Trace metals dynamics in surface sediments investigated by DGT micro-scale measurements

International audienceIn surface sediments, metal mobility is controlled by the recruitment and turn-over of organic matter whereas sulphide is thought to control the concentration of metals in sediment pore water by removing them from the solution. DGT is a dynamic probe that measures the kinetically available fraction of metals or sulphide. DGT uses a credit card size probe inserted into the sediment that provides a snapshot of the metal distribution in the sediment, which can be uncovered by spectrochemical analytical techniques. In-situ vertical profiles and horizontal maps of trace metals at high (mm scale) and ultra-high resolution (100 $\mu$m) together with Fe, Mn and sulphide were generated from DGT probes deployed in surface sediments. Collectively, the results showed that besides vertical gradients, associated with the depletion of oxygen with depth and the degradation of organic matter by a succession of electron acceptors, small scale remobilisation of metals associated with sediment heterogeneity take place

Role of the sediments in scavenging inorganic contaminants in the Syr Daria River and the Small Aral Sea (Kazakhstan)

9-11 july 2008International audienceThis study is focused on the measurement and the behaviour of the inorganic pollutants in the Suspended Particulate Material (SPM) and the Bottom Sediments (BS) of the Syr Daria River in its Kazakh course, including its outlet in the Small Aral Sea. Two field campaigns were carried out during the low and the high water period. The results display that the current Syr Daria River sediments quality is influenced by several phenomena seasonally and locally controlled: carbonate precipitation, absorption by phytoplankton, scavenging by clay mineral, upstream ground leaching during the snow melting, various discharges (drainage canals, urban and industrial waste waters), connection with delta lakes during the flooding period and impact of the geochemical background. The study of the solid – liquid partition coefficients emphasizes the contrasted behaviour of trace elements with respect to different scavenging phases. As a result it appears that in spite of the mitigating action of the dam reservoirs, of the irrigated areas, of the overflow lakes and flood plains, the Aral Sea sediments are not completely preserved from the contamination

Application of an inverse neural network model for the identification of optimal amendment to reduce Copper toxicity in phytoremediated contaminated soils

International audienceArtificial neural network ANN prediction approaches applied to the modeling of soil behavior are often solved in the forward direction, by measuring the response of the soil (outputs) to a given set of soil inputs. Conversely, one may be interested in the assessment of a given set of soil inputs that leads to given (target) soil outputs. This is the inverse of the former problem. In this study, we develop and test an inverse artificial neural network model for the prediction of the optimal soil treatment to reduce copper (Cu) toxicity assessed by a given target concentration of Cu in dwarf bean leaves (BL) from selected soil inputs. In this study the inputs are the soil pH, electrical conductivity (EC), dissolved organic carbon (DOC) and a given target toxicity value of Cu, whereas the output is the best treatment to reduce the given toxicity level. It is shown that the proposed method can successfully identify the best soil treatment from the soil properties (inputs). Two important challenges for optimal treatment prediction using neural networks are the non-uniqueness of the solution of the inverse problem and the inaccuracies in the measurement of the soil properties (inputs). It is shown that the neural network prediction model proposed can overcome both these challenges. It is also shown that the proposed inverse neural network method can potentially be applied with a high level of success to the phytoremediation of contaminated soils. Before large-scale application, further validation is needed by performing several experiments and investigations including additional factors and their combinations to capture the complex soil behavior

Influence géomorphologique et structurale des sols sur le report hydrique : Conséquences sur le fonctionnement hydrique des sols et la biomasse en zone soudano-sahélienne

21-22 nov. 2007Cette étude démontre qu'en zone soudano-sahélienne, en année à pluviosité déficitaire, à différentes échelles, plusieurs facteurs physiques, chimiques et anthropiques des sols régissent le report hydrique, part bénéfique du ruissellement représentant "une irrigation naturelle, complémentaire et simultanée à la pluie qui l'a générée en fonction des conditions topographiques et micro morphologiques avec transfert complémentaire de fertilité ". En condition pluviale, l'encroûtement décroissant le long du versant favorise l'infiltration du report hydrique dans les unités de sol en bas de pente. A l'échelle du champ, il est encore maximisé par le micromodelé concave, par une compaction profonde moins forte et par le labour. L'augmentation des nutriments est de 13,4 à 10% et en éléments fins de 16,3 à 12,5% respectivement sous labour et grattage. Cette maximisation du report hydrique provoque un drainage significatif très localement. Sur les périmètres irrigués la salinisation renforce les facteurs de formation des croûtes et le ruissellement, réduisant drastiquement la capacité d'infiltration des sols. Il en résulte une limitation des réserves en eau utile malgré l'irrigation non contraignante. Ceci provoque une sécheresse édaphique. Dans ces deux conditions culturales cette baisse drastique des stocks hydriques due à la formation d'un encroûtement conduit à la réduction voire à l'annulation du rendement des cultures. Ces résultats montrent la difficulté qu'ont les chercheurs à expliquer la complexité du comportement et du fonctionnement hydrique sur l'ensemble d'un versant et à l'intérieur du champ pour comprendre la variabilité des rendements. L'identification, à l'échelle de la parcelle et sur le versant, des causes du ruissellement et du report hydrique et d'en mesurer l'effet sur la modification des stocks hydriques sur de faibles distances est nécessaire pour conceptualiser un aménagement raisonné, innovant et efficace des flux de surface

