The transport of uranium through a sandy podzolic soil has been investigated in the field and in ... more The transport of uranium through a sandy podzolic soil has been investigated in the field and in column experiments. Field monitoring, numerous years after surface contamination by depleted uranium deposits, revealed a 20 cm deep uranium migration in soil. Uranium retention in soil is controlled by the <50 microm mixed humic and clayey coatings in the first 40 cm i.e. in the E horizon. Column experiments of uranium transport under various conditions were run using isotopic spiking. After 100 pore volumes elution, 60% of the total input uranium is retained in the first 2 cm of the column. Retardation factor of uranium on E horizon material ranges from 1300 (column) to 3000 (batch). In parallel to this slow uranium migration, we experimentally observed a fast elution related to humic colloids of about 1-5% of the total-uranium input, transferred at the mean porewater velocity through the soil column. In order to understand the effect of rain events, ionic strength of the input solu...
Prévoir l'évolution future du climat passe par une meilleure compréhension des processus d'échang... more Prévoir l'évolution future du climat passe par une meilleure compréhension des processus d'échange de matière et d'énergie entre l'atmosphère, l'océan et la biosphère continentale. Ces deux derniers jouent également un rôle essentiel dans le cycle du carbone en absorbant plus de la moitié du dioxyde de carbone émis par les activités humaines.
Radon-222, a radioactive gas naturally produced in the Earth's crust, informs us about the migrat... more Radon-222, a radioactive gas naturally produced in the Earth's crust, informs us about the migration of fluids and is sometimes considered as a potential earthquake precursor. Here we investigate the effects of mechanical and thermal damage on the radon emanation from various granites representative of the upper crust. Radon concentration measurements performed under triaxial stress and pore fluid pressure show that mechanical damage resulting from cycles of differential stress intensifies radon release up to 170 ± 22% when the sample ruptures. This radon peak is transient and results from the connection of isolated micropores to the permeable network rather than new crack surface creation per se. Heating to 850°C shows that thermal fracturing irreversibly decreases emanation by 59-97% due to the amorphization of biotites hosting radon sources. This study, and the developed protocols, shed light on the relation between radon emanation of crustal rocks, deformation, and pressure-temperature conditions.
The Cretaceous Chalk of North-western Europe exhibits a double porosity (matrix and fracture) pro... more The Cretaceous Chalk of North-western Europe exhibits a double porosity (matrix and fracture) providing pathways for both slow and rapid flow of water. The present study aims at understanding and predicting the contaminant transfer properties through a significant section of this formation, with a particular emphasis on diffusion. This requires to study the nature of porosity and to perform diffusion experiments in representative samples using uranium and tritiated water (HTO), respectively taken as a reactive tracer and an inert one. The diffusive parameters, i.e. the accessible porosity and the effective diffusion coefficient were determined. Additional information was obtained with mercury porosimetry, gravimetric water content, textural and mineralogical characterization. The diffusion tests performed with HTO appear to be the best method to measure the total accessible porosity in any type of porous media, especially those having large pore size distributions. Our study demonstrates that classical gravimetric water content measurements are not sensitive to the reduction in pore size as opposed to HTO diffusion tests because capillary water is not extracted by conventional gravimetric method but can still be probed by diffusion experiments. We found effective diffusion coefficients D e (U (VI) ) near 4 Â 10 À10 m 2 s À1 . The slower migration of U (VI) compared to HTO indicates sorption, with R d (U (VI) ) from 100 to 360 mL g À1 . These values are one order of magnitude larger than other determinations of the U (VI) sorption coefficient because only the matrix porosity is concerned here. The migration of U (VI) in chalk is only limited by sorption on ancillary Fe-Pb-bearing minerals. Transport of HTO and U (VI) is independent of the porosity distribution. Uranium diffusion in the chalk matrix porosity is fast enough to allow the total invasion of the pore space within characteristic time scales of the order of 1000 years. This results in a partitioning of uranium velocities in fracture flow and matrix flow proportionally to the respective fracture and matrix porosities.
h i g h l i g h t s ␦ 34 S SO4 vs. [As] and ␦ 18 O SO4 vs. As V /As III correlations provide new ... more h i g h l i g h t s ␦ 34 S SO4 vs. [As] and ␦ 18 O SO4 vs. As V /As III correlations provide new tools even for low [As]. Long-term and high-resolution monitoring shows droughts enhance pyrite dissolution. Major effect of 2003 European heatwave on pyrite dissolution and [As] increase.
