Eric C Gaucher
email: e.gaucher@brgm.fr

Philippe Blanc

Frédérique Bardot

Gilles Braibant

Stéphane Buschaert

Catherine Crouzet

Anne Gautier

Jean-Pierre Girard

Emmanuel Jacquot

Arnault Lassin

Gabrielle Negrel

Christophe Tournassat

Agnès Vinsot

Scott Altmann

MODELING THE POREWATER CHEMISTRY OF THE CALLOVIAN-OXFORDIAN FORMATION AT A REGIONAL SCALE MODELISATION REGIONALE DE LA CHIMIE DES EAUX PORALES DE LA FORMATION DU CALLOVO-OXFORDIEN

Keywords: Équilibre eau/argiles, Eau interstitielle, Argilites, Callovo-Oxfordien, ANDRA clay/water equilibrium, porewater, argillites, Callovian-Oxfordian, ANDRA

Au sein des études menées par l'ANDRA pour caractériser le Callovo-Oxfordien, la chimie de l'eau interstitielle constitue une thématique clé car elle détermine le devenir dans le temps des matériaux introduit sur les sites de stockage (bentonite, béton, métaux, colis de verre). Elle détermine aussi la spéciation (et donc la mobilité) des radionucléides. La méthode développée dans le cadre du projet THERMOAR permet l'acquisition d'un jeu complet de données pour modéliser la chimie des eaux à partir de carottes de roche. Elle nécessite une étude minéralogique approfondie, un modèle de répartition eau libre/eau liée, des expériences de lessivage, des mesures des ions adsorbés, l'acquisition de constantes d'échange d'ions, la mesure des pressions partielles en CO 2 . L'ensemble de ces expériences et mesures a été appliqué à des échantillons provenant du site du laboratoire Meuse/Haute-Marne et des forages régionaux de l'ANDRA. On observe ainsi la stabilité régionale d'un grand nombre de paramètres à l'exception d'une diminution de teneur en Na et Cl suivant un axe nord-est / sud-ouest passant par le laboratoire. Le modèle d'équilibre eau/roche permet de calculer la composition chimique des eaux interstitielle de la formation.

INTRODUCTION

L'Agence Nationale pour la gestion des Déchets Radioactifs (ANDRA) construit un laboratoire de recherche souterrain sur la commune de Bure (NE, France), pour étudier la faisabilité d'un stockage profond de déchets radioactifs au sein de la formation argileuse du Callovo-Oxfordien (COX, ~490m).

Pour connaître la durabilité chimique des matériaux introduits sur le site (verres, aciers, bétons, barrières ouvragées argileuses, …) et la mobilité des radioéléments dans cet environnement, il est nécessaire de connaître la chimie des eaux de la formation. Pour répondre à cet objectif, l'ANDRA a carotté un grand nombre d'échantillons au niveau du laboratoire et dans son environnement régional.

Ces carottes ont permis d'obtenir un grand nombre de données géochimiques utilisables pour définir la chimie des eaux interstitielles de la formation à une échelle régionale. La littérature scientifique dans ce domaine [START_REF] Bath | Water-rock interactions in mudrocks and similar low permeability material[END_REF][START_REF] Sacchi | Porewater extraction from argillaceous rocks for geochemical characterisation. Methods and interpretation[END_REF] montre que la définition de cette chimie est très difficile. L'extraction de l'eau interstitielle par pressage [START_REF] Fernández | Water sampling by squeezing drillcores[END_REF][START_REF] Fernández | Analysis of the pore water chemistry obtained by squeezing from rock samples of the Meuse Haute-Marne underground laboratory[END_REF], ou l'échantillonnage des eaux en forages spéciaux à chambre sous N 2 [START_REF] Griffault | Water sampling and analyses for boreholes and seepages[END_REF][START_REF] Vinsot | Gas equilibration and pore water sampling experiment in the Opalinus Clay at the Mont Terri Rock Laboratory[END_REF][START_REF] Wersin | Results from the Porewater Chemistry (PC) experiment at Mont Terri[END_REF] génèrent des artéfacts modifiant dans des proportions variables cette chimie, l'oxydation des échantillons étant l'artéfact le plus difficile à maîtriser [START_REF] Fernández | Analysis of the pore water chemistry obtained by squeezing from rock samples of the Meuse Haute-Marne underground laboratory[END_REF][START_REF] De Craen | High sulphate concentrations in squeezed Boom Clay pore water: evidence of oxidation of clay cores[END_REF]. Dans le cadre du partenariat BRGM-ANDRA, une méthode a été développée d'abord au sein du laboratoire de recherche du Mont Terri [START_REF] Pearson | Geochemistry of water in the Opalinus Clay Formation at the Mont Terri Rock Laboratory[END_REF] puis appliquée à Bure. A partir d'un jeu complet de données géochimiques, il est possible de construire des modèles d'équilibre eau/argilite sans ajustement de paramètres. Dans les modèles antérieurs, Bradbury et Baeyens [START_REF] Bradbury | A physicochemical characterisation and geochemical modelling approach for determining porewater chemistries in argillaceous rocks[END_REF] étaient obligés d'estimer la pCO 2 , Beaucaire et al., [START_REF] Beaucaire | Regional fluid characterisation and modelling of water-rock equilibria in the Boom clay Formation and in the Rupelian aquifer at[END_REF] négligeaient les argiles 2 :1 et les réactions d'échange d'ions, ou encore Motellier et al., [START_REF] Motellier | Modelling of the ion-exchange properties and indirect determination of the interstitial water composition of an argillaceous rock. Application to the Callovo-Oxfordian low-water-content formation[END_REF] faisait l'impasse sur les teneurs en sulfates.

SITUATION GEOLOGIQUE

La zone étudiée par l'ANDRA [START_REF] Gaucher | ANDRA underground research laboratory: Interpretation of the mineralogical and geochemical data acquired in the Callovian-Oxfordian Formation by investigative drilling[END_REF] concerne un bloc structural homogène ceinturé par des accidents structuraux (Fig. 1). Le COX imperméable est recouvert par l'Oxfordien calcaire et le Kimméridgien qui présentent quelques niveaux producteurs en eau. Il surmonte le Dogger peu productif en eau, le Lias qui est imperméable et les formations triasiques qui contiennent des saumures.

ACQUISITION DES DONNEES EXPERIMENTALES

Les paramètres utilisés pour construire les modèles sont les suivants : (1) une identification exhaustive des minéraux et une qualification de leur état cristallin (Tableau I) ; (2) une qualification de l'état réduit du système compte tenu de la présence de pyrite et d'une capacité réductrice de la roche en moyenne de 4,7 meq/g (meq d'électrons par g de solide sec) ; (3) une mesure de la teneur en eau (Tableau II) ; (4) une mesure des surfaces spécifiques mouillées par l'eau pour calculer la répartition entre eau libre et eau liée tenant compte des phénomènes d'exclusion anionique [START_REF] Horseman | Water, gas and solute movement through argillaceous media[END_REF][START_REF] Pearson | What is the porosity of a mudrock?[END_REF] existant dans les bancs argileux de la formation (Tableau III) ; [START_REF] Griffault | Water sampling and analyses for boreholes and seepages[END_REF] des expériences de lessivage pour connaître les teneurs en anions dissous (Tableau IV, Fig. 2) ; [START_REF] Vinsot | Gas equilibration and pore water sampling experiment in the Opalinus Clay at the Mont Terri Rock Laboratory[END_REF] des mesures de cations adsorbés (Fig. 4) ; [START_REF] Wersin | Results from the Porewater Chemistry (PC) experiment at Mont Terri[END_REF] l'acquisition des constantes d'échange ionique spécifiques de l'argilite (Tableau VI, Fig. 5) ; (8) la mesure des pressions partielles en CO 2 (Fig. 7) ; (9) la mesure des δ 13 C du CO 2 (Fig. 8). Les protocoles expérimentaux correspondant peuvent être trouvés dans [START_REF] Gaucher | ANDRA underground research laboratory: Interpretation of the mineralogical and geochemical data acquired in the Callovian-Oxfordian Formation by investigative drilling[END_REF][START_REF] Gaucher | Specific determination of ion exchange constants: the Opalinus Clay example[END_REF][START_REF] Girard | Stable isotope composition of CO 2 outgassed from cores of argillites: a simple method to constrain δ 18 O of porewater and δ 13 C of dissolved carbon in mudrocks[END_REF][START_REF] Gaucher | A new analytical method to constrain the partial pressure of CO 2 in mudrocks[END_REF].

RESULTATS ET MODELISATION

Les observations montrent une grande homogénéité dans la nature des minéraux rencontrés à l'échelle régionale. Les constantes d'échange ionique correspondent à celles attendues pour des argiles de types illite et smectite [START_REF] Appelo | Geochemical calculations and observations on salt water intrusions. II. Validation of a geochemical model with laboratory experiments[END_REF][START_REF] Griffault | Acquisition and regulation of the water chemistry in clay environment. Project "Archimède[END_REF][START_REF] Fletcher | The chemical modeling of clay/electrolyte interactions for montmorillonite[END_REF][START_REF] Gaucher | A new analytical method to constrain the partial pressure of CO 2 in mudrocks[END_REF]. La composition isotopique du CO 2 montre que la concentration de ce gaz dans l'eau de la formation est régulée de manière constante par l'équilibre entre la solution et les minéraux de la roche comme cela est prévu par Coudrain-Ribstein et al. [START_REF] Coudrain-Ribstein | Temperature-carbon dioxide partial pressure trends in confined aquifers[END_REF]. En revanche les teneurs en Cl -chutent de 100 à 15 mmol/L en suivant un axe NE/SW (du forage EST312 au forage EST322) (Fig. 3). Cette variation s'accompagne d'une évolution des cations adsorbés (Fig. 4). Compte tenu du profil de concentration en Cl -du Dogger, on peut émettre l'hypothèse d'une entrée de solutions salées provenant du Trias par un système de failles situé au NE du forage EST312 et diffusant horizontalement au sein du Dogger (les gradients hydrauliques assez mal connus paraissent très faibles dans cette formation). Le Dogger influence nettement la chimie des eaux du COX.

Le modèle d'équilibre thermodynamique entre la solution, la minéralogie, les surfaces argileuses, et la pCO 2 utilisant le code PHREEQC [START_REF] Parkhurst | User's guide to PHREEQC (Version 2). A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations[END_REF] et la base de données llnl.dat [START_REF] Wolery | EQ3/6, a software package for geochemical modeling of aqueous systems: Package overview and installation guide[END_REF] a permis de calculer plusieurs compositions d'eau compte tenu d'hypothèses sur les valeurs de pCO 2 mesurées. Le modèle de Coudrain-Ribstein et al., [START_REF] Coudrain-Ribstein | Temperature-carbon dioxide partial pressure trends in confined aquifers[END_REF] ne fixant pas la pCO 2 mais régulant ce paramètre par un assemblage de minéraux en équilibre avec la solution a été testé et donne de bons résultats (Tableau VII).

