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Space and time variability of water content in aquifers are fundamental issues to understand complex interactions taking part in the critical zone, such as land use and irrigated agricultural production. Fundamental parameters on aquifer behavior are commonly monitored through hydrogeological methods, such as piezometric levels and pumping tests in boreholes. Precisions on the water quality and residence time are provided by geochemical analyses of samples collected in surface streams and boreholes. Several studies showed how additional data can be obtained from non-invasive hydrogeophysical methods, that reveal structural heterogeneities of hydrogeological parameters filling the gaps between boreholes.We carried out a multimethod geophysical survey in the Berambadi experimental catchment (India) which is part of the M-TROPICS CZO (Multiscale TROPIcal CatchmentS Critical Zone Observatory). Two surveys including seismic, electrical, and electromagnetic methods have been repeated for contrasting piezometric levels (high in December 2019, low in May 2022) corresponding to contrasted water contents. We considered time-lapse imaging using electrical resistivity tomography (ERT) and audio-magneto-tellurics (AMT), which sensitivities apply at complementary scales. Changes in the electrical resistivity from ERT shallow cross-sections and deeper jointly inverted ERT-AMT vertical profiles are compared for the two seasons. Results are discussed in terms of water content and porosity of the regolith as well as uncertainties caused by inherent repeatability issues of time-lapse measurements. Final discussion concerns perspectives of combined time-lapse electrical and seismic velocity models to assess the impact of the spatial variability of regolith properties at the catchment scale.

International audience; Bringing nature into urban areas is largely recognized as one of the multiple benefits of green walls. Among the existing types of living walls, those made from durable building materials have a longer lifespan and require less maintenance than many typical green wall systems. In that context, there is a need to study the bioreceptivity of materials, i.e., their ability to host living organisms. This paper presents a novel method to study the bioreceptivity of materials, by enhancing the biological and physicochemical exchanges in different mixtures of crushed materials and substrates, and by measuring their effect on plant growth. For this experiment, we crushed and sieved four different building materials (3 types of concretes and 1 brick) to increase their contact surface. We then mixed them with substrate with or without compost additions. The mixtures were used as growing media for the cultivation of two wild mural plant species in a glasshouse. Plant growth and the physicochemical evolution of the mixtures were monitored for one year. The study shows the physical and chemical differences between the building materials, their interactions with substrate composition, and their effect on plant growth. Two main results appeared: (i) high pH (>8) was the major constraint against plant growth (ii) compost additions significantly compensated for adverse characteristics of the materials. Assuming that higher plant growth is associated with better bioreceptivity, this experiment introduces a quantitative method that could contribute to the evaluation of the agronomic properties of building materials. The results allow us to make recommendations for Living wall and Technosol design.

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.

Due to high rainfall erosivity and rapid land-use changes, South-East Asia is one of the hot spots of soil degradation worldwide. In recent decades, several studies showed that the expansion of rubber tree (RT) plantations into previously forested areas has caused a major increase in soil erosion. However, the effects of cropland conversion to RT plantations on surface runoff and soil detachment are still unclear. Here we show that mature RT plantations considerably increase runoff and soil detachment compared to annual crops or young RT plantations with intercrop, mostly due to the absence of understorey. Monitoring 1 m2 microplots over four years in mountainous Northern Thailand, we found that runoff and detachment increased with time since the onset of the rainy season and with rainfall height, but more so in mature RT plantations than in young RT plantations and maize. This led to much higher annual soil detachment in mature RT plantations (5.7 kg/m2 on average) than in maize and young RT plantations with intercrop (0.36 kg/m2). We identified two main factors explaining this difference: first, rubber tree leaf litter, although abundant, seemed ineffective in reducing runoff at the end of the rainy season. Secondly, the cover by low-growing plants in mature rubber tree plantation was usually sparse and provided little protection. In particular we showed that increasing the cover by low-growing plants from quasi-null cover to >31% cover decreased runoff coefficient by 32%. Our results demonstrate that afforestation by RT, at least under current management practices and on steep slopes, is overall detrimental to soil conservation but that its effects could be mitigated by the adoption of less intense weeding practices.

