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International audience; Latin America, like other areas in the world, is faced with the problem of high arsenic (As) background in surface and groundwater, with impacts on human health. We studied As biogeochemical cycling by periphyton in Lake titicaca and the mine-impacted Lake Uru Uru. As concentration was measured in water, sediment, totora plants (Schoenoplectus californicus) and periphyton growing on stems, and As speciation was determined by X-ray absorption spectroscopy in bulk and eDtA-extracted periphyton. Dissolved arsenic was between 5.0 and 15 μg L −1 in Lake Titicaca and reached 78.5 μg L −1 in Lake Uru Uru. As accumulation in periphyton was highly variable. We report the highest As bioaccumulation factors ever measured (BAFs periphyton up to 245,000) in one zone of Lake Titicaca, with As present as As(V) and monomethyl-As (MMA(V)). Non-accumulating periphyton found in the other sites presented BAFs periphyton between 1281 and 11,962, with As present as As(III), As(V) and arsenosugars. DNA analysis evidenced several taxa possibly related to this phenomenon. Further screening of bacterial and algal isolates would be necessary to identify the organism(s) responsible for As hyperaccumulation. Impacts on the ecosystem and human health appear limited, but such organisms or consortia would be of great interest for the treatment of As contaminated water. The arsenic (As) geogenic background of surface and groundwater is naturally high in South America, predominantly originating from young volcanic rocks and their weathering products in arid oxidizing conditions 1-4. As a result, about 4.5 million people in South America are chronically exposed to high levels of As (>50 µg L −1) 5 , and certain Andean populations have developed a unique capacity to adapt to As toxicity 6,7. Concerning Andean lakes, extreme As concentrations are observed in hypersaline lakes colonized by extremophile bacterial communities 8 , and lower but still significant concentrations are observed in other, less saline lakes, which are major freshwater resources 5. In many areas of the Altiplano, mining and smelting activities add to natural rock weathering processes in the As budget 9. The biogeochemical cycling of As has been studied in freshwater and marine ecosystems, and in hypersaline environments, but its trophic transfer and speciation in living organisms mainly concerns the marine environment and As contaminated freshwater systems 10. In the high altitude lakes of the Andean Altiplano (above 3500 m asl), shallow zones (

Spatial patterns, cluster or dispersion trends are statistically different from random patterns of trace elements (TEs), which are essential to recognize, e.g., how they are distributed and change their behavior in different environmental processes and/or in the polluted/contaminated areas caused by urban and industrial pollutant located in upstream basins and/or by different natural geological conditions. The present study focused on a statistical approach to obtain the spatial variability of TEs (As, B and Sb) in shallow groundwater (GW) in a high-altitude arid region (Lower Katari Basin, Bolivian Altiplano), using multivariate analysis (PCA and HCA), geochemical modeling (PHREEQC, MINTEQ) and spatial analyses (Moran's 1 and LISA), considering the community supply wells. The results indicate that despite of the outliers there is a good autocorrelation in all cases, since Moran's I values are positive. The global spatial dependence analysis indicated a positive and statistically significant spatial autocorrelation (SA) for all cases and TEs are not randomly distributed at 99% confidence level. The results of hydrochemical modeling suggested the precipitation and stability of Fe (III) phases such as goethite. The re-adsorption of As and Sb on the mineral surface in the aquifer could be limiting the concentrations of both metalloids in southern regions. Spatial autocorrelation was positive (High-High) in northwestern (arsenic), southeastern (boron) and northeastern (antimony) region. The results reflected that the As and Sb are the main pollutants linked to the natural geological conditions, but B is a main pollutant due to the anthropogenic activities. Furthermore, >50% shallow groundwater exceeded the WHO limit and NB-512 guideline values for Sb (87%), B (56%) and As (50%); therefore the spatial distribution and concentrations of these TEs in GW raise a significant concern about drinking water quality in the study area.

Sustainable horticulture depends on the integrity of soil functions, which directly depend on soil architecture affecting aggregation, root growth, as well as liquid and gas permeability. We hypothesised that changes in soil architecture resulting from feedback mechanisms between management, soil organic carbon contents (SOC), biota and vegetation can be captured with X-ray computed tomography (CT), and that these affect the soil filtering function, which thus, can be manipulated through orchard management. We compared the transport of copper, a widely used fungicide, through intact soil cores from vine rows of kiwifruit orchards under organic and integrated management. We first derived 3D-macropore characteristics from CT-images, followed by leaching a pulse of copper and a tracer through the same cores. The organic orchard soil had a significantly higher SOC content than the integrated orchard soil, and this was positively correlated with total porosity. Macropores (>92 mu m) were larger with a higher connectivity, but significantly fewer in the organic than the integrated orchard soil. This resulted in a lower macroporosity and a better copper filtering capacity of the organic than the integrated orchard soil. Copper distribution was reasonably predicted when combining SOC contents, pH and macropore characteristics. Significant relationships between soil parameters and indicators of the strength of preferential flow verified that CT-derived macropore characteristics can be used to predict functional solute transport parameters. The relevance of our results and relationships observed between macropore characteristics, functional indicators of preferential flow and the fate of copper needs verification with samples representing more soils and sites.

