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Mountain regions of the humid tropics are characterized by steep slopes and heavy rains. These regions are thus prone to both high surface runoff and soil erosion. In Southeast Asia, uplands are also subject to rapid land-use change, predominantly as a result of increased population pressure and market forces. Since 1998, the Houay Pano site, located in northern Lao PDR (19.85°N 102.17°E) within the Mekong basin, aims at assessing the long-term impact of the conversion of traditional slash-and-burn cultivation systems to commercial perennial monocultures such as teak tree plantations, on the catchment hydrological response and sediment yield. The instrumented site monitors hydro-meteorological and soil loss parameters at both microplot (1 m2) and small catchment (0.6 km2) scales. The monitored catchment is part of the network of critical zone observatories named Multiscale TROPIcal CatchmentS (M-TROPICS). The data shared by M-TROPICS in Houay Pano are (1) rainfall, (2) air temperature, air relative humidity, wind speed, and global radiation, (3) catchment land use, (4) stream water level, suspended particulate matter, bed particulate matter and stones, (5) soil surface features, and (6) soil surface runoff and soil detachment. The dataset has already been used to interpret suspended particulate matter and bed particulate matter sources and dynamics, to assess the impact of land-use change on catchment hydrology, soil erosion, and sediment yields, to understand bacteria fate and weed seed transport across the catchment, and to build catchment-scale models focused on hydrology and water quality issues. The dataset may be further used to e.g. assess the role of headwater catchments in large tropical river basin hydrology, support the interpretation of new variables measured in the catchment (e.g. contaminants other than fecal bacteria), and assess the relative impacts of both climate and land-use change on the catchment.

In the protection of karstic aquifers, the role played by dolines is of interest as these shallow structures can be preferential infiltration pathways for groundwater recharge. In this study, three depressions in South France (Orniac, Lot) were identified and characterized using combined geophysical methods. Apparent conductivity mapping allows precise definition of dolines corresponding to topographic depressions. It also reveals other dolines that are hidden by soil cover. Electrical resistivity and seismic refraction tomography provide detailed cross‐sections. Even though the dolines studied are close one to another, they exhibit great variability in shape and content. Moreover, time‐lapse data show that the most saturated regions are on the edges of the first doline. Such information helps to evaluate human impacts and forecast potential groundwater contamination.

This study evaluates the use of the geoelectric self-potential (SP) method on a typically thick lateritic cover in the tropical rain forest of the Nsimi watershed of South Cameroon. In this respect, we have deployed SP and Wenner resistivity profiling along 10 tracks, which has enabled us to produce an SP map and a Wenner resistivity profile of the watershed. Because the application of the SP method is not ideal, the interpretation of our profiles is based on correlations between SP, elevation, electrical resistivity and well-constrained granulometric parameters in the watershed. The absence of a correlation between SP and electrical resistivity suggests that these natural potentials would not be the result of a simple circulating current by Ohm's law through the medium, but would be associated with electromotive forces, either electrokinetic or electrochemical potential. The positive correlation observed between SP and elevation enables us to discard an electrokinetic origin for the Nsimi watershed SP and to propose an electrochemical one instead. This is confirmed by a clear linear correlation between SP and the percentage of clay material in the lateritic covers. This further enables us to define loose clay-rich horizons in the laterites.

