Chemical weathering in the drainage basin of a tropical watershed (Nsimi-Zoetele site, Cameroon): Comparison between organic-poor and organic-rich waters

This study deals with the weathering processes operating at the scale of a small catchment (Nsimi-Zoetele, Cameroon) and is focused on the role of organic colloids on mineral weathering and transport of elements in natural waters. Samples of river, spring and groundwaters from Nsimi-Zoetele were filtered through membranes of decreasing pore size (0.22 mu m, 0.025 mu m, or: 300,000 Da, 5000 Da) to separate colloidal fractions from the truly dissolved one. Major and trace elements and dissolved organic carbon (DOG) were analysed in each fraction. Two kinds of waters can be distinguished in the catchment: clear and coloured waters. Clear waters exhibit low concentrations of major and trace elements and DOG. Elements are carried in these solutions in a true dissolved form except Al and rare earth elements (REEs). By contrast, the higher abundances of Al, Fe and trace elements in coloured waters are controlled by the colloidal fraction. Thermodynamic equilibrium calculations show that clear waters an in equilibrium with kaolinite and iron oxi-hydroxide which are major minerals in the weathered soil. For coloured waters, the: aqueous speciation of Ca, Mg, Cu, Fe, Al, La and Th was calculated taking into account the complexes with humic acids. Speciation calculations fur Cu, Fe, Al, La, Th show a strong complexation with humic acids, in good agreement with the results of the filtration experiments. By contrast, although filtration experiments show a strong control of major cations by organic matter (for example 75% for Ca), speciation calculations reveal that their complexes with humic ligands do not exceed a few percent of total dissolved elements. This discrepancy is explained as an artefact induced by the organic colloids and occurring during the filtration procedure. Finally, both filtration experiments and speciation calculations show that organic matter plays an important role in natural DOG-rich waters. Organic acids increase significantly the dissolution rates of silicates and oxi-hydroxides and thus the amounts of solutes and of complexed elements leaving the catchment. (C) 1997 Elsevier Science B.V.