Rapid urbanization and the absence of efficient water management policies are increasingly degrading the water quality of tropical reservoirs in developing countries. The small tropical reservoir of Cointzio, located in the Trans-Mexican Volcanic Belt, is a warm monomictic water body (surface area = 6 km(2) with short water residence time < 1 year) that is strategic to the drinking water supply of the city of Morelia and to downstream irrigation during the dry season (6 months of the year). The reservoir faces two threats: (a) reduced water storage capacity due to sediment accumulation and (b) eutrophication caused by excess nutrients that likely come from untreated wastewaters in the upstream watershed. Intensive field measurements of water and sediment were conducted in 2009 to characterize the trophic status of the reservoir and to estimate nitrogen (N) and phosphorus (P) sources, total suspended sediment (TSS) (N), (P), and carbon (C) loads, and their accumulation or removal in the reservoir. We found that point sources represent the majority of N and P inputs to the reservoir. The trophic status is clearly eutrophic given the high chlorophyll a peaks (up to 70 A mu g L-1) and a long period of anoxia (from May to October). Most of the TSS, C, N, and P were conveyed to the reservoir between June and October during the wet season. The TSS yield from the watershed was estimated at 35 +/- A 19 t km(-2) year(-1), of which more than 90 % was trapped in the reservoir (sediment accumulation rate = 7800 +/- A 2100 g m(-2) of reservoir year(-1)). The export load of C, N, and P downstream at the reservoir outlet was reduced by 31, 46 and 30 % respectively in comparison to the load at the reservoir inlet. This study reveals the effect of climatic seasonality on inputs to tropical reservoirs and accumulation; it also highlights the need to both reduce nutrient input to combat eutrophication and mitigate erosion to maintain the water storage capacity of the reservoir over the long term.

In the Mezquital valley, untreated wastewater (45 m(3) s(-1)) from Mexico City is used for the irrigation of around 900 km(2) of agricultural soil. High concentrations of metals including methylmercury (3.8+/-2.5 ng l(-1)) and lead (0.16+/-0.05 mg l(-1)) were measured in anoxic wastewater canals. Downstream, dissolved, and particulate polymetallic (Hg, Pb, Cr.) concentrations decreased by factors 10 to 1,000 in the Tula River (which received a mix of fresh and wastewater) due to the dilution and oxidation of surface water, and to the decrease of contaminants concentration in wastewater downstream irrigated soils. However, dissolved and particulate methylmercury concentrations (0.06 to 0.33 ng l(-1) and 1.6 to 4.5 g kg(-1), respectively) remained elevated in comparison to other natural hydrosystems. The monitoring of an irrigation event and the distribution of metals in a soil profile irrigated for more than 80 years showed that metals were retained in the draining tilled layer. The oxic conditions and slightly acidic pH (similar to 6.5) in this layer were found favorable for metal adsorption and co-precipitation with redox-sensitive elements (Fe, Mn) and suggestively for mercury demethylation. In the downstream Tula River and groundwater, almost all metallic concentrations remained below guideline thresholds. Only, dissolved As and Pb concentrations remained two to five times above thresholds for drinking water, highlighting a potential health risk for approximately 500,000 people who use groundwater as water supply.

Le sud et le centre du Mexique sont beaucoup plus favorisés que le nord en terme de bilan hydrique. On considère qu'ils regroupent 46 % des terres arables du pays, mais disposent de 93 % des ressources en eau. Cependant, c'est une région qui est aussi bien plus peuplée, du fait de l'ancienneté de la présence des cultures indiennes, et aussi du fait de la fertilité de ses sols (ce fait et l'ancienneté des civilisations ne sont probablement pas étrangers l'un à l'autre) ; les sols volcaniques s...