Appliquée à une échelle régionale, la modélisation a permis de calculer la variabilité de la composition chimique des eaux (Fig. 8).

CONCLUSION

La méthode employée permet de définir la chimie des eaux sans estimation de paramètres. Cependant, différents modèles peuvent être testés compte tenu des informations géochimiques recueillies. Cela permet de donner une gamme de compositions relativement restreinte pour la chimie des eaux au niveau du laboratoire souterrain de Bure. Les variations de teneurs en Na et Cl suivant un axe NE/SW au sein de la formation sont imputables au Dogger. Il conviendrait de construire un modèle 2D rendant compte de cette dynamique de transport.

INTRODUCTION

Context of the study

The French Agency for Nuclear Waste Management (ANDRA) is building an underground research laboratory (URL) in Bure (northeastern France) to study the disposal of radioactive waste in a deep clay formation. The main gallery is located in the Callovian-Oxfordian formation (COX) at an approximate depth of 490 m. To assess the durability of materials (glass, stainless steel, concrete, engineered clay barrier, etc.) and determine the mobility of the radionuclides in the host rock formation, it is necessary to define the chemistry of the COX porewater. ANDRA has therefore drilled a series of boreholes in the vicinity of the URL (Fig. 1). Core drilling provides a vast amount of geochemical data. This paper presents a method enabling the modeling of the porewater chemistry. This method is applied at a regional scale and enables a proper description of the chemical conditions in the formation.

Studying the geochemistry of clay systems

Studying the water chemistry of an argillaceous formation is a challenging task. The recovery of undisturbed water samples by classical methods is almost impossible due to the low permeability and low water content. Recently developed techniques for water extraction (squeezing, leaching, special borehole with gas control) give valuable results that can be used to characterize the water chemistry.

However, each of these techniques introduces experimental and analytical artifacts. A review of studies of the porewater chemistry of mudrocks and extraction techniques can be found in [START_REF] Bath | Water-rock interactions in mudrocks and similar low permeability material[END_REF][START_REF] Sacchi | Porewater extraction from argillaceous rocks for geochemical characterisation. Methods and interpretation[END_REF]. The chemistry of porewater in clay systems is difficult to define for several reasons including the very low porewater content and its strong interactions with clay minerals. The physical interaction between water and minerals means that either high pressure must be used to recover water by squeezing, or long-term sampling must be done to recover water in special boreholes in URLs. The high pressures used for squeezing techniques can dissolve some minerals. Even for conservative anions like Cl, squeezing techniques generate artifacts [START_REF] Fernández | Water sampling by squeezing drillcores[END_REF][START_REF] Fernández | Analysis of the pore water chemistry obtained by squeezing from rock samples of the Meuse Haute-Marne underground laboratory[END_REF]. Sampling in special boreholes requires preventive measures. As the porewater slowly fills the borehole chamber (specifically drilled and/or stored under N 2 to minimize the oxidation) [START_REF] Griffault | Water sampling and analyses for boreholes and seepages[END_REF][START_REF] Vinsot | Gas equilibration and pore water sampling experiment in the Opalinus Clay at the Mont Terri Rock Laboratory[END_REF], it will degas and bacterial development can perturb the system [START_REF] Wersin | Results from the Porewater Chemistry (PC) experiment at Mont Terri[END_REF]. The reducing conditions that prevail in such systems could be another major source of concerns.

Oxidation might significantly modify the mineralogy and the porewater quality [START_REF] Fernández | Analysis of the pore water chemistry obtained by squeezing from rock samples of the Meuse Haute-Marne underground laboratory[END_REF][START_REF] De Craen | High sulphate concentrations in squeezed Boom Clay pore water: evidence of oxidation of clay cores[END_REF]. At the contrary, growing of anaerobic bacteria can strongly reduce the system [START_REF] Wersin | Results from the Porewater Chemistry (PC) experiment at Mont Terri[END_REF].

Development of the THERMOAR method

In a preliminary step to the URL porewater recovering experiments, a strategy to characterize and model the geochemical system was developed within the framework of the THERMOAR project through an ANDRA-BRGM partnership. The method was first developed for Opalinus Clay (Switzerland, [START_REF] Pearson | Geochemistry of water in the Opalinus Clay Formation at the Mont Terri Rock Laboratory[END_REF]). The first samples from Bure came from the EST205 borehole drilled on the URL site where the laboratory's auxiliary shaft has later been dug. Many additional samples from regional boreholes drilled later by ANDRA have been added to the original data set. Most of the geochemical data obtained by ANDRA on the formation have been used to define the porewater chemistry at a regional scale.

The construction of the thermodynamic model enables us to define the porewater chemistry and also to describe reaction pathways. With this tool, we are able to model perturbations (alkaline solution, oxidation, microbial disturbance, etc.). In the THERMOAR method, data for a complete set of physical and chemical parameters were obtained to constrain the model. The originality of this approach is that no parameters are estimated and all major mineral phases are taken into account. In the past, clay porewater models have generally prescribed a hypothetical CO 2 partial pressure (e.g., the model developed by Bradbury and Baeyens for the Opalinus Clay formation (Switzerland) [START_REF] Bradbury | A physicochemical characterisation and geochemical modelling approach for determining porewater chemistries in argillaceous rocks[END_REF]).

Beaucaire et al., [START_REF] Beaucaire | Regional fluid characterisation and modelling of water-rock equilibria in the Boom clay Formation and in the Rupelian aquifer at[END_REF] disregard 2:1 clay and ion exchange reactions in a model for the Boom Clay porewater. Another model for the Callovian-Oxfordian formation developed by Motellier et al., [START_REF] Motellier | Modelling of the ion-exchange properties and indirect determination of the interstitial water composition of an argillaceous rock. Application to the Callovo-Oxfordian low-water-content formation[END_REF] includes ion exchanges for 3 types of sorption sites, but calculates the aqueous speciation partially and does not take into consideration SO 4 2-content.

After a short description of the geology of the area, the data are presented and used to constrain the models. As several options are possible, the results of a series of models are presented and the most realistic model for the URL was applied to the evolving chemical conditions observed northeast and southwest of the URL site.

GEOLOGICAL SETTING

The geology of the site is described in detail in [START_REF] Gaucher | ANDRA underground research laboratory: Interpretation of the mineralogical and geochemical data acquired in the Callovian-Oxfordian Formation by investigative drilling[END_REF]. The Eastern Paris Basin sedimentary formations Some levels in the Kimmeridgian, Oxfordian and Dogger limestone are permeable. In the last two formations, water productivity is low. The hydraulic gradient in the Dogger is known has low. The COX is considered as impermeable. The Lias is impermeable or an aquitard formation. The Triassic is known to contain brines.

MATERIALS AND METHODS

Origin of the core samples

The location of the boreholes is shown in Fig. 1. Boreholes EST205, EST207, EST212 concern the URL and additional data were obtained on samples from the laboratory auxiliary access shaft (PAX).

The distance from the URL to boreholes MSE101, EST312, EST322 is 12.5, 15.5 and 9 km, respectively. They are located, respectively, NW, NE and SW of the URL. All of these boreholes and shafts are located in the same structural block. Borehole EST342 is located beyond the "Marne graben" and "Joinville graben" structural accidents. The depths of boreholes in figures 3 and 6 are normalized to depths corresponding to borehole EST205 (URL site). As the thicknesses are not identical at all locations, the correspondence of depths between boreholes is not perfect.

Borehole EST205 was drilled with an oil-based mud consisting of 80 % (by volume) oil (gas-oil containing 0.4 % aromatics) and 20 % brine (CaCl 2, 350 g/l). The oil and brine mixture forms an emulsion whose high Ca content prevents swelling of the clay minerals. The addition of CaCO 3 to the mud helps to obtain densities above 1.1 necessary for coring in clay formations. This type of drilling mud helped prevent the mechanical evolution of the core samples and the swelling of clay minerals.

However, the brine imposed a very high Cl gradient, which contaminated the cores by diffusion. An immediate treatment of the cores eliminates the edges, which preserves the chemistry of the samples.

This was done for a limited number of borehole EST205 samples. The authors do not recommend the use of Cl data published in [START_REF] Gaucher | ANDRA underground research laboratory: Interpretation of the mineralogical and geochemical data acquired in the Callovian-Oxfordian Formation by investigative drilling[END_REF] which are too high because of the contamination by the oil-based mud.

(The same problem exists for borehole EST211). The samples from the shafts were air drilled without mud. The mud used for boreholes EST212, EST207, EST342, EST312, EST322, MSE101 was a common mixture of bentonite and polymers. No major contamination was detected for this last series of boreholes.

Physical and geochemical parameters

The parameters used in the model are the following:

• Mineral identification and crystallinity of minerals;

• The total reduced capacity of the rock (TRC);

• Water content and argilite texture;

• Anion leaching;

• Cation exchange capacity (CEC) and adsorbed cation population;

• Ion exchange constants (K H/Na , K K/Na , K Ca/Na , K Mg/Na , K Sr/Na );

• CO 2 (g) partial pressure and isotopic signature of the gas.

The analytical techniques used to identify and characterize the minerals, measure the TRC, determine the water content and textural properties, the leaching of anions, the cation exchange capacity and the adsorbed cation population are detailed in [START_REF] Gaucher | ANDRA underground research laboratory: Interpretation of the mineralogical and geochemical data acquired in the Callovian-Oxfordian Formation by investigative drilling[END_REF]. The experimental protocol used to determine ion exchange constants has been published in [START_REF] Gaucher | Specific determination of ion exchange constants: the Opalinus Clay example[END_REF]. The method used to obtain the pCO 2 (g) and the isotopic signature of the gas is presented in [START_REF] Girard | Stable isotope composition of CO 2 outgassed from cores of argillites: a simple method to constrain δ 18 O of porewater and δ 13 C of dissolved carbon in mudrocks[END_REF].

EXPERIMENTAL AND MODELING

4.1 Experimental data

Identification and crystallinity of minerals

An accurate modeling of the water-rock interactions takes into account all minerals present, even at trace levels, and requires a complete mineralogical analysis of the samples. Because several minerals can control the aqueous speciation of an element, it was also necessary to determine some crystallinity criteria. Indeed, any mineral having alteration figures or an unbalanced structural formula is not considered at equilibrium in the model. Table I lists all of the minerals found in the Callovian-Oxfordian Formation with their relative abundance, distribution, and information on their crystallinity.

Table I also indicates whether the phase must be taken into account in the thermodynamic model.

Total reduced capacity of the rock (TRC)

The TRC is an index of the redox potential of the rock. It represents its "oxidizability" and is expressed in term of mole of available electrons. The mean value for the formation is 4.7 ± 1.3 meq/g.