In the past decades, the expansion and modernisation of agriculture in the mountainous areas of Southeast Asia has had severe impacts on biodiversity, as the once species-rich forests were turned into homogeneous fields receiving ample external inputs. A common feature of permanent cropping with annual crops is the frequent change of crop choice, depending on market opportunities or other motives. However, the precise effect of crop shifts on weeds in tropical areas is largely unknown. In this study, we investigated the short-term effect of crop sequences on the diversity of weed communities in smallholder fields in Northern Thailand. Crop choices were upland rice, maize, fallow and young tree plantations with or without intercrop. We counted the number of crop shifts and the number of crops involved during a 3-years period preceding weed sampling. We showed that the number of crop shifts did not affect weed density and biomass. However, herbaceous species number and diversity (measured as Shannon index) increased by 36% and 46% respectively, while herbaceous species dominance decreased by 38%, in fields with yearly crop shifts compared to fields with no shifts in the previous three years. The effect of a particular crop on diversity, or the effect of intercropping with young trees, was weaker. It was likely due to the more variable resources (especially light) in fields with two crop shifts, allowing species with different niches to co-exist. Crop type and frequent crop shifts did not affect shrub and tree species number, diversity or dominance. Some species were strongly associated with fields with no crop shift in the sequence (e.g. the treeAntidesma velutinosum) or to fields with two crop shifts in the sequence (e.g. the herbCentella asiatica, the C4grassDigitaria radicosa). Overall, this study showed that in this agronomical system, maintaining yearly crop shifts does not significantly affect weed abundance, but supports in-field plant species diversity, which is likely to impact the services provisioned by tropical mountainous agro-ecosystemsHighlightsFrequent crop shifts in a crop sequence increased weed richness and diversity.Crop shifts had a stronger effect on weed richness and diversity than the current crop.The number of crop shifts did not affect weed biomass and density.Graphical abstract

Groundwater contribution to streamflow sustains biodiversity and enhances ecosystem services, especially under monsoon-driven climate where stream baseflow is often the only available water resource during the dry season. We assessed how land use change influences streamflow and its groundwater contribution in a small headwater catchment subject to shifting cultivation in Montane Southeast Asia. Continuous time series of rainfall, reference evapotranspiration, groundwater level, stream discharge and electrical conductivity (EC) of surface and groundwater were monitored from 2002 to 2007. With the rainfall-runoff model GR4J, we investigated temporal changes in the hydrological behaviour of the study catchment to verify consistencies with observed land use change. An EC-based hydrograph separation method allowed estimating the groundwater contribution to 104 stormflow events. Mean soil surface crusting rates corresponding to each of the nine land uses identified in the catchment were determined using 236 standard 1-m2 micro-plots. Mean plant cover for each land use was assessed in 10 × 10-m2 plots. Bedrock topography and soil layers’ structure were assessed by electrical resistivity tomography to determine pathways of subsurface storm flows. Our results indicate that an increase in the catchment's areal percentage of fallow from 33% to 71% led to a decrease in the annual runoff coefficient from 43% to 26%. The concurrent reduction of soil crusting rate over the catchment, from 48% to 30%, increased rainwater infiltration. Consecutively, groundwater contribution to storm streamflow increased from 83% to 94%, highlighting the protective role of a dense vegetation cover against flash floods. The overall reduction of the annual basin water yield for inter-storm streamflow from 450 to 185 mm suggests that the potential gain in groundwater recharge was offset by the increased root water uptake for evapotranspiration, as confirmed by the drop in the groundwater level. This analysis illustrates how two different land uses with opposite impacts on soil permeability (i/ extensive soil surface crusting under annual crops resulting in limited runoff infiltration or ii/ fallow regrowth promoting both infiltration and evapotranspiration) both inhibit groundwater recharge. The maintenance of strips of fallow buffers between annual crop plots can slow down runoff and locally promote infiltration and groundwater recharge while limiting evapotranspiration.

