Laurent Barbiero

Geochemistry of water and ground water in the Nhecolândia, Pantanal of Mato Grosso, Brazil: variability and associated processes

A distinctive feature of the Nhecolândia, a sub-region of the Pantanal wetland in Brazil, is the presence of both saline and freshwater lakes. Saline lakes used to be attributed to a past arid phase during the Pleistocene. However, recent studies have shown that saline and fresh water lakes are linked by a continuous water table, indicating that saline water could come from a contemporary concentration process. This concentration process could also be responsible for the large chemical variability of the waters observed in the area. A regional water sampling has been conducted in surface and sub-surface water and the water table, and the results of the geochemical and statistical analysis are presented. Based on sodium contents, the concentration shows a 1: 4443 ratio. All the samples belong to the same chemical family and evolve in a sodic alkaline manner. Calcite or magnesian calcite precipitates very early in the process of concentration, probably followed by the precipitation of magnesian silicates. The most concentrated solutions remain under-saturated with respect to the sodium carbonate salt, even if this equilibrium is likely reached around the saline lakes. Apparently, significant amounts of sulfate and chloride are lost simultaneously from the solutions, and this cannot be explained solely by evaporative concentration. This could be attributed to the sorption on reduced minerals in a green sub-surface horizon in the “cordilhieria” areas. In the saline lakes, low potassium, phosphate, magnesium, and sulfate are attributed to algal blooms. Under the influence of evaporation, the concentration of solutions and associated chemical precipitations are identified as the main factors responsible for the geochemical variability in this environment (about 92% of the variance). Therefore, the saline lakes of Nhecolândia have to be managed as landscape units in equilibrium with the present water flows and not inherited from a past arid phase. In order to elaborate hydrochemical tracers for a quantitative estimation of water flows, three points have to be investigated more precisely: (1) the quantification of magnesium involved in the Mg-calcite precipitation; (2) the identification of the precise stoichiometry of the Mg-silicate; and (3) the verification of the loss of chloride and sulfate by sorption onto labile iron minerals.

Distribution and dynamics of gibbsite and kaolinite in an oxisol of Serra do Mar, southeastern Brazil

Abstract In the Serra do Mar region, in southeastern Brazil, the soil mantle is mainly characterised by (i) a gibbsitic saprolite, (ii) various kaolinitic horizons within the gibbsitic material, (iii) kaolinito-gibbsitic topsoil horizons. This organisation does not match with the thermodynamic stability of gibbsite and kaolinite accompanying the solution percolation through soil profiles. A study of the micromorphological, mineralogical and chemical properties of the soil mantle reveals that this organisation arises from the in situ development of the soil from the crystalline bedrock. The bauxitic weathering of the bedrock, even if it is rich in quartz, can be explained by a fast renewal of the solutions and/or a high solubility of the kaolinite. Recycling of Si and Al by the forest can maintain a dynamic equilibrium of kaolinite in the topsoil horizons, as observed in Amazonia. The kaolinitic compact horizons evolve upslope at the expense of the gibbsitic material. At the contact between kaolinitic and gibbsitic material, dissolution patterns of quartz and gibbsite are observed, indicating that this evolution is in process. These observations and the organisation of the soil mantle set the problem of the apparent stability of gibbsite and kaolinite in this environment. Various assumptions that could explain this organisation of the soil mantle are discussed. Changes in the activity of water due to the pore size diminution and displacement of the gibbsite–kaolinite equilibrium appear insufficient to explain the stability of kaolinite. However, it could be allotted to the slow down of water flows in the soil mantel. Lastly, the eventual role of the complexing organic matter is presented. More investigations on the biogeochemical cycle of Si and Al and on the physico-chemical processes at the soil solution–mineral interface are necessary to explain the stability and dynamics of gibbsite and kaolinite in this environment.