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. Another 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 linked with colloidal organic matter located in the upper horizons of the swamp. Fe has a much more complex behaviour due to its change of redox state which can be independent of organic matter complexation. Concerning the major base cations, their origin can be constrained by the biological cycle (storage or leaching). K is typically influenced by the biological cycle. During the floods, Cl has the same behaviour as K: it is one of the most striking points of this study. However, the Cl annual budget is balanced. These characteristics can be understood as the consequence of the weathering of the minerals present in the saprolite (kaolinite, goethite, zircon, Th-oxide). This chemical weathering allows the leaching of base cations and also Al and Fe. It has been demonstrated that the microbial populations of the swampy zone can play an important role in the mobilization of transition metals (e.g. Fe). This study point out the role of humic acids in the transport and the weathering budget of elements usually considered as immobile in the superficial cycle (e.g. Al, Th, Zr, Fe). It must be mentioned that worldwide the SEW and even the Nyong network waters are among the least concentrated river waters. It means that even if the organic matter plays an important role in the mobilization and transport of some elements in the swampy zone, its action is limited in term of major cation fluxes on the SEW scale. The reason invoked is that the cation fluxes are directly linked to the pedological history and the geomorphology of the watershed. The presence of thick soil layers composed of saprolite and latosol on the hillsides and of hydromorphic soils in the swampy zone with constant mineralogy lead to isolating the bedrock. The long residence time of water close to the weathering front plays a major role in preserving the parent rock from the hydro-chemical outputs. Moreover, the topsoil layers are stabilized by the vegetation cover, which limits mechanical erosion. This should be taken into account for the carbon mass balance calculation because of the wide areas on stable shields concerned by the humid tropical ecosystems. Moreover, comparison between long and short-term weathering allows us to suggest that paleo-climatic conditions did not change since the Miocene (6–20 Ma) in this part of the world.

The effect of organic matter during soil/water interaction is still a debated issue on the controls of chemical weathering in a tropical environment. In order to study this effect in detail, we focused on the weathering processes occurring in a small tropical watershed (Nsimi-Zoetele, South Cameroon). This site offers an unique opportunity to study weathering mechanisms in a lateritic system within a small basin by coupling soil and water chemistry. 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 elevated 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 results suggest the existence of chemical and spatial heterogeneities of waters in the basin: colored waters flooding the swamp zone have much higher concentrations of both organic matter (i.e., DOC) and inorganic ions (e.g., Ca, Mg, Al, Fe, Th, Zr) than those from springs and groundwater from the hills. Nevertheless, these organic-rich waters present cation concentrations (Na, Ca, Mg, K) which are among the lowest compared to that of most world rivers. The main minerals in the soils are secondary kaolinite, iron oxi-hydroxides, quartz, and accessory minerals (e.g., zircon, rutile). We mainly focused on the mineralogical and geochemical study of the swamp zone soils and showed through SEM observations the textural characterization of weathered minerals such as kaolinite, zircon, rutile, and the secondary recrystallization of kaolinite microcrystals within the soil profile. Water chemistry and 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-rich waters, we suggest that organic acids enhance dissolution of minerals such as kaolinite, goethite, and zircon and also favors the transport of insoluble elements such as Al, Fe, Ti, Zr, and REE by chemical complexation. SiO2(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 the 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 in soils that are very depleted in soluble elements. Thus, the low concentration of major elements in these water is a direct consequence of the depleted nature of the soils.

Quantifying the magnitude of soil organic carbon loss linked to the conversion of forest to arable land in tropical environments requires long-term estimates of soil redistribution trends within the catchment. In order to assess the cumulative soil carbon loss that occurred since deforestation and cropping started in 1967, a multidisciplinary study was carried out in a small catchment (67 ha) with a swamp area in its middle part, characterized by steep slopes under shifting cultivation of upland rice, typical for northern Laos's landscapes. Using 137Cs and total organic carbon (TOC) inventories, it was possible to estimate a soil TOC depletion rate of 0.039 kgC m−2 yr−1 in 42 years (1963–2005), corresponding to ca. 21% of its initial content in the top 10 cm. Because farmers preferred clearing more fertile soils located downhill and leave forest on poorer soils on the crests, the redistribution of 137Cs and TOC along slopes of the catchment was mainly controlled by the topographic position of soils and the cultivation frequency. Estimates of soil TOC accumulation in wetlands with δ13C measurements and sediment volumes by electrical resistivity tomography showed that TOC accumulation rates were linked to land use change since the 1960s with three successive periods corresponding to undisturbed forest, deforestation-first rice crop and rice-fallow rotations. Forest clearing triggered a higher soil TOC delivery to the swamp (19.6 ± 5.5 kgC ha−1 yr−1) over 8 years (1967–1975) than the former undisturbed forest (8.5 ± 1.8 kgC ha−1 yr−1) and crop-fallow rotations over 30 years (1975–2005, 6.4 ± 4.8 kgC ha−1 yr−1). Due to the small size of the area covered by the swamp, only a limited fraction of eroded soil organic carbon (ca. 7 wt.%) was stored in wetlands of the catchment. Mineralization apparently played a key role for soil TOC depletion since first forest clearing.

