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This study examines groundwater samples from four areas under different conditions. Variations amongst the areas are due to the distinct topography, demographics, and socio-economic activities. A qualification of the groundwater using the following parameters; pH, electric conductivity and nitrate analysis yielded results depicting the deplorable state of the unconfined aquifer; which is even worst in the densely populated and low-lying areas. These unfavorable conditions are present in the older districts (Bependa and Ndogbong) compared to the younger and least populated (urbanized) districts (Ndogpassi and Logpom). The poor quality of groundwater could be due to contamination by sewage from latrines and surface discharges. Nevertheless, nitrate contents (>241 mg/l) in these districts indicate that pollution has not reached the alarming levels observed in other West African cities (nitrates > 400 mg/l). Relatively low concentrations of nitrate in groundwater of the studied areas could be due to volatilization phenomena common in the unconfined aquifers in Douala. We however remarked that most of the studied wells are not used as drinking water, because they present particular health risks to humans.

This study interprets 11 years (2006 to 2016) and 6 months (March to August in 2017) of respectively monthly and daily isotopic (δD and δ18O) monitoring of rain at Douala (Cameroon), a humid tropical station in Western Africa. The main scope is to analyze the climate controls on precipitation isotopes at different timescales. Firstly, we examine the annual cycles of δ18O. Over the 11 years of survey, the annual cycle exhibits a W shape that is quite reproducible from year to year, with two minima in spring and autumn periods. Based on back trajectory calculations and remote sensing observations of water vapor isotopic composition, we show that the observed depletion in spring and autumn is due to strong convective activity along air mass trajectories. The same effect of convective activity can be observed at the daily timescale. At seasonal and daily time scales, the isotopic composition is also strongly tied to the convective organization and cloud types. More depleted precipitation is associated with larger areas of high clouds. Very low to low clouds are observed in July-August, mid-level to high clouds are dominant in June and high to very high clouds characterize March-April-May, thus explaining the enriched (depleted) values in summer (spring). Finally, this paper highlights the importance of large scale meteorological conditions controls on precipitation stable isotope composition in the Gulf of Guinea.

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