International audience; In South-East Asia, rapid land use changes in recent decades have raised concerns for biodiversity and soil conservation. Weeds provide many ecosystemic services for soil protection and support biodiversity, and could mitigate the negative effects of intensification. We investigated the changes in weed assemblages and weed-soil interactions on a chronosequence from annual crops to mature rubber tree plantations. We sampled five fields for each of four land uses in mountainous northern Thailand (rainfed upland rice, maize, young rubber tree (RT) intercropped with maize, and mature RT). We characterized weed assemblages (abundance, richness) and soil properties (bulk density, water, carbon and nitrogen content). Rice had the most diverse and abundant weed assemblages. Weed assemblages differed between (i) rice, (ii) maize and young RT with maize and (iii) mature RT. Soil water content was the highest in mature RT. Other soil properties varied strongly within and among fields, and did not vary significantly among land uses. Water and nitrogen content increased overall with living soil cover but decreased with weed species richness in mature RT. Such interactions could provide a basis for sustainable weeding practices favourable to soil and biodiversity conservation.

In agricultural studies, estimating and testing differences between treatment regimes in a designed experiment usually determines the subsequent success or failure of inputs or management practices. Fine-scale soil variability of the experimental site can reduce statistical power and should be determined in order to optimize the experiment design. Although randomization is often performed, there is a probability that such randomization does not cover for fine scale soil variability. To determine the effect of this fine scale soil variability, a well characterized test system comprising of biochar's effects on soil microbial communities were studied, in conjunction with Electrical Resisitivity Tomography (ERT) to identify its role on biochar's differential interaction with soil microbial communities. Two main soil profiles were identified by the ERT survey and soil characteristics and soil microbial communities were differently affected by the biochar application with regards to soil profile. Fungal communities (mycorrhization intensity and fungal richness in particular) seemed to be more strongly affected than bacterial communities, however, these effects didn't show a similar trend in between the two studied soil profiles. Consequently, most of the effects of biochar application were masked by the soil variability of the experimental site when analysed in a nested design with randomization. Our results support the relevance of taking fine scale soil variability into account prior to the establishment of a field trial to optimize experimental design.

Lack of access to clean water and adequate sanitation continues to be a major brake on development. Here we present the results of a 12-month investigation into the dynamics of Escherichia coli, a commonly used indicator of faecal contamination in water supplies, in three small, rural catchments in Laos, Thailand and Vietnam. We show that land use and hydrology are major controlling factors of E. coli concentrations in streamwater and that the relative importance of these two factors varies between the dry and wet seasons. In all three catchments, the highest concentrations were observed during the wet season when storm events and overland flow were highest. However, smaller peaks of E. coli concentration were also observed during the dry season. These latter correspond to periods of intense farming activities and small, episodic rain events. Furthermore, vegetation type, through land use and soil surface crusting, combined with mammalian presence play an important role in determining E. coli loads in the streams. Finally, sampling during stormflow revealed the importance of having appropriate sampling protocols if information on maximum contamination levels is required as grab sampling at a fixed time step may miss important peaks in E. coli numbers.

The humid tropics are exposed to an unprecedented modernisation of agriculture involving rapid and mixed land-use changes with contrasted environmental impacts. Afforestation is often mentioned as an unambiguous solution for restoring ecosystem services and enhancing biodiversity. One consequence of afforestation is the alteration of streamflow variability which controls habitats, water resources, and flood risks. We demonstrate that afforestation by tree planting or by natural forest regeneration can induce opposite hydrological changes. An observatory including long-term field measurements of fine-scale land-use mosaics and of hydrometeorological variables has been operating in several headwater catchments in tropical southeast Asia since 2000. The GR2M water balance model, repeatedly calibrated over successive 1-year periods and used in simulation mode with the same year of rainfall input, allowed the hydrological effect of land-use change to be isolated from that of rainfall variability in two of these catchments in Laos and Vietnam. Visual inspection of hydrographs, correlation analyses, and trend detection tests allowed causality between land-use changes and changes in seasonal streamflow to be ascertained. In Laos, the combination of shifting cultivation system (alternation of rice and fallow) and the gradual increase of teak tree plantations replacing fallow led to intricate streamflow patterns: pluri-annual streamflow cycles induced by the shifting system, on top of a gradual streamflow increase over years caused by the spread of the plantations. In Vietnam, the abandonment of continuously cropped areas combined with patches of mix-trees plantations led to the natural re-growth of forest communities followed by a gradual drop in streamflow. Soil infiltrability controlled by surface crusting is the predominant process explaining why two modes of afforestation (natural regeneration vs. planting) led to opposite changes in streamflow regime. Given that commercial tree plantations will continue to expand in the humid tropics, careful consideration is needed before attributing to them positive effects on water and soil conservation.

