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Quimica del agua intersticial de una arcilla continental Paleogena en Espana y una arcilla marina Jurasica en Suiza: metodos de muestreo e interpretacion geoquimica.

Pore water chemistry of a Paleogene continental mudrock in Spain and a Jurassic marine mudrock in Switzerland: Sampling methods and geochemical interpretation

1. Introduction

At present, the deep geological disposal (DGD) is the most promising choice of management of radioactive wastes. It is a technically feasible option and provides the long-term isolation and protection of the radioactive waste (Chapman, 2006). The DGD involves a system of natural and engineered barriers to isolate the wastes from the environment. The disposal strategies include, as one of the possible natural barrier alternatives, argillaceous formations as host rocks. Different countries are investigating various aspects on selected formations in order to evaluate if this alternative is reliable regarding safety purposes.

If a failure of any engineered barrier occurs, the natural barrier plays an important role in retaining radionuclides. In addition, considering the mass balance of the geological system, the natural barrier can dilute strongly the pollution. In argillaceous formations, it has been demonstrated that radionuclide transport will be dominated by diffusion and the migration will be retarded by the high sorption capacity of the clays (e.g. Bonne and Heremans, 1981, Chapman and McKinley, 1987, Horseman, 1994, Savage, 1995, Mazurek et al., 2003). Furthermore, multiple coupled processes of water-clay interactions as well as osmotic behaviour in these clay-rich formations control solubility, speciation and mobility of radionuclides. For this reason, an exhaustive knowledge on the bulk and clay mineralogy of mudrocks, their ion exchange capacity and hydrogeochemical characteristics (pH, redox state, chemistry of the solution, presence of colloids and complexes, etc.) is essential for assessing argillaceous formations as part of a geological disposal system of hazard wastes.

Among the countries that are undertaking or have accomplished research programs on consolidated or plastic argillaceous formations are: France (Toarcian-Domerian Clay at Tournemire and Callovo-Oxfordian Clay at Bure), Belgium (Boom Clay at Mol), Hungary (Boda Clay at Mecsek), Switzerland (Opalinus Clay at Mt Terri and Palfris Formation at Wellenberg), Japan (Wakanai and Koetoi formations at Honorobe), United Kingdom (Oxford Clay) and United States of America (Pierre Shale in South Dakota). Based on these studies, considerable progress in the development of techniques for hydrology, geochemistry and hydrogeochemistry of mudrocks has been accomplished (e.g. Entwisle and Reeder, 1993; Gautschi and Scholtis, 1989; Beaufais et al., 1994; De Windt et al, 1998; Bradbury and Baeyens, 1998; Thury and Bossart, 1999; Sacchi and Michelot, 2000 (review), 2001; Bath, 2001; Gaucher et al., 2002, 2004, 2006; Pearson et al., 2003; Wersin et al., 2004; Vinsot et al., 2005) with important advances in the knowledge of geochemical processes in these systems and their evolution over time (e.g. Reeder et al., 1993; Baeyens and Bradbury, 1994; Beaucaire et al., 2000; Pearson et al., 2003).

The methods for collecting water from mudrocks, either in situ or in the laboratory, may affect the chemistry of the water (Baeyens et al., 1985; McCarthy and Degueldre, 1993; Appelo and Postma, 1996; Pearson et al., 2003; de Craen et al., 2004). The main problems usually found in low permeability geological systems for groundwater sampling and chemical characterisation are related to outgassing and oxidation of the samples (e.g. Almen et al., 1986; Griffault et al., 1996; Degueldre et al., 1999; Laaksoharju, 1999; Arcos et al., 2001; Fernandez et al., 2001a; Pena et al., 2001; Pearson et al., 2003; Wersin et al., 2004), which lead to variations in key parameters such as pH, alkalinity, redox potential or sulphate concentration. On the other hand, the lack of measurements of organic matter either of anthropogenic origin (e.g. drilling machinery, downhole equipment to sample, etc.) or naturally present in the formations can mislead the interpretation of alkalinity measurements (e.g. Merino, 1979; Eichinger and Wersin, 2004). Cation exchange reactions exert an important control on the chemistry of water in clay-rich rocks as well. To model water chemistry in such rocks requires knowledge of the concentration of exchangeable cations in the formation. However, measurement of concentrations of individual exchangeable cations is experimentally difficult for a number of reasons; for example, the presence in the clayey systems of minerals such as carbonates and/or sulphates. When placed in water, these minerals could react to some extent. The thereby dissolved cations react with the exchangeable cations originally present in the rock and change their relative concentrations (Thomas, 1982; Fernandez et al., 2001b; Waber et al., 2003).

All these facts make it difficult to correctly interpret measurements made in the field or in the laboratory for safety assessment purposes. The uncertainties can be checked and rationalized assuming geochemical constraints, which are supported by a complete characterisation of the system based on collection of samples at different points and long-term monitoring (e.g. Horseman et al., 1992; Griffault et al., 1996; Arthur and Wang, 2000; Pearson et al., 2003). At this point, geochemical modeling is the most common tool to interpret and understand the processes controlling the evolution of water chemistry (e.g. Beaucaire et al., 1995; Pearson et al., 1998; Waber et al., 1998; Tempel and Harrison, 2000; Arcos et al., 2001; Pearson et al., 2003; Gaucher et al., 2006). The use of geochemical modeling tools helps to determine the physicochemical and compositional reconstruction of the formation pore waters as well as their evolution.

The hydrogeochemical characterization of an Oligocene-Miocene Clay from N Spain has been an objective of the Spanish research program. Furthermore, Spain takes part in the Swiss research program studying the Opalinus Clay, and is involved in the characterization of the Callovo-Oxfordian Clay and the Boom Clay through the Integrated Project Fundamental Processes of Radionuclide Migration (Sixth Framework Programme of the European Commission).

This work presents a summary of the methodologies and approaches applied to study the pore water of two sedimentary formations (Opalinus Clay from Switzerland and an Oligocene-Miocene Clay from Spain) with rather different characteristics as much from the point of view of the methodology applied to the study as for the geological history of both formations. The methodological advances and results of the two argillaceous formations are presented. The work is part of a research program funded by ENRESA/NAGRA, on one hand, and, ENRESA on the other, in the context of their R&D programs.

The Swiss clay is studied through the participation in the Mont Terri international research project (1), which is part of the strategy of development of methodologies to characterize in situ argillaceous formations, and study the geochemical aspects of these formations. The project is structured as a series of individual experiments in the fields of geology, hydrogeology, geochemistry and rock mechanics. The experimental objectives cover, on one hand, the general testing of tools and methods for the investigations in clay-rich formations and, on the other hand, the characterisation specifically of the Opalinus Clay formation. After the lessons learned in the Mont Terri Rock Laboratory concerning the methods to investigate the water chemistry and the chemical evolution of claystones (Pearson et al., 2003), attention is focused in this paper to the results obtained from the experiments devoted to water sampling and analysis by different in situ and laboratory methods, with the goal of minimizing the alteration of the samples due to outgassing and oxidation during sampling. We focus specifically on water samples collected from the DI-B experiment, in its part dedicated to in situ hydrogeochemical characterisation.

Concerning the Spanish clay, the work shows the development of an integrated methodology to characterize a clay formation from core samples belonging to one borehole named S1. The study integrates all the available data on pore water composition, based on squeezing and leaching experiments, with the mineralogy and geochemistry studied from the mentioned core samples. A conceptual model on the evolution of the water-rock reactions is proposed. Then geochemical modelling is employed with two main objectives: to check the consistency of the pore water analytical data and to determine the relative importance of the main processes governing the chemistry of the pore water.

2. Geological aspects of the studied sites

2.1. Opalinus Clay (Switzerland)

The studies on the Opalinus Clay were carried out in an underground laboratory (the reconnaissance gallery of a motorway) constructed in the Jura Mountains, in north western Switzerland. The stratigraphic section includes rocks with Triassic-Jurassic age (Fig. 1). The Opalinus Clay is an indurated clay, upper Aalenian (180-170 Ma), deposited in a shallow marine environment. It is underlain by marls-limestones (Toarcian) and overlain limestones (upper Aalenian/lower Bajocian). The geological history is described in Tripet et al. (1990), Bossart and Wermeille (1999) and Bath and Gautschi (2003). The porosity ranges from 5 to 20 % (Mazurek, 1999), including total (water content) and geochemical/transport porosity. The hydraulic conductivity values of the intact Opalinus Clay are in the range of 2 x [10.sup.-14] - 2 x [10.sup.-12] m/s (Marschall et al, 2004).