First events of illicit trafficking of nuclear and radiological materials occurred 50 years ago. ... more First events of illicit trafficking of nuclear and radiological materials occurred 50 years ago. Nuclear forensics expertise are aiming at determining the use of seized material, its industrial history and provenance (geographical area, place of production or processing), at assisting in the identification and dismantling of illicit trafficking networks. This information is also valuable in the context of inspections of declared facilities to verify the consistency of operator's declaration. Several characteristics can be used to determine the origin of uranium ore concentrates such as trace elemental impurity patterns (Keegan et al., 2008 ; Varga et al., 2010a Varga et al., , 2010b or uranium, oxygen and lead isotopic compositions (Tamborini et al., 2002a(Tamborini et al., , 2002b Wallenius et al., 2006; Varga et al., 2009). We developed analytical procedures for measuring the isotopic compositions of uranium (234U/238U and 235U/238U) and oxygen (18O/16O) and levels of elemental impurities (e.g. REE, Th) from very small amounts of uranium ore concentrates (or yellow cakes). Micrometer particles and few milligrams of material are used for oxygen isotope measurements and REE determination, respectively. Reference materials were analyzed by mass spectrometry (TIMS, SF-ICP-MS and SIMS) to validate testing protocols. Finally, materials of unknown origin were analyzed to highlight significant differences and determine whether these differences allow identifying the origin of these ore concentrates.
With the increase of contaminant flux of radionuclides in the environment, there is a need to und... more With the increase of contaminant flux of radionuclides in the environment, there is a need to understand and model the processes that control the distribution of uranium and its daughter products during transport within aquifers. We have used U-series disequilibria as an analogue for the transport of anthropogenic nuclides in a small watershed of a chalk aquifer in Eastern France. We have measured naturally occurring U and Th isotopes in rock and water phases in order to determine transport mechanisms at water/rock interface and quantify parameters controlling the migration of radionuclides. The studied aquifer has a double porosity and is characterised by a saturated matrix up to 40m above the water-table and variably saturated fractures. Measurements of U and Th isotopic composition in the water phase were performed on a MC-ICPMS which enable in particular high precision measurements of (230Th/232Th) activity ratio. Water samples were obtained from wells both in low and high water-table conditions. Chalk samples were collected at the same wells and were analysed also for U-series and Sr isotopes. We observed seasonal precipitation of uranium ((234U/238U) activity ratio above 1) in the zone of water-table fluctuation linked with the oversaturation of water with respect to calcite during low water-table period. (230Th/232Th) activity ratios in water are lower than in the substratum meaning that the time of water infiltration is not long enough to allow an equilibration of groundwater with rock. Water acquires its Th isotopic composition when interacting with another phase of the aquifer (soil) and joins the water-table mainly through fractures. The complete data set obtained on solid and liquid phases of the aquifer allow us mass balance calculations and to model the behaviour of uranium and thorium during transport. The 1-D model we have developed considers a transient behaviour of radionuclides in the aquifer and for the first time allows the calculation of the evolution of rock composition with time. We have then calculated a global uranium weathering rate within the aquifer of 2.5 10-6 y-1.