Rubber tree (Hevea brasiliensis) is of major economic importance in Southeast Asia and for small land holders in Thailand in particular. Due to the high value of latex, plantations are expanding into unsuitable areas, such as the northeast province of Thailand where soil fertility is very low and therefore appropriate management practices are of primary importance. Arbuscular mycorrhizal fungi (AMF) contribute to plant growth through a range of mechanisms and could play a key role in a more sustainable management of the rubber plantations. We described the diversity of AMF associated with rubber tree roots in Northeast Thailand in relation to tree age and soil parameters along a chronosequence of rubber tree plantations. Cassava fields were included for comparison. Rubber tree and cassava roots harbored high diversity of AMF (111 Virtual Taxa, VT), including 20 novel VT. AMF VT richness per sample was consistently high (per site mean 16 to 21 VT per sample) along the chronosequence and was not related to soil properties. The composition of AMF communities differed between cassava and rubber tree plantations and was influenced by soil texture and nutrient content (sand, K, P, Ca). AMF community composition gradually shifted with the age of the trees. Our results suggest that the high diversity of AMF in this region is potentially significant for maintaining high functionality of AMF communities.

Rubber tree is a very important crop in Thailand, representing an essential source of income for farmers. In the past two decades, rubber tree plantations have been greatly expanding in unfavorable areas, where climate conditions are difficult and soil fertility is very poor. To optimize latex yields, mineral fertilizers have been widely used. A better understanding of the roles of the biological compartment in soil fertility is essential to determine alternative management practices to sustain soil fertility and optimize latex yields. Arbuscular mycorrhizal fungi (AMF) are widely recognized as beneficial for plants, mainly through their role in improving plant nutrient uptake. The objective of this study was to assess the AMF populations in rubber tree plantations and the impact of both soil characteristics and plantation age on these communities. Our results showed that all rubber trees were highly colonized, regardless of the soil structure and nutrient contents. AMF colonization was not affected by the age of the trees, suggesting that maintaining the symbiosis is likely to be beneficial at all stages. A better understanding and management of the microbial communities would contribute to maintaining or restoring soil fertility, leading to a better tree growth and optimized latex yield.

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.

The humid tropics are exposed to an unprecedented modernization of agriculture involving rapid and highly-mixed land-use changes with contrasted environmental impacts. Afforestation is often mentioned as an unambiguous solution for restoring ecosystem services and enhancing biodiversity. One consequence of afforestation is the alteration of streamflow variability controlling habitats, water resources and flood risks. We demonstrate that afforestation by tree planting or by natural forest regeneration can induce opposite hydrological changes. An observatory including long-term field measurements of fine-scale land-use mosaics and of hydro-meteorological variables has been operating in several headwater catchments in tropical Southeast Asia since 2001. The GR2M water balance model repeatedly calibrated over successive 1 year periods, and used in simulation mode with specific rainfall input, allowed the hydrological effect of land-use change to be isolated from that of rainfall variability in two of these catchments in Laos and Vietnam. Visual inspection of hydrographs, correlation analyses and trend detection tests allowed causality between land-use changes and changes in seasonal flows to be ascertained. In Laos, the combination of shifting cultivation system (alternation of rice and fallow) and the gradual increase of teak tree plantations replacing fallow, led to intricate flow patterns: pluri-annual flow cycles induced by the shifting system, on top of a gradual flow increase over years caused by the spread of the plantation. In Vietnam, the abandonment of continuously cropped areas mixed with patches of tree plantations led to the natural re-growth of forest communities followed by a gradual drop in streamflow. Soil infiltrability controlled by surface crusting is the predominant process explaining why two modes of afforestation (natural regeneration or planting) led to opposite changes in flow regime. Given that commercial tree plantations will continue to expand in the humid tropics, careful consideration is needed before attributing to them positive effects on water and soil conservation.