The mineralogical composition consists of an average of 45-76 % of phyllosilicates (illite, illite/smectite mixed layers, chlorite and kaolinite), along with 6-39 % of calcite and 6-30 % of quartz. Other minerals present are siderite (-1-4%), plagioclase and K-feldspar (-2-4%), and other minerals in trace amounts such as aragonite, biotite, moscovite, apatite, celestite, zircon and monazite. The contents of pyrite and organic matter vary between 0.7-3.2 wt.% and 0.1-1.5 wt.%, respectively (Gaucher et al. , 2003). The cation exchange capacity ranges from 9 to 13 meq/100g (Waber et al., 2003). Table 1 shows a synthesis of the main characteristics of the formation.

The in situ experiment for water sampling is located at the end of the DI niche within the New Gallery of the Mont Terri Rock Laboratory (Fig. 2). The rock in this zone is part of the lowest Opalinus Clay units, the so-called shaly facies; and is situated a few meters away from the underlying formation (Jurensis Marl, Toarcian). The salinity in this zone is the highest in the shale formation with 12 g/L of chloride (Fig. 3), and the physical porosity and water content of the rock are around 17% and 7 wt.%, respectively (Pearson et al., 2003).

2.2. Oligocene-Miocene Clay (Spain)

The clays investigated are alluvial to lacustrine sediments of Oligocene-Miocene age and occur throughout North-Central Spain, in a continental and mainly endorheic environment. It is a plastic clay formation, with a thickness of around 300 m, occurring at depths below 80 m. The total porosity ranges from 15 to 40 % (Villar, 2003) and the hydraulic conductivity is estimated from one borehole testing to be 2 x [10.sup.-11] - 1 x [10.sup.-12] m/s (LopezGeta et al, 1994).


The material for the study consists of one well, named S1 (650 m depth) (Fig. 4), which comprises a 106 m thick basal lutite sequence dominated by red and green lutites, with some sandstone alternations deposited by fluvio-alluvial processes. The upper part is a 198 m thick lutite to marl sequence, with gypsum and carbonates increasing towards the top, deposited in a predominantly lacustrine environment. It is underlain by terrigenous facies and overlain by marls/limestones with gypsum (lower part).

The composition of the clays is briefly described in Pelayo et al. (2003). Mineralogical composition of the basal sequence of the clay formation consist of 10-30 % quartz, 6-30 % calcite, 1-4 % dolomite, 50-70 % clay minerals (0-2 % smectite, 85-90 % illite and 10-12 % chlorite/kaolinite) and accessory feldspars, gypsum, barite, celestite, ilmenite, iron oxy-hydroxides and pyrite; the upper part of the clay consists of 2-9 % quartz, 6-38% calcite, 0-8 % dolomite, 45-65 % clay minerals (45-60

% illite, 30-50% smectite and 3-7 % chlorite/kaolinite), 2-4 % gypsum and accessory feldspars, barite, celestite, iron oxy-hydroxides, ilmenite and pyrite.

Gypsum has been identified to occur dispersed in the matrix in all the samples analyzed and anhydrite occurs in the deepest samples (e.g. Fig. 5). Most of the iron analyzed in the solid samples is in the <2 urn fraction, as part of the illite structure (e.g. (Si3.375Al0.625)(Al1.595Fe(m)0.255 [Mg.sub.0.26]) [O.sub.10] [(OH).sub.2]([Ca.sub.0.03][K.sub.0.49]), or in the smectite (e.g. [Al.sub.0.605])([Al.0.69]Fe[(III).sub.0.32][Mg.sub.1,55]) [O.sub.10][(OH).sub.2]([Ca.sub.0.11],[K.sub.0.24])) and in chlorite. Only a small part of the iron is present as iron oxides (e.g. goethite was detected in the deepest samples) or pyrite, usually altered, mainly in the upper part of the stratigraphic column.

Isotopic results on gypsum and carbonates from borehole S1 and another borehole in the area (borehole S2, not included in this work) (Fig. 6) fits to the meteoric water line, and indicates the gap between the results of the upper and the lower part in the borehole. This fact suggests that no connection between both parts exists. Thus, we assume that the istopic data reflect equilibrium conditions from the moment in which deposits were buried. This situation should be maintained nowadays.

Table 1 shows a synthesis of the main features of the formation.

3. Field and laboratory sampling and analysis of pore water

The pore water was extracted directly from boreholes with specific downhole equipment (case of Opalinus Clay; Fernandez et al, 2006) or from clay cores by means of squeezing (case of Oligocene-Miocene Clay). Additional analyses of aqueous extracts, cation exchange capacity and geochemical modelling allowed draw up the geochemical behaviour of the systems.




3.1. Field methods for extracting and analyzing water (Mont Terri Rock Laboratory)

One of the main objectives of the participation in the program on geochemical studies at the Mont Terri Rock Laboratory through the Mont Terri project was to collect water representative of the on site conditions by sampling a borehole interval. After considering the recommendations by Pearson and Bath (2003) for obtaining reliable samples from boreholes, samples were collected and analysed after developing equipments and methodologies to avoid oxygen or air contact with the formation samples. The goal was sampling with a minimum perturbation of the clay pore water critical parameters: pH, Eh, pC[O.sub.2], Fe(II) and alkalinity.

For this purpose, the borehole BDI-B1 (20 m length) was drilled two meters from the floor towards the roof of the DI niche within the New Gallery of the Mont Terri Rock Laboratory. It was drilled with pressurized air as drilling fluid and with a single core barrel. A certain time was required to isolate the borehole to obtain reliable pore water samples. During that time, besides drilling works, the borehole was open to air sporadically, until its definitive closing. At this point, the borehole was equipped with a downhole single packer in order to maintain it isolated from air until designing the definitive downhole system for long-term monitoring and sampling. The tritium measurements made in the water sampled in this period fell from 3.8 T.U. to <0.7 T.U. (Fig. 7). Tritium was analysed in other boreholes (Pearson et al., 2003) and results were <0.8 T.U., indicating a residence time for the sampled water to be higher than 60 years.

The downhole equipment was specially designed to obtain samples of water seeping at very low rates from the rock. As a complement, surface equipment controled the functions of the downhole equipment and made it possible to take unaltered samples.

The downhole equipment was made up of the following components (Fig. 8): (1) An inflatable packer placed roughly at 7.75 m from the borehole mouth, which isolates the water sampling interval, located between the packer and the end of the borehole. This sampling interval was equipped with seven external rigid PVC tubes with 200 um slots. Due to the low water volume expected, a tube of POM (Polyoxymethylene Tecaform Delrin) was placed inside the PVC tube to collect the scarce seepage water, thus avoiding the dead volumes inside the PVC tube. At the end of this central tube, four valves were connected to different PFA (Perfluoroalkoxy) tubing (an inert material impervious to [O.sub.2] diffusion) for water sampling, water pressure monitoring in the section, gas inflow (the test interval was flushed with argon) and gas outflow; and (2) An inflatable packer positioned closing the borehole mouth. The space between the two packers was filled with six rigid tubes of PVC to avoid the collapse of the borehole and to increase the hydraulic gradient in the sampling interval. The end-packer was connected up to the mouth of the borehole with a 1 m long PVC rigid tube. The four PFA tubing were guided from the test interval through this PVC tube, which was sealed with epoxy resin at the end, in the tunnel wall.

