Search

Author(s): Tournassat, C; Steefel, CI; Gimmi, T | Abstract: Molecular diffusion of dissolved species is a fundamental mass transport process affecting many environmental and technical processes. Whereas diffusive transport of single tracers can be described by Fick's law, a multicomponent approach based on the Nernst-Planck equation is required for charge-coupled transport of ions. The numerical solution of the Nernst-Planck equation requires special attention with regard to properties that are required at interfaces of numerical cells when using a finite difference or finite volume method. Weighted arithmetic and harmonic averages are used in most codes that can solve the Nernst-Planck equation. This way of averaging is correct for diffusion coefficients but inappropriate for solute concentrations at interfaces. This averaging approach leads to charge balance problems and thus to numerical instabilities near interfaces separating grid volumes with contrasting properties. We argue that a logarithmic-differential average should be used. Here this result is generalized, and it is demonstrated that it generally leads to improved numerical stability and accuracy of concentrations computed near material interfaces. It is particularly relevant when modeling semipermeable clay membranes or membranes used in water treatment processes.

Fluid inclusions in quartz are known to mod- ify their shapes and microstructures (textures) during weak plastic deformation. However, such changes have not been experimentally demonstrated and criteria are not available to relate them to paleostress conditions. To address these issues, quartz crystals containing natural CO2-H2O-NaCl fluid inclusions have been experimen- tally subjected to compressive deviatoric stresses of 90-250 MPa at 700C and *600 MPa confining pres- sure. Strains of up to 1% cause the inclusions to develop irregular shapes and to generate microcracks in crystal- lographic planes oriented subperpendicular to the major compression axis, r1. The uniform alignment of the microcracks imparts a planar fabric to the samples. The microcracks heal and form swarms of tiny satellite inclusions. These new inclusions lose H2O by diffusion, thereby triggering plastic deformation of the surrounding quartz via H2O-weakening. Consequently, the quartz samples deform plastically only in domains originally rich in inclusions. This study shows that fluid inclusions deformed by deviatoric stresses may indeed record information on paleostress orientations and that they play a key role in facilitating crystal-plastic deformation of quartz.

