干旱区生态环境知识资源中心

Deuterium, delta O-18, major ions and dissolved silica in groundwater from semi-arid Mayo-Tsanaga river basin in the Far North Province, Cameroon were used to trace hydrogeochemical processes that control their concentrations and to explore for usability of the water. Electrical conductivity ranges from 57-2,581 mu s/cm with alternating low and high values along the hydraulic gradient. Waters from piedmont alluvium show low concentrations in major cations, which peak in Mg within basalt, Na within plain alluvium, and Ca within basalt and the sandy Limani-Yagoua ridge. The initial dominant groundwater composition is CaHCO3, which did not evolve within the basalt and piedmont alluvium, but evolved to NaHCO3 in the granite and plain alluvium. The main processes controlling the major ions composition include the following: (1) dissolution of silicates and fluorite; (2) precipitation of fluorite and carbonate; (3) cation exchange of Ca in water for Na in clay; (4) and anthropogenic activities. The delta D and delta O-18 ratios vary from -35 to 0.7 and -5.3 to 1.1aEuro degrees, respectively. The lowest and highest isotope ratios are observed in groundwater within the downstream sandy Limani-Yagoua ridge and the upstream graintes respectively. Variation in isotope ratios depends on altitude effect of -0.48aEuro degrees per 100 m between 600 and 850 m asl, and on evaporation, which had insignificant effect on the water salinity. Seventy percent of the groundwater shows poor drinking quality and 90% is suitable for irrigation.