Theses values are similar to those measured in the Opalinus Clay Formation, 4.8 ± 0.9 meq/g [START_REF] Waber | Aqueous leachates and cation exchange properties of Mont Terri claystones[END_REF].

TRC values do not vary a lot in the clay formation. The lower values are mainly due to calcareous beds (1.6 meq/g). TRC is linked to the organic matter content and mainly to sulfide minerals and to a lesser extent to siderite. Conditions in the formation are clearly reducing. In the model, the redox potential can be imposed by pyrite.

Water content and argilite texture

The Callovian-Oxfordian Formation has a very low water content. Water content ranges from 6 % to 9 % (mass water content dry base). Low contents (1.5 %) correspond to levels rich in fine-grained carbonate cement. There is a very good correlation between high carbonate content (> 65 %) and very low water content (< 4 %). At the regional scale, variations in water content seem to be insignificant (Table II).

The water in the clay is of four different types [START_REF] Horseman | Water, gas and solute movement through argillaceous media[END_REF]:

• Interlayer water within the sheet structure of clay minerals. This water is strongly adsorbed;

• another strongly adsorbed water at the clay mineral surface is the (structured) water participating in the hydration shells of the cations (which locally balance the mineral surface charges);

• loosely-held water in the diffuse-layer, and;

• free water.

For geochemical purposes, it is convenient to distinguish only between "bound" water (which includes interlayer, strongly adsorbed and some diffuse layer waters) and "free" water (which includes not only Horseman's free water [START_REF] Horseman | Water, gas and solute movement through argillaceous media[END_REF], but also some of the diffuse layer and surface-sorbed waters as well) [START_REF] Pearson | What is the porosity of a mudrock?[END_REF].

Note that the concept of geochemical porosity has been developed for clay to take into account anionic repulsion due to the negatively charged surface of the clay. Anionic repulsion limits the access of anions to part of the porosity. Geochemical porosity is the porosity that is accessible to mobile anions (such as Cl or Br, for example). In a carbonaceous rock, anionic repulsion is limited and anions have access to all of the porosity. This distinction between geochemical porosity and total porosity for argillaceous rock is approximate, however practical. Thus, it is possible to quantify the two types of water by measuring the specific wetted surface using water sorption isotherms ( H2O BET S

) [START_REF] Gaboriau | Characterization and distribution of porewater in low permeable clayey formations: contribution of physical methods[END_REF]. For the argilite, the "bound" water is estimated to be about 2 to 3 water layers thick. Knowing the H2O BET S and the thickness of the water layer, the volume of "bound" water is estimated to be about 50 % of the total water volume (Table III). The ionic concentrations can be calculated either with the total water content (TW) or only with the "geochemical porosity" (GP). The advantages of the "geochemical porosity" concept will be demonstrated in the next paragraph.

Leaching of anions

Leaching of ions is a classic technique to obtain Cl -and Br -(and, in some cases, I -and SO Examples of concentrations of Cl -, Br -, SO 4 2-in the total water (TW) and in the geochemical porosity (GP) are given in table IV for borehole EST205 (external part of the samples immediately cut after drilling to avoid contamination).

An interesting observation was made during the calculation of Cl -concentrations in TW and GP. If we know the Cl -concentration in mg/kg of rock, we can calculate the Cl -concentration in the porewater. For the samples from the auxiliary shaft, the mean concentration of Cl -in the total porosity is 2.37×10 -2 ± 1.22×10 -2 mol/L. If a distinction is made between the argillaceous layers and the carbonaceous layers (water content < 4 %), two mean TW concentration values are observed (table V).

The Cl -concentration in the carbonaceous layers is about twice as great as in the argillaceous layers. If we consider that the concept of geochemical porosity is applicable only to the argillaceous layers, the Cl -concentration values in the two media are very similar. The Cl -content in the formation is rather homogenous at the URL site and a value of 3×10 -2 mol/L can be used in the model.

The Cl -contents were determined for all of the boreholes (Fig. 3). Cl -concentrations were calculated in the geochemical porosity for the argillaceous layers and in the total porosity for the calcareous layers. Dogger porewater was sampled in the regional boreholes and the Cl -concentrations are given on Fig. 3, which shows the strong influence of this formation on the Cl -content in the Callovian-Formation. The Cl -content varies with depth in borehole EST312. A straight line can be drawn from the Dogger at 110 mmol/l to the top of the formation at 40 mmol/L. For borehole MSE101 and at the URL site, there is less vertical variation-between 50-60 and 30 mmol/L. Although MSE101 is located northeast of the URL, no Cl -variations can be detected between the two locations. For the two boreholes located southwest of the site, the vertical variation is negligible (EST 322: 17 ± 10 mmol/L, EST342: 26 ± 23 mmol/L). There is, therefore, a regional variation with a decreasing concentration of Cl -from northeast to southwest.

One possible explanation for this might be that saline water migrates from the Triassic formation via a fault system located northeast of borehole EST312. Since there is a Cl -concentration gradient in the Dogger formation, diffusion or a very slow advection might explain the horizontal migration of Cl -.

The Dogger clearly influences the chemistry of porewater in the Callovian-Oxfordian Formation. Exchangeable Sr was not systematically determined at the regional scale. Only the concentrations of exchangeable K, Na, Mg, and Ca will, therefore, be discussed. No significant vertical variations were observed for the different boreholes. This is also true for EST312, which presents the major Cl evolution. From NE to SW, there are large variations in the exchangeable cation distribution (Fig. 4).

Cation exchange capacity (CEC) and adsorbed cationic population

CaX 2 values are identical in borehole EST312 and in the URL (EST205/PAX), and MgX 2 increases to the detriment of alkaline ions. For example, there is only half as much KX. Southwest of the URL site at borehole EST322, the proportion of MgX 2 is identical to that at the URL, there is a greater proportion of CaX 2 and only one-third as much NaX. Borehole EST342, located beyond the Marne and Joinville fault systems, has a greater proportion of NaX, to the detriment of MgX 2 .

Considering the ion exchange populations of the regional boreholes, we can conclude that, for the COX porewater:

• the proportion of Na decreases from NE to SW following the decrease of the Cl -content;

• the proportion of K also decreases, in a lesser extent, and;

• Ca and Mg concentrations are stable considering the electroneutrality.

Ion exchange constants (K H/Na

, K K/Na , K GT NH4\Na , K Ca/Na , K Mg/Na , K Sr/Na )

The work of many authors [START_REF] Pearson | Geochemistry of water in the Opalinus Clay Formation at the Mont Terri Rock Laboratory[END_REF][START_REF] Bradbury | A physicochemical characterisation and geochemical modelling approach for determining porewater chemistries in argillaceous rocks[END_REF][START_REF] Motellier | Modelling of the ion-exchange properties and indirect determination of the interstitial water composition of an argillaceous rock. Application to the Callovo-Oxfordian low-water-content formation[END_REF][START_REF] Gaboriau | Characterization and distribution of porewater in low permeable clayey formations: contribution of physical methods[END_REF][START_REF] Plummer | Geochemical modeling of the Madison aquifer in part of Montana, Wyoming and South Dakota[END_REF][START_REF] Appelo | Geochemical calculations and observations on salt water intrusions. II. Validation of a geochemical model with laboratory experiments[END_REF][START_REF] Griffault | Acquisition and regulation of the water chemistry in clay environment. Project "Archimède[END_REF][START_REF] Walraevens | Preferential pathways in an Eocene clay: Hydrogeological and hydrogeochemical evidence[END_REF] shows that there is a large consensus that ion exchange reactions are essential if we wish to model the porewater chemistry of clay-rich formations.

We therefore determined a complete set of ion exchange constants (K GT H\Na , K GT K\Na , K GT NH4\Na , K GT Ca\Na , K GT Mg\Na , K GT Sr\Na ) on the < 2 µm clay fractions using the Gaines-Thomas [START_REF] Gaines | Adsorption studies on clay minerals. II. A formulation of the thermodynamics of exchange adsorption[END_REF] approach. As opposed to [START_REF] Motellier | Modelling of the ion-exchange properties and indirect determination of the interstitial water composition of an argillaceous rock. Application to the Callovo-Oxfordian low-water-content formation[END_REF][START_REF] Jacquier | The ion-exchange properties of the Tournemire argillite: I. Study of the H, Na, K, Cs, Ca and Mg behaviour[END_REF], which use H + , our cation of reference is Na + . This gives constants that can be compared to data presented in the other studies after formalism adaptation [START_REF] Thellier | Quaternary cation exchange on Silver Hill illite[END_REF][START_REF] Fletcher | The chemical modeling of clay/electrolyte interactions for montmorillonite[END_REF].

For a detailed description of the following experimental method, see [START_REF] Gaucher | Specific determination of ion exchange constants: the Opalinus Clay example[END_REF].

Batch experiments were done using the purified clay fraction (< 2 µm) from a sample (K119 at 476 m) from borehole EST205. The clay (0.5 g) was equilibrated (72 h) with a solution (25 ml) of Na x A 1-x Cl 5×10 -3 mol/L (A = K or NH 4 + ) or Na x B 0.5-x Cl 5×10 -3 mol/L (B = Ca, Mg, Sr). The discrete variations of x enabled a progression of the ion exchange reactions. After separation (centrifuging and filtration), this first solution was analyzed and the equivalent fractions of aqueous species were determined. The clay was re-dispersed (24 h) in a concentrated solution of cobaltihexamine (4 g/l). After separation (centrifuging and filtration), this second solution was analyzed and the equivalent fractions of adsorbed species were calculated. During this calculation, the molar quantity of aqueous species remaining in the residual porewater (after the first centrifugation) was corrected. The equivalent fractions of aqueous and adsorbed Na and A (or B) were used to construct isotherms (Fig. 5).

Using the Gaines-Thomas approach [START_REF] Gaines | Adsorption studies on clay minerals. II. A formulation of the thermodynamics of exchange adsorption[END_REF], it is possible to calculate for the exchange sites X i -, the selectivity coefficient K i GT M\N (eq. 1) for an exchange between two cations M and N of charge m + and n + respectively. E M and E N (eq. 2) are the equivalent fractions of the adsorbed species. In equation 2, EC i is the exchange capacity of the X i -sites.

( ) ( ) ( ) ( ) n m m n m N n M N M GT i M N E E K + + = \ eq. (1) ( ) [ ] ( ) [ ] N M i i n n i N i m m i M E E EC EC N X n E EC M X m E + = = = + - + -

eq. (2)

[…]: Concentration of adsorbed species Isotherm data are processed from equation ( 1) using a non-linear regression [START_REF] Marquardt | An algorithm for least squares estimation of parameters[END_REF]. By modeling the Na/K isotherm, we were able to identify illite-type and smectite-type sites. For the Opalinus Clay, a similar fit was done with 2 types of sites [START_REF] Gaucher | Specific determination of ion exchange constants: the Opalinus Clay example[END_REF]. However, in this study, the global approach considering the ion exchange sites as a whole was tested for the modeling of porewater in neutral conditions. Therefore, in this paper, only the global constants are given in table VI.

