Anion-accessible Porosity Research Articles

Porewater profiles of Cl and Br in boreholes penetrating the Mesozoic sequence in northern Switzerland

The analysis of tracer profiles of porewaters is a valuable tool to understand transport processes in argillaceous rocks and to unravel the paleo-hydrogeology of a site. In this contribution, anionic tracers (Cl, Br) from eight boreholes located in three study areas (∼20 km apart) penetrating the ∼800 m thick Mesozoic sequence in northern Switzerland, were analysed. A specific focus was on the Opalinus Clay, a ∼100 m thick homogeneous claystone formation, foreseen as host rock for radioactive waste disposal in Switzerland, as well as its mostly clay-rich confining units. Using porewater extraction methods, such as high-pressure squeezing, advective displacement and aqueous extraction, a unique dataset of spatially highly resolved Cl and Br profiles could be obtained. These show systematic and comparable patterns suggesting common paleo-hydrogeological evolution paths for all three study areas. The scatter in the tracer profiles based on the aqueous extraction data are mostly related to the uncertainty in the estimation of the anion-accessible porosity fraction on one hand and in the water content data in the case of low-porosity calcareous rocks on the other. Differences between the study areas are in line with differences in aquifer characteristics. The current shape of the anion profiles is dominated by diffusive exchange with the bounding aquifers over the last several ten thousands to millions of years, as supported by numerical modelling. The Br/Cl signals also suggest the preservation of older signals related to halite dissolution and highly evolved evaporitic porewaters occurring in evaporite-bearing units of the Triassic. Overall, the adopted methodology has enabled to obtain a unique dataset of anionic tracers at regional scale providing a solid basis for understanding the regional paleo-hydrogeology of siting areas for a potential nuclear waste repository.

Impact of Fe(II) on 99Tc diffusion behavior in illite

A comprehensive understanding of the geochemical behavior of 99Tc is of great importance for safe disposal of radioactive waste and remediation of contaminated environmental sites. Illite is one of the most common constituents of clay rocks, and thus used in this work as a model system for studying the retention and transport of 99Tc in clay-rich systems. In this study, a through-diffusion technique was applied to investigate the diffusion behavior of Tc in compacted illite clay under oxic and anoxic conditions. Particular focus of this investigation was on the role of Fe(II) on the redox state and mobility of Tc in clay. As the diffusion cells contained stainless steel filters for confining the clay plug, 99Tc-diffusion in the filters was assessed first, followed by 99Tc diffusion in the illite column with or without Fe-loading. Two types of Fe(II) loadings in illite were considered, surface complexed Fe(II) on illite edge sites and Fe(II) added as pyrite grains. COMSOL Multiphysics was used to analyze the diffusion data. The measured filter porosity was about 0.2 and the effective diffusion coefficient, De, for Tc (as TcO4−) in the filter was 0.59 × 10−10 m2/s. Tc diffusion in illite under ambient conditions showed a typical anion diffusion behavior with De in the range 0.38-0.44 × 10−10 m2/s and the anion accessible porosity ε of approximately 0.2. In the presence of Fe(II), De for Tc migration in illite was one order of magnitude lower, showing that Fe(II) has a significant effect on the migration of 99Tc. Analysis of the Tc distribution in the system suggests that most Tc was retarded at the filters, especially the ones connected to the high concentration reservoir. The remaining Tc quantities were immobilized at sample boundaries next to the filters with higher concentrations observed in the domains close to the reservoirs. Almost no Tc was immobilized in the middle part of the sample, where the Fe(II) was preloaded. This observation contradicts the anticipation, because Tc was expected to be immobilized in the middle of the sample preloaded with Fe(II). A possible explanation is that a delocalized redox reaction may occur, which means that electrons from the oxidation of the preloaded Fe(II) in the central part of the cell are transferred to the filter via the walls of the steel diffusion cell. Tc reacts with the electrons in the filter at a relative slow rate, which results in most of the Tc retarded in the filter, while some small amounts of it may diffuse through the clay.

