Monthly δ18O, δD and Cl− characteristics of precipitation in the Ndop plain, Northwest Cameroon: Baseline data

Knowledge of stable isotopes and chemical tracers in meteoric water has been used as a valuable tool in various environmental studies. A systematic monthly sampling of precipitation, in the Ndop plain, was carried out to generate basic data on δ18, δD and Cl−, determine their seasonal variations and controlling factors to be used as baseline data in hydrological and climatological studies. The δ18O-δD relationship of rainfall gives a regression line: δD = 7.93 δ18O + 13.26 (R2 = 0.99), which represents the Ndop Meteoric Water Line (NMWL). The slope is similar to the Global Meteoric Water Line (GMWL), but, with a high d-excess, which suggests a contribution of continental recycled moisture to precipitation. Precipitation shows a wide variation, throughout the year (2012) in δ18O and δD, from +3.86 to +38.62‰ in January to −7.98 and −53.18‰ in September, respectively. The volume-weighted mean of precipitation, which plots close to June rainfall, is −5.61‰ and −31.93‰ for δ18O and δD, respectively. Light convective pre-monsoon and post-monsoon showers, under low relative humidity conditions, are isotopically more enriched than heavy orographic monsoon rain, under high relative humidity conditions. Chloride varies from 1.56 mg/l in the pre-monsoon to 0.06 mg/l during the monsoon rains; like δ18O and δD, it shows high values in early and late rains. Apart from January, all d-excess values in monthly rains are >10‰. The high d-excess values at the beginning and end of the rainy season suggest the contribution of recycled moisture to precipitation during these periods, under low relative humidity conditions. Assuming a constant condensation temperature, the observed seasonal variation in isotopic composition of precipitation is probably as a result of the (1) rain out of Atlantic moisture, (2) monthly rainfall amounts, (3) addition of inland recycled moisture, (4) rain formation mechanism, and (5) varied relative humidity. The varied Cl− concentrations are due to anthropogenic and oceanic sources, and rainfall amounts.