CO 2(g) partial pressure and isotopic signature of the gas

The most original component of the THERMOAR project is the direct analysis of CO 2 partial pressure on core samples [START_REF] Gaucher | A new analytical method to constrain the partial pressure of CO 2 in mudrocks[END_REF]. The core samples were placed in a rigid container immediately after they had been drilled and the edges sawed off. Then the container was filled with an inert gas (N 2 , He, or Ar).

The results are slightly different, depending on the inert gas used. The best results were obtained when the initial total pressure in the container was sufficiently high (P tot > 800 mbar). The temperature and pressure in the cell were monitored continuously. The container was placed in a ventilated cupboard thermostated at a temperature of 25°C. The gases released from the container were analyzed by gas chromatography until the pCO 2 stabilized. At this point, analyses of CO 2 isotopes were done [START_REF] Girard | Stable isotope composition of CO 2 outgassed from cores of argillites: a simple method to constrain δ 18 O of porewater and δ 13 C of dissolved carbon in mudrocks[END_REF].

The mean pCO 2 values for the reference borehole EST205 are between 3.10 and 7.17 mbar (Log pCO 2 : -2.51 / -2.15). These results are in the range of data compiled by Coudrain-Ribstein et al., [START_REF] Coudrain-Ribstein | Temperature-carbon dioxide partial pressure trends in confined aquifers[END_REF] for sedimentary confined aquifers (1 to 100 mbar at 25 °C). Fig. 6 presents the data obtained at a regional scale in the best analytical conditions (P tot > 800 mbar). Values range between 1.4 and 4.1 mbar (Log pCO 2 : -2.9 / -2.4). No variation as a function of depth or at the regional scale can be observed. This can be a good indication of an internal control of this parameter by the mineralogy, the nature of which is constant throughout the formation. The isotopic composition of the gas confirms this. It shows (Fig. 7) that the CO 2 has an original isotopic composition independent of the mantle, the atmosphere, organic matter or bacterial activity. Considering the gas δ 13 C values, we were able to determine the isotopic composition of the bicarbonate ions at equilibrium with the gas. The values are between -2 and +2 ‰. The δ 13 C signatures of the bicarbonates and the carbonate cements of the clay rocks (mainly calcite and some dolomite and siderite) are similar. This proves that the pCO 2 is controlled by the mineralogical assemblage (carbonates) and not by degradation of organic matter or diffusion of gas coming from others geological formations.

Modeling porewater chemistry at a regional scale

Introduction

In the Callovian-Oxfordian Formation, the nature of the mineralogy and the pCO 2 regulation mechanisms can be considered as invariable at a regional scale. As the nature of the clay is also stable, the exchange selectivity coefficients can be used for the entire formation. The regional variation of the porewater chemistry was observed for Cl -and the ion exchange population. The model was initially constructed to define the porewater chemistry at the URL and, in a second step, was adapted to regional variations.

Constrains to model the porewater at the URL

Due to the stability of the mineralogy and the geochemical data, the porewater chemistry was considered to be constant in the clay formation at the URL location. All calculations were done at thermodynamic equilibrium. Due to the very low permeability of the clay formation and the probable residence time of the water, it is realistic to suppose that thermodynamic equilibrium can be reached in these geological media. The numerical model of the porewater chemistry involves two steps. First, seawater is diluted by pure water, and then the resulting water is equilibrated with the mineralogical assemblage and the exchange sites (X) of the clay. The dilution is defined so as to obtain the Cl - concentration determined in §4.1.4. The entire mineral assemblage ( §4.1.1) controls the following elements: e -, S, C, Si, Ca, Mg, Na, K, Sr, Fe, Al. Concerning the mineralogy, some simplifications have been considered: Pure phases (dolomite and siderite) and end members for chlorite solid-solution (Daphnite [Fe] and Clinochlore [Mg]) have been introduced in the modeling, in place of the observed minerals. Some geochemical subsystems can be distinguished in the model. Na is only determined by ion exchange (NaX). A subsystem Fe, S, e -, Sr, accounts for daphnite (Fe-Chlorite), celestite, pyrite, and ion exchange (SrX 2 ). A subsystem Si, Al, K is determined by quartz, illite, and ion exchange (KX). A subsystem C, Ca, Mg, is controlled by pCO 2 , calcite, and dolomite. The ion exchange equations for Ca and Mg are considered in the equation system. However, they are not the determining factors for the water chemistry. They can only adjust the distribution of cations on the exchanger. The pH is calculated by iteration to obtain electroneutrality of the solution.

The calculations are performed with the PHREEQC code [START_REF] Parkhurst | User's guide to PHREEQC (Version 2). A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations[END_REF] using the llnl.dat database [START_REF] Wolery | EQ3/6, a software package for geochemical modeling of aqueous systems: Package overview and installation guide[END_REF]. Three cases are presented in table VII. The simulations use the Cl -concentrations determined in the geochemical porosity. "A" simulation uses the maximum measured pCO 2 value and "B" simulation, a value in the best analytical range ( §4.1.7). "Chlo" corresponds to test cases where the pCO 2 is not fixed. Mg-chlorite was added to the phase assemblage as in the model of Coudrain-Ribstein et al., [START_REF] Coudrain-Ribstein | Temperature-carbon dioxide partial pressure trends in confined aquifers[END_REF]. In this model, the pCO 2 is not controlled externally and is determined by the equilibrium between calcite, dolomite, and the Mg-Chlorite.

Results: Composition of the porewater at the URL

Erreur ! Source du renvoi introuvable. shows that the solutions are relatively concentrated with an ionic strength of 10 -1 mol/L. The pH values are between 7.00 and 7.28. The Eh values are low (-176 mV < Eh SHE < -156 mV). The waters have a Cl/SO 4 facies. Na is twice as concentrated as Ca and Mg and four times as concentrated as K. The alkalinity is moderate, between 1 and 3 mmol/kg w . The Al and Si contents agree with a system controlled by silicates and aluminosilicates. The Fe contents (determined by the equilibrium with daphnite) are quite high. However, the model calculates a quasi equilibrium with siderite, which has been found in places in the formation. The logpCO 2 value calculated with the "Chlo" model (-1.96) is similar to the maximum measured logpCO 2 values (-2.15). This is a good indication that the model converges to a representative chemistry. Another possible verification is the proportion of adsorbed cations that are accounted for by model. The Table VII shows that the results are close to the measured values. On the contrary, if microcline is tested in the model instead of illite, the percentage of adsorbed cations is significantly different. This indicates that this phase does not participate in the regulation of the porewater chemistry.

Modeling porewater at the regional scale

To model the porewater at a regional scale, the "B" model described in §4.2.2 was adjusted using the Cl -concentrations obtained for boreholes EST312 and EST322 ( §4. 1.4). The corresponding adsorbed ion concentrations are considered in the model ( §4.1.5). Erreur ! Source du renvoi introuvable.

shows the evolution of the porewater chemistry, which is related mainly to a decrease in salinity (NaCl) from NE to SW in the structural block in which the URL is located. The decrease is of about 1 order of magnitude for the Cl and Na concentrations. The other concentrations are determined by the mineralogy and do not evolved significantly.

CONCLUSIONS

The THERMOAR method enables a detailed characterization of the geochemistry of the COX. The data set is complete and a model can therefore be developed without any estimated parameters. The different models presented in this paper for the ANDRA URL give an indication of the range of variation that can be calculated for the porewater chemistry. At the regional scale, the Cl -contents change as well as the adsorbed cations. It was also possible to model the porewater chemistry for the different regional boreholes. The challenge is now to model the transport of a saline porewater originating in the northeast and migrating horizontally in the Dogger Formation and then vertically and probably slowly in the Callovian-Oxfordian Formation.

The porewater chemistry model will also enable us to interpret future data obtained experimentally, directly in the ANDRA underground research laboratory.

  slightly dip (1 to 2°) to the West within a simple monocline related to subsidence of the central part of the basin. Except for major regional fault zones (the Bray, Vittel and Metz faults) and the Gondrecourt and Marne faults delimiting the area investigated by ANDRA (Fig.1), there is very little evidence of deformation.At Bure, the Callovian-Oxfordian (COX) argillite (conventionally referred to as C2) is about 130 m thick. Five lithostratigraphic units have been identified and labeled C2a, C2b1, C2b2, C2c and C2d, from base to top. The sedimentary series is primarily clayey in the bottom two-thirds, and becomes progressively siltier, and then increasingly calcareous towards the top. A remarkable sedimentary surface is associated with the maximum clay zone (MCZ, 489-486 m): the maximum flooding surface (MFS) of the Callovian-Oxfordian cycle. This cycle marks the transition from transgression at the base to a regression, which led to the installation of the Oxfordian carbonate platforms. The top of the clayey formation is located at a depth of around 417 m, overlapped by 243 m of Middle and Late Oxfordian limestone, followed by 175 m of Kimmeridgian marl and limestone, capped by the Barrois limestone of Kimmeridgian age. The clayey formation is separated from the Dogger limestone by a regional paleo-erosion surface. Under the Dogger limestone or marl (Bathonian, Bajocian, Aalenian), the Lias is marl-rich and capped the Triassic formations (sandstone and evaporite).

  in clay porewater. Information concerning cations, aqueous silica, carbonates and fluorides is generally disturbed by the dissolution of minerals. When plotted, concentrations of leached conservative anions give straight lines as a function of the solid/liquid ratio of the batch systems. The dissolution of a mineral gives a hyperbole (Fig. 2), and the oxidation of rock samples produces scattered values. The behavior of SO 4 2-during the leaching experiments with a regression coefficient (r 2 ) of 0.997 shows a perfect conservative character. The experiments seem to be undisturbed by pyrite oxidation. The Sr concentration value indicates that celestite is not significantly dissolved during leaching. The SO 4 2concentrations obtained by leaching represent probably the maximum values present in the porewater.

Figures

  Figures

Fig. 1 :Fig. 2 :.Fig. 3 :

 123 Fig. 1: Geological map of the study area with location of the Bure underground research laboratory (ANDRA URL) and regional boreholes. (Lambert Coordinates) Fig. 1: Carte géologique de la zone d'étude avec implantation du laboratoire souterrain de l'ANDRA (URL) et des forages régionaux. (Coordonnées Lambert)

Fig. 4 :Fig. 5 :Fig. 6 :

 456 Fig. 4: Variation in the proportion of adsorbed cations from northeast (EST 312 NE) of the URL (EST205/PAX) to southwest (EST 322 and EST 342, SW). Fig. 4: Evolution de la proportion des cations adsorbés du nord-est (EST 312 NE) du laboratoire souterrain (EST205/PAX) au sud-ouest (EST322 et EST 342, SW)

Fig. 7 :

 7 Fig. 7: Isotopic composition of CO 2 obtained by degassing of core samples from northeast (NE) to southwest (SW) of the URL. Comparison with various sources of CO 2 and with the isotopic compositions of calcites of the formation. Isotopic References: PDB: Peedee formation Belemnite, U.S.A ; SMOW: Standard Mean Ocean Water.

