The effect of organic matter on chemical weathering: Study of a small tropical watershed: Nsimi-Zoetele site, Cameroon

The effect of organic matter during soil/water interaction is; still a deba ted issue on the controls of chemical weathering in a tropical environment. In order to study this effect in detail, we focused on the weathering proc esses occurring in a small tropical watershed,(Nsimi-Zoetele, South Cameroo n). This site offers an unique opportunity to study weathering mechanisms i n a lateritic system within a small basin by coupling soil and water chemis try. The lateritic cover in this site can reach up to 40 m in depth and show two pedological distinct zones: unsaturated slope soils on the hills and/or el evated areas; and: water-saturated soils in the swamp zone which represent 20% of the basin surface. The study present chemical analysis performed on water samples collected monthly from different localities between 1994-1997 and on: soil samples taken during a well drilling in December 1997. The re sults suggest the existence of chemical and spatial heterogeneities of wate rs in the basin: colored waters flooding the swamp zone have much higher co ncentrations of both organic matter (i.e., DOG) and inorganic ions (e.g., C a, Mg, Al, Fe, Th, Zr) than those from springs and groundwater from the hil ls. Nevertheless, these organic-rich waters present cation concentrations ( Na, Ca, Mg, K) which are among the lowest compared to that of most world ri vers. The main minerals in the soils are secondary kaolinite, iron oxi-hydr oxides, quartz, and accessory minerals (e.g., zircon, rutile). We mainly fo cused on the mineralogical and geochemical study of the swamp zone soils an d showed through SEM observations the textural characterization of weathere d minerals such as kaolinite, zircon, rutile and the secondary recrystalliz ation of kaolinite microcrystals within the soil profile. Water chemistry a nd mineralogical observations suggest that hydromorphic soils of the swamp zone are responsible for almost all chemical weathering in the basin. Thus, in order to explain the increase of element concentration in the organic-r ich waters, we suggest that organic acids enhance dissolution of minerals s uch as kaolinite, goethite, and zircon and also favors the transport of ins oluble elements such as Al, Fe, Ti, Zr, and REE by chemical complexation. S iO2(aq) concentrations in these waters are above saturation with respect to quartz. Dissolution of phytholithes (amorphous silica) may be responsible for this relatively high SiO2(aq.) concentration. Al/Mg ratios obtained for the soil and the Mengong river waters show that a significant amount of Al does not leave the system due to kaolinite recrystallisation in the swamp zone soils. Geochemical data obtained for this watershed show the important Contribution of vegetation and organic matter on chemical weathering in th e swamp zone. Quantitatively we propose that the increasing amount in total dissolved solid (TDS) due;to organic matter and vegetation effect is about 35%. In summary, this interaction between soils and waters occurs mostly i n soils that are very depleted in soluble elements. Thus, the low concentra tion of major elements in these water is a direct consequence of the deplet ed nature of the soils. Copyright (C) 1999 Elsevier Science Ltd.