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 r ole of organic colloids on mineral weathering and transport of element s in natural waters. Samples of river, spring and groundwaters from Ns imi-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 f ractions from the truly dissolved one. Major and trace elements and di ssolved organic carbon (DOG) were analysed in each fraction. Two kinds of waters can be distinguished in the catchment: clear and coloured w aters. Clear waters exhibit low concentrations of major and trace elem ents and DOG. Elements are carried in these solutions in a true dissol ved form except Al and rare earth elements (REEs). By contrast, the hi gher abundances of Al, Fe and trace elements in coloured waters are co ntrolled by the colloidal fraction. Thermodynamic equilibrium calculat ions show that clear waters an in equilibrium with kaolinite and iron oxi-hydroxide which are major minerals in the weathered soil. For colo ured waters, the: aqueous speciation of Ca, Mg, Cu, Fe, Al, La and Th was calculated taking into account the complexes with humic acids. Spe ciation calculations fur Cu, Fe, Al, La, Th show a strong complexation with humic acids, in good agreement with the results of the filtratio n experiments. By contrast, although filtration experiments show a str ong control of major cations by organic matter (for example 75% for Ca ), speciation calculations reveal that their complexes with humic liga nds do not exceed a few percent of total dissolved elements. This disc repancy is explained as an artefact induced by the organic colloids an d 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 an d thus the amounts of solutes and of complexed elements leaving the ca tchment. (C) 1997 Elsevier Science B.V.