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Arsenic (As) is one of the main toxic elements of geogenic origin that impact groundwater quality and human health worldwide. In some groundwater wells of the Sologne region (Val de Loire, France), drilled in a confined aquifer, As concentrations exceed the European drinking water standard (10 µg L-1). The monitoring of one of these drinking water wells showed As concentrations in the range 20-25 µg L-1. The presence of dissolved iron (Fe), low oxygen concentration and traces of ammonium indicated reducing conditions. The δ34SSO4 was anticorrelated with sulphate concentration. Drilling allowed to collect detrital material corresponding to a Miocene floodplain and crevasse splay with preserved plant debris. The level that contained the highest total As concentration was a silty-sandy clay containing 26.9 mg kg-1 As. The influence of alternating redox conditions on the behaviour of As was studied by incubating this material with site groundwater, in biotic or inhibited bacterial activities conditions, without synthetic organic nutrient supply, in presence of H2 during the reducing periods. The development of both AsV-reducing and AsIII-oxidising microorganisms in biotic conditions was evidenced. At the end of the reducing periods, total As concentration strongly increased in biotic conditions. The microflora influenced As speciation, released Fe and consumed nitrate and sulphate in the water phase. Microbial communities observed in groundwater samples strongly differed from those obtained at the end of the incubation experiment, this result being potentially related to influence of the sediment compartment and to different physico-chemical conditions. However, both included major Operating Taxonomic Units (OTU) potentially involved in Fe and S biogeocycles. Methanogens emerged in the incubated sediment presenting the highest solubilised As and Fe. Results support the hypothesis of in-situ As mobilisation and speciation mediated by active biogeochemical processes.

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Due to global warming and local anthropogenic pressures, sustainable groundwater resource exploitation in coastal cities is increasingly threatened. For example, the fifth largest Brazilian city, Recife, is considered as a representative hot spot for these issues and illustrates the great challenges facing many urban areas in the southern hemisphere. There, recharge as well as surface water and groundwater quality are altered by frequent droughts and poorly planned environmental management since decades. To maintain access to water, thousands of private wells were dug in order to pump water from the multi-layered aquifer system found under the city. This massive exploitation is causing a chronic lowering of the water levels, as well as seawater intrusion and contaminations by wastewater or polluted surface waters. Through hydrochemical characterization, mainly Cl/Br ratio and Cl concentrations, of wells sampled throughout the metropole, this study first characterizes the main environmental impacts on the resource, i.e. waste waters and seawater. Combining this evaluation with lithological, land-use and socio-environmental data, it was then possible to build decision trees identifying combinations of multiple factors possibly having an impact on contamination types. The well and population densities, the waste and sewage management, as well as the absence of sanitary facilities in houses appeared as critical parameters to target in order to reduce the risk of contamination of the water resource and ensure its preservation. Based on these factors, we created a risk map for contamination types that should help in identifying areas where groundwater resource may present an environmental (and then health) issue for people. Besides, this study shows that the combination of hydrochemical, geomorphological and socio-environmental characterizations of these urban systems featuring very contrasted situations between neighborhoods is a relevant tool to propose further groundwater management strategies.

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The Chalk aquifer used for drinking-water production in the southwest of the Lille European Metropolis is threatened by the presence of chlorinated aliphatic hydrocarbons (CHCs), their concentrations in groundwater regularly exceeding the regulatory limits for drinking water in France. This hinders its use for drinking-water production. Understanding the dynamics and spatial distribution of CHC in the aquifer is a key factor for resource sustainability. For that purpose, an intensive monitoring was undertaken in several well fields and at different depths over eight years. To assess a possible migration and/or degradation of the compounds, the water column in several wells was sampled at various depths with passive samplers. Furthermore, CHC degradation mechanisms were investigated with compound-specific carbon-isotope analysis. The CHC concentrations and their distributions in the area depend on past and current industrial activity, causing plumes emphasized by pumping in the wells, such plumes being multi-source with no identified origin in most wells. In the south area of Les Ansereuilles, reductive dechlorination of tetrachloroethylene from a former industrial laundry highly impacted the surrounding area with its main degradation product cis-1,2-dichloroethylene. The same area is also affected by tetrachlroroethylene from several industrial laundries, textile factories and dyeing industries with also an anaerobic degradation. In the northern part of Les Ansereuilles, tetrachloroethylene, trichloroethane, trichloroethylene and 1,1-dichloroethylene were found as primary products, whereas cis-1,2-dichloroethylene appears to be an anaerobic degradation product of TCE. The other well fields (Houplin-Ancoisne, Seclin and Emmerin) are less impacted by CHC pollution, and it was shown that no CHC degradation occurred in the wells. However, the stratification of CHCs in the well-water columns, their constant concentration values over time caused by the large amount of available CHCs, and the minor degradation occurring in wells are of concern for water operators in the future.

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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.

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Coastal water resources are a worldwide key socio-environmental issue considering the increasing concentration of population in these areas. Here, we propose an integrative transdisciplinary approach of water resource, water management and water access in Recife (NE Brazil). The present-day water situation is conceptualized as an imbricated multi-layered system: a multi-layered water resource, managed by a multi-layered governance system and used by a multi-layered social population. This allows identifying processes of quantitative, qualitative, and sanitary conflicts between governance and population strategies regarding water supply, as well as the institutional and individual denials of these conflicts. Based on this model, we anticipate future water-related problematic fates. Concerning the water resource system, the rapid groundwater level decrease due to unsustainable water predatory strategies, and the very low recharge rate have drastically modified the aquifer system functioning, inducing hydraulic connection between shallow groundwater (contaminated and locally salty) and deep ones (mostly fresh, with local inherited salinity), threatening the deep strategic water resource. Concerning the water governance system, the investments to increase the capacity storage of surface water, the water regulation agencies and the public/private partnership should shortly improve the water supply and wastewater issue. Nevertheless, the water situation will remain highly fragile due to the expected water demand increase, the precipitation decrease and the sea-level increase. Concerning the water access system, the population variably perceives these current and further effects and the possible mitigation policies, and develops alternative individual strategies. Authorities, policymakers and water managers will have to implement a well-balanced water governance, taking into account the specificities of the PPP, public and private groundwater users, and with a strong political willingness for a sustainable water management to ensure water supply for all the population. In other words, an anticipatory and integrated vision is necessary to reduce the discrepancies in this complex system.

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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 15m 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.

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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 paleoseawater intrusion, most probably the 120 kyB.P. Pleistocene marine transgression, and cationic exchange are clearly evidenced in the most salinized parts of the Cabo and Beberibe aquifers. All (87)Sr/(86)Sr 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 behavior, 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 the 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.