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

The knowledge of the biogeochemical cycle of chlorine (Cl) is important sin ce this element is used as a tracer of geochemical and hydrological process es in oceanic or continental environments. More specifically, Cl can be use d to correct surface water composition from atmospheric contribution in ord er to calculate precise chemical weathering rates in watersheds. Beyond the problem of potential Cl sources in a given watershed, which is directly re lated to the lithology, vegetation, and industrial activities, the Cl norma lization is based on the assumption that this element behaves conservativel y during surface processes (e.g., chemical weathering, adsorption/desorptio n 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 t his 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 s tudied period (1995-1996) but Cl behavior in Mengong River draining the wat ershed suggests a non-conservative behavior. Indeed, Cl concentrations in t he Mengong River are low during dry seasons and high during wet seasons, wh ich is the reverse tendency to what is usually observed taking into account dilution and evaporation processes. As Cl concentrations in the Mengong Ri ver 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 balanc e is equilibrated over the whole-year, Cl should be adsorbed and released a t a seasonal scale. The results we obtained for this small watershed were n ot generalized for a larger studied basin (i.e., Nyong River basin). Even i f these results should be followed by further investigations, this study su ggests that Cl normalization should be used with caution to avoid under- or over-estimation of chemical weathering rates.