Solute transport in fractured porous media presents a large variability in space and time due to ... more Solute transport in fractured porous media presents a large variability in space and time due to variations in water content, in flow pathway and regime, and chemical reactions. The dynamic of these systems in response to external stimuli such as meteorological, mechanical, hydrogeological and chemical disturbances remains poorly known. To build reliable predictive models, there is a strong need to obtain data from complex but well characterized natural observatories. We set up an experiment in a 150 m long tunnel where drippings give access to ground water flowing vertically from 50 m of variably saturated fractured gneiss. This tunnel is located close to and above the artificial Lake Roselend (France) where large variations in level induce reproducible deformation and hydrogeological disturbances. This area is also characterized by contrasted infiltration regimes with alternating snow, rain, and dry periods. In addition, we performed a tracer test with the surface injection of NaCl, 50 m vertically above the dripping water collecting points. In order to get water representative of different contributions of matrix porosity and fractures, we monitored several zones with different structures and flow rates, and looked for changes in flow rates and ion concentrations in water over several years. In addition, we monitored in the tunnel atmosphere the radon activity, independently known to respond to deformation events. Water composition reflects a yearly equilibrium. In spring, concentrations decrease by inputs of rain and melted snow along fractures at high flow rates with limited reactivity. In summer, concentrations increase by diffusion of saline water from matrix porosity and increased dissolution at low flow rates. We observed transient increases of SO4 and Mg concentrations associated with radon bursts, and that are not related to meteorology. Transient enhancement of conductance, with discharges of saline water and radon from the matrix porosity to the fractures, is thus a mechanism able to produce both ground-water and gas anomalies in response to hydrogeological or mechanical processes such as increases in pore pressure or changes in crack geometry. Before the tracer injection, Na, Mg, Ca and SO 4 are contributed by the host rock, whereas Cl and K are contributed by meteoric sources, soils and surface biota. After the surface injection of tracers, NaCl breakthrough (Na being late relative to Cl) is associated with an important release of K, Ca, Mg, Ba, and F that were not injected and are contributed by the host rock by ion exchange reactions. By contrast, SO4 is generated by oxidative dissolution of pyrite enhanced by air entry on drought regime and subsequent flush on recharge. Natural and artificial stimuli, even minor, are thus able to trigger major changes in solute transport in fractured rocks, with transient, seasonal or long-term evolutions.
Important problems in earth sciences concern the deformation of the crust. Such a deformation is ... more Important problems in earth sciences concern the deformation of the crust. Such a deformation is quantifiable through the observations of geophysical and hydrogeological phenomena that are associated to it. Determining the link between these phenomena and their driving forces is of great importance and can improve our understanding of geological and geophysical processes. These observations can be made via laboratory experiments, but it might not be ideal to understand crustal-scale phenomena. The Roselend experiment, set in the French Alps near an artificial lake, provides a field-scale setting for a multidisciplinary research project to study transfer processes and transient phenomena within a highly dynamic system (Provost et al., EOS Vol 85 No 12, 2004). In this setting the water level variations in the lake are acting as a natural press on the surrounding massif. The possibility to systematically measure and record variations of tilt in a neighboring tunnel, as well as water le...
Groundwater geochemical and radon emanation responses to mechanical forcing are studied in a tunn... more Groundwater geochemical and radon emanation responses to mechanical forcing are studied in a tunnel located next to an artificial lake having large seasonal water level variations. Dripping waters have been monitored over one year in two tunnel zones, one highly fractured the other poorly fractured, characterized by flow rates different by a factor of 100. The waters percolate 50 m of unsaturated gneiss with a residence time from 25 to 60 days. In the poorly fractured Host Gneiss Zone, concentrations are higher but show smaller variations, whereas in the highly fractured Fracture Zone concentrations are lower but show larger variations. In addition to the long-term seasonal dilution by rain and snowmelt infiltrations, transient increases of sulfate and magnesium concentrations, observed simultaneously in both zones, are not related to meteorology but are associated with radon bursts into the tunnel air and lake loading rate changes. This suggests that deformation-induced connectivit...
Groundwater geochemical response to mechanical forcing is studied in a tunnel located next to an ... more Groundwater geochemical response to mechanical forcing is studied in a tunnel located next to an artificial lake having large seasonal water level variations. Dripping waters have been monitored over one year in two tunnel zones, one highly fractured the other poorly fractured, characterized by flow rates different by a factor of 100. The waters percolate 50 m of unsaturated gneiss with a residence time from 25 to 60 days. In the poorly fractured Host Gneiss Zone, concentrations are higher but show smaller variations, whereas in the highly fractured Fracture Zone concentrations are lower but show larger variations. In addition to the long-term seasonal dilution by rain and snow infiltrations, transient increases of sulfate and magnesium concentrations, observed simultaneously in both zones, are not related to meteorology but are associated with radon bursts into the tunnel air and lake loading rate changes. This suggests that deformation-induced connectivity enhancements of the doub...