All the tubing for the water and gas lines were guided towards the equipment installed at surface, which is composed of the following elements (Fig. 9): (1) A packer system control unit equipped with: a three-way valve and a manual manometer for packer inflation pressure control, and a DRUCK transducer connected to a data acquisition system for controlling that the packer keeps the pressure. (2) A vacuum pump system for generating an under-pressure in the sampling interval in order to increase the hydraulic gradient towards the sampling zone. A vacuum transducer is installed in the gas outflow tube to measure the vacuum pressure in the sampling interval for controlling interval tightness. (3) A gas pressure regulator system with two functions: to flush the gas (Ar+1 %C[O.sub.2] mix used in the first campaigns or Ar used afterwards) in the sampling interval through the gas tubing and to keep the anoxic conditions in the glove box. A semi-automatic gas control unit with a gas inlet with maximum pressure of 250 bar and a pre-regulated gas outlet of 8 bar is used for gas flushing and feeding the anoxic glove box. Another gas regulator system is used for the regeneration process of the anoxic glove box with an Ar+1% H2 gas mixture. (4) A water pressure measurement control unit measures the water pressure in the sampling interval. It is equipped with a three-way valve, a manual manometer and a DRUCK PMP 4070 pressure transducer (amplified output, 0-10 bar abs., [+ or -] 0.04% accuracy). (5) A data acquisition system, which is connected to a dial-up router, records the water and the packer pressures. This configuration makes possible access to the operation of the equipment via modem. (6) An anoxic glove box (JACOMEX EURAM type) with Ar atmosphere was used to avoid that water has contact with air. The sampling and measuring of pH, Eh, electrical conductivity (E.C.), Fe(II) and alkalinity in situ was made inside the anoxic glove box to preserve the anoxic conditions of the samples, as described below.




3.1.1. In situ measurements

The redox potential and the pH were measured inside the anoxic glove box by means of an ORION 720A pH-meter equipped with a Metrohm 6.0451.100 Pt electrode and a Metrohm 6.0234.100 pH combination electrode. The pH electrode was calibrated at the sample temperature using Merck pH 4 and pH 7 buffer solutions. A redox buffer solution of Fe[(CN).sub.6.sup.4-] / Fe[(CN).sub.6.sup.3-] ([10.sup.-3] M), at +250 [+ or -] 5 mV was used for the standardizations of the potential measurements.

Electrical conductivity (EC) was measured using an ORION 115 conductimeter. The conductivity cell was checked/calibrated with a standard solution of 12.6 mS/ cm (NaCl) at the same temperature as the sample.

Fe(II) was analyzed using the specific procedure described in Sanchez et al. (2004). The determination of the absorbance was performed at 562 nm against a blank using a Spectroquant Vega 400 Merck portable spectrophotometer, equipped with cells of 1 cm light path.

The alkalinity (expressed as mg/L of HC[O.sub.3]-) was analyzed by a potentiometric Titroprocessor Metrohm 716 DMS. The analyses were made by means of a specific Dynamic Equivalence point Titration (DET) method specifically developed for analyzing 2 mL samples (Sanchez et al., 2003).

3.1.2. Sampling procedure

Water samples were collected inside the anoxic box, from the packed-off interval after leading water to seep over several months. All samples were conventionally filtered through 0.45 [micro]m syringe filters to remove a large amount of particulate material, and collected in polyethylene bottles for complete chemical characterization. Preservation was undertaken according to the constituents under analysis. Ultrapure hydrochloric acid was used to bring the water samples to pH<2 for determinations of Fe(II) at the laboratory. Major cations and trace elements were determined after bringing the samples to pH<2 with 2 vol% of 60% HN[O.sub.3]. Non-acidified samples were used to determine anion and silica concentrations.

Water and gas samples were collected inside TEDLAR gas sampling bags for chemical and gas analyses. Norbert Jockwer from GRS (Germany) performed the analyses of gases by using a gas chromatograph VARIAN CP 4900.

3.2. Laboratory methods for extracting water (Oligocene-Miocene Spanish Clay)

Pore water samples and analyses were obtained from 7 cores from the borehole S1 drilled in Oligocene-Miocene clays of Spain. The maintenance of the chemical integrity of the samples from in situ conditions to laboratory is difficult. In fact, when the samples are analysed in the laboratory, the extent to which water-rock reactions have proceeded depends on the way of collection and conservation of the samples until analyses (Pearson et al., 2003 and references therein). For this reason, as will be shown further on, pore water analyses need to be checked in order to assess if suitable samples and data have been obtained.

In the case of the Oligocene-Miocene Clay, once borehole was drilled, the core samples were isolated from the atmosphere by means of PVC tubes sealed with paraffin to minimize oxidation or degassing, and were stored in a room with high relative humidity and constant temperature until analyses of both rock and pore waters.

The pore water samples were obtained from the cores using the compression of the rock sample technique (squeezing), similar to that developed by Peters et al. (1992) and Entwisle and Reeder (1993). A detailed description of the method is given elsewhere (Cuevas et al. 1997). The compaction chamber is made of type AISI 329 stainless steel (due to high tensile strength and resistance to corrosion) with an internal diameter of 70 mm. It has been designed to allow a one-dimensional compression of the sample (Cuevas et al., 1997; Fernandez et al., 2001b) by means of an automatic hydraulic ram operating downwards being the squeezed water expelled into vacuum vials at both top and bottom of the cell (Fig. 10). The pore water of the Oligocene-Miocene Clay was extracted by applying a constant pressure of 64 MPa onto the sample, under laboratory conditions (25[degrees]C), and avoiding contact with atmospheric air. The filtration system (0.5 um stainless steal AISI 316L porous disk in contact with the sample) allows the extraction of interstitial water by drainage at the top and at the bottom of the sample. The main problem of obtaining accurate information on the composition of pore waters based on this technique is a possible water modification due to irreversible oxidation and degassing during the handling of the samples (Pearson and Bath, 2003; Fernandez et al., 2003). The accuracy of the pore water chemical data was assessed through charge balance calculations, and geochemical modelling was used to check if the squeezed waters are altered.

Before squeezing, sub-samples of each sample were taken for different physical determinations. The grain density was measured by the pycnometer method. The dry density ([[rho].sub.d]) of the samples was determined by the mercury displacement method. The gravimetric water content was determined by weighting the sample before and after oven-drying at 150[degrees]C for 24 hours. The moisture content of the individual samples was calculated from the ratio between water weight loss after heating sample to 110[degrees]C for 24 hours, and the weight of the dried clay, expressed as percentage.

The Chapman displacement method was used to determine the exchangeable cations by means of successive washing with 1N ammonium acetate at pH=8.2, after flushing the soluble salts (Thomas, 1982). To determine the cation exchange capacity, the exchange sites of the sample were saturated with sodium by means of successive washing with1N sodium acetate at pH=8.2. The adsorbed sodium was displaced by successive extractions with ammonium acetate 1N at pH = 8.2 (Rhoades, 1982).

3.3. Analytical techniques in laboratory

Common anions (Cl-, Br-, N[O.sub.3]-, N[O.sub.2]-, P[O.sub.4.sup.3]-, S[O.sub.4.sup.2]-) were analyzed by ion chromatography (Dionex DX-4500i). An ORION 901 microprocessor ion-analyzer, equipped with ion selective electrodes, was used for F- and I- determinations. The alkalinity (expressed as mg/L of HC[O.sub.3]-) was analyzed by a potentiometric Titroprocessor Metrohm 785. The analyses were made by means of a specific Dynamic Equivalence point Titration (DET) method specifically developed for analyzing 2 mL samples (Sanchez et al., 2003). The main cations and some trace elements were analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES), in a Jobin Yvon JY48+JY38 spectrometer. Sodium, lithium and potassium were determined by flame emission spectroscopy in a Perkin Elmer 2280 spectrometer. Trace and ultratrace elements were determined using a Finningan Mat SOLA quadrupole ICP-MS apparatus. Total silica was determined by the ammonium molybdate colorimetric method using a UV-VIS spectrophotometer (EPA 370.1 Method). The tritium content was determined on 1 L of sample by means of liquid scintillation (LS) spectrometry after electrolytic enrichment in CIEMAT laboratories.


3.4. Geochemical modelling

Geochemical modelling was used in this work as a tool to check the representativity of the samples and the analytical data and to interpret and discuss the processes affecting the water chemistry. Further, the physicochemical and compositional reconstruction of the formation pore waters was made.

The geochemical data were interpreted with the geochemical code PHREEQC (Parkhurst and Appelo, 1999) and the WATEQ4F modified thermodynamic database (Ball and Nordstrom, 1991) and NAGRA/PSI thermody namic database (Hummel et al., 2002).