Resistivity investigations were carried out on an elementary watershed in SW Cameroon, firstly to assess the applicability of direct-current (DC) resistivity methods to solve various pedological problems in intertropical regions, and subsequently to determine the relationships between electrical resistivities and pedological properties of lateritic soil systems. The survey included measurements in pits with a small Wenner fixed-spacing array (SWA), vertical electrical soundings (VES) and vertical electrical “quick soundings” (VEQS) both using the Schlumberger configuration. The VES data were interpreted using a conventional multilayer inversion program to obtain best-fit models. Constraints to the interpretation of these data were provided by SWA and pedological information from existing observation pits. The results of the interpretation reveal five distinct geoelectrical layers overlying a resistive bedrock. The first is a thin organo-mineral upper layer with low resistivities in the range 250–450 Ωm. The second layer corresponds to micro-aggregated clayey materials and is more resistive (1300–1800 Ωm). The third represents the main part of ferruginous materials and is even more resistive (2000–4500 Ωm). The fourth corresponds to unsaturated saprolite and the last to saturated saprolite (ground water) with resistivities ranging from 800 to 1500 Ωm and from 150 to 250 Ωm, respectively. Estimates of soil volumes for the entire study area were obtained from VEQS interpretations. Most of the soil cover corresponds to saprolite (74%, 1 4 being saturated by ground water), while topsoil and ferruginous materials represent 14 and 12%, respectively. Finally, geophysical results based upon 1-D inversion provide a satisfactory approximation of the various lateritic components' 3-D geometry over the watershed. The study provides original quantitative results concerning the behaviour of intertropical soil systems as well as some geomorphological keys for soil mapping at a regional scale.

The comparison between contemporary and long-term weathering has been carried out in the Small Experimental Watershed (SEW) of Nsimi, South Cameroon in order to quantify the export fluxes of major and trace elements and the residence time of the lateritic weathering cover. We focus on the hillside system composed of a thick lateritic weathering cover topped by a soil layer. This study is built on the recent improvements of the hillside hydrological functioning and on the analyses of major and trace elements. The mass balance calculation at the weathering horizon scale performed with the parent rock as reference indicates (i) strong depletion profiles for alkalis (Na, K, Rb) and alkaline earths (Mg, Ca, Sr, Ba), (ii) moderate depletion profiles for Si, P, Cd, Cu, Zn, Ni and Co, (iii) depletion/enrichment profiles for Al, Ga, Ge, Sn, Pb, LREE, HREE, Y, U, Fe, V, Cr, Mn. It is noteworthy that (i) Mn and Ce are not significantly redistributed according to oxidative processes as it is the case for Fe, V and Cr, and (ii) Ge is fractionated compared to silica with enrichment in Fe-rich horizons. The calculations performed for the topsoil with iron crust as parent material reference reveal that the degradation of the iron crust is accompanied by the loss of most of the constituting elements, among which are those specifically accumulated as the redox sensitive elements (Fe, V, Cr) and iron oxide related elements like Th. The overall current elemental fluxes from the hillside system at the springs and the seepage zones are extremely low due to the inert lateritic mineralogy. Ninety-four percent of the whole Na flux generated from the hillside corrected from atmospheric deposits (77 mol/ha/yr) represents the current weathering rates of plagioclase (oligoclase) in the system, the other remaining 6% may be attributed to the dissolution of hornblende. The silica hillside flux is 300 mol/ha/yr and can be mostly attributed to the plagioclase and kaolinite dissolution. Al and Ga are exported from the lateritic regolith and maybe due to the dissolution of kaolinite crystals. Compared to the other immobile elements (Zr, Hf, Nb and Th), Ti is significantly exported. Among redox-sensitive elements (Fe, V, Cr, Mn, Ce), only Ce and Mn are exported out of the hillside system. The other elements (Fe, V, Cr) are likely able to be mobilized but over a short distance only. Rb, Sr, Ba, Ni, Cu, Zn are affected by export processes. LREE and Y are exported but in very low amounts (in the range from μmol/ha/yr to mmol/ha/yr) while HREE and U are exported in negligible quantities. A first attempt is carried out to compare the mature ridge top profile from Nsimi SEW with the immature ridge top weathering profile from the Mule Hole SEW (South India), developed on similar granodioritic basement, in order to get deeper insight into (i) the contemporary saprolite production rates and (ii) the combined effect of precipitation (in terms of Mean Annual Rainfall, MAR) and evapotranspiration on the aggressiveness of the draining solutions. Considering (i) the contemporary Na flux as representative of the dissolution of plagioclase crystals and conservative during saprolitization processes and (ii) steady state of the inter-annual recharge (R) over a 10 years period, the current saprolite production rates (σr) are of 22 mm/kyr for Mule Hole SEW and 2 mm/kyr for Nsimi SEW, respectively. Even with a very low R/MAR ratio (0.04) compared to Nsimi, the chemical weathering at Mule Hole is active and related to the groundwater exports. At Mule Hole, plagioclase crystals are still present in the saprolite and the soil cover leading to a diffuse weathering front. The high Nsimi R/MAR ratio (0.2) allows the solution to be still aggressive with respect to the plagioclase and other weatherable minerals at the bedrock interface resulting in their complete breakdown in a few centimetres (sharp weathering front) leading to a mature saprolite. For the Nsimi SEW, if we consider (i) the low contemporary saprolite production rate (2 mm/kyr), (ii) the Miocene age (average 15 Myr) of the South Cameron Plateau landscape and (ii) the limited movement of Africa continent since Eocene, the long term saprolite production rate should have remained in its lower range, from 2 to 10 m/Myr. This suggests that, for thick weathering profiles the migration of the weathering front into the bedrock occurs at a relatively uniform rate regardless of present-day climatic conditions. Climate variation leading to the alternation of setup of savanna or humid forest will have an effect on physical erosion rather than chemical erosion for such deep weathering profiles.

