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This study investigates the spatiotemporal variability of major and trace elements, dissolved organic carbon (DOC), total dissolved solids (TDS), and suspended particulate matter (SPM) in surface waters of several hydrosystems of the Loire River watershed in France. In particular, this study aims to delineate the impact of the abovementioned water physicochemical parameters on natural iron and manganese physical speciation (homoaggregation/heteroaggregation) among fine colloidal and dissolved ( 10 nm), colloidal (10-450 nm) and particulate ( 450 nm) phases in Loire River watershed. Results show that the chemistry of the Loire River watershed is controlled by two end members: magmatic and metamorphic petrographic context on the upper part of the watershed; and sedimentary rocks for the middle and low part of the Loire. The percentage of particulate Fe and Mn increased downstream concurrent with the increase in SPM and major cations concentration, whereas the percentage of colloidal Fe and Mn decreased downstream. Transmission electron microscopy analyses of the colloidal and particulate fractions (from the non-filtered water sample) revealed that heteroaggregation of Fe and Mn rich natural nanoparticles and natural organic matter to the particulate phase is the dominant mechanism. The heteroaggregation controls the partitioning of Fe and Mn in the different fractions, potentially due to the increase in the ionic strength, and divalent cations concentration downstream, and SPM concentration. These findings imply that SPM concentration plays an important role in controlling the fate and behavior of Fe and Mn in various sized fractions. Graphical abstract Physical speciation by heteroaggregation of (Fe-Mn) compounds: high [SPM] → [Fe-Mn] particulate faction; low {SPM] → [Fe-Mn] colloid-dissolved fraction.

Successive transport experiments of TiO2 nanoparticles (NP) suspension through fractured hard-rock column were done in laboratory. A low ionic strength (IS) water (0.8-1.3 10-3 M) at pH ∼4.5 was used, corresponding to the chemical composition of groundwater where the rock was collected (Naizin, France). The surface charge of TiO2 NP was positive while that of rock was negative favoring NP deposition. SEM/EDX reveals that NP were retained on a broad distribution of mineral collectors along the preferential flow pathways (i.e., fractures). However, a non-negligible amount of NP (∼10%) was transferred through the rock. Divalent cation (Ca2+) was responsible for the reduction of the negative charge of the rock and thus contributed to limit the NP deposition as attested by DLVO model. Blocking of rock surfaces by NP favored NP transfer while the ripening process and the size exclusion of aggregates decreased NP mobility. Decrease of water flow favored the exchange of solutes from the immobile to the mobile water in the porous medium, which in turn favored the aggregation of the NP and led to their natural attenuation. The result evidences how slight modifications of the environmental conditions can strongly influence the fate of NP in groundwater.

Successive transport experiments of TiO

10 p.; International audience; This study characterises the heavy-metal content in leachates collected from eight landfills in France. In order to identify heavy metal occurrence in the different size fractions of leachates, a cascade filtration protocol was applied directly in the field, under a nitrogen gas atmosphere to avoid metal oxidation. The results of analyses performed on the leachates suggest that most of the metals are concentrated in the

International audience; Water flow and TiO2 nanoparticle (NP) transfer in a fractured hard-rock aquifer were studied in a tracer test experiment at a pilot site in Brittany, France. Results from the Br tracer test show that the schist aquifer can be represented by a two-layer medium comprising i) fractures with low longitudinal dispersivity in which water and solute transport is relatively fast, and ii) a network of small fissures with high longitudinal dispersivity in which transport is slower. Although a large amount of NPs was retained within the aquifer, a significant TiO2 concentration was measured in a well 15 m downstream of the NP injection well, clearly confirming the potential for TiO2 NPs to be transported in groundwater. The Ti concentration profile in the downstream well was modelled using a two-layer medium approach. The delay used for the TiO2 NPs simulation compared to the Br concentration profiles in the downstream well indicate that the aggregated TiO2 NPs interacted with the rock. Unlike Br, NPs do not penetrate the entire pore network during transfer because of electrostatic interactions between NP aggregates and the rock and also to the aggregate size and the hydrodynamic conditions, especially where the porosity is very low; NPs with a weak negative charge can be attached onto the rock surface, and more particularly onto the positively charged iron oxyhydroxides coating the main pathways due to natural denitrification. Nevertheless, TiO2 NPs are mobile and transfer within fracture and fissure media. Any modification of the aquifer’s chemical conditions is likely to impact the groundwater pH and, the nitrate content and the denitrification process, and thus affect NP aggregation and attachment.