MINERALOGY AND GENESIS OF SMECTITES IN AN ALKALINE-SALINE ENVIRONMENT OF PANTANAL WETLAND, BRAZIL

Smectite formation in alkaline-saline environments has been attributed to direct precipitation from solution and/or transformation from precursor minerals, but these mechanisms are not universally agreed upon in the literature. The objective of this work was to investigate the mineralogy of smectites in the soils surrounding a representative alkaline-saline lake of Nhecolândia, a sub-region of the Pantanal wetland, Brazil, and then to identify the mechanisms of their formation.Soils were sampled along a toposequence and analyzed by X-ray diffraction, transmission electron microscopy-energy dispersive X-ray analysis, and inductively coupled plasma-mass spectrometry. Water was collected along a transect involving the studied toposequence and equilibrium diagrams were calculated using the databases PHREEQC and AQUA.The fine-clay fraction is dominated by smectite, mica, and kaolinite. Smectites are concentrated at two places in the toposequence: an upper zone, which includes the soil horizons rarely reached by the lake-level variation; and a lower zone, which includes the surface horizon within the area of seasonal lake-level variation. Within the upper zone, the smectite is dioctahedral, rich in Al and Fe, and is classified as ferribeidellite. This phase is interstratified with mica and vermiculite and has an Fe content similar to that of the mica identified. These characteristics suggest that the ferribeidellite originates from transformation of micas and that vermiculite is an intermediate phase in this transformation. Within the lower zone, smectites are dominantly trioctahedral, Mg-rich, and are saponitic and stevensitic minerals. In addition, samples enriched in these minerals have much smaller rare-earth element (REE) contents than other soil samples. The water chemistry shows a geochemical control of Mg and saturation with respect to Mg-smectites in the more saline waters. The REE contents, water chemistry, and the presence of Mg-smectite where maximum evaporation is expected, suggest that saponitic and stevensitic minerals originate by chemical precipitation from the water column of the alkaline-saline lake.

Residual alkalinity as tracer to estimate the changes induced by forage cultivation in a non-saline irrigated sodic soil

Soil alkalinisation generally constitutes a major threat to irrigated agriculture in the semi-arid regions of West Africa. The improvement of sodic soils is generally difficult and expensive. However, a recent study in the Niger valley in NIGER, reveals that a natural de-alkalinisation is possible under natural conditions in a semi-arid climate. Transformation of non-saline sodic soil into brown steppe soil type was recorded. On the same site, the cultivation of a Sahelian fodder grass, locally known as “Burgu” was used on the sodic soil/brown steppe soil transition zone to accelerate this natural de-alkalinisation and characterise its mechanisms. The geochemical properties of both soil types were monitored before cultivation and one year after continuous crop cultivation. After cultivation and regular irrigation, the chemical properties of the former sodic soils were close to those of the surrounding brown steppe soils, which are better suited for agriculture. This modification of the sodic soil properties can be attributed to (i) the large amount of water supplied during cultivation that induced salt leaching. This is the main phenomenon responsible for the changes observed; (ii) the root activity that modified the acid-base equilibrium and consumes alkalinity. The Residual Alkalinity (RA) concept was used to select chemical tracers of the concentration/dilution of the soil solution. Here, sodium amount and calcite + fluorite residual alkalinity (RAcalcite+fluorite) were the most adequate ones. These two tracers decreased proportionally under the influence of leaching, but the exchanges between cations and protons changed the RAcalcite+fluorite, without modifying the sodium amount. Their combined use allowed us to separate and quantify the uptake of the plant from the leaching in the de-alkalinisation process. This study highlighted that reclamation of this type of sodic soils is feasible. The use of the RA concept is advisable to design a sustainable management system for irrigated sodic or saline soils.

Organisation of the soil mantle in tropical southeastern Brazil (Serra do Mar) in relation to landslides processes.

