotopes (18O, 2H and 3H) indicate mixing between recent and old groundwaters; the latter were recharged under more humid climatic conditions than that at present or from higher elevations. Groundwater in Mbanga, Njombe and Penja are of meteoric origin, young and are still in the early stage of geochemical evolution.

► Main water types are Ca–HCO3 and Ca–Na–HCO3.
► Silicate minerals dissolution and anthropogenic activities control hydrogeochemistry.
► Recharge origin of groundwater is meteoric.
► Most of the groundwaters were recharged on or after 1960.
► Groundwater is still in the early stage of geochemical evolution.