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.

Only three independent measurements of apparent resistivity are theoretically sufficient to retrieve the parameters of a geoelectrical model with two layers, i.e. the resistivities of the two layers and the thickness of the first one. The Three-Point Method (TPM) presented here is a rapid electrical sounding field procedure based on that principle. It consists of measuring the apparent resistivity using only three interelectrode spacings instead of some tens as for a usual vertical electrical sounding. To invert these data and check out the limits of this simplified field procedure, we use a Bayesian probabilistic formalism. Such an approach appears more effective than other traditional inversion methods because it allows to calculate exhaustively a probabilistic description of the equivalent solutions. Both synthetic and field data validate TPM when a model with two layers correctly describes the geoelectrical structure of the ground. Particularly, TPM combined with the Bayesian inversion method is a tool well-designed to rapidly characterise the overburden thickness in sedimentary contexts, or the water table depth in the case of sufficient resistivity contrast between saturated and nonsaturated materials.

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.

Near-surface inhomogeneities (NSIs) can lead to severe problems in the interpretation of apparent resistivity pseudosections because their effects significantly complicate the image aspect. In order to carry out a more efficient and reliable interpretation process, these problematic features should be removed from field data. We describe a filtering scheme using two-sided half-Schlumberger array data. The scheme was tested on synthetic data, generated from a simple 2D resistivity model contaminated by NSIs, and is shown to be suitable for eliminating such contaminations from apparent resistivity data. Furthermore, the original model without NSIs can be recovered satisfactorily from the inversion of filtered apparent resistivity data. The algorithm is also applied efficiently to a real data set collected at Nsimi, in southern Cameroon, along a 200-m shallow depth profile crossing a complex transitional zone. For this case, the filtering scheme provides accurate structural and behavioural interpretations of both the geometry of the major soil constituents and the groundwater partitioning.

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.

A multi-method geophysical survey was carried out with the aim of reassessing the structure of the Rochechouart-Chassenon astrobleme and the geometry of the impact-related formations, referred to as fallout breccias, in the vicinity of Chassenon. Gravity data extracted from the BRGM InfoTerre database were reprocessed and revealed both the shape of the astrobleme and the presence of a small negative gravity anomaly close to Chassenon, due to the low density of impact-related rocks; a 2D gravity model across the fallout deposits was provided. Geological settings, well-logging and geo- physical soundings (direct current (DC), time-domain electromagnetic (TDEM) and frequency- domain electromagnetic (FDEM)) were carried out on the fallout breccias at Chassenon. The 1D resistivity models based on TDEM and DC soundings, resulting from a mutually constrained inver- sion and coupled with 1D models of FDEM data, were constrained by borehole data and provided 2D resistivity models across the fallout deposits of Chassenon. The combination of these methods allows the description of both the impact-related rocks geometry and the global structure of fallout breccia remains at Chassenon. The study also shows that the fallout breccia has a complex structure, consisting of thin layers of lithic breccia and suevite, the total thickness of which is larger at Chassenon than previously estimated by means of geological analysis.

