『Abstract
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 (0.22μm, 0.025μm, or: 300,000 Da, 5000 Da) to separate
colloidal fractions from the truly dissolved one. Major and trace
elements and dissolved organic carbon (DOC) 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 DOC. 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
are 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 for 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 DOC-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.
Keywords: Chemical weathering; Drainage basin; Tropical watershed;
Nsimi-Zoetele site; Cameroon; Organic-poor waters; Organic-rich
waters』
1. Introduction
2. Description of the natural system and water sampling
2.1. The Nyong and Sanaga hydrographical systems
2.2. Presentation of the Nsimi-Zoetele watershed
2.3. The parent rock and the soil system
2.4. The hydrographical systems (groundwaters, springs and brook)
3. Field and laboratory techniques
3.1. Solution sampling and filtration
3.2. Chemical analyses
4. Results
4.1. Distribution of elements in the 0.22μm filtrate, ‘dissolved’
fraction
4.1.1. Major elements: Na, Mg, K, Ca and Si
4.1.2. Trace elements
4.1.2.1. Mn and Co
4.1.2.2. Ni, Cu and Zn
4.1.2.3. As and Ge
4.1.2.4. Rb and Ba
4.1.2.5. U
4.1.2.6. Al and Fe
4.1.2.7. Ga
4.1.2.8. The REEs
4.1.2.9. Ti, Th and Zr
4.1.3. Dissolved organic carbon
4.2. Effect of successive filtration (300,000 Da, or 0.025μm,
and 5000 Da) on element distribution
4.2.1. Major elements: Na, Mg, K, Ca and Si
4.2.2. Trace elements
4.2.3. Dissolved organic carbon
5. Discussion
5.1. Clear waters
5.1.1. Control of Fe, Al and Si by mineral phases
5.1.1.1. Kaolinite, gibbsite
5.1.1.2. Quartz
5.1.1.3. Goethite, hematite
5.1.2. Control of Y and REE
5.2. Coloured waters
5.2.1. Successive filtration experiments
5.2.1.1. Th: tracer of the colloidal pool in rivers
5.2.2. Speciation calculations
5.2.2.1. Conditional stability constants
5.2.2.2. Results of the speciation calculation
5.2.3. Comparison between the results obtained by the filtration
experiments and by speciation calculations
6. Consequences of present-day chemical weathering
6.1. Geochemical behaviour of elements within the swampy
zone
Acknowledgements
References