Fig

  Fig. 7:Composition isotopique du CO 2 obtenu par dégazage des carottes du nord-est (NE) au sud-ouest (SW) du laboratoire souterrain. Comparaison avec diverses sources de CO 2 et avec les compositions isotopiques des calcites de la formation. Références isotopiques : PDB : Bélemnite de la formation de Peedee, U.S.A ; SMOW : Standard Mean Ocean Water.

Fig. 8 :

 8 Fig. 8: Schoeller diagram: Porewater chemistry modeling. Comparison of "B" model (URL) with the models developed for EST312 (NE) and EST322 (SW). Fig. 8: Diagramme de Schoeller: Modélisation de la chimie des eaux interstitielles. Comparaison du modèle « B » (Laboratoire souterrain) avec les modèles réalisés pour EST312 (NE) et EST322 (SW).

  At the URL (EST205/PAX data), the mean CEC of the Callovian-Oxfordian Formation is 17.4 ± 6.1 meq/100 g. The lowest values are measured in the more calcareous beds (2.1 meq/100 g) and the

	highest values in the I/S R0 zone (25.1 meq/100 g at 463.74 m). The I/S R1 (less rich in smectite) has a
	very stable average value of 16.1 ± 0.6 meq/100 g.
	As Bradbury and Baeyens [10] pointed out, the adsorbed cation population is a reflection of the
	solution in equilibrium with the mineral assemblage. Exchange reactions are very rapid and adsorbed
	cations are immediately in equilibrium with the porewater. The proportion of exchangeable cations is,

therefore, very useful information. At the URL site, the distribution of exchangeable cations slowly evolves. In addition to the 4 % of exchangeable Sr 2+ , the adsorption sites are taken up mainly by 45.3 ± 1.6 % of Ca, 25.0 ± 2.1 % of Mg, 17.7± 1.9 % of Na, and 8.1 ± 1.1 % of K.

Table V :

 V Comparison of calculated concentrations of Cl in the calcareous and argillaceous layers as a function of the accessible porosity. Tableau V :Comparaison des concentrations en Cl calculées au sein des bancs argileux ou calcaires en fonction de l'accessibilité de la porosité.

	Est205 Sample & Depth (m)	H 2 O Specific Surface (m 2 /g) H 2 O size: 10,6 Å	Total water content (g/100 g) (dry base)	Free water (g/100 g) (dry base)	free water (%)
	K100 424m	30,0			6,13	3,79	61,8
	K108 447m	35,6			6,28	3,50	55,8
	K113 462m	36,0			6,18	3,34	54,1
	K119 477m	56,1			8,98	4,60	51,3
	Table III:		Results of H 2 O specific surface and calculation of the
				% of free water.
	Tableau III :	Résultats de surface spécifique à l'eau et calcul du %
				d'eau libre.
	EST205		Depth		Cl -	Br -	SO 4 2-
	Sample		(m)		(mol/L)	(mol/L)	(mol/L)
				TW	GP	TW	GP	TW	GP
	K100 424m	423.68-424.48	1.79×10 -2 2.90×10 -2	3.2×10 -5	5.1×10 -5	5.17×10 -2 8.35×10 -2
	K119 477m	476.57-476.97	1.54×10 -2 3.01×10 -2	3.4×10 -5	6.5×10 -5	3.22×10 -2 6.28×10 -2
	Table IV:		Concentrations of Cl -, Br -, SO 4	2-in the total of the
				porosity (TW) and in the geochemical porosity (GP)
				calculated from leaching experiments.
	Tableau IV :	Concentrations de Cl -, Br -, SO 4	2-dans la porosité
				totale (TW) ou dans la porosité géochimique (GP)
				calculées à partir des expériences de lessivage.
	Rock		Anions accessible		Cl -Concentration in the	Cl -Concentration in the
			Porosity (Geochemical		total porosity	geochemical porosity
			porosity concept).			(TW)	(GP)
							(mol/L)	(mol/L)
	Carbonates Argilites		100 % 50 %		3.48×10 -2 ± 1.96×10 -2 1.65×10 -2 ± 5.80×10 -3	3.48×10 -2 ± 1.96×10 -2 3.61×10 -2 ± 1.68×10 -2
		Model 1 type of sites	Constant	Standard Deviation	log10 Constant
			H\Na		13,0		2,63	1,11
			K\Na		2,78		0,12	0,44
			NH4\Na		2,98		0,14	0,47
			Ca\Na		3,17		0,10	0,50
			Mg\Na		2,41		0,09	0,38
			Sr\Na		4,75		0,19	0,68

Table VI :

 VI Callovian-Oxfordian Formation (fraction < 2µm). Gaines-Thomas constants for a global type of exchange sites. Tableau VI : Formation du Callovo-Oxfordien (fraction < 2µm). Constantes de Gaines-Thomas pour un type global de sites d'échange.

	Model	A	B	Chlo
	Porewater chemistry	mol/kg w	mol/kg w	mol/kg w
	pH	7,10	7,28	7,00
	Eh SHE (mV)	-163	-176	-156
	Alk	1,97E-03	1,29E-03	2,46E-03
	Ionic strength	1,16E-01	1,16E-01	1,17E-01
	Cl	3,01E-02	3,01E-02	3,01E-02
	S	3,39E-02	3,40E-02	3,39E-02
	Na	3,21E-02	3,20E-02	3,21E-02
	K	7,09E-03	7,07E-03	7,10E-03
	Ca	1,49E-02	1,48E-02	1,50E-02
	Mg	1,41E-02	1,41E-02	1,42E-02
	Sr	1,12E-03	1,12E-03	1,12E-03
	Si	9,41E-05	9,43E-05	9,40E-05
	Al	7,36E-09	8,62E-09	6,85E-09
	Fe	2,14E-04	9,40E-05	3,32E-04
		saturation index (si)
	Log(pCO 2(g) ) bar	-2,15*	-2,51*	-1,96
	si_Illite	0	0	0
	si_Montmor-Ca	-0,177	-0,143	-0,195
	si_Microcline	0,5581	0,652	0,509
	si_Calcite	0	0	0
	si_Dolomite	0	0	0
	si_Fe-Chlorite	0	0	0
	si_Quartz	0	0	0
	si_Siderite	0,0906	-0,235	0,261
	si_Pyrite	0	0	0
	si_Celestite	0	0	0
	si_Gypsum	-0,0085	-0,0086	-0,0085
	si_ Mg-Chlorite	0,85	2,48	0
		ion exchange
	SrX 2	4 %	4 %	4 %
	KX	8 %	8 %	8 %
	NaX	14 %	14 %	14 %
	MgX 2	28 %	28 %	28 %
	CaX 2	45 %	45 %	45 %

Table VII :

 VII Model results (PHREEQC, llnl.dat). Various test cases of porewater chemistry: Cl -concentration in the geochemical porosity, "A" simulation use the maximum measured pCO 2 value and "B" a pCO 2 value in the best analytical range, "Chlo" corresponds to test cases where the pCO 2 is not fixed. In this model, the pCO 2 is determined by the equilibrium between calcite, dolomite, and Mg-chlorite.Tableau VII : Résultats de modélisations (PHREEQC, llnl.dat): Différents cas tests pour la chimie de l'eau interstitielle : Concentration en Cl -dans la porosité géochimique, la simulation "A" utilise la valeur mesurée de pCO 2 maximum, "B" utilise la valeur de pCO 2 obtenue dans les meilleures conditions analytiques, "Chlo" correspond à un cas test où la pCO 2 n'est pas fixée. Dans ce dernier modèle, la pCO 2 est déterminée par équilibre avec la calcite, la dolomite et une chlorite-Mg.

  . 7: Composition isotopique du CO 2 obtenu par dégazage des carottes du nord-est (NE) au sud-ouest (SW) du laboratoire souterrain. Comparaison avec diverses sources de CO 2 et avec les compositions isotopiques des calcites de la formation. Références isotopiques : PDB : Bélemnite de la formation de Peedee, U.S.A ; SMOW : Standard Mean Ocean Water.

AKNOWLEDGEMENT

This work was funded within the framework of the ANDRA -BRGM partnership (THERMOAR project 1998(THERMOAR project -2005)). We thank H. Rebours (ANDRA/EST) and the ERM team (Poitiers) for their contributions. The reviews of Dr Pierre de Cannière (CEN/SCK, Belgique) and of Dr Gilles Berger (CNRS, France) have been very helpful. Table II: Mean values of mass water content (%) in the Callovian-Oxfordian formation at the regional scale Tableau II :

Valeurs moyennes (%) des teneurs massiques en eau de la formation du Callovo-Oxfordien à l'échelle régionale.


Eric C Gaucher
email: e.gaucher@brgm.fr.

Philippe Blanc
email: p.blanc@brgm.fr.

Jean-Michel Matray
email: jean-michel.matray@irsn.fr

Nicolas Michau
email: nicolas.michau@andra.fr

Modeling diffusion of an alkaline plume in a clay barrier

Keywords: Clay barrier, alkaline plume, concrete, diffusion, modeling

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

The French agency for radioactive waste management (ANDRA) is constructing an underground research laboratory at a depth of about 500 m in the Callovian-Oxfordian Formation of the eastern Paris Basin in the Meuse and Haute-Marne Departments [START_REF] Andra | Référentiel Géologique, Site de Meuse/Haute Marne[END_REF][START_REF] Gaucher | ANDRA underground research laboratory: Interpretation of the mineralogical and geochemical data acquired in the Callovian-oxfordian Formation by investigative drilling[END_REF]. The purpose of this laboratory is to study the feasibility of deep storage of radioactive waste using the current concept of multiple barriers: the waste itself, the waste container surrounded by backfill, and the geosphere.

Concrete will be used extensively in this design both to build the repository chambers and as waste containment material. The large excavated concrete chambers that will hold the containers of radioactive waste will be separated from the repository access galleries by bentonite plugs several meters thick. The volume of concrete is likely to be much greater than that of the bentonite plugs. After being sealed, the repository will become saturated with interstitial waters from the Callovian-Oxfordian Formation, which will produce high-pH solutions through interaction with the concrete. The aggressiveness of these alkaline solutions, when in contact with the bentonite, could possibly weaken the bentonite's confinement properties with respect to long-lived radionuclides.