International audience; At bioreactor landfills, biodegradation parameters are carefully controlled to achieve faster stabilization of the waste mass. For example, liquid (leachate) injection into the waste mass is used to increase waste moisture content and thereby enhance waste degradation. The objective of this Study is to validate the use of non-intrusive two-dimensional electrical resistivity imaging (ERI) for monitoring a liquid injection system, and to determine the relationship between measured resistivity and water content variations in waste. Temporal variations of waste electrical resistivity, during leachate recirculation, are mainly linked to water and ionic content variations. Two-dimensional ERI monitoring was carried Out at a municipal solid waste landfill using a multi-channel resistivity meter to follow transient resistivity variations during leachate recirculation in the waste mass. Resistivity variations mainly correspond to variations in water content, and to temperature variations resulting from the injection of cold leachate into the warm waste mass. Laboratory experiments can be used to calibrate water content variations in the waste as a function of variations in electrical resistivity. Although Such an approach is realistic for short measurement periods (hours), it is not reliable over longer time periods (months) since other parameters, Such as biodegradation of the waste, may also influence its electrical resistivity.

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.

A bioreactor landfill is designed to manage municipal solid waste, through accelerated waste biodegradation, and stabilisation of the process by means of the controlled addition of liquid, i.e. leachate recirculation. The measurement of electrical resistivity by Electrical Resistivity Tomography (ERT) allows to monitor water content present in the bioreactors. Variations in electrical resistivity are linked to variations in moisture content and temperature. In order to overcome this ambiguity, two laboratory experiments were carried out to establish a relationship between temperature and electrical conductivity: the first set of measurements was made for leachate alone, whereas the second set was made with two different granular media saturated with leachate. Both experiments confirm a well known increase in conductivity of about 2%degrees C-1. However, higher suspended matter concentrations lead to a lower dependence of electrical conductivity on temperature. Furthermore, for various porous media saturated with an identical leachate, the higher the specific surface of the granular matrix, the lower the effective bulk electrical conductivity. These observations show that a correct understanding of the electrical properties of liquids requires the nature and (in particular) the size of the electrical charge carriers to be taken into account. (c) 2006 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.