Rôle de l'encroûtement superficiel des sols sur leur fonctionnement hydrique : Conséquences hydrologiques et hydro-pédologiques

National audienceTous les sols, du climat tempéré à tropical, sont susceptibles de s'encroûter superficiellement pour de très larges conditions d'utilisation et d'environnement. Des études à différentes échelles montrent que ces croûtes contrôlent les flux hydriques superficiels des éco(agro)systèmes. En zone soudano sahélienne, elles sont à l'origine de la variabilité, à différents niveaux scalaires, des transferts hydriques, de fertilité, de matériaux érodés et de la salinité fortement modifiée par le changement climatique. Sur les périmètres irrigués, la formation des croûtes sous les pluies et sous le ruissellement est grandement accélérée par la salinisation croissante due à une eau du fleuve Niger déséquilibrée dans le pôle alcalin (Niger) ou de plus en plus chargée en sels dans le fleuve Syr Daria (Kazakhstan). L'encroûtement provoque alors la sécheresse édaphique avec chute des rendements. Aucune classification taxonomique pédologique ne prend en compte l'existence de ces croûtes et de leur effet hydropédologique ni hydrologique. On comprendra le parti que pourrait tirer la recherche pédologique et agronomique par la refondation d'une nouvelle classification taxonomique des sols prenant en compte ces processus en accord avec leur susceptibilité intrinsèques à s'encroûter

Aquifer Biothermoremediation using Heat Pumps : sound theoretical basis and results on thermal, geochemical and biological impacts on aquifers

International audienceGeothermal heat pumps have been operated on aquifer over the past thirty years. Currently, these systems constitute a major sector of renewable energy development. However, this development is accompanied by several difficulties at scientific, technical and administrative levels, to describe short and long term interactions with aquifers, at both the local and regional scale, especially close to urbanized or industrialized areas. As a typical example, the long-term use of groundwater heat pumps for air conditioning of homes or buildings can induce significant increases in temperature of the aquifer, if, for given groundwater flow direction and rate, there is an imbalance between the demands for cooling and heating. Therefore, the aquifer (groundwater and sediments) can be affected. In fact, it is expected that a wide variety of geochemical reactions and microbiological changes with variable intensity should occur which depends on the sediment characteristics and hydrogeological background. The presence of organic pollutants in the aquifer can amplify these phenomena. The predictable results may progressively be: (i) precipitation, which reduces the porosity of the aquifer and/or the well productivity, (ii) an inappropriate temperature for the use of groundwater heat pumps for air conditioning, but also and especially (iii) pollutant bioremediation, which has a beneficial impact on the aquifer. Under certain conditions, this bio-thermo-remediation can be considered as an attractive option for some contaminated aquifers, taking into account the fact that, administratively in European countries, any action is subject to authorization. Notably in the framework of the collaborative project BIOTHERMEX (French Region Centre, convention n° 200800034163), exploratory studies were focused on three objectives: i) Improving the understanding of the concept of bio-thermo-remediation of aquifers by using effects induced by geothermal heat pumps. ii) examining realistic configurations, on a laboratory scale, pilot-scale and in-situ, in order to identify the most determining factors for possible (undesirable or helpful) aquifer impacts, e.g. temperature increase, bacterial development, organic pollutants degradation iii) evaluating if those configurations can present interests for bio-thermo-remediation of polluted aquifers by using heat pumps. A literature survey led to the consolidation of the basic principles of bio-thermo-remediation of most organic pollutants: increase in volatilization, solubility, degradation kinetics and mobility of the residual pure phases by reduction of their viscosity. The different heat pump systems and processes are discussed in the article, to identify what is suitable for bio-thermo-remediation. To reproduce the underground thermal conditions encountered in groundwater heat pumps systems (temperature range of water production from 10 to 25 °C, temperature range of water injection between 20 and 35 °C), batch-experiments, at laboratory and pilot scale have been carried out to investigate the impacts of the different parameters on the geochemical equilibrium (solubility, mobility, precipitation, dissolution, volatilization,) on the aquifer inorganic content and organic pollutants as well as on microbial populations activities. Two different sediments were used. Depending on the nature of the sediments and the water composition, the impacts are more significant: (i) thermally, with a low water flow rate, (ii) geochemically, with the high bicarbonate and Ca/Mg contents in water and (iii) biologically, with a organic contaminants contents in water. Some results from in-situ measurements (especially with an aquifer polluted by chlorinated solvents) are used to corroborate the conclusions. Even if the most part of the work is still in progress, the obtained results are validated by biogeochemical modelling. The combination of a low-enthalpy geothermal system using aquifer heat pumps (water pumping and re-injection) and remediation approaches appears to be an appropriate method to treat aquifers polluted by organic contaminants without use of any other way than the thermal energy