Editorial; International audience; Porewaters in aquitards have origins and geological histories ranging from freshwaters emplaced with clay sediments [1], primary diagenetic fluids in marine systems [2, 3], and systems mixed over geological time [4]. Determining theirorigin and fate and the transport of dissolved constituents in these porewaters is complex. Research on fluids and gases in aquitards has increased over the past two decades, largely driven by the need for nuclear agencies to find suitable sites for deep geological repositories to isolate nuclear waste.Understanding the characteristics of aquitards as barriers to contaminant transport to shallow aquifers is also key to thedevelopment of unconventional oil and gas reserves. These regulatory and economic drivers have amplified industrialsupport for research on aquitard fluids and accelerated the rate of peer-reviewed publications from less than one peryear just three decades ago to more than 15 per year today, highlighting advances in methods for porewater analysis(cf. [5]), pore gases analysis [6, 7], and the development of diffusion parameters [8]. This special issue presents newresearch that describes methods to characterize porewaters and gases in aquitards, the diffusive characteristics ofaquitards, and methods to characterize aquitard porewaters and solute profiles to simulate diffusive transport in thesecritical barriers. Analyzing porewater composition is a critical first step in characterizing aquitard fluids and remains a challenge where the essential condition of low-permeability complicates the task. Two papers address this issue. M. Celejewski et al.present a new method using absorption by capillary tension to extract porewaters onto cellulosic paper that is sealed between two faces of a split core sample. Their findings highlight the process of anion exclusion in the mobile fraction of porewaters in the Ordovician shales of the Michigan Basin where hydraulic conductivities are as low as 10−14 m/s. U.Mäder uses artificial porewater to displace in an advective manner the mobile water fraction of the natural porewater under confining pressure from aquitard rocks with similarly low hydraulic conductivities. The advantage of this technique lies in the fact that the first few mL of displaced porewater are unaffected by induced reactions with the rock or atmosphere. This allows complete chemical characterization of the porewater including pH and redox. Like M. Celejewski et al., this method highlights the small difference between Cl−and Br− transport in systems where anion exclusion becomes important. In situ porewater pressure is an important feature ofaquitard systems, one which reflects the state of openness of the formation to porefluid migration and transport. I.Morales-Arredondo et al. present insights into the origin of the overpressured diagenetic fluids developed in the CerroPrieto basin of the Salton Sea, relating the geochemistry of these brines to the formation and alteration of mineral phases that preclude dewatering and that maintain overpressured conditions. Fluid-mineral interactions maintaining an overpressured system contrasts with the more mobile fluids of a normal-pressured sediment system presented by Q. Ge et al. from the North Jiangsu coastal plain, a region that hosts much younger Pleistocene to Holocene remnant seawater with freshwater inputs. S. D. Normani et al. present a contrasting site that completes this spectrum, where shales and argillaceous limestones in the easternmargin of the Michigan Basin in southern Ontario are underpressured by up to 300m below hydrostatic. Their 1D hydromechanically coupled modelling of fully saturated conditions shows the resilience of this system to glacial perturbations during the Pleistocene and suggest that the stable underpressured conditions are likely remnant from post-Mesozoic exhumation. The diffusion properties of argillaceous sediments arecritical to defining transport in aquitard barriers. E. Jacops et al. explore a different parameter—the effect of the size of nonreactive solutes on diffusion parameters. Using Eigenbilzen Sands, Opalinus Clay,Callovo-Oxfordian Clay, and the Boom Clay, they measure and model through-diffusion of gases from He and H2 to Xe and ethane. S. Savoye et al. investigate the case of diffusion under partial gas saturation using the Callovo-Oxfordian claystone, comparing marginally conservative 22Na and reactive Cs with HTO. A. Pazdniakou and M.Dymitrowska examine advective free-gas transport in watersaturated clays using a hydraulic-mechanical model and show the importance of localized dilatant pathways in the Callovo-Oxfordian claystone. All three studies address the important case of gas transport in aquitards. Case studies remain an important area of research which allow us to apply and assess advanced field-testing methods to aquitard study sites. S. C. Priestley et al. use traditional conservative tracers, Cl− and 4He, to constrain vertical transport through the aquitard underlying the Great Artesian Basin in central Australia, showing how these porewater profiles record paleohydrological conditions and impacts of past climates on recharge. K. Nakata et al. detail the challenges in reconstructing porewater ages in a case study where 4He ingrowth and 36Cl decay for over 1My demonstrate near closed conditions since uplift. A greater focus on redox reactions is presented by C. Lerouge et al. in a case study of the Tégulines Clay in the Eastern Paris Basin where porewater chemistry and mineralogy show aprogressive trend towards oxidation in the shallow aquitard. The breadth of research presented in this special issue demonstrates the considerable interest and significant advances in the study of aquitard fluids and gases. We believethe high-quality of recent contributions to the science of aquitard hydrogeology, including those presented in this specialissue, meets the challenge of demonstrating the integrity of aquitard barriers for the protection of freshwater resources.

International audience; Reactive transport modelling was used to simulate solute transport, thermodynamic reactions, ion exchange and biodegradation in the Porewater Chemistry (PC) experiment at the Mont Terri Rock Laboratory. Simulations show that the most important chemical processes controlling the fluid composition within the borehole and the surrounding formation during the experiment are ion exchange, biodegradation and dissolution/precipitation reactions involving pyrite and carbonate minerals. In contrast, thermodynamic mineral dissolution/precipitation reactions involving alumo-silicate minerals have little impact on the fluid composition in the time-scale of the experiment. With the accurate description of the initial chemical condition in the formation in combination with kinetic formulations describing the different stages of bacterial activities, we succeeded in reproducing the evolution of important system parameters, such as the pH, redox potential, total organic carbon, dissolved inorganic carbon and sulphate concentration. Leaching of glycerol from the pH-electrode may be the primary source of organic material that initiated bacterial growth, which caused the chemical perturbation in the borehole. Results from these simulations are consistent with data from the over-coring and demonstrate that the Opalinus Clay has a high buffering capacity in terms of chemical perturbations caused by bacterial activity. This buffering capacity can be attributed to the carbonate system as well as to the reactivity of clay surfaces.

The geochemistry of an argillaceous rock sequence from a deep borehole in NE-Switzerland was investigated. The focus was to constrain the porewater chemistry in low permeability Jurassic rocks comprising the Liassic, the Opalinus Clay formation, the „Brown Dogger‟ unit and the Effingen Member (Malm). A multi-method approach including mineralogical analysis, aqueous and Ni-ethylenediamine extraction, squeezing tests and pCO2 measurements as well as geochemical modelling was applied for this purpose. A consistent dataset was obtained with regard to the main solutes in the porewaters. A fairly constant anionaccessible porosity of 50% of the total porosity was deduced for all analysed samples which displayed variable clay-mineral contents. Sulphate concentrations were shown to be constrained by a sulphate-bearing phase, presumably by celestite or a Sr-Ba sulphate. Application of a simple equilibrium model, including cation exchange reactions, calcite and celestite equilibrium showed good agreement with squeezing data, indicating the suitability of the modelling approach to simulate porewater chemistry in the studied argillaceous rocks. The modelling highlighted the importance of correct determination of the exchangeable cation population. The analysis corroborates that squeezing of the studied rocks is a viable and efficient way to sample porewater.