The bioreactor is a new concept of landfill where parameters of biodegradation can be controlled in order to stabilize the waste mass. A high waste moisture, generally brought by leachate recirculation, favours biodegradation and methanogenesis reactions. This study conducted by CreeD, Paris 6 University and IRD aims to establish the validity of geophysical measurement methods and the link between the geophysical measurements and the water content variations in the waste mass. Electrical resistivity is mainly linked to water and ionic content. This is the reason why geophysics methods measuring electrical resistivity are used in this study. The originality of this study will consist in in situ and laboratory measurements: On an ONYX site where a leachate recirculation is performed by vertical wells. In order to follow the leachate flow in the cell, an electrical 4D imaging is carried out with fast resistivitymeter. Results during leachate injection trials are very encouraging. The recirculation has begun in 2003 and should be carried out during several years. The study of electrical properties of leachate is conducted in order to characterise the evolution of electrical resistivity versus temperature (before studying this evolution in waste). Introduction For several years a new waste treatment approach is developed for landfilling wastes: the bioreactor. This concept consists in accelerating the waste biodegradation and stabilization process through controlled additions of liquid i.e. leachate recirculation by vertical boreholes or by horizontal trenches (Figure 1). The increase of moisture content enhances the growth of bacteria responsible of the solid waste decomposition (Warith, 2002). The bioreactor allows to accelerate the waste biodegradation and also decrease the waste stabilization time which limits environmental risks. The biogas production is enhanced during landfill operation, thus it is easier and economically feasible to recover it. Today, several research programs are developed to be able to monitor a bioreactor landfill site. Actually, for example, it is very important to control the quantity and the diffusion of leachate which have been injected in order to obtain an homogeneous and optimal moisture content in the whole waste mass of the bioreactor cell. Leachate injection (horizontal trenches) Biogas collecting Leachates storage Biogas collecting and reclaiming Leachate collecting Leachate injection (wells) Figure 1: Horizontal and vertical leachate injection system (from Onyx Waste Services) The geophysical methods measuring the electrical resistivity have proved to be efficient to monitor a bioreactor (Grellier, 2003, Moreau, 2003). In fact electrical resistivity depends partly on water content, and allows to obtain a general view of sections of the waste cells without invasive measurement. Methodology Geophysical methods measuring the electrical resistivity have been used to follow the leachate flows (2D electrical imaging, Dahlin, 2001). These methods have been well studied and show that electrical current has no difficulty in flowing through the waste mass (Carpenter et al., 1991; Haker et al., 1997; Berstone et al., 2000). However all the measurements are carried out on landfill covered with low resistive material (compacted top clay layer). In a bioreactor a geomembrane is located at the top of the landfill to remain the tightness. With such a structure electrical current can not flow through the cover. The Electrodes have to be placed just beneath the geomembrane to make measurement possible and keep the tightness of the cover. Their installation requires also precaution to be sure that the contact between electrodes and electrical line is correct. To carry out electrical imaging of the bioreactor we use a resistivity-meter from IRIS Instruments called the Syscal Pro. This is a resistivity-meter developed by the French environmental project RITEAU and designed for high productivity survey. We used to make electrical measurements with the standard resistivity-meter Syscal R1+ (IRIS Instruments). The number of electrodes (48 for each instrumented line on the bioreactor cell) requires a fast resistivity-meter in order to record several electrical imagings during a leachate recirculation time (which lasts from 45 minutes to 2h30). With a standard resistivity-meter (Syscal R1+), a wenner-α sequence of 48 electrodes takes at least 40 minutes. With the Syscal Pro, the same sequence can last at least 4’. It can save a more considerable time with other arrays like pole-dipole. This time saving is based on quick mode with a pulse duration of 240 ms constituted by one positive pulse and one negative pulse only. To save a maximal time, sequences have to be optimized in order that the maximum number of channels (10 max) for the reception can be used. Consecutive measurements can be made at the same time if they used the two same current electrodes (C1 and C2) and one common potential electrode (P1 or P2). Site study The study is conducting on one site located in the west of France. This site is the first official French ONYX bioreactor. The injection system is constituted by vertical wells. Its operation has begun in 2001, June. An important geophysical instrumentation was installed on the site (Figure 3): 200 electrodes (three horizontal 2D electrical imaging of 48 electrodes and one of 32 electrodes; four vertical 2D electrical lines of 8 electrodes. in each case the electrode spacing is 1 m). Moreover temperature will be monitored by means of 12 thermal sensors placed on 3 out of the 4 vertical 2D electrical lines. All this instrumentation allows us to monitor the bioreactor in 4D (3 spatial dimensions and 1 time dimension).

Geophysical methods have already shown their interest for the continuous characterisation of soils over landscapes, rapidly and, non-intrusively. But in bottomland areas, difficulties are encountered in relating geophysical properties to soil spatial distribution due to large variations in the depth, texture and/or water content of soils. Indeed, respective variations of these parameters can result in ambiguous geophysical responses. For example, a decrease in soil water content, which causes an increase in electrical resistivity, may be offset by an increase in soil clay content, inducing a decrease in resistivity. The objective of this study was to improve the continuous characterisation of soils affected by an excess of water by using a combination of geophysical techniques. Three techniques, the radio-magnetotelluric (RMT), the ground penetrating radar (GPR) and the electrostatic quadrupole (ESQP) were implemented along eight representative transects where soils were extensively described. The soil cover shows a succession from downslope to upslope consisting in fibric Fluvisols, gleyic Fluvisols, and Albefluvisols. None of the geophysical methods allows us to distinguish all soil limits and to estimate the geometry of soil horizons. The ESQP discriminates Fluvisols from Albefluvisols, whereas the RMT above all reveals differences in soil material thickness, which do not permit to discriminate between these soils. In complement, the GPR allows the estimation of the geometry of organic horizons and anthropic structures, such as ditches. Finally, the combination of these three techniques allows us to assess the main features of soil spatial distribution in bottomlands. To cite this article: V. Chaplot et al., C. R. Geoscience 336 (2004).