3.4.1. Checking the representativity of the samples and the analytical data

Different aspects can perturb collection of pore water from clays, both by means of squeezing and from boreholes, causing the water chemistry alteration. Pearson and Bath (2003) and Fernandez et al. (2003) made an exhaustive review on the factors that have important effects on the chemistry of a clay formation. Some of these effects are considered to check the logics and the quality of the chemical data obtained and used in this research. These processes are: (1) Uncertainties related to the small amount of water available for analysing, that can bring to analytical errors; (2) mineral oxidation (e.g. sulphide oxidation by intrusion of oxygen in the system) by air contact during drilling, handling of cores, storage, or during squeezing, which should cause decrease of pH and dissolution of minerals such as calcite; (3) Outgassing (or ingassing) of C[O.sub.2] if system is exposed to air, affecting pH and calcite saturation state; (4) Contamination by metallic materials (e.g. filters used at the top and bottom of the compaction chamber during squeezing); and (5) Pressure effects of squeezing by increasing the solubility of minerals.

According to the mentioned aspects, a number of criteria can be used based on the analytical data, in order to assess data quality. The criteria employed in this work are: (1) Charge balance calculations as indicator of analytical errors in major components or omission of an ion with significant abundance (Freeze and Cherry, 1979; according to this convention, the error in the charge balance should be <10%); (2) Comparing with existing analyses at other points of the same formation and observing deviation in trends; (3) Anomalous concentration of a solute or anomalous values of pH that might suggest oxidation and/or degassing; it should be interesting to detect the potential impact of C[O.sub.2] loss or oxidation as sources of inconsistency in the analytical dataset obtained in situ and/or in the laboratory; (4) Assume, as expected, pore water-rock equilibrium with respect to particular minerals in the clay rock.


Additionally, during the storage period of the core samples for squeezing (case of Oligocene-Miocene Clay) some variations in the saturation degree due to evaporation could occur. If so, exchange with [O.sub.2] and C[O.sub.2] from atmosphere should be checked. This effect was also evaluated.

4. Results and discussion

4.1. Water chemistry of Opalinus Clay from borehole BDI-B1

With the downhole equipment described above and the system completely closed to the atmosphere, borehole samples were collected periodically from 2003 to 2006. Hydraulic conductivities were measured in the sampling interval for this time and an average value in the range of [10.sup.-14] m/s was obtained (Fernandez et al, 2006), which is somewhat similar to that measured by Marschall et al. (2004) as average value for the Opalinus Clay matrix. Table 2 summarizes the data on physicochemical parameters and chemical analyses of the water in various campaigns during this period. The charge balances agreed within less than 10% in all the samples. The water is of sodium-chloride type, with high concentrations of sulphate, calcium and magnesium. The ionic strength is around 0.4 mol/L and the electrical conductivity is 28.7 [+ or -] 3.4 mS/cm.


In situ Eh and pH values versus time are presented in figure 11. A trend of increasing pH and decreasing Eh with time can be observed. This evolution is attributed to a tendency towards the stabilization of the system, starting from the borehole drilling to its complete isolation and recovering to the original conditions, with the packer system. During the first stages oxygen entered the borehole and, consequently, the pyrite present in the Opalinus Clay (e.g. Fig. 12) was oxidized, which can be described according to the following reactions:

Fe[S.sub.2] + 7/2 [O.sub.2] + [H.sub.2]O [??] [Fe.sup.2+] + 2S[O.sub.4.sup.2-] + + 2[H.sup.+] eq. 1

Fe[S.sub.2] + 1/2 [O.sub.2] + 5/2 [H.sub.2]O [??] F[(OH).sub.3(s)] + 2[H.sup.+] eq. 2

During the first campaigns a slightly reddish colour was observed in the water samples, which was attributed to the precipitation of iron oxy-hydroxides, according to the described processes, as also observed in other boreholes previously (Gaucher et al., 2003). In fact, calculations of saturation indices for Fe[(OH).sub.3] indicate oversaturation with respect to this mineral. In this respect it is noteworthy that in the last sampling campaign all the iron measured was Fe(II), but that was not the case for the first sampling campaigns, when oxygen was present in the borehole, and part of the iron was Fe(III) (see Table 2).

The oxidation led to an acidification, which was neutralized by the pH buffering of the carbonates, according to the reaction:

2[Fe.sup.2] + 1/2[O.sub.2] + [5[H.sub.2] O + 4CaC[O.sub.3] [right arrow] [right arrow] 2Fe[(OH).sub.3] + 4[Ca.sup.2+] + 4HC[O.sub.3.sup.-] eq. 3

Indeed, the higher alkalinities are measured in the first sampling campaigns (Table 2) and trend to decrease with time, as well as the calium concentration.

The sulphate concentration (Table 2) reflects also the oxidation process during the first stages. Figure 13 depicts the variation of pH and S[O.sub.4.sup.2-] with time. There is a good correlation between the low pH and the high concentration of sulphate due to pyrite oxidation, which causes H+ and S[O.sub.4.sup.2-] release (see eq. 1).

The evidences of exchange of oxygen with the atmosphere (oxidation) during the first sampling campaigns might also lead to think in an additional effect of exchange of C[O.sub.2] (outgassing or ingassing). The calculations of pC[O.sub.2] (based on the measured values for pH and alkalinity) are in Table 2. A decrease of the pC[O.sub.2] regarding that calculated by means of modeling by Pearson et al. (2003) for the Opalinus Clay, at the closest point to BDI-B1 (pC[O.sub.2] < [10.sup.-3] bar), should confirm an exchange of C[O.sub.2]. Indeed, this progressive decreasing of pC[O.sub.2] is correlated with precipitation of carbonate, as shown in Table 2 (strongly oversaturation with respect to calcite is observed).

Up to now the pC[O.sub.2] values for Opalinus Clay were obtained from calculations by using the in situ measurements of pH and alkalinity at different points in the formation, or pH and alkalinity measured from pore water extracted by squeezing of cores, assuming alkalinity equal to HC[O.sub.3.sup.-]. The interpretation of the calculated pC[O.sub.2] values was based in certain statements: on one hand, pC[O.sub.2] [greater than or equal to] [10.sup.-3.5] could be representative of the formation water and lower values (mainly those obtained from squeezing data) are attributed to interaction with the atmosphere during the storage of the cores before being analysed (Pearson et al, 2003).


Therefore, there are some uncertainties linked to the interpretation of the carbonate system. The dataset corresponding to the last sampling campaign in borehole BDI-B1 included C[O.sub.2] measurements from samples taken inside the anoxic box in Tedlar gas sampling bags. The measured pC[O.sub.2] is [10.sup.-19] bar, which is in disagreement with the calculations made for this campaign by means of modeling, based on the in situ measured values for pH and alkalinity (see Table 2, pC[O.sub.2] = [10.sup.-355]).

The discrepancies can be explained in different ways: (1) The in situ pH measurements tend to increase with time, which could reflect a continuous C[O.sub.2] degassing, and the water would tend to equilibrate via calcite precipitation; (2) Microbial activity was detected in studies made on this borehole (Mauclaire and McKenzie, 2006). It is an important source of C[O.sub.2] according to C[H.sub.2]O + [O.sub.2] [right arrow] C[O.sub.2], and should contribute to the increase of C[O.sub.2] observed from the measurements made from samples taken in the anoxic globe box. The reason for the calcite precipitation could be linked to the production of C[O.sub.2] in the sampling sections by microbial activity and a later degassing during sampling; (3) The alkalinity values measured in situ and at CIEMATs laboratory are in rather good agreement (Fig.14). It is logic considering that the alkalinity measurements are independent of outgassing, unless carbonate precipitates. While degassing of C[O.sub.2] could not contribute to alter the alkalinity, the presence of microbial activity could have affected, as investigated by Eichinger and Wersin (2004), Wersin et al. (2004) and Mauclaire and McKenzie (2006) for the long-term pore water experiment at Mont Terri.

Each of these aspects could mislead the interpretation of the carbonate system and it is difficult to evaluate specific contributions to the alteration, so there are still a number of uncertainties in the parameters related with the carbonate system.