L'eau est rare au nord du Mexique, et la gestion de cette ressource limitée est devenue une priorité nationale. Dans ce contexte, la Sierra Madré occidentale, château d'eau pour tout le nord du pays, est un espace particulièrement convoité. La relative abondance en eau y a entraîné le développement de pâturages et d'exploitations forestières, provoquant une surexploitation des milieux et des ressources. La déglaciation des sols liée au surpâturage et au déboisement conduit de fait inévitablement à celle de l'eau et menace aujourd'hui le potentiel de toute cette région. À travers des études de cas précises, cet ouvrage montre comment la gestion de l'eau est liée à celle de l'espace et nécessite la prise en compte des besoins de l'ensemble des usagers, du bûcheron au gardien de troupeaux. Cette gestion patrimoniale, qui permet de régler les nombreux conflits d'usage, forme un modèle de développement dont de nombreux pays de montagne pourraient s'inspirer. à Henri Barral

International audience; El problema de migración de plaguicidas en los campos agrícolas se estudia tradicionalmente a través del muestreo periódico de suelos, aguas y cultivos. Existen métodos más avanzados que permiten reducir el costo de los análisis en campo y pronosticar el riesgo de contaminación del medio ambiente. Estos métodos están basados en la simulación matemática del movimiento de plaguicidas. Se estudió el proceso de migración de atrazina en un suelo volcánico Andosol y un Vertisol de México en experimentos de laboratorio en columnas empacadas. En los experimentos se aplicaron diferentes dosis de atrazina y se estableció un flujo insaturado de agua bajo régimen permanente. Se utilizaron los marcadores del agua 18O y Br– para comparar los procesos de movimiento del agua y la atrazina. Se midieron los componentes del balance de masa de atrazina y agua, y su dinámica de elusión. Se aplicó la teoría del movimiento de sustancias químicas para interpretar los resultados experimentales y obtener las propiedades principales de transporte de atrazina en ambos suelos. Los resultados señalan que la atrazina se mueve en el Andosol 10-16 veces más lentamente que el agua y en el Vertisol el movimiento es 2,5 veces más lento. Se determinaron valores de cinco propiedades de transporte de atrazina que permiten predecir la migración del plaguicida, minimizar el muestreo en el campo y estimar el riesgo de contaminación del suelo y del acuífero aplicando los métodos de cálculo existentes en la literatura. The problem of pesticide migration in agricultural lands is traditionally studied by means of periodical soil, water and plant sampling. More advanced methods are available today, which permit to reduce the costs of field analysis and to predict environmental contamination risk. These methods are based on mathematical simulation of pesticide movement. The process of atrazine migration in a volcanic soil Andosol and a Vertisol of Mexico was studied by means of laboratory experiments in packed columns. Different atrazine doses and steady state unsaturated water flow were applied in the experiments. The 18O and Br– water tracers were used to compare atrazine and water movement. The atrazine and water mass balance components were measured, as well as the dynamics of elution. The chemical substances movement theory was applied to interpret the experimental results and to obtain the principal properties of atrazine transport in both soils. The results show that atrazine moves in the Andosol 10-16 times slower than water, and 2.5 times slower in the Vertisol. Five properties of atrazine transport were determined, which can be used to predict the migration of this pesticide so as to minimize the sampling effort in the field, and to assess the risk of soil and aquifer contamination by calculation methods reported in the literature.

Les travaux réalisés ont montré que la vitesse de transfert du nitrate pouvait être réduite par rapport à celle de l'eau du fait de sa rétention sur les charges électropositives des oxyhydroxides de fer et d'aluminium. Cette rétention, relativement faible dans les horizons de surface, peut devenir importante dans les horizons de profondeur, notamment quand la concentration en nitrate est réduite. Dans le cas de pesticides, l'adsorption sur la phase organo-minérale du sol contribue à limiter les transferts de ces produits vers les lentilles d'eau douce. Toutefois, la dégradation très lente de ces produits dans le sol des îles Loyauté, comparativement à d'autres sols, et la faible épaisseur du sol induisent des risques de contamination importants. (D'après résumé d'auteur)