International audience; In the coastal multilayer aquifer system of a highly urbanized southern city (Recife, Brazil), where groundwaters are affected by salinization, a multi-isotope approach (Sr, B, O, H) was used to investigate the sources and processes of salinization. The high diversity of the geological bodies, built since the Atlantic opening during the Cretaceous, highly constrains the heterogeneity of the groundwater chemistry, e.g. Sr isotope ratios, and needs to be integrated to explain the salinization processes and groundwater pathways. A palaeoseawater intrusion, most probably the 120 ky B.P. Pleistocene marine transgression, and cationic exchange are clearly evidenced in the most salinized parts of the Cabo and Beberibe aquifers. All 87Sr/86Sr values are above the past and present-day seawater signatures, meaning that the Sr isotopic signature is altered due to additional Sr inputs from dilution with different freshwaters, and water-rock interactions. Only the Cabo aquifer presents a well-delimitated area of Na-HCO3 water typical of a freshening process. The two deep aquifers also display a broad range of B concentrations and B isotope ratios with values among the highest known to date (63-68.5‰). This suggests multiple sources and processes affecting B behaviour, among which mixing with saline water, B sorption on clays and mixing with wastewater. The highly fractionated B isotopic values were explained by infiltration of relatively salty water with B interacting with clays, pointing out the major role played by (palaeo)-channels for the deep Beberibe aquifer recharge. Based on an increase of salinity at end of the dry season, a present-day seawater intrusion is identified in the surficial Boa Viagem aquifer. Our conceptual model presents a comprehensive understanding of the major groundwater salinization pathways and processes, and should be of benefit for other southern Atlantic coastal aquifers to better address groundwater management issues.

International audience; Inter-well tracer tests were conducted at the Soultz-sous-Forets (France) site of the European Hot Dry Rock Program at less than 3900m depth before the year 2000, and at about 5000m depth in the period 2000-2005. The paper discusses the results and conclusions drawn from tracer tests performed in 2000-2005 during hydraulic stimulation operations and a short-term circulation test in wells GPK-2, GPK-3 and GPK-4. The tracer tests were conducted using organic compounds such as Na-benzoate, 1,5-, 2,7-, 1,6- and 2,6-naphthalene disulfonate (nds), and fluorescein. These studies indicated that the tracers behaved conservatively during a long period (at least 5 months for fluorescein and up to 4 years for Na-benzoate and 1,5-nds) under the harsh conditions of the Soultz reservoir (up to 200 ◦C and high fluid salinity). Combined with the monitoring of conservative species such as dissolved chloride, the tracer tests allowed us to estimate the fractions of native geothermal brine and injected fresh water in the fluids discharged during the short-term production tests. These tests consistently indicated that only low amounts of the injected fresh water were recovered and that the proportion of native brine was relatively high in the produced fluids. It was estimated that more than 750,000m3 of native brine mixed with approximately 142,000m3 of fresh water that had been injected since 2000. The mean natural flux for the native brine was estimated at about 1-1.2m3/h. The tracer tests gave evidence of a fast and relatively direct hydraulic connection between GPK-3 and GPK-2 (short loop) but also indicated the existence of another larger and slower hydraulic connection between the wells (large loop). Hydrodynamic characteristics (mean transfer time, swept volume, tracer recovery, andmean fluid velocity) were estimated for both circulation loops. The hydraulic connection between GPK-3 and GPK-4 appeared to be poor. Differences observed between experimental and modeled data at the end of the fluorescein-recovery curves suggest the existence of another quasi-infinite loop connecting both GPK-2 and GPK-4 to GPK-3.

The measurement of gas concentrations and fluxes in the soil and atmosphere is a powerful tool for monitoring geological carbon capture and storage (CCS) sites because the analyses are made directly in the biosphere in which we live. These methods can be used to both find and accurately quantifying leaks, and are visible and tangible data for public and ecosystem safety. To be most reliable and accurate, however, the measurements must be interpreted in the context of natural variations in gas concentration and flux. Such baseline data vary both spatially and temporally due to natural processes, and a clear understanding of their values and distributions is critical for interpreting near-surface gas monitoring techniques. The best example is CO2 itself, as the production of this gas via soil respiration can create a wide range of concentrations and fluxes that must be separated from, and not confused with, CO2 that may leak towards the surface from a storage reservoir. The present article summarizes baseline studies performed by the authors at various sites having different climates and geological settings from both Europe and North America, with focus given to the range of values that can result from near surface processes and how different techniques or data processing approaches can be used to help distinguish a leakage signal from an anomalous, shallow biogenic signal.