The Br-/Cl- and S[O.sub.4.sup.2-]/Cl- ratios were compared with those found in other water sampling experiments at Mont Terri (Degueldre et al., 2003; Pearson et al, 2003). These ratios are close to the seawater, as in all the Opalinus clay water analysed (Fig. 15), although depend on the location of water sampled within the Opalinus Clay formation (e.g. the proximity to the surrounding aquifers).

A Br/Cl- ratio similar to that of seawater, suggests a synsedimentary marine origin of the water (Bath and Gautschi, 2003). On the other hand, the observed chloride and bromide concentrations are lower than the seawater values (~half of the seawater values). The infiltration of meteoric waters could have induced the dilution of the ancient Opalinus Clay pore water. Uplift and erosion during the Pliocene/Quaternary initiated the influx of meteoric water to the sequence (Bossart and Wermeille, 2003). Infiltration to the Liassic limestone would have started in the Quaternary, around 350 thousand years ago. Groundwater flow would have been redirected northwards by the low permeability of the Opalinus Clay (Bath and Gautschi, 2003). Nevertheless, the lower content of the conservative elements seems to be related to a diffusion mechanism from the Opalinus Clay towards the overlying (Lower Dogger limestone) and underlying formations (Jurensis Marls). The chloride and sodium concentration versus the location of the boreholes in the underground laboratory in a NW-SE profile depicts a trend that seems to indicate an effect of solute out-diffusion, specially towards the most permeable overlying Dogger limestones (see in Fig. 3 the chloride trend). The Lower Dogger is a recharge and karstic zone containing dilute meteoric water, which establishes a chemical gradient in relation to the clay formation with high saline contents. This leads to a long-term large-scale process of mass transfer by diffusion (Arcos et al., 2001; Degueldre et al, 2003; Pearson et al., 2003).

4.2. Water chemistry of Oligocene-Miocene Clay from borehole cores

The composition of all the pore waters collected by squeezing is given in Table 3. In all the samples analyzed, charge balances were better than 10%, thus analytical errors can be discarded. Most of the waters are sodium-sulphate type waters, with high concentration of chloride, calcium and magnesium. According to Hanor (1987) the waters can be classified as brackish (TDS ranging from 1000 to 10000 mg/L). pH values range from neutral to slightly alkaline (6.9 to 8.1). The ionic strength is around 0.2 mol/L for all the samples and the electrical conductivity varies from 7.9 to 13.7 mS/cm. The total cation exchange capacity ranges from 16 to 49 meq/100g, being the exchangeable cation population dominated by calcium and sodium (Table 4).


As factors that might have contributed to the alteration of the samples the following were considered: (1) the conservation and storage of the samples and, for instance, the degree of atmosphere isolation; and (2) the handling of the samples before squeezing and during analysis.

Table 5 shows a number of parameters used to check if the results are indicative of the in situ geochemical conditions of the formation. Some of them, such as saturation indices and pC[O.sub.2] were calculated from the analytical data, with the geochemical code PHREEQC. Each sample was checked separately. Notice that there is no data on in situ pore water extracted from boreholes to compare with.

In core sample C201 and C205 the values of pH, pC[O.sub.2] and saturation index with respect to calcite do not seem indicate any anomaly.

The water degree of saturation calculated for core sample C203 is the lowest ([S.sub.r] = 0.62), which may indicate evaporation during the storage. The pH, pC[O.sub.2] and saturation index with respect to calcite indicate a slight C[O.sub.2] outgassing.

In core sample C253 the values of pH, pC[O.sub.2] and saturation index with respect to calcite indicate an oxidation process. The B content is high as well, which could affect the alkalinity by overestimation of the inorganic carbon concentration. This should reduce even more the saturation index of calcite, so it should not discard the oxidation process.

Regarding the core sample C257, the data indicate that this sample is oversaturated with respect to calcite. The pH and pC[O.sub.2] values reflect C[O.sub.2] outgassing.

The pore water extracted from core sample C283 has the lower pH and the highest pC[O.sub.2], alkalinity and sulphate concentration. This sample is slightly oversaturated with respect to gypsum, so it seems to reflect an oxidation process of pyrite. However, there is oversaturation regarding the calcite and the S[O.sub.4.sup.2-] / Cl- is the lowest, which is in contradiction with pyrite oxidation. To evaluate the possible oxidation process of this sample geochemical modeling was used. A kinetic process of pyrite oxidation, maintaining calcite equilibrium was assumed. For that, the Singer and Stumm (1970) model was applied. It considers the oxidation rate of pyrite controlled by the oxidation of Fe(II) to Fe(III) according to:


where t is time (s), [a.dun.OH]- is the hydroxide activity, [m.sub.Fe2+] is the molality of the ferrous iron in the solution and P[O.sub.2] is the oxygen partial pressure (atm). The precipitation of amorphous ferric iron was assumed. The result is similar to the measured one, so the sample is considered unaltered. The higher sulphate content could be just related with the sedimentation environment, which is different to the samples analyzed above, more than with artifacts during sampling or analyses.

In the core sample C290 the pore water has the highest pH and a pC[O.sub.2] similar to the atmospheric one. The alkalinity is the lowest and the sample is oversaturated regarding the calcite. This points to C[O.sub.2] outgassing. On the other hand, the iron and zinc concentration are quite anomalous compared with the other samples. It could be associated to a metallic contamination by filters or pressure effects when squeezing, since the pressure increases the solubility of minerals, affecting the mineral-solution equilibria (Fernandez et al., 2003).

There are no data on redox potential for any sample and the confidence for measured iron concentrations is low due to the mentioned evidences on oxidation processes and contamination. There is some correlation between the variations in the [[delta].sup.34]S content and the variations in the sulphate concentration and organic matter content of the cores (Table 6). This fact can be interpreted as sulphur oxidation/reduction processes, the latest microbially-mediated, which control the redox in the system (e.g. Fig. 16).


4.2.1. Synthesis of assumed geochemical processes

Geochemical equilibrium calculations performed from the present formation pore water compositions indicated that the sampled pore water closely approaches saturation with respect to the sulphate minerals celestite, anhydrite, gypsum and barite. Since the water degree of saturation is lower than 1 in all the samples (Table 5) an evaporation process, with exchange of [O.sub.2] and C[O.sub.2], might occur. An oxidation process by the supply of atmospheric oxygen into the samples during the storage period might have led to the sulphide oxidation. In such a case, the system exchanged not only [O.sub.2] but also C[O.sub.2] with the atmosphere, and sulphide and [Fe.sup.2+] were oxidized to sulphate and [Fe.sup.3+], precipitated as Fe[(OH).sub.3] (oversaturated as indicated in the calculations made with PHREEQC). These hypothesis were considered in terms of modelling.

Calcium required for gypsum precipitation was assumed to be present from cation exchange and calcite dissolution, the last triggered by the [H.sup.+] produced by pyrite oxidation, buffering this acid production.

Furthermore, carbonate minerals analyses indicate that these minerals are being dissolved because of pore water-undersaturated conditions; SEM studies provided evidences on calcite partially dissolved and with corroded edges.

The excess of sodium concentration with respect to chloride was attributed to cation exchange processes. High potassium concentrations might be derived from silicates transformation and equilibrium with the exchanger population. Magnesium is consumed in the process of chlorite formation (there is a positive correlation between [Mg] and % wt. of chlorite). Calcium is controlled by calcite dissolution and cation exchange.

4.2.2. Reconstructing the pore water composition

It is assumed that the pore water composition represents meteoric water contained in the sediments at the time of deposition, evolved until the present by diagenetic water-rock reactions and diffusive transport, and modified in terms of evaporation, as a consequence of storage and handling of samples.

The geochemical modeling used is a simple equilibrium modelling of aqueous species and water-rock reactions. It is thought that the pore water chemistry is controlled by chemical equilibrium with respect to most of the reactive minerals contributing to the water-rock interaction processes in the clay formation, with the exception of certain free or non-reacting constituents. This seems reasonable from various constraints such as the fine-grained nature of the Oligocene-Miocene Clay, assuming a long residence time of the pore water within the rock, inferred by isotopic studies on carbonates and gypsum, and petrography evidence from minerals such as calcite, dolomite, gypsum, pyrite and iron hydroxides, suggesting that they are the main reactive phases.