Abstract The Serra do Mar complex in southeastern Brazil is subject to frequent landslides especially on the Atlantic coast. The three-dimensional organisation of the soil was studied in a 56-ha representative catchment. Upslope in situ soil material has developed from the parent rock, and downslope the soil has resulted from landslide processes. The soil mantle upslope is mainly characterised by slowly permeable kaolinitic horizons within a thick gibbsitic weathering horizon. The field and microscopic relationships between the two suggest that the kaolinitic horizon has evolved at the expense of the gibbsitic material. Resilication of gibbsite to form kaolinite is postulated in this environment. The kaolinitic horizons are overlain by a microaggregated horizon. The difference in porosity and water movement between these two horizons was estimated by density measurements, mercury porosimetry, image processing and calculation of water retention and shrinkage curves. The results indicate that water is retained briefly within the microaggregated horizon during intensive rainfall. Because of the inclination of the kaolinitic horizon, any excess water within it flows laterally downslope and accumulates in the lowest part of the slope. This leads to landslipping, the main process of landform development in the region.

Salt distribution in the Senegal middle valley : analysis of a saline structure on planned irrigation schemes from N'Galenka creek

In the middle Senegal valley, the saline soil distribution is not related to the present faint topography. The absence of a relationship is one of the major constraints in establishing new irrigation schemes. The salt distribution was studied to understand its variability, and to describe its structure and spatial arrangement. Saline areas were delineated by measuring the electromagnetic soil conductivity (ECm), a rapid technique with a portable instrument (EM38). The results indicate that the saline soils are distributed as strips. A detailed examination revealed that the major strip is actually composed of two parallel minor strips, and a comparison with aerial photographs showed that one lies in a former creek bed, and the other fringes it on the southern bank. The strip is intersected by an actual creek bed, indicating that the salt distribution is ancient, related to previous geo-morphology, and does not result from a recent remobilisation of the marine salt deposits incorporated in the soil. The identification of this relationship between the present saline soil distribution and previous geo-morphology allowed us to survey the whole N’Galenka region (about 6000 ha) using ECm measurements on selected transects. # 2001 Elsevier Science B.V. All rights reserved.

Natural arsenic in groundwater and alkaline lakes at the upper Paraguay basin, Pantanal, Brazil

A distinctive feature of Nhecolândia, a sub-region of the Pantanal wetland in Brazil, is the presence of both saline (alkaline) and freshwater lakes. Saline lakes were attributed to a past arid phase during the Pleistocene, but recent studies have shown that the geochemistry of the saline lakes arises from the current concentration process of fresh waters that is supplied to the Pantanal every year. The region is mainly used for cattle farming, and more recently eco-tourism is becoming widespread and water conservation agencies have begun to worry about As contents in surface and shallow groundwater. The study was carried out along a 600-m-long transect that linked a freshwater lake to a saline lake. The spatial distribution of As is studied, based on soil morphology, according to pH, redox conditions (Eh) and dissolved organic carbon, and also compared to distribution of major elements. Three main processes, responsible for the chemical variability, were identified: (1) the concentration of the solution under the influence of evaporation and the associated precipitations (Mg-calcite, illite, sodium carbonate); (2) oxidation of sulphides included in clay layers and subsequent development of acid conditions and (3) buffering of acid conditions by clay dissolution. Arsenic contents are not affected by the last two processes (2 and 3), but evolve in proportion to the concentration. Arsenic concentrations in water ranged from 0.11 μg/l to 3.68 mg/l, i.e., 368 times higher than the World Health Organization guidelines for drinking water. The As speciation, as calculated from field conditions, indicated that the As(V) redox state dominated in the groundwater and in the lakes, whereas some proportion of As(III) could occur on the shore and in the sediments of the saline lake. Arsenic(III) from the sediment can be remobilized and dispersed into the water following the disturbance of the sediments by cattle that come to drink water. Arsenic contents increased with most of the other dissolved species; hence, competitive adsorption prevents dissolved As regulation by solid phase reaction. Moreover, the pH values increased above 8 with increasing As, favouring As(V) desorption. As a consequence, the good correlation established between As contents and electrical conductivity (r2 = 0.97) should help to indirectly evaluate the As contents in shallow groundwater and lakes of the region.