International audience; Soil gas and CO2 flux measurements were conducted above and in the vicinity of the Weyburn oil field during the period 2001-2005 and in 2011 to determine baseline values and distributions, and to monitor for surface leaks, above this well-established and intensely studied CO2-Enhanced Oil Recovery (CO2-EOR) project in southern Saskatchewan, Canada. Multiple sites were studied which had sample spacing that ranged from 25 to 200 m, including a 360 point regional grid above the CO2 injection field, a background site off the oil field, and a new site where a landowner claimed CO2 leakage on his property from the storage reservoir. Typically 400-500 points were sampled during each of the seven field campaigns and analysed for a wide range of components, thus yielding a large and varied database collected during different seasons and years. Results show no sign of leakage of the injected CO2. Spatial and seasonal trends and measured values from discrete sampling of soil gas CO2, O2 + Ar, N2, δ13C-CO2, He, Rn, and CH4, from continuous monitoring of soil gas CO2 and Rn, and from discrete sampling of CO2 flux can all be explained by the interplay between near-surface biochemical processes, seasonal environmental conditions, and soil properties. Other light hydrocarbon gases, like C2H4 and C2H6, were generally near or below the instrument detection limit. Lessons learned during the research are described to help improve future near-surface gas geochemistry surveys for site assessment, baseline definition, and leakage monitoring at active CCS sites.

International audience; Natural analogues studies have received much interest over past years through the CO2 capture and storage applications. In this paper we focus on one natural analogue in leakage situation in order to describe the nature of the gas leakage, its interaction with surrounding rocks and aquifers and its behaviour over time. Soil gas and water monitoring techniques are used to gather point and continuous records. Leakage occurs along discrete sections relying on the nature of surface formations and the permeability of discontinuities. Main gas vents are perennial, relative proportions of different gas phases (CO2, 222Rn and 4He) being dependent from the interaction with surface deposits and the distance to main tectonic pathways. Mineral waters were also monitored as they represent integrative bodies that exist above nearly all CCS sites. Constraints on chemical processes occurring at depth are brought by characterisation of the dissolved constituents and by related isotopic systematics. Such determination of mixing processes and their influence is important, as leakage from CCS site may be evidenced through the mixing of the water from the storage complex and overlying aquifers. Longer term monitoring was conducted for physico-chemical parameters highlighting noticeable variations for pH, dissolved oxygen, redox potential and dissolved CO2.

International audience; One of the options considered to mitigate greenhouse gas concentrations in the atmosphere is underground storage of CO2. There is a strong need for enhancing and developing methods that would help throughout the duration life of such underground storage, to ensure the safety and able to monitor the evolution of the injected CO2 plume. Among these, geochemical methods can play an important role. Here, we describe results acquired under the research programme “Géocarbone-Monitoring”, partially funded by the French National Research Agency, on the Montmiral natural analogue in South-Eastern France. Other results obtained under the same research programme in the French Massif Central are reported elsewhere in this volume.Spot sampling methods allowing a great geographical coverage and continuous measurements on selected points were undertaken in 2006 and 2007, in order to determine soil gas concentrations and fluxes as well as carbon isotope ratio determinations. One important result is that without any evidence of deep CO2 leakage, both CO2 concentrations and fluxes appear to be higher than can be explained only by biological activities. Further investigations are thus needed to understand the gas evolution better throughout the year.

International audience; Sodium-K, Na-Ca, Na-Mg, and Na-Sr exchange isotherms were performed at a total Cl concentration of about 0.005 mol L−1 on the fine fraction (

International audience; Na-K, Na-Ca and Na-Mg exchange isotherms were performed on the fine fraction (< 2 µm) of Imt-2 illite samples at a total normality of about 0.005 mol/L in anionic chloride medium. The derived selectivity coefficients for Na-K, Na-Ca and Na-Mg were found to vary as a function of the exchanger composition and compared well with the data collected in the literature for similar experimental conditions. Two models were built to reproduce the data: the first was a multi(2)-site model with constant Gaines and Thomas selectivity coefficients; the second was a one-site model taking into account surface species activity coefficients. The results of the models were in rather good agreement with both our data and literature data. The multi-site model proved to be efficient in predicting the exchanger composition as a function of the Na-Ca-Mg-K concentrations in solution, whereas the one-site model proved to be a better approach to derive the Na-Ca-Mg-K concentrations in solution based on the knowledge of the exchanger composition and the total normality of the solution. The interest of this approach is illustrated by the need for major cation solute concentration predictions in compacted clay for the characterization of nuclear deep disposal host rock repositories.