For applying corrections based on water-rock interaction processes, exchange with atmospheric gases, and ionic exchange, all the data obtained by means of squeezing were considered. The aim was to correct the processes occurred in the sample due to storage and handling. The corrections were made following different steps: (1) Estimate the humidity of the samples by assuming a complete water saturation degree, to rectify the evaporation effect; and (2) Correct the composition of the water considering equilibrium with the minerals present in the formation, the interaction with atmosphere gases, ionic exchange processes and water-rock reactions during storage and handling of samples.

(1) The humidity assuming complete water saturation of samples was calculated from:


where w is the humidity, pd, dry density, n is porosity and p.e. is the specific weight or particle density (Table 7). Then, the volume of water lost by evaporation was calculated as:


[w.sub.Sr=1] / w means the quantity of water necessary to achieve the complete saturation, by unity of water volume, assuming that water density is 1 g/[cm.sup.3]. Table 7 shows the results. So, the corrections to the chemical composition of the pore water, were made by adding [V.sub.evap] to the solution following the equation 6:


where [w.sup.-1] is the coefficient used to recalculate the water added from mol/[kg.sub.clay] l to mol/[kg.sub.solution].

(2) If evaporation affected to the solid phases (e.g. pyrite and calcite) and the exchange population of the clayey fraction, the equilibrium of the solution with respect to these minerals and the exchange population should be considered. The calculations of the redox potential were made considering that in the lower unit the redox is controlled by the equilibrium between the iron measured in the solution and the mineral phase Fe[(OH).sub.3]; and in the upper unit the redox is controlled by the equilibrium gypsum/pyrite (S(VI)/S(-II)).

The results are shown in Table 8. The pH in all the samples is around 7.6 (except in sample 283) and the average pe is around -3.5. Calcium, from calcite dissolution, contributes to the modification of the cation exchanger population in the clay and to the cation composition of the pore water.

4.2.3. Modelling the redox evolution of the system

This section presents an example of the redox calculations made with the sample C201. Some slow kinetic processes, such as organic matter degradation, are difficult to adjust to equilibrium. Because the clay formation is rich in organic matter (0.7 - 4.1 weight percent C), it was assumed that the redox system could be controlled by the kinetics of microbially-mediated degradation reactions of organic matter.


In this sense, there is some evidence of the occurrence of sulphate reduction in the system (e.g. Fig. 16). Moreover, analysis of the [[delta].sup.34]S of gypsum compared with the sulphate concentrations measured in pore water and the chemical composition of the clay indicates two different zones (upper and lower zone, as defined in section 2.2.). The presence of two zones is further confirmed by a correlation between enriched [[delta].sup.34]S in gypsum, lower sulphate concentration in pore water, and higher ferrous iron concentrations in the upper zone of the clay with respect to the lower one, which clearly indicate the presence of sulphate reduction (Table 6). After these initial considerations, the PHREEQC geochemical code was used to simulate the evolution of the redox system.

Once demonstrated that alteration of the samples from in situ to laboratory conditions occurred and a long storage period lead to the exchange with atmospheric oxygen and C[O.sub.2] a modelling exercise, selecting sample C201, considered to be initial pore water in equilibrium with atmospheric oxygen, was made. The simulation starts in equilibrium with calcite, used to correct for the degassing of C[O.sub.2], and with dolomite, gypsum, and ferrihydrite. The rate of oxidation of pyrite is based on the rate law from Williamson and Rimstidt (1994):


where r is the rate of pyrite dissolution in mol [m.sup.-2][s.sup.-1]. Equilibrium was used for precipitation of pyrite.

The formulation for microbially-mediated degradation of organic matter was based on a Monod approach with only electron acceptor limitation. The following rate expression was used:

r = [r.sub.m] (C/ [K.sub.m] + C] eq. 9

where r is the rate of organic matter degradation in mol [m.sup.-2][s.sup.-1], [r.sub.max] is the maximum reaction rate, [K.sub.m] is the half saturation constant for electron acceptor and C is the concentration of electron acceptor. Only aerobic respiration and sulphate reduction were considered as degradation processes.

Figure 17 illustrates the evolution of pe in a time period of 105 years. Four different periods, with changes in the redox controlling species can be described. The first one is controlled by the pair O(-II)/O(0). Pyrite oxidation and resulting ferrihydrite precipitation, together with processes such as organic matter degradation by aerobic respiration (until pore water is depleted in dissolved oxygen) and sulphate reduction, guide the next step in the redox evolution.

Once dissolved oxygen is almost completely consumed, only sulphate reduction controls the redox evolution as a non-equilibrium process. In this stage, the redox condition is regulated by the Fe(II)/ferrihydrite pair. When complete reductive dissolution of previously formed ferrihydrite is achieved, dissolution of gypsum and precipitation of pyrite starts. Because the solution is depleted in iron and no reasonable iron source in the solid is found, the S[O.sub.4]/HS- pair is likely to buffer the latest redox state of the system.

5. Comparison of pore water chemistry of different mudrocks

A comparison on pore water chemistry at different locations (e.g. Opalinus Clay and Oligocene-Miocene Clay (this work), and Boom Clay from Belgium (Dierckx, 1997) and Callovo-Oxfordin Clay from France (Andra, 2005) have been made. The differences between the water chemical composition are well observable in the Schoeller's diagram shown in figure 18, which illustrates the pattern of the total concentration of major ions of the pore waters. All the waters have rather high salinity, being sulfate and chloride the dominant anions, except for the Boom Clay. However, the graph shows that the main differences are expressed in the contents of sulfates and bicarbonates, suggesting that processes related to carbonate and redox systems are critical in defining the composition of these waters. As shown for Opalinus Clay and Oligocene-Miocene Clay the variations can be explained by principles of chemical equilibrium and cation exchange. Although the origin itself of the waters is different for both clays, chemical perturbations (e.g. oxidation and/or C[O.sub.2] exchange) induce the same response, with a tendence of the system to recover the "unperturbed state". In this sense, the geochemical model considered for each individual system assumes that the equilibrium of "key minerals" commonly present in these mudrocks, such as calcite, siderite, pyrite, and chalcedony, and the cation exchange processes, are enough to explain the nature of the pore waters.

6. Conclusions

The participation in the International Project Mont Terri during the last 10 years allowed us to obtain an invaluable background about the knowledge of the pore water chemistry in argillaceous formations. Several water-sampling campaigns were performed in borehole BDI-B1 by means of a quite sophisticated equipment, developed to obtain reliable samples and measurements of the pore water from the Opalinus Clay at Mont Terri Rock Laboratory. Avoiding the contact of samples with air showed to be necessary, since this argillaceous rock is extremely susceptible to pyrite oxidation. The successive sampling campaigns and analyses demonstrate the evolution of the physicochemical parameters of the pore water towards a stabilization of the system. Some of the main parameters measured in situ, such as pH, Fe(II), alkalinity and pC[O.sub.2], were comparable with previous results obtained by modelling other points of the Opalinus Clay. The modelling results show a reasonable agreement with the measured chemical data. This is true for all constituents except for carbonate concentration and pH, a possible explanation for this disagreement could be related to a C[O.sub.2] shift of pore water samples during sampling.


The characterization of the pore waters of clays collected from a borehole of 650 m made in the Oligocene-Miocene Clay, and the vertical distribution of dissolved con stituents was made in order to identify sources of waters and water-rock interaction processes. The study of the samples included mineralogical analyses of the rock, cation exchange capacity and exchanger population, chemical analyses of porewater samples and integration of results and modelling using the PHREEQC geochemical code. The evaluation of the fluid composition indicated that oxidation and degassing processes during handling and storage of the samples could have occurred. Taking this into account, the original porewater of the formation was reconstructed by means of modelling, which served as a powerful tool to reconstruct the original water of the formation, taking into account these water-rock interaction processes derived from observations and analyses. Further, a conceptual model on the redox evolution in the Oligocene-Miocene Clay is described as a partial equilibrium process, where the kinetics of sulphate reduction led redox evolution from one equilibrium redox pair to another.