International audience; This paper presents a hydrogeochemical modeling code HYDROS, which combines the multi-component transport model with equilibrium speciation module MINTEQA2. The processes of adsorption, aqueous speciation and mineral precipitation/dissolution are represented in the model. The numerical model uses a sequential iterative approach for solving the solute transport and the equilibrium geochemistry modules. Further the transport part is solved using an operator split approach wherein a finite volume method is used for solving the advective equations while a classical finite difference method is employed for solving the dispersive equations. The model performance is evaluated by comparing it with MINTOX for a literature problem. HYDROS is then applied to the case study of the transfer of transition metals with organic colloids in the swamp groundwater system of the experimental Nsimi watershed, representative of the humid tropical ecosystem of the South Cameroon Plateau. Field observations at the site swamp system suggest that the carbon is mainly transferred as organic colloids (i.e., dissolved organic carbon) produced by the slow biodegradation of the swamp organic matter. Using HYDROS, the behaviour of Al(III) and Fe(III) elements in the base flow system is simulated during inter rain events of a short rainy season (May-June 1996). The elemental time-series for Al, Fe, Cl, pH compare well with the simulation results. The colloids are found to have a strong impact on the mobilization and transfer of Al(III) and Fe(III), which are considered to have low mobility in weathering environment.

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The aim of this study is threefold: (1) to evaluate the effect of the physical properties of weathered granitic material (geometry and spatial distribution of hydraulic conductivity) on water fluxes at the outlets of two small and nearby agricultural catchments, Kerbemez and Kerrien (0. 12 and 0.095 km(2)), underlain by granite bedrock; (2) to explain the variations of nitrate concentration in streamwater in relation to the spatial distribution of dissolved nitrate in the groundwater; (3) to investigate the origin of the groundwater nitrate by analysing the reaction times of groundwater to variations of nitrate concentration in water recharge. These objectives were attained by developing a flow and nitrate transport model for the two catchments from geophysical measurements on the geometry and hydraulic conductivity of the weathered granite layer, and using data collected from soil surveys. The models were calibrated and validated from spatial and temporal observations of hydraulic heads and nitrate concentrations in groundwater and stream water. The flow models appeared to be less sensitive to the geometry of the weathered granite layer than the spatial distribution of hydraulic conductivity. Model results show that seasonal patterns of nitrate concentrations in streamwater result partly from the spatial distribution of nitrate concentration in the groundwater. The horizontal distribution of nitrate concentrations in groundwater derives from denitrification in the downslope domain. An analysis of the groundwater reaction times for both catchments shows that, following a variation in the recharge nitrate concentration; the time to reach equilibrium in the whole groundwater body is more than 14 years. In the Kerbemez catchment, the vertical distribution observed in groundwater nitrate concentrations appears to be caused by a temporal decrease of the nitrate concentration in the water recharge over the last 15 years. (c) 2005 Elsevier B.V. All rights reserved.