Evolution of HTO and 36Cl− diffusion through a reacting cement-clay interface (OPC paste-Na montmorillonite) over a time of six years

Cement and clays are proposed as sealing materials in underground repositories for radioactive waste. When cement and clay come into contact, chemical gradients between their very different porewater compositions lead to diffusive exchange of solutes, which can result in mineral transformations and alterations of transport properties at the materials interface. Small samples of cement-clay interfaces were prepared and let react over a period of six years. During this time, the changes in transport properties of the samples were periodically monitored by means of through-diffusion experiments. This technique allows studying the evolution of the diffusive flux across the reacting interface and characterizing the variation of the corresponding effective diffusion coefficient (De) over time. All experiments were performed on samples consisting of a hardened high porosity OPC paste and Na-montmorillonite. These model materials were chosen in order to simplify the mineralogy of the system. The experiments allowed obtaining relevant information regarding the development of the diffusive properties and the reactivity of such a cement-clay interface system. HTO and 36Cl− were used as tracers to study the evolution of both the total and the anion accessible porosity. After six years of reaction a considerable reduction of the flux for both HTO and 36Cl− was observed. The flux of HTO did, however, not approach zero, which means that connected porosity for diffusive transport of water is still available. The periodic monitoring of the sample evolution showed a strong reduction of the effective diffusion coefficient De of the samples within the first 1.5 years of the experiment and a less prominent decrease in the period between 1.5 and 6 years. The De of 36Cl− showed a stronger reduction compared to that of HTO; for some cells no chloride flux at all could be measured anymore at t > ~4 a. Using additional information on the extension of porosity changes from a neutron imaging study performed in parallel on the same samples, the diffusive properties of each component of the interface, or of a clay zone with reduced porosity could be estimated. For HTO the relation between the evolution of De and of the porosity in the clay part can be well described with Archie's empirical law. For chloride large uncertainties regarding the accessible porosity do not allow a precise correlation. Whether a complete porosity clogging will take place or some fraction of connected pore space will persist in the sample over reaction times ≫ than 6 years remains an open question.

Effect of the pore water composition on the diffusive anion transport in argillaceous, low permeability sedimentary rocks

The effect of the pore water composition on the diffusive anion transport was studied for two different argillaceous, low permeability sedimentary rocks, Opalinus Clay (OPA) and Helvetic Marl (HM). The samples were saturated with different solutions with varying molar concentration and different main cations in the solution: NaCl based pore solutions and CaCl2 based pore solutions. The total porosity was measured by through-diffusion experiments with the neutral tracer HTO. Experiments performed in NaCl solutions resulted in a porosity of 0.12 for OPA and 0.03 for HM, and are consistent with results of the experiments in CaCl2 solutions. The total porosity was independent of the molar concentration, in contrast to the measured anion porosity, which increased with increasing molar concentration. It could further be observed that the pore solution based on the bivalent cation calcium shielded the negative surface charge stronger than the monovalent cation sodium, resulting in a larger measureable anion-accessible porosity in the case of CaCl2 solutions.The data was modelled based on an adapted Donnan approach of Birgersson and Karnland (2009). The model had to be adjusted with a permanent free, uncharged porosity, as well as with structural information on the permanent anion exclusion because of so-called bottleneck pores. Both parameters can only be evaluated from experiments. Nevertheless, taking these two adaptions into account, the effect of varying pore water compositions on the anion-accessible porosity of the investigated argillaceous rocks could be satisfactorily described.

A comparative anion diffusion study on different argillaceous, low permeability sedimentary rocks with various pore waters

Through-diffusion experiments with tritiated water (HTO) and 36Cl− as a function of pore water concentration (0.01–5 M) were performed on two Ordovician-age argillaceous rock samples from the Blue Mountain Fm and Queenston Fm shales of the Paleozoic intracratonic Michigan Basin in Canada. This study reveals that the effect of ionic strength on the anion-transport porosity is similar, and only the minimal anion excluded porosity is higher in the Blue Mountain Fm shale. The differences in rock sample mineralogy cannot explain this effect. It is hypothesized that the structure of the Blue Mountain Fm shale samples has led to pore space openings sufficiently small that they behave as interlayers. Such pores are defined as interlayer equivalent (ILE) pores. These ILE pores, as in the case of interlayer pores, can act to permanently limit the anion-accessible porosity. Pore-size distribution measurements provide further evidence of increased potential for ILE pores within the Blue Mountain Fm samples. A Donnan model, which includes consideration of both ILE and uncharged pores, is shown to describe the effect of molar concentration on the anion-accessible porosity in the argillaceous rocks investigated.