The combination of a precise analytical work together with the geochemical modelling has proven to be useful to characterise these low porosity media and to establish the main sources of uncertainty both in pore water samples obtained in situ from boreholes and from squeezing borehole cores.


The work is part of a research program funded by ENRESA (Spain) / NAGRA (Switzerland), on one hand, and ENRESA on the other, in the context of their R&D programs. The authors are grateful to Aitemin (Spain) by the support in designing the downhole equipment for BDIB1 experiment, and to A.E. Gonzalez and the chemical department of CIEMAT by the analytical work. We also thank Heinz Steiger, Nicolas Badertscher, Christophe Nussbaum and Olivier Meier, from the Geotechnical Institute, for their help in the operation of the experiment in Mont Terri. Thanks are due to Eric Gaucher and David Arcos for their useful comments and suggestions that improved the original manuscript. We will forever be grateful to Pedro Rivas for many years of learning and fruitful collaboration.

Received: 25/05/06 / Accepted: 10/09/06


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(1) Run as a consortium of partners: NAGRA (Switzerland), ANDRA and IRSN (France), JNC and Obayashi (Japan), SCK-CEN (Belgium), BGR and GRS (Germany), ENRESA (Spain). The investigations within the project are carried out under the patronage of the Swiss Geological Survey.

M.J. Turrero [1] *, A.M. Fernandez [1], J. Pena [1], M.D. Sanchez [2], P. Wersin [3], P. Bossart [4], M. Sanchez [2] A. Melon [1], A. Garralon [1], A. Yllera [1], P. Gomez [1], P. Hernan [5]

[1] Dpto. de Medioambiente, CIEMAT, Avda. Complutense 22, 28040 Madrid, Spain

[2] Dpto. de Tecnologia, CIEMAT, Avda. Complutense 22, 28040 Madrid, Spain

[3] NAGRA, Hardstrasse 73, CH-5430 Wettingen, Switzerland

[4] Geotechnical Institute Ltd, CH - 2882 St-Ursanne, Switzerland

[5] ENRESA, Emilio Vargas, 7, 28043 Madrid, Spain

* corresponding author:
Table 1.- Main characteristics of the clays selected for this study.

Tabla 1.- Principales caracteristicas de las arcillas seleccionadas
para este estudio.

Mudrock                              Oligocene-Miocene Clay (1)

Location                                 North-Central Spain

Classification                   Plastic red to green massive clays
                                        with sandy admixtures

Depth                                        82 - 145 m

Country                                         Spain

Age                                       Oligocene-Miocene

Origin                                       Continental

Thickness                                    300 - 350 m

Overlain Fm                        Low-permeable marls with gypsum

Underlain Fm                            More permeable marls

Clay minerals                                 30 - 80%
                               (smectite, illite, chlorite, kaolinite)

Carbonates                                     7 - 48%
                                         (calcite, dolomite)

Other minerals                      Q (1 - 30%), gypsum, pyrite,
                                           organic matter

CEC                                       16 - 49 meq/100g

Dry density                            1.7 - 2.1 g/[cm.sup.3]

Total porosity                                24 - 39%

Permeability                        [approximately equal to] 2 x
                                 [10.sup.-11] - 1 x [10.sup.-12] m/s

Dominant transport mechanism                  Diffusion

Specific Surface Area              29 - 58 [m.sup.2]/g (external)

Water type                              Na-S[O.sub.4.sup.2-]

Ionic strength                             0.2 - 0.3 mol/L

[Cl-] mg/L                                      1000

Organic matter                      0.7 - 4.1 % (C[O.sub.2] org.)

Mudrock                                 Opalinus Clay (2)

Location                                    Mt Terri

Classification                   Indurated dark-gray silt/sandy

Depth                                      230 - 320m

Country                                    Switzerland

Age                                 Jurassic (180 - 178 m.a.)

Origin                                       Marine

Thickness                                     160 m

Overlain Fm                             Dogger Limestones

Underlain Fm                      Liassic marls and limestones

Clay minerals                               55 - 80%
                                    (illite, illite/smectite,
                                      kaolinite, chlorite)

Carbonates                                   6 - 31%
                                  (calcite, dolomite/ankerite,

Other minerals                   Q (15 - 30%), feldspar, pyrite,
                                         organic matter

CEC                                      9 - 13 meq/100g

Dry density                          2.2 - 2.4 g/[cm.sup.3]

Total porosity                              12 - 19%

Permeability                      [approximately equal to] 2 x
                               [10.sup.-12] - 2 x [10.sup.-14] m/s

Dominant transport mechanism                Diffusion

Specific Surface Area            24 - 37 [m.sup.2]/g (external)

Water type                                Na-[Cl.sup.-]

Ionic strength                           0.1 - 0.4 mol/L

[Cl-] mg/L                                 4000-10000

Organic matter                           5 - 15 mg/L DOC

(1) Lopez-Geta et al., 1994, Pelayo et al., 2003, Pena et al., 2003,
Proyecto AFA, 1995; 2 Bossart and Thury, 1998, Mazurek, 1998, 1999,
Cottour et al., 1999, de Windt and Palut, 1999, Thury and Bossart,
1999a, Bock, 2001, Pearson et al., 2003.

Table 2.- Physicochemical parameters and chemical and isotopic
composition of the pore waters extracted from borehole BDI-B1.

Tabla 2.- Parametros fisicoquimicos y composicion quimica e
isotopica del agua intersticial extraida del sondeo BDI-B1

Sampling Date                   April 2003   June 2003   October 2003

Electrical Cond (mS/cm)               29.8        26.8           28.1
pH in situ                            n.d.         6.8            6.9
pH lab                                 7.3         7.5            7.4
Electrical Neutrality (%)             3.17       -0.90           3.32
Ionic Strength (M)                    0.42        0.43           0.40
Eh in situ (mV)                       n.d.          83             77
Tritium (T.U.)                        3.82        1.93           n.d.
[F.sup.-] (mg/L)                      0.30        0.30           0.30
[I.sup.-] (mg/L)                       2.2         2.2            1.8
N[H.sub.4.sup.+] (mg/L)               14.2        12.7           14.0
[Br.sup.-] (mg/L)                       31          28             28
[Cl.sup.-] (mg/L)                    11000       12000          11000
N[O.sub.3.sup.-] (mg/L)                < 1         < 1            < 1
S[O.sub.4.sup.2-] (mg/L)              2100        1833           1700
Alkalinity in situ (mg/L)             n.d.         334           n.d.
Alkalinity in lab (mg/L)               344         347            313
Fe Total (mg/L)                        2.2         4.3            4.1
Fe(II) in situ (mg/L)                 n.d.         1.6           n.d.
Fe(II) in lab (mg/L)                   1.3         1.6            1.0
Si[O.sub.2] (mg/L)                     7.4         9.2            4.2
K (mg/L)                                88          89             93
Al (mg/L)                              0.1         0.1            0.1
Li (mg/L)                              0.8         0.8           n.d.
B (mg/L)                               0.1         3.5            3.1
Ca (mg/L)                              922        1033            886
Mg (mg/L)                              611         574            456
Na (mg/L)                             6480        6300           6650
Sr (mg/L)                               43          40             41
Mn (mg/L)                              0.4         0.3            0.3
Saturation [index.sub.calcite]        0.19        0.32           0.27
log pC[O.sub.2]                      -1.45       -1.52          -1.59

