S. Savoye

Effect of carbonate ions on pyrite (FeS2) dissolution

Neutralization by carbonate of acidification generated by pyrite (FeS2) oxidation was investigated by both solution (iron and sulfur speciation, pH and Eh) and solid (FT-IR) characterizations. Batch dissolution experiments were carried out in contact with atmospheric oxygen (20 %) in four different bicarbonated solutions ((NaHCO3)=10 -3 , 1,12.10 -2 ,1 0 -1 and 1 mol/L). Five different contact duration were selected : 6 hours, 1, 3, 8 and 30 days. Ferrous carbon- ate complexes (FeOHCO 3 and Fe(CO 3 ) - 2 2 ) tend to maintain iron in solution (up to 152.2 µmol/L in (NaHCO3)=1 mol/L solution) and to increase pyrite oxidation rate by preventing surface coating. Acidification is thus more intense in di- luted and concentrated carbonate medium ((NaHCO 3 ) =10 -3 and 1 mol/L) with respectively ∆ pH=5.06 and ∆ pH=1.99 at 30 days whereas pH remains buffered in () NaHCO 3 =1.12.10 -2 and 0.1 mol/L solutions. Siderite appears to be the first solid precipitating, transforming into gœthite, oxyhydroxy ferric sulfate incorporating sulfite and thiosulfate, and then lepidocrocite. Sulfur chemistry controls the acidification observed. Thiosulfate is the first sulfoxyanion released in solution and its oxidation into sulfite then sulfate seems to be the key of acidification production. Thus, carbonate pH buffer properties seem to be limited and effective for moderated carbonate concentrations.

Diffusion as the main process for mass transport in very low water content argillites: 2. Fluid flow and mass transport modeling

[1] On the basis of chloride concentrations of pore water in the Tournemire massif (part 1), a conceptual model for mass transport in argillites by diffusion is proposed. From this conceptual model and current knowledge of the geological history of the massif, one-dimensional numerical simulations are formulated for chloride transport in Tournemire massif over the past 53 Ma. Good agreement between experimental data and calculated values for both diffusion coefficients and concentrations of chloride confirms that diffusion is the main process for mass transport in the massif. This model is also tested using deuterium contents of pore water, applying variable concentrations to meteoric water (circulating in system boundary layers) based on the thermal dependency of its isotopic composition. These simulations reveal the likely important role of lithologic heterogeneities, such as fractures, in the horizontal distribution of tracer concentrations.

Geochemistry and14C dating of groundwaters from Jurassic aquifers of North Aquitaine Basin (France)

Twenty-seven samples from a confined Lower-Middle Jurassic aquifer and an unconfined Oxfordian aquifer of the North Aquitaine Basin (France) have been analysed for their major elements, Br−,18O,2H,13C and14C contents. Hydrochemistry indicates (1) a dissolution of carbonate and anhydrite near the recharge zone and (2) a dilution of a saline water derived from a seawater/halite mixing in the deeper part of the aquifer. The mixing is also visible in a δ18O vs Cl− diagram in which two different groups appear: recent waters and old waters indicating a mixing process between fresh and saline groundwaters. The composition of the saline water is likely to be 34,100±11,200 ppm in Cl, 70±20 ppm in Br and more than −3.5±07‰ vs SMOW in18O.13C contents indicate (1) a C exchange with CaCO3 matrix for groundwaters near the recharge zone and (2) a participation of organic matter in the deep part of the aquifer. Residence times for waters near the area of the aquifer outcrop correspond to Holocene and Late Pleistocene periods. The depletion in stable isotopes of 10 to 15,000 y B.P. waters show a late glacial period infiltration to the aquifer. After a distance of about 10 km in the aquifer, the14C activities are 0 pmc showing the presence of ‘old’ groundwaters.

How mobile is iodide in the Callovo–Oxfordian claystones under experimental conditions close to the in situ ones?

