Evidence for non-conservative behaviour of chlorine in humid tropical environments

The knowledge of the biogeochemical cycle of chlorine (Cl) is important since this element is used as a tracer of geochemical and hydrological processes in oceanic or continental environments. More specifically, Cl can be used to correct surface water composition from atmospheric contribution in order to calculate precise chemical weathering rates in watersheds. Beyond the problem of potential Cl sources in a given watershed, which is directly related to the lithology, vegetation, and industrial activities, the Cl normalization is based on the assumption that this element behaves conservatively during surface processes (e.g., chemical weathering, adsorption/desorption processes). The purpose of the present study is to forecast the geochemical behavior of Cl in a forested ecosystem located under humid tropical environment. For this reason, we have analyzed the Cl (and also Ca and Na) concentrations of surface waters (rainwater, groundwater, river water) over a two-year period in the Nsimi-Zoetele watershed (Cameroon). The Cl mass balance for the watershed appears to be equilibrated over the studied period (1995-1996) but Cl behavior in Mengong River draining the watershed suggests a non-conservative behavior. Indeed, Cl concentrations in the Mengong River are low during dry seasons and high during wet seasons, which is the reverse tendency to what is usually observed taking into account dilution and evaporation processes. As Cl concentrations in the Mengong River are lower than those measured in all the feeding reservoirs, Cl should be adsorbed onto the soils of the watershed. However, as the Cl mass balance is equilibrated over the whole-year, Cl should be adsorbed and released at a seasonal scale. The results we obtained for this small watershed were not generalized for a larger studied basin (i.e., Nyong River basin). Even if these results should be followed by further investigations, this study suggests that Cl normalization should be used with caution to avoid under- or over-estimation of chemical weathering rates.