The dominant transport mechanism in these relatively impermeable formations is assumed to be diffusion [START_REF] Lehikoinen | Evaluation of factors affecting diffusion in compacted bentonite[END_REF]. The aim of this study was to determine the critical processes affecting the pore water, the minerals and the clay-mineral surfaces of a bentonite plug during the diffusion of an alkaline plume. The calculations were carried out in two steps: re-equilibration of the system after saturation of the dry bentonite barrier with the interstitial pore water from the Callovian-Oxfordian Formation, and diffusion of the alkaline plume from the concrete through the bentonite plug. Calculations were done for a simulated period of 100,000 years, at 25 °C and 1bar, using PHREEQC (version 2), a code well-suited to coupled chemistrydiffusion modeling [START_REF] Parkhurst | User's guide to PHREEQC (version 2) -A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations[END_REF]. The reader should keep in mind that this type of calculation is more a tool to understand and assess the impact of the perturbation than an entirely realistic model. The limitations comes from (i) uncertainties on thermodynamic data for clays, zeolites and cement phases, (ii) the fact that it is difficult to take into account the chemical evolution of concrete over long periods and (iii) impossible to take into account the heterogeneous transport behavior in the clay barrier, and (iv) the absence of kinetic data for all of the phases considered (dissolution and precipitation). Concerning this last point, we chose to consider only the thermodynamic equilibrium. The low diffusion coefficients in the clayey media are conducive to a favorable context in which equilibrium should be reached if sufficient time is considered. This work contributes to current international efforts on this subject (for example, [START_REF] Savage | Modelling reactions between cement pore fluids and rock: implications for porosity change[END_REF], [START_REF] Braney | A study of the effects of an alkaline plume from a cementitious repository on geological materials[END_REF], [START_REF] Steefel | Diffusion and reaction in rock matrix bordering a hyperalkaline fluid-filled fracture[END_REF], [START_REF] Adler | High-pH alteration of argillaceous rocks: an experimental study[END_REF], [START_REF] Bateman | Experimental simulation of the alkaline disturbed zone around a cementitious radioactive waste repository: numerical modelling and column experiments[END_REF], [START_REF] Savage | Modelling the interaction of bentonite with hyperalkaline fluids[END_REF][START_REF] Soler | Reactive transport modelling of the interaction between a high-pH plume and a fractured marl: the case of Wellenberg[END_REF]).

THERMODYNAMIC DATABASE

A thermodynamic database for aqueous species and mineral solubility products adapted to this problem was prepared in PHREEQC format using the llnl.dat database [START_REF] Wolery | EQ3/6, a software package for geochemical modelling of aqueous systems: Package overview and installation guide[END_REF].

To maintain internal consistency in the database, and because of the scarcity of other such constants in the literature, we used the hydrolysis constant given for a pure montmorillonite (Na 0.33 Mg 0.33 Al 1.67 Si 4 O 10 (OH) 2 ). Except for Fe, this formula is similar to that of [START_REF] Lehikoinen | Determination of porewater chemistry in compacted bentonite[END_REF]. Some phases were added to the database to account for the possible precipitation of cement phases close to the interface. Most of the composition and solubility products were taken from [START_REF] Bourbon | Chemical conceptual model for cement based materials, mineral phases and thermodynamic data[END_REF], who has done a critical review of all of the thermodynamic data available for these minerals.

The thermodynamic data for cement phases, clays and zeolites are, however, scarce and uncertain. Our results give only general trends. They do not take into account the precipitation of gels and illdefined phases that may persist over long periods.

The database was completed when the volume of a given phase was not in the [START_REF] Wolery | EQ3/6, a software package for geochemical modelling of aqueous systems: Package overview and installation guide[END_REF] database (Table 1).

The ion-exchange constants for Na + /Ca 2+ , Na + /Mg 2+ , Na + /K + and Na + /H + (Table 2) were recalculated to conform to the formalism of [START_REF] Gaines | Absorption studies on clay minerals. II. A formation of the thermodynamics of exchange absorption[END_REF] using the values provided by [START_REF] Fletcher | The chemical modelling of clay/electrolyte interactions for montmorillonite[END_REF]. Due to a total absence of data for CaOH + and MgOH + , these species were not considered in the exchange reactions, even though their role is probably significant in the waters modeled. [START_REF] Bourbon | Chemical conceptual model for cement based materials, mineral phases and thermodynamic data[END_REF]; Ba, [START_REF] Ball | User's manual for WATEQ4F, with revised thermodynamic data base and test cases for calculating speciation of major, trace, and redox elements in natural waters[END_REF]; F, [START_REF] Fischer | The crystal structure determination of the zeolite, gismondite, CaAl 2 Si 2 O 8 •4H 2[END_REF]; H, [START_REF] Hatches | The Harwell/Nirex Thermodynamic Database for Chemical Equilibrium Studies[END_REF]; M [START_REF] Murphy | Geochemical Investigations Related to the Yucca Mountain Environment and Potential Nuclear Waste Repository[END_REF]; R, [START_REF] Rinaldi | Zeolites of the phillipsite family. Refinement of the crystal structures of phillipsite and harmotome[END_REF]; Ro, [START_REF] Roberts | Encyclopaedia of Minerals[END_REF]; S, [START_REF] Sacerdoti | The crystal structure of katoite and implications within the hydrogrossular group of minerals[END_REF]; W, Wolery [1995]

DESCRIPTION OF THE INITIAL SYSTEM

The first system simulated was a dry MX80type compacted clay, equilibrated with interstitial water from the Callovian-Oxfordian Formation, in contact with an Ordinary Portland Cement (OPC) barrier.

Input data taken from the literature were converted, where appropriate, to a molal scale using a dry density of 1.8 g/cm 3 . The bentonite used in the model was a Wyoming montmorillonite commercially designated MX80. The properties of this clay were taken from [START_REF] Lehikoinen | Determination of porewater chemistry in compacted bentonite[END_REF] (Table 3).

These authors give a cation exchange capacity (CEC) value of 81 meq per 100 g of dry clay for MX80, with the proportions of exchangeable cations being: Na: 80.6%, K: 1.4%, Ca: 12.9%, Mg: 5%.

The Cl concentrations in the Callovian-Oxfordian interstitial water were obtained by squeezing a core sample [START_REF] Andra | Référentiel Géologique, Site de Meuse/Haute Marne[END_REF]. Other elements were calculated assuming equilibrium with the bentonite minerals (Table 3) and with the surface of the montmorillonite.

Adler et al. (1999) demonstrated the importance of the partial pressure of CO 2 for another clay pore water system. This parameter is set free in all calculations, implying that the mineral buffering will be strong enough to avoid fixing it a priori. The buffering capacity of mineral assemblages with calcite, dolomite and a Mg-aluminosilicate was demonstrated in Coudrain-Ribstein et al. (1998).

The chemical composition of the concrete water (Table 4), taken from [START_REF] Bourbon | Chemical conceptual model for cement based materials, mineral phases and thermodynamic data[END_REF], corresponds to the pore water of moderately evolved concrete. It has a pH of 12.5 and the calcium concentration indicates a control by portlandite Ca(OH) 2 . Sulfate and dissolved silica concentrations are given by the equilibrium with ettringite and CSH phases, respectively. Other minerals: feldspars, micas, ...

13.0

The mineralogical and chemical evolution of the concrete was not taken into consideration in this study because 1) it is not well documented over long periods of time, and 2) the huge volume of concrete compared to that of bentonite in the planned repository allowed us to consider the concrete medium to be infinite.

We know, however, that the chemistry of the concrete will evolve with time (see, for example, [START_REF] Berner | Evolution of pore water chemistry during degradation of cement in a radioactive waste repository environment[END_REF]. Using a solution with a pH 12.5, which represents a "moderately evolved" concrete pore water, was consistent with the approximation of an infinite concrete medium, 12.5 being a mean value between that of young evolved water (pH > 13) characterizing the first stages of concrete evolution, and pore water from aged concrete (pH < 11) characterizing long-term reaction [START_REF] Bateman | Experimental simulation of the alkaline disturbed zone around a cementitious radioactive waste repository: numerical modelling and column experiments[END_REF].

SIMULATION OF THE RE-SATURATION PHASE

The time required for the barrier to become resaturated with groundwater was not taken into consideration because water/rock equilibrium calculations using PHREEQC can only start when the barrier is water saturated. We simulated equilibration of the pore water of the host formation with the minerals of the MX80 bentonite and the exchange sites of the montmorillonite. The composition of the pore water in the host formation was taken from [START_REF] Gaucher | Simulation de la chimie des eaux du Callovo-Oxfordien[END_REF]. Some of the bentonite minerals (feldspars, micas) were considered to be non-reactive. The results of the simulation are given in Table 5.

The major changes in the MX80 bentonite system are related to partial dissolution of gypsum and equilibration of the carbonate system. Equilibration of the exchanger causes changes in the concentrations of the alkali and alkaline-earth cations, both in the solution and at the surface of the exchanger. Only a very small amount of pyrite is assumed to be initially present since quantification of this phase remains uncertain even though it has been observed in MX80. An interesting feature was the precipitation of a small amount of smectite that was different from the original Na-Montmorillonite, i.e., Na-Nontronite (ferric smectite) and Na-Saponite (magnesian smectite). These new phases are necessary to perform a complete assemblage in equilibrium with the solution.

SIMULATION OF THE ALKALINE PLUME

When simulating the alkaline plume, the transport of solutes into the clay barrier was considered to be a pure diffusion process in a saturated medium with interconnected porosity. The PHREEQC code, however, models one-dimensional transport and allows us to use only one effective diffusion coefficient. Despite the fact that it has been experimentally demonstrated that each aqueous species has its own diffusion coefficient, writing the conservation equations in terms of total concentration is possible only when the diffusion coefficients for all species are the same [START_REF] Steefel | Diffusion and reaction in rock matrix bordering a hyperalkaline fluid-filled fracture[END_REF]. Therefore, a single diffusion coefficient with a value of 10 -11 m 2 /s was selected based on published data for compacted MX80 [START_REF] Lehikoinen | Evaluation of factors affecting diffusion in compacted bentonite[END_REF]. Diffusion was assumed to be homogeneous throughout the barrier, although researchers have recently observed in the Mol underground rock laboratory that some intact islets of rock were preserved near the clay/concrete interface. The mineralogical impact of diffusion is, in fact, heterogeneous ( [START_REF] Cannière | Communication Ecoclay II final meeting[END_REF].

The boundary conditions were those of a semi-infinite medium for the clay barrier, and constant fixed fluid concentrations in the concrete. The barrier was first modeled with a grid of eighty 0.1-m cells for the clay and one 0.1-m cell for the concrete. Because mineralogical transformations occur close to the concrete/clay interface, this grid was then modified in the following way: one 0.1-m cell for the concrete, then ten 0.025-m cells, ten 0.05-m cells and finally seventy-five 0.1-m cells. This enabled a more precise description of mineralogical evolution near the interface. The simulation covered a period of 100,000 years in 10,000-year steps.