Variabilité structurale et physico-chimique des croûtes superficielles des sols de la vallée du Syr Daria (Kazakhstan)

International audienceToutes les régions arides et semi-arides sont le lieu de nombreux types de croûtes physiques et micro ou cryptobiologiques (Afrique, Israël, Australie, USA, Chili). Les micro horizons durs formés sur la surface du sol sont le résultat de processus naturels principalement l'impact des gouttes de pluies, le ruissellement et le dessèchement et anthropiques tel que l'irrigation et le travail du sol. Une prospection réalisée au Kazakhstan, en 2003, a démontré, pour la première fois l'existence d'un tel encroûtement sur les sols cultivés ou non de la vallée du Syr Daria. L'étude de ces croûtes fait apparaître leur extrême variabilité physique, chimique et microbiologique. Toutefois le milieu spécifique du Kazakhstan conduit à la formation de nombreuses variantes et même à celle de croûtes apparemment spécifiques. L'analyse en composantes principales fait apparaître que seulement 4 variables sont suffisantes pour expliquer la variabilité des sites (longitude, épaisseur et largeur des fentes et couvert végétal). Les trois premiers axes expliquent 92,7% de l'inertie totale des données. Le premier axe, climatique, explique 54,4% de la variance des échantillons et le second, physique, explique 27,7%. Les quatorze échantillons de croûtes étudiés sont très bien classés par ces quatre facteurs physiques, géographiques et biotiques (R=0,90). Les analyses au RX indiquent une forte salinité de ces croûtes qui croit vers l'ouest, à l'exemple des sols. Les images microscopiques (MEB et classique) montrent une forte hétérogénéité structurale avec des micropores variables d'un site à l'autre. Les mesures porosimétrique confirment bien cette variabilité qui traduit les caractéristiques spécifiques de cette région. Cette variabilité des croûtes confère aux champs une variabilité des conditions hydrologiques, géochimiques et d'érodibilité qui devront être prises en compte dans le cadre de réhabilitation des sols des périmètres irrigués comme dans le contrôle d'un pâturage rationnel. Ces premiers résultats montrent que les politiques d'aménagement des périmètres irrigués et des zones de parcours d'élevage doivent intégrer le rôle spécifique des diverses croûtes en fonction de l'importance de leur effet dans la stabilité du milieu. Pour identifier ce rôle, leur étude doit être entreprise de façon systématique car leur présence est généralisée et dominante. Pour ce faire il faut démarrer une étude des mécanismes d'encroûtement et de la salinisation pour la compréhension et la prédiction des processus de ce type de dégradation des différents agro/écosystèmes

Sustainable Irrigated Agricultural Production of Countries in economic Transition: Challenges and Opportunities (A case study of Uzbekistan, Central Asia)

For the fulfillment of the thirsty ambition of self-sufficiency of the Soviets for cotton production, the arid Central Asian region and in particular Uzbekistan has been extensively exploited. In fact, vast tracts of deserts have been converted into irrigated agricultural lands without proper consideration to environment and technical standards. As a result trends in natural resource degradation (soil salinity, desertification, water quality) as well as declining crop yields have dramatically increased. The agricultural sector is the backbone for employment, food security and export revenues of the Central Asian countries. Since the independence of the Central Asian countries (after the breakup of the former Soviet Union) the situation has changed dramatically in terms of institutional, political and technical systems. Political transition, which is defined as a shift from once planned centralized economy to a market-driven one, has introduced 'new' concepts like land tenure, water rights and different kinds of ownership. All of such transformations have impacted the agricultural production in Central Asia. The institutional change can be described as decentralization of the farming systems i.e., transition from the former state collective farms into the smaller forms of private farms. The institutional interventions are aimed to increase agricultural production through improving water management. It is arguable that private production systems are the most effective business driven forces but the situation is quite different in Central Asia due to the irrigated agriculture. The biggest challenge for a sustainable irrigated agricultural production lies in the recent reforms of water management sector in Central Asia and Uzbekistan. The water users associations have been established for replacing the former collective farming systems for irrigation water distribution and maintenance of irrigation infrastructures at on-farm level. The intention of the national government was to shift the operation, maintenance and management of irrigation infrastructures to non government institutions (decentralization). However, these institutions have not fulfilled their promising tasks because of i) a rapid increase of number of private farms along canals; ii) the cropping structure is mosaic with different crop water requirements against the former monoculture; iii) a poor financial, trained and technical capacities of new established institutions; iv) a state ordered agricultural production quota system (for cotton and wheat). This paper analyzes the historical aspects of transformation in the farming production institutions in Central Asia with special focus on Uzbekistan and comprehensively overviews the main current challenges facing the farming system and potential opportunities for reversing the situation