The purpose of tbis study was to determine the risk of groundwater pollution from intensified agricultural practices on Maré, in the Loyalty Islands of New Caledonia. To achieve this goal, the water and nitrogen balances were established during three quite different wet seasons, and under different agricultural treatments and soil-cover conditions. Maré is an uplifted coral atoll which is build upon an underlying volcanic structure. Underneath, fresh water lenses are located inside the coral rock and these are floating on the underlying sea water. These lenses supply potable water to the indigenous Kanak people. The Maréan soil is a ferralitic oxisol derived from weathered volcanic ejecta and ash. It is a shallow and very permeable soil, with a saturated conductivity in excess of 100 mm h-1 . This soil possesses a certain degree of anion exchange capacity, that should retard to some degree the leaching of nitrate. The weather of Maré is characterized by a wetter season during which there are frequent tropical storms of high intensity, so the soil water fluxes can be very high. Intensification of agriculture on Maré offers the opportunity to develop economically sustainable enterprises for the Kanak population. However the fragile nature of the ecosystem requires careful land management strategies to ensure that such intensive agriculture is also environmentally sustainable. The impact of land management on the quality of underlying water contained in the fresh water lenses is critical. To calculate drainage and nitrate leaching under the rootzone, non-destructive devices such as Time Domain Reflectometry were used to measure the soil water content, tensiometers for the soil water pressure, and suction cups to extract the soil solution. An infiltrometer provided us with the soil's hydraulic characteristics, and we obtained some hydro-dispersive parameters with soil column experiments in the laboratory. About 100 kgN ha*1 per year was applied to a cornfield, a perennial pasture and a bare soil plot. The results show that pollution risks from fertilizers are significant under this permeable soil in this humid tropical climate. Furthermore, mineralisation of organic nitrogen is high thereby increasing the amount of N-NO3 present in the soil. In 1996, we obtained 200 % losses of nitrate under a fertilized bare soil, as compared to the amount of N-NO3 applied. However, even though the 1996 season was wet compared to 1995, the amount of nitrate leaching on corn was reduced from 130 % (1995) to 48 % (1996) of the N-NO,' applied. In 1996, the nitrate fertilizer was applied in two split applications 50 that the plants were able to take it up more efficiently. In 1995, ail the N-NO3 fertilizer was applied on the day of corn sowing. The grass plot showed that a perennial plant is able to consume most of the applied nitrate throughout the 1996 season, and this decreased the leaching to about 20 % of the NNO3 applied. Despite possessing anion exchange capacity, the high rainfall rates and penneable nature of the soil conspire to render nitrate easily leached. The pollution risk can however be strongly reduced with split applications such that root uptake of nitrate can proceed before the nutrient is leached beyond the rootzone where it can be tenned a pollutant. Sorne fertilization is still considered necessary because these ferralitic oxisols lack a range of nutrients of which nitrogen is one of the most important. Therefore careful application strategies will need to be developed. The soil parameters detennined on the field and in the laboratory allowed us to employa mechanistic model for water and solute transport, and to assess its ability to reproduce and predict our data. The model WAVE (Water and Agrichemicals in the Vadose Environment) was used. The measurements for the three years under different climatic conditions were quite weil reproduced. By the model now validated, it can thus be used to develop efficient and sustainable fertiliser management practices for the farmers on Maré., Le but principal de cette thèse est la détermination des risques de pollution de l'eau souterraine dus à des pratiques agricoles intensives sur l'île de Maré (Nouvelle-Calédonie). Maré est un atoll corallien surélevé où la principale ressource en eau douce provient de lentilles situées à l'intérieur du soubassement corallien. Le sol est un sol ferrallitique allitique très pérméable, et possédant une capacité importante d'échange anionique. Le climat est caractérisé par une saison humide, où des événements pluvieux de fortes intensités peuvent survenir. Pour calculer le drainage et la lixiviation du nitrate en-dessous de la zone racinaire, une métrologie non destructive a été mise en oeuvre sur plusieurs parcelles soumises à différentes pratiques culturales. Les caractéristiques hydrauliques du sol ont été obtenues par infiltrométrie et les paramètres de transfert de solutés grâce à des expériences réalisées au laboratoire, sur colonnes de sol. Environ 100 kg N ha-1 ont été apportés sur une parcelle sol nu, de maïs et de graminées. Bien que la saison 1996 fut plus humide qu'en 1995, la quantité de nitrate livixié sous maïs a diminué de 130 % (1995) à 48 % (1996) du nitrate apporté. En 1996, l'engrais a été apporté en deux fois alors qu'en 1995, il a été apporté en totalité le jour du semis. Malgré sa capacité d'échange anionique, la perméabilité du sol ainsi que les fortes intensités de pluie concourent à rendre le nitrate facilement lixiviable. Cependant, le risque de pollution peut être fortement réduit avec des applications fractionnées d'engrais de manière à ce que la consommation du nitrate par les plantes soit améliorée. Les paramètres du sol déterminés sur le terrain et au laboratoire nous ont permis d'utiliser le modèle mécaniste WAVE et d'évaluer sa capacité de reproduction et de prédiction des données. Les mesures lors des trois saisons aux conditions climatiques différentes ont été reproduites avec satisfaction.