The simulation allows not only the original minerals in the barrier to precipitate or dissolve, but also the 47 mineral species in table 1, once their saturation indexes have been reached.

We chose not to take kinetic dissolution or precipitation of the mineral phases into account. Because our calculations concerned a very long period of diffusion (10,000 to 100,000 years), we assumed that the impact of kinetic phenomena was minimized. Moreover, a lot of kinetic data are missing for the minerals considered in this study.

RESULTS OF THE SIMULATION

One of the most expected chemical changes in the barrier is the increase in the pH of the pore water. However, after 100,000 years, a very strong buffering of the pH was observed and the pH front evolved little between 10,000 and 100,000 years (Fig. 1). As stated earlier, the pCO 2 was not fixed in our calculations. Carbon dioxide was produced by the system itself and the partial pressure of CO 2 varied little from its value in the host rock, except very close to the interface (Fig. 2). This indicates that the mineralogical assemblage was rich enough to fix the pCO 2 internally, which in turn exerted a strong control on pH. Coudrain-Ribstein et al. (1998) demonstrated the efficiency of a mineralogical buffer, depending on the nature of the phases at equilibrium with the solution. There was a noticeable decrease in pH at 7.5 and 22.5 cm after 10,000 and 100,000 years, respectively. In this zone, calcite was entirely dissolved, decreasing the pH buffering capacity. Dissolution/precipitation reactions also acted as mineral buffers and limited the increase in pH. Tobermorite precipitation, for example, affected pH via the following reaction (for pH < 12.9, where CaOH + was not the dominant species of Ca (aq) ): Another important change involved the dissolution/precipitation of mineral phases (Fig. 3) after a diffusion period of 100,000 years. ♦ Between 0.7 and 8 m from the interface, the only modification caused by the alkaline plume in the bentonite was the total or partial dissolution of gypsum. This mineral was totally dissolved in the first 5.0 m of the barrier because of the low sulfate content in the concrete water. The diffusion front slightly modified the composition of the species at the exchanger surface. ♦ Between 0.40 and 0.70 m from the interface, the dissolved potassium concentration increased because the concrete pore water was more potassic than the host water. This favored the precipitation of a potassic zeolite, K-clinoptilolite.

Kclinoptilolite precipitated at the expense of Na-montmorillonite, which produced aluminum. However, the Si/Al ratio is 2.4 in montmorillonite compared to 4.2 in Kclinoptilolite. The additional amount of silica needed for clinoptilolite precipitation was provided by cristobalite, which dissolved entirely at 0.44 m from the interface. Partial dissolution of montmorillonite provided magnesium that was incorporated in Nasaponite.

Within this zone, Camontmorillonite progressively replaced Namontmorillonite since concrete pore water is slightly richer in calcium than the host rock pore water. Exchange reactions do not seem to be efficient enough to attenuate the calcium gradient. Precipitation of Ca-montmorillonite depleted the dissolved calcium, which was compensated for by the partial dissolution of calcite. Calcite disappeared completely at a distance of 0.54 m from the concrete barrier. ♦ Between 0.10 and 0.40 m from the interface, both Ca and Na-montmorillonite disappeared in favor of illite. Illitization of montmorillonite under alkaline conditions is well-documented. [START_REF] Howard | Development of layer charge and kinetics of experimental smectite alteration[END_REF] and [START_REF] Eberl | Synthesis of illite-smectite from smectite at earth surface temperatures and high pH[END_REF] observed partial illitization of a smectite phase, obtaining interstratified illite/smectite (IS) minerals as final products. IS minerals are known to represent an intermediate stage between smectite and illite during the diagenetic evolution of shale [START_REF] Inoue | Chemical and morphological evidence for the conversion of smectite to illite[END_REF]. At the Khushaym Matruk natural analogue, illitization is associated with the precipitation of zeolites [START_REF] Rassineux | Mineralogical evolution of clay-bearing rock during alkaline alteration[END_REF]. Zeolites became the predominant phases, with the precipitation of gyrolite and gismondine and the continuing presence of clinoptilolite. Zeolites replaced illite, which dissolved entirely at a distance of 0.10 m from the concrete. [START_REF] Adler | High-pH alteration of argillaceous rocks: an experimental study[END_REF] reported the presence of Ca-zeolite as a product of the alteration of a smectite-containing clay rock by a K-Na-Ca hydroxide solution. Calculations performed by [START_REF] Savage | Modelling the interaction of bentonite with hyperalkaline fluids[END_REF] produced similar results. Magnesium, released by illite dissolution, is included in Na and Ca-saponite. Experimental precipitation of saponite from muscovite altered by an alkaline solution (pH 9.2) has been observed by [START_REF] Nagy | Alteration of Muscovite to a Mg-Clay, Goldschmidt Conf[END_REF]. In their study, SIMS analyses indicated that Al was leached from the muscovite surface and replaced by Mg, suggesting that the phyllosilicate changed from a dioctahedral into trioctahedral structure.

In this zone, where the amount of illite is at a maximum, the total volume of minerals decreases from 3770 to 2940 cm 3 . Therefore, we would expect the porosity to be greater because illite replaces montmorillonite. ♦ Between 0.00 and 0.10 m from the interface, cement phases (Afwillite, Tobermorite, Katoite, Hydrotalcite, Ettringite) precipitated instead of zeolite phases. [START_REF] Claret | Experimental investigation of the interaction of clays with high-pH solutions. A case study from the Callovo-Oxfordian formation, Meuse-Haute Marne underground laboratory (France)[END_REF] and [START_REF] Adler | Experiment vs modelling: Diffusive and advective interaction of high-pH solution in argillaceous rock at 358C. Goldschmidt Conf[END_REF] reported the formation of CASH phases when a smectite-containing rock was weathered by an alkaline solution. Tobermorite and ettringite formation has been observed in a marl naturally weathered by flowing hyperalkaline water at Maqarin, Jordan [START_REF] Milodowski | Mineralogical alteration by hyperalkaline groundwater in northern Jordan[END_REF][START_REF] Cassagnabère | Mineralogical evolution of bituminous marl adjacent to an alkaline water conducting feature at the Maqarin analogue site[END_REF].

The Maqarin and Khushaym Matruk natural analogues are roughly 100,000 years old and the perturbation does not extend beyond a few centimeters. The calculations of [START_REF] Savage | Modelling the interaction of bentonite with hyperalkaline fluids[END_REF] also indicate precipitation of tobermorite near the concrete interface. The CASH gels observed by [START_REF] Adler | Experiment vs modelling: Diffusive and advective interaction of high-pH solution in argillaceous rock at 358C. Goldschmidt Conf[END_REF] and [START_REF] Claret | Experimental investigation of the interaction of clays with high-pH solutions. A case study from the Callovo-Oxfordian formation, Meuse-Haute Marne underground laboratory (France)[END_REF] may be considered to be metastable phases with respect to the katoite observed at the concrete interface. Because our calculations were done for a 100,000-year period, we chose not to take into account these metastable phases because thermodynamic data are lacking and also because metastable phases tend to evolve into stable phases with time. Our results are, however, similar to experimental observations. The conversion of zeolites into cement phases is related to an increase in the Ca/Si ratio in the solid phases as we get near the concrete interface. [START_REF] Savage | Rate and mechanism of the reaction of silicates with cement pore fluids[END_REF] mentioned that CSH phases with relatively high Ca/Si ratios (tobermorite, Ca/Si = 0.83) may re-dissolve to form minerals with lower Ca/Si ratios in response to a decreasing fluid Ca/Si ratio (gismondine, Ca/Si = 0.5). The increase in pH favored the precipitation of calcite. Adler et al (1998) observed a similar phenomenon during experiments on Swiss Opalinus Clay.

Concerning the Mg-rich phases, clinochlore followed Ca-saponite because clinochlore formation consumes protons and clinochlore is stabilized at a higher pH than saponite, according to the reaction:

O 7.34H 1.67Al 2Mg (OH) O Si Al Mg Ca 8.68H 0.67SiO 0.165Ca (OH) O Si Al Mg
+ + + = + + + + + + +
Clinochlore is a Mg-chlorite that contains brucite [Mg(OH) 2 ] in the interlayer hydroxide sheet.

The comparison of figures 3 and 4 shows that, as expected, a shorter diffusion time caused a decrease in the extent of the mineralogical transformations whereas mineralogical processes remained essentially the same. Finally, the total volume increased due to the precipitation of cement phases near the interface. [START_REF] Savage | Modelling the interaction of bentonite with hyperalkaline fluids[END_REF] reported a similar increase near the concrete interface. The change in porosity would modify transport conditions. A porosity collapse and an increase of the mechanical stress can be expected in this zone due to this important increase of mineralogical volume. The diffusion coefficient will probably decrease near the interface. Because it is not possible to consider this effect with PHREEQC, our simulations over-estimated the extent of the perturbation.

STATUS OF THE EXCHANGED SPECIES BEARING PHASE

In the previous simulation, the cationic exchange capacity (CEC) was not related to any specific mineral phase. CEC remained constant throughout the simulation, even if the montmorillonite, the mineral responsible for the exchange capacity of the barrier, was entirely dissolved. In an effort to simulate plume diffusion more realistically, we did additional simulations taking into account the relationship between CEC and the amount of montmorillonite (new option of PHREEQC). This phase was assumed to be the only mineral with a significant CEC. Due to difficulties in numerical convergence when using this option, the system was simplified by removing all of the Fe-bearing phases.

The grid was also simplified, becoming a regular mesh of 80 0.1-m cells. A second simulation was then done with exactly the same characteristics, except for the relationship between montmorillonite and CEC.

Figure 5 shows the evolution in the amount of 3 main phases of the clay barrier, calculated with both approaches, for a diffusion time of 100,000 years. There are only very minor differences in the two simulations, which means that surface reactions are not critical for the mineralogical transformation.

INFLUENCE OF THE DIFFUSION COEFFICIENT

Influence of the diffusion coefficient has been tested with different values. New simulations were done with D=10 -10 m²/s and D=10 -9 m²/s.

The grid was divided into 80 0.1-m cells for D=10 -10 m²/s and 52 0.25-m cells for D=10 -9 m²/s. Results obtained for a diffusion time of 100,000 years are shown in figure 6 for D=10 - 9 m²/s. Note: the horizontal scale of this figure is different from that of figure 3. There are no significant differences in the mineralogical transformation. However, the extent of the mineralogical transformation is multiplied by approximately one order of magnitude.

There is, therefore, a phenomenological relationship between the extent of mineralogical transformation of the host rock, the diffusion coefficient and the diffusion time.

