This work is a part of the french program PEGI (Programme d'Etude de la Géosphère Intertropicale) devoted to the study of the biogeochemical cycles in the tropical ecosystems. The studied site (Nsimi-Zoetele) is a small watershed located in Africa (South Cameroon). It was chosen for its representativity of the lateritic cover developed in central Africa under tropical rain forest. The general purpose of this study is to understand the transfer of elements within the hydrological system of this watershed.
The distribution of major and trace elements was established in the natural waters between the particulate, colloidal and truly dissolved phases. Spring, stream and groundwaters were sampled in different points of the site. On the field, the water samples were sucessively filtered through membranes of decreasing pore sizes (0.20 µm/ 300 000 daltons/ 5000 daltons) using both frontal and tangential filtration systems. Each filtrated fraction were analysed for major cations, trace elements and dissolved organic carbon (DOC) by ionic
chromatography, ICP-MS and carbon analyser, respectively. In this study are presented the results obtained for the river Mengong which drains the catchment. This river presents a low suspended matter content (<5 mg/l) consisting of particulate organic carbon (POC), a low pH value (4.62) and high DOC concentrations (up to 24 mg/l). The 0.20 µm filtered fraction contains about 9 mg/l SiO2 and notable concentrations of Th, Ti and Zr. These chemical characteristics are typical of "black rivers" encountered under tropical climates. Three different behaviors can be distinguished upon filtration:
-DOC, Al, Fe, Y, REEs, Ti, Th and Zr are progressively removed from the solution with decreasing pore size filtration. These metals are linked to the colloidal organic matter. It is interesting to note the strong mobility of Th or Zr often used as invariant to calculate mass balance in weathered lateritic soils.
-Na, K, Mg, Ca, V, Mn, Co, Ni, Cu, Zn, Ge, As, Rb, Sr, Ba and U are partially removed by filtration (from 30 to 75 % of their dissolved concentrations).
-Silica concentration is not affected by filtration and apparently independant of organic matter.
To interprete these results, the aqueous speciation of some characteristic metals (Ca, Mg, Cu, Fe, Al, Sm and Th) has been calculated taking account of metal-humic acid complexation and using the computer code "Balance" (Akinfiev, 1986). In this calculation, it was assumed that humic acids account for the measured DOC (Thurman, 1985). Metal-humic acid association constants were either selected from available literature data or approximated from correlation with the first hydrolysis constant. It was found that all dissolved (<0.20 µm) Al, Fe, Sm and Th are present as complexes with humic compounds. In contrast, for Ca and Mg, complexes with humic ligands do not exceed few percent of total dissolved elements. Thus the decreases of Ca and Mg concentration observed in the filtration experiments can not be explained by their complexation with humic ligands. These elements are probably retained by humic substances during the filtration to balance their net charge.
Akinfiev, N.N., Geochemistry 6, 882-890 (1986).
Thurman, E.M., Martinus Nijhoff / DR W.Junk Publishers (1985).