Importance of Interlayer Equivalent Pores for Anion Diffusion in Clay-Rich Sedimentary Rocks.

The anion exclusion behavior in two different clay stones, Opalinus Clay (OPA) and Helvetic Marl (HM), was studied using a well-established experimental through-diffusion technique. The ionic strength of the pore water was varied between 0.01 and 5 M to evaluate its effect on the diffusion of HTO and 36Cl-. The total porosity determined by HTO-diffusion was independent of the ionic strength, while the anion accessible porosity varies with the ionic strength of the pore water. In the case of Opalinus Clay, the anion accessible porosity increases from 3% at low ionic strength (0.01 M) up to 8.4% at high ionic strength (5 M), whereas the anion accessible porosity of Helvetic Marl increases from 0.6% up to only 1.1%. The anion exclusion effect in HM is thus more pronounced than that in OPA, even at high ionic strength. This observation can be correlated to differences in mineralogy and to the fact that HM has a larger fraction of interlayer equivalent pores. Interlayer equivalent pores are small pores in compressed clay stones that are small enough to have, because of overlapping electric double layers, properties similar to those of interlayers and are therefore rather inaccessible for anions.

Characterization of Pore Water, Ion Transport and Water-rock Interaction in Claystone by Advective Displacement Experiments

Displacement of preserved pore water from claystones by imposing a hydraulic gradient with an artificial pore water yields early extracts characteristic of the pore water. Long-term tracer breakthrough behavior provides transport properties and anion-accessible porosity, whereas elution of major components are controlled by ion exchange and mineral solubility. A single long-term experiment provides a comprehensive system understanding.

Constraining porewater chemistry in a 250 m thick argillaceous rock sequence

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 anion-accessible 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.

Diffusion behavior of selenite in purified bentonite

Selenium (Se) is an important element for assessing the safety of high-level waste disposal. In this study, the diffusion coefficients of selenite in purified bentonite were investigated. A Japanese purified sodium bentonite, Kunipia-F, which contains approximately 99 wt% montmorillonite, was used in this experiment. Bentonite powder was compacted into cylinders (diameter, 10 mm; height, 10 mm; dry density, 0.8–1.6 Mg m−3). Each compacted bentonite specimen was inserted into an acrylic resin column and saturated with 0.01 M–1.0 M of NaCl (aq.) for 30 days. One side of the bentonite was spiked with 10 μL of tracer solution containing 0.13 M Na2SeO3 and it was kept at 10, 25, 40, or 55 °C for 1–29 days. The apparent diffusion coefficients of selenite in compacted bentonite were 2.5 × 10−11 to 1.9 × 10−13 m2 s−1 and decreased with the increase in dry density. The ionic strength showed no significant effect on the apparent diffusion coefficient of selenite for bentonite densities higher than 1.4 Mg m−3. However, the apparent diffusion coefficients for 1 M NaCl were several times larger than those in 0.1 or 0.01 M NaCl for densities lower than 1.2 Mg m−3. This increase in diffusion coefficient would be caused by a change in anion accessible porosity. The apparent diffusion coefficient would be explained by Archie's law and is proportional to exponentiation of porosity. It is desired to measure exact anion accessible porosity. The activation energy of selenite in purified bentonite was 20 ± 4 kJ mol−1 and was similar to the activation energies of oxo-anions in free water. This also indicates that selenite diffuses in free pore water in the bentonite.

Applying the squeezing technique to highly consolidated clayrocks for pore water characterisation: Lessons learned from experiments at the Mont Terri Rock Laboratory