The iodide behaviour towards the Callovo-Oxfordian claystone was studied using batch and diffusion experiments under conditions which limited the artefacts cited in the literature to be responsible for the iodide uptake (i.e. the experiments were carried out under anoxic conditions with N(2)/CO(2) atmosphere with a monitoring of the iodine redox-state). The results show that all the radioactive iodine was (125)I(-), with no measurable activity for (125)IO(3)(-), which is known to have a higher affinity for the rock than iodide. Moreover, the batch experiments revealed no sorption, independently of the initial iodide concentration (from 10(-6) to 10(-3) mol L(-1)) and the contact time (up to 106 days). Conversely, the diffusion experiments indicated a weak but measurable retention. The through-diffusion experiments led to distribution ratio values only significant (R(D)~0.05 mL g(-1)) for initial iodide concentration ≤ 10(-4) mol L(-1). Higher R(D) values were estimated from out-diffusion experiments, ranging from about 0.05 mL g(-1) for an initial concentration of 10(-3) mol L(-1) to 0.14 mL g(-1) for the lowest one. A retention phenomenon that could be reversible and kinetically-controlled was proposed to explain the differences in the extent of the iodide retardation of the two types of diffusion experiments.

A laboratory experiment for determining both the hydraulic and diffusive properties and the initial pore-water composition of an argillaceous rock sample: A test with the Opalinus clay (Mont Terri, Switzerland)

Argillaceous formations are thought to be suitable natural barriers to the release of radionuclides from a radioactive waste repository. However, the safety assessment of a waste repository hosted by an argillaceous rock requires knowledge of several properties of the host rock such as the hydraulic conductivity, diffusion properties and the pore water composition. This paper presents an experimental design that allows the determination of these three types of parameters on the same cylindrical rock sample. The reliability of this method was evaluated using a core sample from a well-investigated indurated argillaceous formation, the Opalinus Clay from the Mont Terri Underground Research Laboratory (URL) (Switzerland). In this test, deuterium- and oxygen-18-depleted water, bromide and caesium were injected as tracer pulses in a reservoir drilled in the centre of a cylindrical core sample. The evolution of these tracers was monitored by means of samplers included in a circulation circuit for a period of 204 days. Then, a hydraulic test (pulse-test type) was performed. Finally, the core sample was dismantled and analysed to determine tracer profiles. Diffusion parameters determined for the four tracers are consistent with those previously obtained from laboratory through-diffusion and in-situ diffusion experiments. The reconstructed initial pore-water composition (chloride and water stable-isotope concentrations) was also consistent with those previously reported. In addition, the hydraulic test led to an estimate of hydraulic conductivity in good agreement with that obtained from in-situ tests.

Effect of Water Saturation on the Diffusion/Adsorption of 22Na and Cesium onto the Callovo-Oxfordian Claystones

The diffusion and adsorption behaviors of sodium and cesium were investigated in the Callovo-Oxfordian claystones (France) under unsaturated conditions. Through-, out-, and in-diffusion laboratory experiments were performed on intact and compacted samples. These samples were partially saturated using an osmotic method for imposed suction up to 9 MPa. This specific technique enabled us to obtain water saturation degree ranging from 81% to 100% for intact samples and from 70% to 100% for compacted materials. The results showed a very low impact of water saturation on the extent of adsorption for 22Na and cesium, onto intact and compacted materials. Such observations suggest that the saturation degrees were not low enough to limit the access of cations to adsorption sites on clay surfaces. At full saturation, enhanced diffusion for 22Na and cesium was clearly evidenced onto intact and compacted samples. Under unsaturated conditions, the diffusion behavior for Cs and 22Na was not only slower but also distinct as compared to fully saturated samples. For the intact rock and under suction of 1.9 MPa, the Cs diffusivity was reduced by a factor of 17, whereas for sodium, it was reduced by a factor of 5. Explanation was then proposed to explain such a difference.