Biogeohydrodynamic in the forested humid tropical environment: the case study of the Nsimi small experimental watershed (south Cameroon)

Abstract

Abstract. -This paper summarizes a six-year study of the Nsimi Small Experimental Watershed (SEW), considered as a model for the South Cameroon humid tropical ecosystem. When this small watershed was set up, no similar survey of input/output hydrobiogeochemical fluxes in granitoid rocks in stable cratonic environment was available, to our knowledge, on any site close to the Equator. Moreover, this is the first attempt, world-wide, to combine different approaches in hydrology, (bio)geochemistry, mineralogy, crystallography, microbiology, geophysics and pedology. Research is based on (1) regular hydrobiogeochemical surveys in various water reservoirs of the SEW ecosystem (atmospheric deposits, groundwater and stream), (2) surveys related either to the organisation and composition of different reservoirs in the superficial layers (basement rocks, saprolite, soils) or to various hydrological, biological and geochemical processes. These surveys aim at (1) finding the main parameters involved in the chemical and physical erosion processes of the humid tropical ecosystem, (2) understanding the source of a particular chemical composition in groundwater and rivers, (3) documenting accurately the different exportation processes of chemical elements in water and soil (4) investigating the possible relation between the biodegradation of soil organic matter and the leaching of metals (especially iron) and (5) comparing the long and short term weathering rates using mass balance calculations. An other important objective of this study is to provide a new scientific and engineering database for the future development of South Cameroon, which is still nowadays a relatively preserved ecosystem.One of the major results is the essential role played by the biological cycle (vegetation and soil organic matter) in the fractionation, exportation or storage of the chemical elements in humid tropical environments. Moreover we are able to propose a model of the current erosion for this SEW from the database obtained on (1) the mineralogy of the basement rocks and the soil layers, (2) the geochemistry of the soluble and colloidal phases of waters and (3) the hydrology within the different reservoirs of the hydrosystem. This model has been confirmed and extended on a regional scale (Nyong river basin). It emphasized the behaviour of the main elements of the tropical soil layers (Fe, Al, Si), the nutrients (C, Ca, Mg, K, Sr) and specific tracers of the weathering processes either with strong mobility (Cl, Na) or on the contrary with an extremely low mobility (Zr, Th, REEs).On the SEW scale, a strong geochemical contrast occurs between the different groundwater zones flooding (1) the hill slope lateritic profiles, (2) the weathering front (interface between the saprolite and the basement rocks), and (3) the swampy zone in which the Mengong brook flows. High DOC contents (15 mg/L) but also high Fe, Th, Al, Zr contents characterize the swampy zone waters. Na and Si have mainly a deep origin (exfiltration), Al, Th, Zr and REEs are strongly link...

Processus et bilan des flux hydriques d’un bassin versant de milieu tropical de socle au Bénin (Donga, haut Ouémé)

“…Ionic ratios show little changes through time and, for river waters, elements related to chemical weathering (Si, Ca, Mg, Na, and HCO 3 ) vary proportionally to EC ( Fig. 5A); potassium is an exception, probably because of its sensitivity to biochemical cycles [4]. Changes in ionic fluxes appear to mimic the peak flow with a time-lag of a few hours ( Fig.…”

Partitioning carbon sources in a tropical watershed (Nyong River, Cameroon) between wetlands and terrestrial ecosystems – Do CO<sub>2</sub> emissions from tropical rivers offset the terrestrial carbon sink?

“…The low weathering rates are mainly attributed to the thickness of the lateritic cover on hills and slopes, which considerably slows the advance of percolating water in the bedrock (Boeglin et al, 2005). This was confirmed by the low mineral dissolved load in the aquifer of the Nyong basin (Braun et al, 2002) and by the dissolved silica fluxes in rivers that were significantly lower compared to the annual rainfall (e.g., White and Blum, 1995). Accordingly, in the Nyong basin, forest groundwater is therefore a limited source of TA for surface waters, as reported in other monolithic catchments draining only silicate rocks (Meybeck, 1987).…”

Impacts of Quarrying Activities on Human Health in Boki Area Cross River State Nigeria

“…These geohazard challenges can be mitigated by proper management of farm and forest lands in the area [8]. According to [18], vegetation and soil organic matter play essential role in the fractionation, distribution and storage of the chemical elements in the humid tropical environment in the southern parts of Cameroon. They used biogeohydrodynamics, which combined approaches in mineralogy, crystallography, biogeochemistry, microbiology, hydrology, geophysics and pedology to study parts of a humid environment.…”