Knowledge of the pore water chemistry in clayrock formations plays an important role in determining radionuclide chemical speciation and migration in the context of nuclear waste disposal. Among the different in situ and ex situ techniques for pore water sampling in clay sediments and soils, the squeezing technique dates back 115years. Although different studies have been conducted on the reliability and representativeness of squeezed pore waters, most of these involved high-porosity, high water content and unconsolidated clay sediments. Very few of them analysed squeezed pore water from low-porosity, low water content and highly consolidated clayrocks.In this paper, a specially designed and fabricated one-dimensional compression, two-directional fluid flow cell was used to extract and analyse the pore water composition of Opalinus Clay core samples from Mont Terri (Switzerland) with water contents between 6.2 and 7.8wt.%. A study of the influence of the squeezing pressure on the chemistry of the pore water was performed at pressures of up to 200MPa. To validate and demonstrate the validity of the squeezing technique for obtaining reliable pore water, different tests were performed at various pressures and the squeezed waters were compared with those obtained by other methods, such as in situ collected borehole waters. The reproducibility and quality of the squeezing method was also checked, as well as different artifacts which could affect the reliability of the pore water data.The results show that the reproducibility of the technique is good and no membrane effect (ionic ultrafiltration, chemical fractionation, anion exclusion) was found in Opalinus Clay at Mont Terri in the range of pressures analysed (70–200MPa). Pore waters extracted in this range of pressures do not decrease in concentration as a function of pressure, which indicates that a dilution of water by mixing of the free pore water and the outer layers of diffuse layer water does not occur and a threshold (safety) squeezing pressure of 175MPa could be established to avoid membrane effects. Furthermore, a direct comparison against in situ collected borehole waters shows that the pore waters extracted at these pressures are representative of the Opalinus Clay Formation at Mont Terri. Extraction artifacts such as temperature effect, oxidation and degassing can be avoided or minimised by taking special precautions when preparing and handling the core sample and extracting the pore water. Finally, we show that it is possible to obtain insightful information on anion-accessible porosity by combining squeezing data with aqueous leaching data to characterise the pore water composition and solute distribution for different rock porosities.

Profiles of chloride and stable isotopes in pore-water obtained from a 2000 m-deep borehole through the Mesozoic sedimentary series in the eastern Paris Basin

Water stable isotopes and chloride profiles in pore-water through more than 800m of sediments were obtained from a 2000m-deep borehole (EST 433) drilled by Andra in the eastern Paris Basin. Vapour exchange method and aqueous leaching were used to obtain the stable isotope and chloride concentrations of pore-water from 24 rock samples. Petrophysical measurements included water contents, grain densities and porosities of the studied formations. Pore-water and some groundwater samples collected during the drilling are mainly of meteoric origin: they plot near the Global Meteoric Water Line, distributed between heavy-isotope depleted Oxfordian groundwater and enriched Triassic groundwater, in good agreement with previous data. The δ2H and δ18O values describe curved profiles in the Callovo-Oxfordian formation, and show an increase with depth below this formation (Dogger and Liassic). Similar trends were observed for the chloride concentrations, except in the Liassic formation where they are more or less constant. The low chloride concentrations in the basal Jurassic layers indicate that the source of salinity to the Dogger aquifer is likely the middle Liassic formation and not the Triassic salt as previously suggested. A preliminary modelling exercise showed that currently available diffusion parameters (diffusion coefficients and accessible porosities) might be used to properly simulate these exchanges for deuterium. This is not the case for chloride, perhaps because the used values for anion accessible porosity were not relevant and/or the applied modelling conditions were unsuitable.

Consistent interpretation of the results of through-, out-diffusion and tracer profile analysis for trace anion diffusion in compacted montmorillonite

Literature data for anion diffusion in compacted swelling clays contain systematic inconsistencies when the results of through-diffusion tests are compared with those of out-diffusion or tracer profile analysis. In the present work we investigated whether these inconsistencies can be explained by taking into account heterogeneities in the compacted samples; in particular increased porosities at the clay boundaries. Based on the combined results of out-diffusion, tracer profile analysis and the spatial distribution of the electrolyte anion in the clay, we conclude that the inconsistencies can indeed be resolved by taking into account a heterogeneous distribution of the total and the anion-accessible porosity. This, by definition, leads to a position dependence of the effective diffusion coefficient. Neglecting these effects results in a rather subordinate systematic error in the determination of effective diffusion coefficients of anions from through-diffusion tests with clay thicknesses in the centimetre range. However, stronger errors in terms of absolute values and conceptual interpretation may be introduced in out-diffusion tests and profile analyses of the diffused tracer. We recommend that anion diffusion tests should be accompanied by measurements of the total and anion-accessible porosity as a function of position in the direction of diffusion.