Geochemistry of clay dunes and associated pan in the Senegal Delta (Mauritania)

Since the construction of the anti - salting Diama dam , a resumption of aeolian deflation has been observed in the Senegal delta and particularly in Diawling National Park . It results in the formation of many clay dunes with high salt concentrations . The soft powdery structure observed on the soil surface , affected by aeolian defla tion is due to rapid salt crystallization . In order to identify the mineralogy of the salt involved in this environment , the chemistry of soil and groundwater was studied along a transect from a pan to the associated clay dune . Equilibria computation and field and scanning electron microscope (SEM) observations indicated that the changes observed in the chemical facies of the solution were due to the following precipitation sequence: calcite > gypsum > halite . A progressive but significant fixing of Na+ onto the clay particles was observed . The calcite deposition in the soil was limited by exhibition of the potential acidity arising from the former man grove site soils...

Organic Control of Dioctahedral and Trioctahedral Clay Formation in an Alkaline Soil System in the Pantanal Wetland of Nhecolândia, Brazil.

Recent studies have focused on the formation of authigenic clays in an alkaline soil system surrounding lakes of the Nhecolândia region, Pantanal wetland. The presence of trioctahedral Mg-smectites (stevensite and saponite types), which requires low Al and Fe contents in the soil solution for its formation, contrasts with the neoformation of dioctahedral Fe-mica (glauconite, and Fe-illite), which instead requires solutions relatively enriched in Al and Fe. This study aims to understand the conditions of co-existence of both, Mg-smectite and Fe-mica a common clay association in former or modern alkaline soil systems and sediments. The study was carried out along an alkaline soil catena representative of the region. The soil organization revealed that Mg-smectite occur in top soil close to the lake, whereas Fe-mica dominate in the clay fraction of deeper greenish horizons a few meters apart. We propose here that this spatial distribution is controlled by the lateral transfer of Fe and Al with organic ligands. Alkaline organic rich solutions (DOC up to 738 mg L-1) collected in the watertable were centrifuged and filtered through membranes of decreasing pore size (0.45 μm, 0.2 μm, 30 KDa, 10 KDa, 3 KDa) to separate colloidal and dissolved fractions. Fe, Al, Si, Mg and K were analysed for each fraction. Although the filtration had no influence on Si and K contents, almost 90% of Fe (up to 2.3 mg L-1) and Al (up to 7 mg L-1) are retained at the first cutoff threshold of 0.45μm. The treatment of the same solutions by oxygen peroxide before filtration shows that a large proportion of Fe and Al were bonded to organic colloids in alkaline soil solution at the immediate lake border, allowing Mg-smectite precipitation. The fast mineralization of the organic matter a few meters apart from the lake favors the release of Fe and Al necessary for Fe-mica neoformation. In comparison with chemical and mineralogical characteristics of alkaline environments described in the literature, the study suggests that the co-existence of trioctahedral Mg-smectite and dioctahedral Fe-mica should be regarded as a standard occurrence in alkaline soil systems with organic rich waters.

Field Study and Simulation of Geochemical Mechanisms of Soil Alkalinization in the Sahelian Zone of Niger

In some soils of the Niger valley, alkalinization and sodization are related to the concentration of alteration products of calco‐alkaline gneiss to biotite, which releases significant amounts of cations and alkalinity. The result is an oversaturation of the soil solution with respect to calcite, which precipitates with a positive calcite residual alkalinity. Ca2+ molality decreases, while alkalinity and pH increase; Ca2+ desorption is accompanied by adsorption of Na+on the exchange complex. K+, Mg2+, and Na+concentrations in the soil solution are successively controlled by the formation of illites and smectite‐type silicates, while kaolinite dissolves. These processes were successfully simulated according to the analytical data. They agreed with the significant increase of the alkaline reserve in the soils according to chemical, physical, and mineralogical alterations. These geochemical mechanisms were found to be involved in the formation of alkali soils at three sites of the region.