International audience; In 2D-multielectrode electrical surveys using the pole-pole array, the distance to 'infinite electrodes' is actually finite. As a matter of fact, the available cable length generally imposes a poor approximation of theoretical location of these electrodes at infinity. This study shows that in most of the cases, the resulting apparent resistivity pseudosection is strongly distorted. Numerical simulation validated by field test also shows that a particular finite array provides results that are as close as possible to the ones of the ideal pole-pole array. This is achieved when two conditions that are weaker than an Ž. infinite location are fulfilled: i the 'infinite electrodes' are placed symmetrically on both sides of the in-line electrodes with Ž. a spread angle of 308 and ii the length of 'infinite lines' is at least 20 times the greatest distance between in-line electrodes. The electrical 2D image obtained with this enhanced array is the least distorted one with respect to the pole-pole image. The apparent resistivities are generally underestimated, but this deviation is almost homogeneous. Though the shift cannot be determined a priori, the interpretation of such an image with direct or inverse software designed for pole-pole data provides an accurate interpretation of the ground geometry.

The Ground Penetrating Radar (GPR) has demonstrated its particular ability for many sub-surface investigations. Nevertheless, even in the case of an optimal response from the ground, the image is sometimes disturbed by unwanted features (Sun and Young, 1995). Data processing is then a mandatory way to enhance the useful information inside the radargram. Aerial features are one of the noises encountered using non-shielded antennas.

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.

In Mountainous South-East Asia, rapid agricultural intensification and the expansion of rubber tree plantations in the past decades led to drastic biodiversity losses and intense soil degradation. Weeds provide diverse ecosystem services, including soil protection and support for biodiversity at higher levels. However, the determinants of weed communities in mountainous areas of Southeast Asia, and the interactions of these communities with soil characteristics and erosion processes, are still largely unknown. We conducted soil characterisation and botanical inventories in Huai Lang, Northern Thailand, in 20 fields from different land uses along the transition from annual crops to mature rubber tree plantations twice a year from 2016 to 2018. Using 1m2 erosion microplots, we also measured runoff and soil erosion in different land uses, and in rubber tree plantations under different weeding practices. We show that weed communities composition was mostly determined by land use and landscape characteristics, while crop temporal variability was an important determinant of plant diversity. Rubber tree plantations had specific weed communities and had exceptionally high soil detachment rates compared to maize or young plantations, but weed cover was effective in decreasing these erosion rates. Altogether, our results suggest that soil and plant biodiversity conservation in mountainous Southeast Asia should involve both on-field adaptations (e.g. less intense weeding practices, especially under rubber tree plantations) and landscape-scale management programs, with a focus on maintaining landscape diversity.; L’intensification et l’expansion rapide des plantations d’Hevea brasiliensis dans les régions montagneuses d’Asie du sud-est ont conduit à d’importantes dégradations de la biodiversité et des sols. Les adventices remplissent de nombreux services écosystémiques, par exemple en favorisant la protection du sol. Toutefois, les facteurs déterminants les communautés adventices et les interactions entre ces communautés et les processus d’érosion du sol sont mal connus. Deux fois par an entre 2016 et 2018, nous avons réalisé des inventaires botaniques et des caractérisations de sol à Huai Lang, au nord de la Thaïlande, dans 20 champs appartenant à plusieurs types d’usage du sol correspondant à la transition entre les cultures annuelles et les plantations d’hévéa. Nous avons également mesuré l’effet du type d’usage du sol et des pratiques de désherbage sur le ruissellement et le détachement de sol dans des plots d’érosion de 1m2. Les résultats montrent que la composition des communautés végétales est principalement déterminée par le type d’usage du sol et les caractéristiques paysagères, tandis que la variabilité temporelle des cultures dans un champ favorise la diversité végétale. Les plantations d’hévéa matures supportent des communautés végétales très spécifiques et favorisent des ruissellements et détachements de sol très élevés, qui peuvent être atténués par la présence d’un couvert végétal. Cette étude suggère que la conservation du sol et de la biodiversité en Asie du sud-est devrait être fondée à la fois sur une évolution des pratiques à l’échelle du champ (par ex. désherbage moins intense) et sur un maintien de la diversité spatiale des agroécosystèmes.