The transport of uranium through a sandy podzolic soil has been investigated in the field and in ... more The transport of uranium through a sandy podzolic soil has been investigated in the field and in column experiments. Field monitoring, numerous years after surface contamination by depleted uranium deposits, revealed a 20 cm deep uranium migration in soil. Uranium retention in soil is controlled by the <50 microm mixed humic and clayey coatings in the first 40 cm i.e. in the E horizon. Column experiments of uranium transport under various conditions were run using isotopic spiking. After 100 pore volumes elution, 60% of the total input uranium is retained in the first 2 cm of the column. Retardation factor of uranium on E horizon material ranges from 1300 (column) to 3000 (batch). In parallel to this slow uranium migration, we experimentally observed a fast elution related to humic colloids of about 1-5% of the total-uranium input, transferred at the mean porewater velocity through the soil column. In order to understand the effect of rain events, ionic strength of the input solu...
Prévoir l'évolution future du climat passe par une meilleure compréhension des processus d'échang... more Prévoir l'évolution future du climat passe par une meilleure compréhension des processus d'échange de matière et d'énergie entre l'atmosphère, l'océan et la biosphère continentale. Ces deux derniers jouent également un rôle essentiel dans le cycle du carbone en absorbant plus de la moitié du dioxyde de carbone émis par les activités humaines.
Radon-222, a radioactive gas naturally produced in the Earth's crust, informs us about the migrat... more Radon-222, a radioactive gas naturally produced in the Earth's crust, informs us about the migration of fluids and is sometimes considered as a potential earthquake precursor. Here we investigate the effects of mechanical and thermal damage on the radon emanation from various granites representative of the upper crust. Radon concentration measurements performed under triaxial stress and pore fluid pressure show that mechanical damage resulting from cycles of differential stress intensifies radon release up to 170 ± 22% when the sample ruptures. This radon peak is transient and results from the connection of isolated micropores to the permeable network rather than new crack surface creation per se. Heating to 850°C shows that thermal fracturing irreversibly decreases emanation by 59-97% due to the amorphization of biotites hosting radon sources. This study, and the developed protocols, shed light on the relation between radon emanation of crustal rocks, deformation, and pressure-temperature conditions.
The Cretaceous Chalk of North-western Europe exhibits a double porosity (matrix and fracture) pro... more The Cretaceous Chalk of North-western Europe exhibits a double porosity (matrix and fracture) providing pathways for both slow and rapid flow of water. The present study aims at understanding and predicting the contaminant transfer properties through a significant section of this formation, with a particular emphasis on diffusion. This requires to study the nature of porosity and to perform diffusion experiments in representative samples using uranium and tritiated water (HTO), respectively taken as a reactive tracer and an inert one. The diffusive parameters, i.e. the accessible porosity and the effective diffusion coefficient were determined. Additional information was obtained with mercury porosimetry, gravimetric water content, textural and mineralogical characterization. The diffusion tests performed with HTO appear to be the best method to measure the total accessible porosity in any type of porous media, especially those having large pore size distributions. Our study demonstrates that classical gravimetric water content measurements are not sensitive to the reduction in pore size as opposed to HTO diffusion tests because capillary water is not extracted by conventional gravimetric method but can still be probed by diffusion experiments. We found effective diffusion coefficients D e (U (VI) ) near 4 Â 10 À10 m 2 s À1 . The slower migration of U (VI) compared to HTO indicates sorption, with R d (U (VI) ) from 100 to 360 mL g À1 . These values are one order of magnitude larger than other determinations of the U (VI) sorption coefficient because only the matrix porosity is concerned here. The migration of U (VI) in chalk is only limited by sorption on ancillary Fe-Pb-bearing minerals. Transport of HTO and U (VI) is independent of the porosity distribution. Uranium diffusion in the chalk matrix porosity is fast enough to allow the total invasion of the pore space within characteristic time scales of the order of 1000 years. This results in a partitioning of uranium velocities in fracture flow and matrix flow proportionally to the respective fracture and matrix porosities.
h i g h l i g h t s ␦ 34 S SO4 vs. [As] and ␦ 18 O SO4 vs. As V /As III correlations provide new ... more h i g h l i g h t s ␦ 34 S SO4 vs. [As] and ␦ 18 O SO4 vs. As V /As III correlations provide new tools even for low [As]. Long-term and high-resolution monitoring shows droughts enhance pyrite dissolution. Major effect of 2003 European heatwave on pyrite dissolution and [As] increase.
