Composition of the different reservoir waters in a tropical humid area: example of the Nsimi catchment (Southern Cameroon)

The Nsimi experimental catchment (area: 0.60 km(2), latitude: 3degrees10'N, 120 km SSE from Yaounde) has been selected for a study of the humid tropical area dynamics. With a mean rainfall of 1700 mm/yr, and a slightly degraded vegetal cover, this site is representative of the South-Cameroon forest domain. Whereas the slopes correspond to a thick lateritic profile, the swampy depression (representing 20% of the surface) is recovered by a clayey-sandy level enriched in organic matter on its upper part. Successive chemical analyses (major cations and anions, total iron and aluminium, dissolved silica, dissolved organic carbon (DOC)) of water samples from different reservoirs, from springs and outlets enabled us to have a better understanding of the dynamics of this small watershed. (1) Two types of water can be distinguished: clear waters, collected from the slope water tables and from the springs; coloured waters, which have flowed through the depression. This difference has been imputed to the DOC content (near 16 mg/l in the coloured waters). (2) The strong DOC ratio in the depression water table may be responsible for its relatively high cation concentration: In these slow flowing waters, the abundance of dissolved organic matter has a notable effect on the solubility of some mineral species. (3) Nevertheless, all these waters present extremely low dissolved concentrations indicating that chemical weathering is very slow within the lateritic profile, even at its bottom. (C) 2003 Published by Elsevier Ltd.