The study of biogeochemical and hydrological cycles in small experimental watersheds on silicate rocks, common for the Temperate Zone, has not yet been widely applied to the tropics, especially humid areas. This paper presents an updated database for a six-year period for the small experimental watershed of the Mengong brook in the humid tropics (Nsimi, South Cameroon). This watershed is developed on Precambrian granitoids (North Congo shield) and consists of two convexo-concave lateritic hills surrounding a large flat swamp covered by hydromorphic soils rich in upward organic matter. Mineralogical and geochemical investigations were carried out in the protolith, the saprolite, the hillside lateritic soils, and the swamp hydromorphic soils. Biomass chemical analyses were done for the representative species of the swamp vegetation. The groundwater was analysed from the parent rock/saprolite weathering front to the upper fringe in the hillside and swamp system. The chemistry of the wet atmospheric and throughfall deposits and the Mengong waters was monitored. In the Nsimi watershed the carbon transfer occurs primarily in an organic form and essentially as colloids produced by the slow biodegradation of the swamp organic matter. These organic colloids contribute significantly to the mobilization and transfer of Fe, Al, Zr, Ti, and Th in the uppermost first meter of the swamp regolith. When the organic colloid content is low (i.e., in the hillside groundwater), Th and Zr concentrations are extremely low (

Magnetic resonance sounding (MRS) is distinguished from other geophysical tools used for ground water investigation by the fact that it measures a magnetic resonance signal generated directly from subsurface water molecules. An alternating current pulse energizes a wire loop on the ground surface and the MRS signal is generated; subsurface water is indicated, with a high degree of reliability, by nonzero amplitude readings. Measurements with varied pulse magnitudes then reveal the depth and thickness of water saturated layers. The hydraulic conductivity of aquifers can also be estimated using boreholes for calibration. MRS can be used for both predicting the yield of water supply wells and for interpolation between boreholes, thereby reducing the number of holes required for hydrogeological modeling. An example of the practical application of MRS combined with two-dimensional electrical imaging, in the Ker-bernez and Kerien catchments area of France, demonstrates the efficiency of the technique.

This study assesses the causes of the high spatial variability of the mineral content of groundwater in crystalline bedrock of Southern Madagascar. Although many kilometres from the coast and at a mean altitude of 400 m a.s.l, wells drilled in this area produce water with electrical conductivities in the range of 300-30,000 mu S cm(-1) with a high spatial variability. Chemical and isotopic data are used to identify the processes involved in the groundwater mineralization. It is shown that the chemical composition of the groundwater in this region has its origin in (i) normal silicate and carbonate weathering reactions and (ii) input of marine salts, probably via rainfall recharge, modified by evapo-concentrative processes probably including precipitation and re-dissolution of secondary evaporites in the unsaturated zone. To obtain a better understanding of the spatial salinity distribution, well parameters such as yields, weathered zone thickness, weathered materials and morphological positions (upper slope, mid-slope, lower slope or valley bottom) are scrutinized. A correlation was found between high salinity and low flow, shallow groundwater environments (flat hill tops, valley bottoms, weakly developed and clayey weathered zones) and between low salinity and high flow environments (granular, well-developed weathered zones and situation on valley slopes). (C) 2005 Elsevier B.V. All rights reserved.