First events of illicit trafficking of nuclear and radiological materials occurred 50 years ago. ... more First events of illicit trafficking of nuclear and radiological materials occurred 50 years ago. Nuclear forensics expertise are aiming at determining the use of seized material, its industrial history and provenance (geographical area, place of production or processing), at assisting in the identification and dismantling of illicit trafficking networks. This information is also valuable in the context of inspections of declared facilities to verify the consistency of operator's declaration. Several characteristics can be used to determine the origin of uranium ore concentrates such as trace elemental impurity patterns (Keegan et al., 2008 ; Varga et al., 2010a Varga et al., , 2010b or uranium, oxygen and lead isotopic compositions (Tamborini et al., 2002a(Tamborini et al., , 2002b Wallenius et al., 2006; Varga et al., 2009). We developed analytical procedures for measuring the isotopic compositions of uranium (234U/238U and 235U/238U) and oxygen (18O/16O) and levels of elemental impurities (e.g. REE, Th) from very small amounts of uranium ore concentrates (or yellow cakes). Micrometer particles and few milligrams of material are used for oxygen isotope measurements and REE determination, respectively. Reference materials were analyzed by mass spectrometry (TIMS, SF-ICP-MS and SIMS) to validate testing protocols. Finally, materials of unknown origin were analyzed to highlight significant differences and determine whether these differences allow identifying the origin of these ore concentrates.
With the increase of contaminant flux of radionuclides in the environment, there is a need to und... more With the increase of contaminant flux of radionuclides in the environment, there is a need to understand and model the processes that control the distribution of uranium and its daughter products during transport within aquifers. We have used U-series disequilibria as an analogue for the transport of anthropogenic nuclides in a small watershed of a chalk aquifer in Eastern France. We have measured naturally occurring U and Th isotopes in rock and water phases in order to determine transport mechanisms at water/rock interface and quantify parameters controlling the migration of radionuclides. The studied aquifer has a double porosity and is characterised by a saturated matrix up to 40m above the water-table and variably saturated fractures. Measurements of U and Th isotopic composition in the water phase were performed on a MC-ICPMS which enable in particular high precision measurements of (230Th/232Th) activity ratio. Water samples were obtained from wells both in low and high water-table conditions. Chalk samples were collected at the same wells and were analysed also for U-series and Sr isotopes. We observed seasonal precipitation of uranium ((234U/238U) activity ratio above 1) in the zone of water-table fluctuation linked with the oversaturation of water with respect to calcite during low water-table period. (230Th/232Th) activity ratios in water are lower than in the substratum meaning that the time of water infiltration is not long enough to allow an equilibration of groundwater with rock. Water acquires its Th isotopic composition when interacting with another phase of the aquifer (soil) and joins the water-table mainly through fractures. The complete data set obtained on solid and liquid phases of the aquifer allow us mass balance calculations and to model the behaviour of uranium and thorium during transport. The 1-D model we have developed considers a transient behaviour of radionuclides in the aquifer and for the first time allows the calculation of the evolution of rock composition with time. We have then calculated a global uranium weathering rate within the aquifer of 2.5 10-6 y-1.