Remobilisation of uranium from contaminated freshwater sediments by bioturbation

International audiencePrevious studies have demonstrated that benthic macro-invertebrate bioturbation can influence the remobilization of uranium initially associated with freshwater sediments resulting in a high release of this pollutant through the overlying water column. Giving the potential negative effects on aquatic biocenosis and the global ecological risk, it appeared crucial to improve our current knowledge concerning the uranium biogeochemical behaviour in sediments. The present study aimed to assess the biogeochemical modifications induced by Tubifex tubifex (Annelida, Clitellata, Tubificidae) bioturbation within the sediment permitting to explain such a release of uranium. To reach this goal, uranium distribution between solid and solute phases of a reconstructed benthic system (i.e. in mesocosms) inhabited or not by T. tubifex worms was assessed in a 12 day laboratory experiment. Thanks notably to fine resolution (mm-scale) measurements (e.g. DET gels probes for porewater, bioaccumulation in worms) of uranium and main chemical species (iron, sulfate, nitrate, nitrite), this work permitted (i) to confirm that the removal of bottom sediment particles to the surface through the digestive tract of worms greatly favours the oxidative loss of uranium in the water column, and (ii) to demonstrate that both uranium contamination and bioturbation of T. tubifex substantially influence major microbial-driven biogeochemical reactions in sediments (e.g. stimulation of denitrification, sulfate-reduction and iron dissolutive reduction). This study provides the first demonstration of biogeochemical modifications induced by bioturbation in freshwater uranium-contaminated sediments

Phytoremediation of polluted soils : recent progress and developments.: Phytostabilisation of a copper contaminated topsoil aided by basic slags: assessment ofCu mobility and phytoavailability

International audiencePurposeBasic slags are alkaline by-products of the steel industry with potential properties to ameliorate nutrient supply and metal stabilisation in contaminated soils. This study aimed at investigating the potential of a P-spiked Link Donawitz slag and a conventional basic slag called Carmeuse for the aided phytostabilisation of a Cu-contaminated soil at a wood treatment site. The effects of basic slag addition on Cu fractionation, mobility and (phyto) availability were assessed.Materials and methodsBoth slags were incorporated at 1 % w/w into the Cu-contaminated soil phytostabilised with Cu-tolerant plants, using either outdoor lysimeters or a field plot. Untreated soil (UNT), amended soils with the P-spiked Link Donawitz slag (PLDS) and the conventional slag (CARM) respectively, and a control soil (CTRL) were sampled, potted and cultivated with dwarf bean. Physico-chemical analysis, determination of total soil elements and a Cu-sequential extraction scheme were carried out for all soils. Physico-chemical characteristics of soil pore water and Cu speciation (rhizon, ion selective electrode and diffusive gradient in thin film (DGT)) were determined. Shoot dry weight yield and leaf ionome (i.e. all inorganic ions present in the primary leaves) of dwarf beans were investigated.Results and discussionThe slag incorporation at only 1 % w/w increased the soil pH from 1.5 to 2 U and electrical conductivity in soil pore water by three times. The residual Cu fraction increased for both slag amended soils compared to the UNT soil by six times in parallel to the decrease of the Cu oxidisable fraction (1.5 times) and to a less extent the reducible fraction. The incorporation of both slags did not reduce the total dissolved Cu concentration in the soil pore water but significantly reduced the real dissolved Cu concentration ca five times, the Cu labile pool as measured by DGT (at least two times) and the Cu phytoavailability. The dwarf bean total biomass was also improved with the slag addition especially for the P-spiked Linz–Donawitz slag.ConclusionsThe addition of both slags in the contaminated soil increased Cu concentration in the residual fraction and thus reduced its potential mobility. Though the total dissolved Cu soil pore water concentration remained identical, its speciation changed as the real dissolved fraction diminished and lowered the Cu bioavailability. The addition of small amount of P-spiked Linz–Donawitz and Carmeuse slags was beneficial for this Cu-contaminated soil in the context of aided phytostabilisation