International audience; Bentonite clay is considered as a potential buffer and backfill material in subsurface repositories for high-level nuclear waste. As a result of its low permeability, transport of water and solutes in compacted bentonite is driven primarily by diffusion. Developing models for species transport in bentonite is complicated, because of the interaction of charged species and the negative surface charge of clay mineral surfaces. The effective diffusion coefficient of an ion in bentonite depends on the ion’s polarity and valence, on the ionic strength of the solution, and on the bulk dry density of the bentonite. These dependencies need to be understood and incorporated into models if one wants to predict the effectiveness of bentonite as a barrier to radionuclides in a nuclear repository. In this work, we present a benchmark problem for reactive transport simulators based on a flow-through experiment carried out on a saturated bentonite core. The measured effluent composition shows the complex interplay of species transport in a charged medium in combination with sorption and mineral precipitation/dissolution reactions. The codes compared in this study are PHREEQC, CrunchFlow, FLOTRAN, and MIN3P. The benchmark problem is divided into four component problems of increasing complexity, leading up to the main problem which addresses the effects of advective and diffusive transport of ions through bentonite with explicit treatment of electrostatic effects. All codes show excellent agreement between results provided that the activity model, Debye-Hückel parameters, and thermodynamic data used in the simulations are consistent. A comparison of results using species-specific diffusion and uniform species diffusion reveals that simulated species concentrations in the effluent differ by less than 8 %, and that these differences vanish as the system approaches steady state.

© 2015, Springer International Publishing Switzerland. Changes of porosity, permeability, and tortuosity due to physical and geochemical processes are of vital importance for a variety of hydrogeological systems, including passive treatment facilities for contaminated groundwater, engineered barrier systems (EBS), and host rocks for high-level nuclear waste (HLW) repositories. Due to the nonlinear nature and chemical complexity of the problem, in most cases, it is impossible to verify reactive transport codes analytically, and code intercomparisons are the most suitable method to assess code capabilities and model performance. This paper summarizes model intercomparisons for six hypothetical scenarios with generally increasing geochemical or physical complexity using the reactive transport codes CrunchFlow, HP1, MIN3P, PFlotran, and TOUGHREACT. Benchmark problems include the enhancement of porosity and permeability through mineral dissolution, as well as near complete clogging due to localized mineral precipitation, leading to reduction of permeability and tortuosity. Processes considered in the benchmark simulations are advective-dispersive transport in saturated media, kinetically controlled mineral dissolution-precipitation, and aqueous complexation. Porosity changes are induced by mineral dissolution-precipitation reactions, and the Carman-Kozeny relationship is used to describe changes in permeability as a function of porosity. Archie’s law is used to update the tortuosity and the pore diffusion coefficient as a function of porosity. Results demonstrate that, generally, good agreement is reached amongst the computer models despite significant differences in model formulations. Some differences are observed, in particular for the more complex scenarios involving clogging; however, these differences do not affect the interpretation of system behavior and evolution.