We chose the montmorillonite dissolution (>5 %) as an index of the mineralogical transformation. The law is "phenomenological" because it is based solely on a comparison of the results obtained for the three diffusion coefficients. After fitting various numerical expressions, the results of the simulations gave the following equation:

D t A ations transform cal mineralogi the of Extent ⋅ ⋅ =
where the "extension of the mineralogical transformations" is in m, "A" a numerical coefficient, "t" is the diffusion time (years) and "D" is the diffusion coefficient (m²/s). ; Fitting was done with the Solver add-in of EXCEL TM . Results are given in table 6. 100000 5.75 5.55 (1) Sum of the squared differences. (2) Standard error between the recalculated and the PHREEQC transformation extension.

In spite of the fact that for short diffusion times strict thermodynamic conditions cannot be assumed and advection may be taken into account when considering the Engineered Damage Zone (EDZ) near the waste repository, the identified relationship could nevertheless be used to assess the impact of the alkaline perturbation on the clay barrier as a function of time. 7.00 6.50 6.00 5.50 5.00 4.50 4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00

The simulation of the diffusion of an alkaline plume from concrete into an MX80 clay barrier using PHREEQC revealed the changes in the mineralogical and chemical parameters of the system as diffusion from the cement occurred.

The results show that the mineralogical composition of the barrier is profoundly altered near the interface with the concrete.

The extent of mineral transformations is proportional to the square root of the diffusion time and the diffusion coefficient. From the clayey host rock to the concrete, the transformations begin with an illitization of the montmorillonite. The illite then dissolves as zeolite phases are precipitated. These are finally dissolved to produce cement phases near the concrete interface as a result of an increase in the Ca/Si ratio in the solid phases. This transformation sequence causes a decrease in the total volume of the rock as illitization of the montmorillonite progresses and a strong increase in the total volume as cement phases precipitate, which would lead to a decrease in porosity near the concrete interface. Exchange reactions between montmorillonite and the solution are of minor importance. It is clear that some of these results are related to the high number of solid phases retained in the database, which leads to a control of the alkaline and alkaline-earth elements by solid phases rather than by exchange reactions. Using a smaller database would produce different results. The simulations presented here also demonstrate the efficiency of the mineralogical buffering of the pH. The extent of the perturbation for the reference diffusion coefficient (10 -11 m 2 /s) is similar to the order of magnitude of the Jordan analogues (a few centimeters). Finally, calculating the extent of the perturbation as a function of the diffusion coefficient showed that this perturbation will remain in the vicinity of the nuclear waste repository and will not significantly concern the clayey Callovian-Oxfordian Formation host rock.

Fig

  Fig. 1 Change in the pH of the barrier (MX80) pore water

Fig

  Fig. 3 Mineralogical composition of the barrier after 100,000 years, diffusion coefficient = 10 -11 m²/s

Fig. 5

 5 Fig. 5 Comparison of two different ways to take the CEC into account

Table 1 Thermodynamic constants of the mineral phases

 1

	Phase	Formula	Log K	Molar volume
				(cm 3 /mole)
		Clay phases
	Clinochlore	Mg 5 Al 2 Si 3 O 10 (OH) 8	67.24 W	207.11 W
	Montmorillonite-Na	Na .33 Mg .33 Al 1.67 Si 4 O 10 (OH) 2	2.48 W	220 W
	Montmorillonite-Ca	Ca .165 Mg .33 Al 1.67 Si 4 O 10 (OH) 2	2.50 W	220 W
	Beidellite-Na	Na .33 Al 2.33 Si 3.67 O 10 (OH) 2	5.65 W	220 W
	Beidellite-Ca	Ca .165 Al 2.33 Si 3.67 O 10 (OH) 2	5.59 W	220 W
	Saponite-Ca	Ca .165 Mg 3 Al .33 Si 3.67 O 10 (OH) 2	26.29 W	220 W
	Saponite-Na	Na .33 Mg 3 Al .33 Si 3.67 O 10 (OH) 2	26.35 W	220 W
	Nontronite-Na	Na .33 Fe 2 Al .33 Si 3.67 H 2 O 12	-11.53 W	220 W
	Nontronite-Ca	Ca .165 Fe 2 Al .33 Si 3.67 H 2 O 12	-11.58 W	220 W
	Illite	K .6 Mg .25 Al 2.3 Si 3.5 O 10 (OH) 2	9.03 W	138.94 W
		Carbonates
	Calcite	CaCO 3	1.85 W	36.93 W
	Dolomite	(Ca, Mg)(CO 3 ) 2	3.56 P	64.12 W
	Siderite	FeCO 3	-0.19 W
	Monocarbo-aluminate	Ca 4 Al 2 CO 9 ,10H 2 O	80.33 B
	Hemicarbo-aluminate	Ca 8 Al 4 CO 16 ,21H 2 O	182.33 B
		Zeolites
	Natrolite	Na 2 Al 2 Si 3 O 10 ,2H 2 O	18.52 W
	Analcime	Na .96 Al .96 Si 2.04 O 6 ,H 2 O	6.14 W
	Laumontite	CaAl 2 Si 4 O 12 ,4H 2 O	13.67 W
	Lawsonite	CaAl 2 Si 2 O 7 (OH) 2 ,H 2 O	22.21 W
	Wairakite	CaAl 2 Si 4 O 10 (OH) 4	18.08 W
	Clinoptilolite-Na	Na 2 Al 2 Si 10 O 24 ,8H 2 O	-6.71 M	633.1 W
	Clinoptilolite-K	K 3.467 Al 3.45 Fe .017 Si 14.533 O 36 ,10.922H 2 O	-10.95 W	633.1 W
	Clinoptilolite-Ca	Ca 1.7335 Al 3.45 Fe .017 Si 14.533 O36,10.922H 2 O	-7.01 W	633.1 W
	Mordenite	Ca .2895 Na .361 Al .94 Si 5.06 O 12 ,3.468H 2 O	-5.20 W
	Phillipsite		-19.87 Ba	304.56 R
	Okenite	CaSi 2 O 4 (OH) 2 ,H 2 O	10.38 W
	Hillebrandite	Ca 2 SiO 3 (OH) 2 ,0.17H 2 O	36.82 W
	Gismondine	Ca 2 Al 4 Si 4 O 16 ,9H 2 O	41.72 W	315.08 F
	Gyrolite	Ca 2 Si 3 O 7 (OH) 2 ,1.5H 2 O	22.91 W	136.85 W
		Cement phases (CSH, CASH and MAH)
	Tobermorite	Ca 5 Si 6 H 10 O 22	65.50 B	300.81 B
	Jennite	Ca 9 Si 6 H 22 O 32	150.00 B
	Afwillite	Ca 3 Si 2 O 4 (OH) 6	50.00 B	129.53 W
	CSH_0.8	Ca 0.8 SiO 5 H 4.4	11.03 H
	CSH_1.1	Ca 1.1 SiO 7 H 7.8	16.66 H
	CSH_1.8	Ca 1.8 SiO 9 H 10.4	32.41 H
	Hydrogrenat	Ca 3 Al 2 O 12 H 12	78.00 B	149.52 W
	C4AH13	Ca 4 Al 2 O 20 H 26	103.00 B
	Katoite	Ca 3 Al 2 SiO 12 H 8	69.40 B	142.86 S
	Straëtlingite	Ca 2 Al 2 SiO 7 ,8H 2 O	49.50 B
	Hydrotalcite	Mg 4 Al 2 O 7 ,10H 2 O	75.00 B	301.51 Ro
		Other phases
	Portlandite	Ca(OH) 2	22.80 B
	Brucite	Mg(OH) 2	16.70 B
	Ettringite	Ca 6 Al 2 (SO 4 ) 3 (OH) 12 .26H 2 O	54.30 B	725.1 Ro
	Monosulfo-aluminate	Ca 4 Al 2 SO 10 .12H 2 O	71.00 B
	Gypsum	CaSO 4 ,2H 2 O	-4.38 B	74.69 W
	Pyrite	FeS 2	-24.65 W	23.94 W
	Goethite	FeOOH	0.54 W
	Cristobalite	SiO 2	-3.45 W	25.74 W
	B,

Table 2 Exchange constants (log K 25°C) for Wyoming montmorillonite

 2

	Na/K	0.255
	Na/H	0.1003
	Na/Ca	0.687
	Na/Mg	0.687

Table 3 Mineralogical composition of the MX80 clay.

 3

	Mineral	Structural formula weight
			%
	montmorillonite	(Si 4.00 )(Al 1.67	75.0
		Mg 0.33 )O 10 (OH) 2 N
		a 0.33
	cristobalite	SiO 2	10.0
	calcite	CaCO 3	1.4
	gypsum	CaSO 4 :2H 2 O	0.6
	pyrite	FeS 2	1.6 10 -4

Table 4 Chemical composition (mol/kg w ) adopted for the concrete pore water

 4

	pH	12.50
	pE	-8.6
	Na (mol/kg w )	8.02 10 -3
	K (mol/kg w )	2.21 10 -2
	Ca (mol/kg w )	2.19 10 -2
	Si (mol/kg w )	3.01 10 -5
	S (mol/kg w )	1.20 10 -4

Table 5 Results of the re-saturation phase simulation

 5

		Pore water of the
	Element	MX80 after re-saturation	Cation adsorbed	(mol/kg w )
		(mol/kg w )
	pH	7.39	NaB	5.02
	Eh	-192 mV	KB	8.69 10 -2
	Na	3.21 10 -1	CaB 2	5.25 10 -1
	K	3.31 10 -3	MgB 2	1.59 10 -1
	Mg	5.35 10 -3	HB	1.34 10 -6
	Ca	1.40 10 -2	Phase	(mol/kg w )
	Al	8.26 10 -10	Montmorillonit e	16.34
	Si	3.43 10 -4	Saponite-Na	8.69 10 -2
	Fe	2.69 10 -7	Nontronite-Na	3.73 10 -4
	C	2.46 10 -3	Calcite	1.12
	S	1.48 10 -1	Cristobalite	4.44
	Cl	6.46 10 -2	Gypsum	0.25
			CO 2	pCO 2 = 3.7 10 -3 atm

Table 6 Phenomenological law derived from PHREEQC simulations with 3 different diffusion coefficients

 6

			PHREEQC	Recalculated
	D (m²/s) Time (years)	transformation	transformation	Parameters of the fit
			extension (m)	extension (m)
	10 -11 10 -11	10000 100000	0.15 0.59	0.18 0.55	A = 555
	10 -10 10 -10	10000 100000	0.40 1.40	0.55 1.75	SSR (1) = 0.257
	10 -9 10 -9	10000	1.50	1.75	Std Err (2) = 0.201

Acknowledgments

The authors would like to thank Dr F.J. Pearson (GWG) for his review. Financial support was provided by ANDRA, the Commission of the European Communities (Contract ECOCLAY II: FIS5-1999-00145), and BRGM. N. Michau (Project leader of the EcoclayII project) is specially thanked for his strong support. Pr. Mäder and Dr. Savage are thanked for their constructive review of this work.