International audience; An important part of the CO2 capture and storage projects concerns monitoring methods. Here we are focusing on the geochemical monitoring methods that may be deployed at depth to ensure early warning in case of unwanted CO2 leakages from a storage site. Independently from the nature of the reservoir (saline aquifer, depleted oil/gas reservoir), aquifers are ubiquitous in the overlying sedimentary pile. Before deploying water monitoring methods at depth, where long-term deployment is still a challenging task, one must be confident with results that are obtained in surface environments. An ideal way to test such monitoring methods is to deploy them on a site where a CO2 leak is provoked. Pressure effects will certainly not be significant, but deployment in surface environments is a mandatory task to test the real capacity of equipment. This is the case of the Svelvik site located in Norway. Under the auspices of the EUROGIA+ CO2FieldLab project, this site has been instrumented by French, English and Norwegian research teams in order to monitor a CO2 leak using both geophysical and geochemical approaches. Here we focus on the water monitoring methods on these heterogeneous fluvio-glacial deposits with stratified water bodies. Water stratification is a consequence of the presence of infiltrating surface waters, groundwaters, and brackish to saline waters that are influenced by the close Drammen fjord. The leak was performed at a 20 m depth at the center of a 20 m x 20 m monitoring grid. Water was monitored at 4 different depths, from -15m to -2m. It was also possible to monitor more distant water wells (-5 to -6m depth). Monitoring included low flow water pumping for all the wells and boreholes that allowed following, at a short time scale, physico-chemical parameters of the water (temperature, electrical conductivity, redox potential, dissolved oxygen and pH). Frequent sampling was operated in order to both determine alkalinity (expressed as bicarbonate content) and to get water for laboratory analyses (dissolved elements and isotopes). Gas contents were also quantified on site by gas chromatography analysis. The influence of the CO2 leak was mainly noticeable on the eastern part of the site, as a result of a skewed injection plan compared to the initial one. Changes were evaluated by comparison with 1 week baseline data acquired prior to the CO2 injection. The CO2 influence was observed at all depths. The magnitude of the induced variations depended upon both the depth and the time of sampling. The main changes concerned water pH with a decrease of up to 2 pH units, electrical conductivity with a 3 to 4 times increase, and bicarbonate contents with again a 3 to 4 times increase. These parameters, plotted as a function of time, allowed, in the most favorable cases, to perfectly identify the CO2 breakthrough that occurred between 2 to 7 days after the beginning of the injection, depending on the sampling location. Similarly, dissolved gas showed a significant modification of abundance ratios, O2 and N2 species being replaced by CO2 during the paroxysm of the leak. Laboratory analyses performed on one of the most affected level (-10 m depth, eastern part of the site) revealed strong modifications of both major and trace elements contents. Apart from strong a bicarbonate increase, Ca and Fe species did also display an increase of their contents, while Cl and Mg contents were less abundant as a result of local water chemistry and geology. More importantly, regarding the water quality thematic, remobilization of some undesirable trace elements such as Mn or As was also observed together with strong enrichment factors (up to x60 for Mn). Such enrichments were nevertheless only observed during the paroxysm of the leak, going back to lower values once the leak was stopped. Concentration decreases were element-dependent and more rapid for trace elements than for major elements. In parallel with these spot acquisitions, continuous recording of water physico-chemical parameters was done in 1 surface well (-2 m depth) using an Idronaut CTD probe. As this equipment was not deployed where the leak showed a strong influence, it did not allow to monitor significant (e.g. pH) changes. Nevertheless, surprisingly, all parameters were affected by the leak as their records exhibited noisy signals even if mean values remained comparable to those determined during baseline acquisitions. For example, the mean pH value was 7.28 ± 0.03 pH units during the baseline; during the leak, the mean pH value is similar (7.37) but with a greater standard deviation (± 0.33 pH unit). Such records are in agreement with continuous gas records performed at the same location. During the leak, gas records did not show significant deviations from baseline data, but carbon isotope ratios (13C) were clearly modified due to the injection process. This is crucial for monitoring protocols that will ensure the safety of future CO2 storages. Care should be taken as a mean value may not be changed as such, but deviation from this mean value may.