Clay mineral-rich sedimentary formations are currently under investigation to evaluate their potential use as host formations for installation of deep underground disposal facilities for radioactive waste (e.g. Boom Clay (BE), Opalinus Clay (CH), Callovo–Oxfordian argillite (FR)). The ultimate safety of the corresponding repository concepts depends largely on the capacity of the host formation to limit the flux towards the biosphere of radionuclides (RN) contained in the waste to acceptably low levels. Data for diffusion-driven transfer in these formations shows extreme differences in the measured or modelled behaviour for various radionuclides, e.g. between halogen RN (36Cl, 129I) and actinides (238,235U, 237Np, 232Th, etc.), which result from major differences between RN of the effects on transport of two phenomena: diffusion and sorption. This paper describes recent research aimed at improving understanding of these two phenomena, focusing on the results of studies carried out during the EC Funmig IP on clayrocks from the above three formations and from the Boda formation (HU). Project results regarding phenomena governing water, cation and anion distribution and mobility in the pore volumes influenced by the negatively-charged surfaces of clay minerals show a convergence of the modelling results for behaviour at the molecular scale and descriptions based on electrical double layer models. Transport models exist which couple ion distribution relative to the clay–solution interface and differentiated diffusive characteristics. These codes are able to reproduce the main trends in behaviour observed experimentally, e.g. De(anion) Funmig results have clarified understanding of how clayrock mineral composition, and the corresponding organisation of mineral grain assemblages and their associated porosity, can affect mobile solute (anions, HTO) diffusion at different scales (mm to geological formation). In particular, advances made in the capacity to map clayrock mineral grain-porosity organisation at high resolution provide additional elements for understanding diffusion anisotropy and for relating diffusion characteristics measured at different scales. On the other hand, the results of studies focusing on evaluating the potential effects of heterogeneity on mobile species diffusion at the formation scale tend to show that there is a minimal effect when compared to a homogeneous property model. Finally, the results of a natural tracer-based study carried out on the Opalinus Clay formation increase confidence in the use of diffusion parameters measured on laboratory scale samples for predicting diffusion over geological time–space scales. Much effort was placed on improving understanding of coupled sorption–diffusion phenomena for sorbing cations in clayrocks. Results regarding sorption equilibrium in dispersed and compacted materials for weakly to moderately sorbing cations (Sr2+, Cs+, Co2+) tend to show that the same sorption model probably holds in both systems. It was not possible to demonstrate this for highly sorbing elements such as Eu(III) because of the extremely long times needed to reach equilibrium conditions, but there does not seem to be any clear reason why such elements should not have similar behaviour. Diffusion experiments carried out with Sr2+, Cs+ and Eu(III) on all of the clayrocks gave mixed results and tend to show that coupled diffusion–sorption migration is much more complex than expected, leading generally to greater mobility than that predicted by coupling a batch-determined Kd and Fick´s law based on the diffusion behaviour of HTO. If the Kd measured on equivalent dispersed systems holds as was shown to be the case for Sr, Cs (and probably Co) for Opalinus Clay, these results indicate that these cations have a De value higher than HTO (up to a factor of 10 for Cs+). Results are as yet very limited for very moderate to strongly sorbing species (e.g. Co(II), Eu(III), Cu(II)) because of their very slow transfer characteristics.

Abundant veins filled by calcite, celestite and pyrite were found in the core of a 719 m deep borehole drilled in Oftringen near Olten, located in the north-western Molasse basin, close to the thrust of the Folded Jura. Host rocks are calcareous marl, argillaceous limestone and limestone of the Dogger and Malm. The δ18O values of vein calcite are lower than in host rock carbonate and, together with microthermometric data from fluid inclusions in vein calcite, indicate precipitation from a seawater-dominated fluid at average temperatures of 56–68°C. Such temperatures were reached at the time of maximum burial of the sedimentary pile in the late Miocene. The depth profile of δ13C and 87Sr/86Sr values and Sr content of both whole-rock carbonate and vein calcite show marked trends towards negative δ13C, high 87Sr/86Sr, and low Sr content in the uppermost 50–150 m of the Jurassic profile (upper Oxfordian). The 87Sr/86Sr of vein minerals is generally higher than that of host rock carbonate, up to very high values corresponding to Burdigalian seawater (Upper Marine Molasse, Miocene), which represents the last marine incursion in the region. No evidence for internally derived radiogenic Sr (clay minerals) has been found and so an external source is required. S and O isotope composition of vein celestite and pyrite can be explained by bacterial reduction of Miocene seawater sulphate. The available data set suggests the vein mineralization precipitated from descending Burdigalian seawater and not from a fluid originating in the underlying Triassic evaporites.

Fluid inclusions in quartz are known to modify their densities during shear deformation. Modifications of chemical composition are also suspected. However, such changes have not been experimentally demonstrated, their mechanisms remain unexplained, and no criteria are available to assess whether deformed inclusions preserve information on paleofluid properties. To address these issues, quartz crystals containing natural CO2–H2O–NaCl fluid inclusions have been experimentally subjected to compressive deviatoric stresses of 90–250 MPa at 700°C and ~600 MPa confining pressure. The resulting microcracking of the inclusions leads to expansion by up to 20%, producing low fluid densities that bear no relation to physical conditions outside the sample. Nevertheless, the chemical composition of the precursor inclusions is preserved. With time the microcracks heal and form swarms of tiny satellite inclusions with a wide range of densities, the highest reflecting the value of the maximum principle stress, σ 1. These new inclusions lose H2O via diffusion, thereby passively increasing their salt and gas contents, and triggering plastic deformation of the surrounding quartz via H2O-weakening. Using microstructural criteria to identify the characteristic types of modified inclusions, both the pre-deformation fluid composition and syn-deformation maximum stress on the host mineral can be derived from microthermometric analysis and thermodynamic modelling.