Solute transport in fractured porous media presents a large variability in space and time due to ... more Solute transport in fractured porous media presents a large variability in space and time due to variations in water content, in flow pathway and regime, and chemical reactions. The dynamic of these systems in response to external stimuli such as meteorological, mechanical, hydrogeological and chemical disturbances remains poorly known. To build reliable predictive models, there is a strong need to obtain data from complex but well characterized natural observatories. We set up an experiment in a 150 m long tunnel where drippings give access to ground water flowing vertically from 50 m of variably saturated fractured gneiss. This tunnel is located close to and above the artificial Lake Roselend (France) where large variations in level induce reproducible deformation and hydrogeological disturbances. This area is also characterized by contrasted infiltration regimes with alternating snow, rain, and dry periods. In addition, we performed a tracer test with the surface injection of NaCl, 50 m vertically above the dripping water collecting points. In order to get water representative of different contributions of matrix porosity and fractures, we monitored several zones with different structures and flow rates, and looked for changes in flow rates and ion concentrations in water over several years. In addition, we monitored in the tunnel atmosphere the radon activity, independently known to respond to deformation events. Water composition reflects a yearly equilibrium. In spring, concentrations decrease by inputs of rain and melted snow along fractures at high flow rates with limited reactivity. In summer, concentrations increase by diffusion of saline water from matrix porosity and increased dissolution at low flow rates. We observed transient increases of SO4 and Mg concentrations associated with radon bursts, and that are not related to meteorology. Transient enhancement of conductance, with discharges of saline water and radon from the matrix porosity to the fractures, is thus a mechanism able to produce both ground-water and gas anomalies in response to hydrogeological or mechanical processes such as increases in pore pressure or changes in crack geometry. Before the tracer injection, Na, Mg, Ca and SO 4 are contributed by the host rock, whereas Cl and K are contributed by meteoric sources, soils and surface biota. After the surface injection of tracers, NaCl breakthrough (Na being late relative to Cl) is associated with an important release of K, Ca, Mg, Ba, and F that were not injected and are contributed by the host rock by ion exchange reactions. By contrast, SO4 is generated by oxidative dissolution of pyrite enhanced by air entry on drought regime and subsequent flush on recharge. Natural and artificial stimuli, even minor, are thus able to trigger major changes in solute transport in fractured rocks, with transient, seasonal or long-term evolutions.
Important problems in earth sciences concern the deformation of the crust. Such a deformation is ... more Important problems in earth sciences concern the deformation of the crust. Such a deformation is quantifiable through the observations of geophysical and hydrogeological phenomena that are associated to it. Determining the link between these phenomena and their driving forces is of great importance and can improve our understanding of geological and geophysical processes. These observations can be made via laboratory experiments, but it might not be ideal to understand crustal-scale phenomena. The Roselend experiment, set in the French Alps near an artificial lake, provides a field-scale setting for a multidisciplinary research project to study transfer processes and transient phenomena within a highly dynamic system (Provost et al., EOS Vol 85 No 12, 2004). In this setting the water level variations in the lake are acting as a natural press on the surrounding massif. The possibility to systematically measure and record variations of tilt in a neighboring tunnel, as well as water le...
Groundwater geochemical and radon emanation responses to mechanical forcing are studied in a tunn... more Groundwater geochemical and radon emanation responses to mechanical forcing are studied in a tunnel located next to an artificial lake having large seasonal water level variations. Dripping waters have been monitored over one year in two tunnel zones, one highly fractured the other poorly fractured, characterized by flow rates different by a factor of 100. The waters percolate 50 m of unsaturated gneiss with a residence time from 25 to 60 days. In the poorly fractured Host Gneiss Zone, concentrations are higher but show smaller variations, whereas in the highly fractured Fracture Zone concentrations are lower but show larger variations. In addition to the long-term seasonal dilution by rain and snowmelt infiltrations, transient increases of sulfate and magnesium concentrations, observed simultaneously in both zones, are not related to meteorology but are associated with radon bursts into the tunnel air and lake loading rate changes. This suggests that deformation-induced connectivit...
Groundwater geochemical response to mechanical forcing is studied in a tunnel located next to an ... more Groundwater geochemical response to mechanical forcing is studied in a tunnel located next to an artificial lake having large seasonal water level variations. Dripping waters have been monitored over one year in two tunnel zones, one highly fractured the other poorly fractured, characterized by flow rates different by a factor of 100. The waters percolate 50 m of unsaturated gneiss with a residence time from 25 to 60 days. In the poorly fractured Host Gneiss Zone, concentrations are higher but show smaller variations, whereas in the highly fractured Fracture Zone concentrations are lower but show larger variations. In addition to the long-term seasonal dilution by rain and snow infiltrations, transient increases of sulfate and magnesium concentrations, observed simultaneously in both zones, are not related to meteorology but are associated with radon bursts into the tunnel air and lake loading rate changes. This suggests that deformation-induced connectivity enhancements of the doub...
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