Sampling Date                   July 2004   March 2005   June 2005

Electrical Cond (mS/cm)              24.3         26.2        31.8
pH in situ                            7.0          7.6         8.4
pH lab                                7.6          7.6         8.0
Electrical Neutrality (%)           -5.75        -4.76       -5.06
Ionic Strength (M)                   0.43         0.39        0.40
Eh in situ (mV)                        -7        -24.2         -75
Tritium (T.U.)                      < 0.7         n.d.        n.d.
[F.sup.-] (mg/L)                     0.19         0.51        0.90
[I.sup.-] (mg/L)                      2.0          2.6        n.d.
N[H.sub.4.sup.+] (mg/L)              13.0         13.0        11.0
[Br.sup.-] (mg/L)                      19           22          21
[Cl.sup.-] (mg/L)                   13000        11800       12100
N[O.sub.3.sup.-] (mg/L)               < 1          < 1         < 1
S[O.sub.4.sup.2-] (mg/L)             1700         1600        1700
Alkalinity in situ (mg/L)            n.d.          299         175
Alkalinity in lab (mg/L)              303          280         161
Fe Total (mg/L)                       3.3          1.3         1.9
Fe(II) in situ (mg/L)                n.d.          1.3        n.d.
Fe(II) in lab (mg/L)                  3.1          1.2         1.9
Si[O.sub.2] (mg/L)                    5.8          8.3        14.2
K (mg/L)                               73           80          69
Al (mg/L)                            0.12       < 0.05      < 0.25
Li (mg/L)                            n.d.         n.d.        n.d.
B (mg/L)                              3.0          2.5         3.6
Ca (mg/L)                             880          806         629
Mg (mg/L)                             505          395         545
Na (mg/L)                            6300         6000        6100
Sr (mg/L)                              36           37          40
Mn (mg/L)                            0.22         0.12        0.25
Saturation [index.sub.calcite]       0.33         0.87        1.22
log pC[O.sub.2]                     -1.69        -2.35       -3.49

Sampling Date                   March 2006

Electrical Cond (mS/cm)               34.3
pH in situ                             8.6
pH lab                                 7.9
Electrical Neutrality (%)             1.42
Ionic Strength (M)                    0.40
Eh in situ (mV)                        -71
Tritium (T.U.)                        n.d.
[F.sup.-] (mg/L)                      0.96
[I.sup.-] (mg/L)                      n.d.
N[H.sub.4.sup.+] (mg/L)               n.d.
[Br.sup.-] (mg/L)                       31
[Cl.sup.-] (mg/L)                    10500
N[O.sub.3.sup.-] (mg/L)                < 1
S[O.sub.4.sup.2-] (mg/L)              1600
Alkalinity in situ (mg/L)              214
Alkalinity in lab (mg/L)               253
Fe Total (mg/L)                        0.7
Fe(II) in situ (mg/L)                 1.86
Fe(II) in lab (mg/L)                   1.0
Si[O.sub.2] (mg/L)                     7.5
K (mg/L)                                65
Al (mg/L)                             0.10
Li (mg/L)                             n.d.
B (mg/L)                               3.2
Ca (mg/L)                              772
Mg (mg/L)                              449
Na (mg/L)                             6100
Sr (mg/L)                               34
Mn (mg/L)                             0.18
Saturation [index.sub.calcite]        1.66
log pC[O.sub.2]                      -3.55

Table 3.- Physicochemical parameters and chemical composition of
the pore waters extracted by squeezing from Oligocene-Miocene
Clay cores from borehole S1.

Tabla 3.- Parametros fisicoquimicos y composicion quimica del
agua intersticial extraida de los testigos del sondeo S1 de la
arcilla del Oligoceno-Mioceno.

Squeezed Sample             C 201    C 203    C 205    C 253

Depth (m)                    197      199      202      249
Humidity (%)                 19,9     18,4     24,4     15,5
Vol extracted (mL)            76      31,7    74,85      38
pH                           7,53     7,88     7,65     7,17
Electrical Cond (mS/cm)      11.7     11.6     13.7     14.6
Ionic Strength (M)          0,221    0,231    0,219    0,238
Electrical Neutrality (%)   -0,68     5,12     0,59    -4,25
[Cl.sup.-] (mg/L)            940      1030     940      1100
S[O.sub.4.sup.2-] (mg/L)     7700     7800     7500     8200
[Br.sup.-] (mg/L)             <1       <1       <1       <1
N[O.sub.3.sup.-] (mg/L)      2,5       1        <1      1,3
Alkalinity (mg/L)             89     152,72    123      110
[F.sup.-] (mg/L)              <1       <1      2,1       <1
[I.sup.-] (mg/L)            0,675      17       16      1,9
Si[O.sub.2] (mg/L)           12,7      --      15,4     12,8
Al (mg/L)                   <0,002   0,081    0,0047    0,94
Ca (mg/L)                    425      470      375      385
Mg (mg/L)                    400      540      390      265
Na (mg/L)                    3000     3250     3100     3400
K (mg/L)                      65       --       32       32
Sr (mg/L)                    7,9      9,3      7,3      9,0
Li (mg/L)                    0,89      --     0,041     2,2
B (mg/L)                     0,78     1,1      0,90      21
Ba (mg/L)                   <0,06     0,10     0,08     <0,3
Fe Total (mg/L)              0,09     0,08     <0,5     <0,5
Fe (II) (mg/L)               <0,5     <0,5     <0,5     <0,5
Mn (mg/L)                    0,28     0,17     0,12     <0,3

Squeezed Sample             C 257    C 283    C 290

Depth (m)                    253      278      286
Humidity (%)                 20,9     11,4     14,2
Vol extracted (mL)           84,9      --       22
pH                           7,72     6,90     8,13
Electrical Cond (mS/cm)      13.7      --       --
Ionic Strength (M)          0,229    0,296    0,274
Electrical Neutrality (%)    8,77     0,98     2,61
[Cl.sup.-] (mg/L)            899      2200     1600
S[O.sub.4.sup.2-] (mg/L)     7400     9300     9000
[Br.sup.-] (mg/L)             <1       <1       <1
N[O.sub.3.sup.-] (mg/L)      2,1      1,7       <1
Alkalinity (mg/L)            252      410       66
[F.sup.-] (mg/L)              <1       --       --
[I.sup.-] (mg/L)              --       --       --
Si[O.sub.2] (mg/L)            22       --       --
Al (mg/L)                   0,0022   0,188     <0,2
Ca (mg/L)                    440      870      755
Mg (mg/L)                    345      480      370
Na (mg/L)                    3700     4200     4000
K (mg/L)                      19       35       32
Sr (mg/L)                    8,4       17       11
Li (mg/L)                   0,043      --       --
B (mg/L)                     1,4      <0,3     0,64
Ba (mg/L)                    0,09    <0,11     <0,1
Fe Total (mg/L)              0,2      0,34     123
Fe (II) (mg/L)               <0,2      --      33,2
Mn (mg/L)                    0,25     1,8      1,2

Table 4.- Sum of the exchangeable cations and exchange population
analyzed from Oligocene-Miocene Clay cores from borehole S1.

Tabla 4.- Capacidad total de intercambio cationico y cationes de
cambio analizados en los testigos del sondeo S1 de la arcilla del

Sample   Depth (m)   [Al.sup.3+]   [Ca.sup.2+]   [Mg.sup.2+]

C201        197         0.03          10.53         7.61
C203        199          --           15.18         6.38
C205        202          --           20.04         4.94
C253        249         0.09          11.73         4.11
C257        253          --           14.30         4.32
C283        278          --           13.71         3.50
C290        286         0.02          12.38         2.06

Sample   [Na.sup.+]    [Sr.sup.2+]    [K.sup.+]    [Ba.sup.2+]

C201        7.61          0.13          1.28          0.02
C203        5.44          0.09          1.15          0.04
C205        3.59          0.27          1.60          0.02
C253        4.89          0.06          1.92          0.08
C257        5.98          0.06          1.41          0.03
C283        4.35          0.06          1.15          0.07
C290        3.48          0.03          1.60          0.03

Sample   [Mn.sup.2+]   [Fe.sup.3+]   CEC

C201         --           0.01       27
C203         --            --        28
C205         --            --        30
C253         --           0.03       23
C257         --            --        26
C283        0.005          --        23
C290        0.005          --        20

Table 5.- Parameters considered to check the representativity of
the samples and the analytical data obtained from the
Oligocene-Miocene Clay cores.

Tabla 5.- Parametros considerados para valorar la
representatividad de las muestras y de los datos analiticos
obtenidos de los testigos de la arcilla del Oligoceno-Mioceno.

Sample                                C201                 C203

S[O.sub.4.sup.2-]/[Cl.sup.-]          3.03                 2.80
pH                                    7.53                 7.88
Calculated log pC[O.sub.2]           -2.72                -2.86
Electrical Neutrality (%)            -0.68                 5.12
Humidity (%)                          19.9                 18.4
Ionic Strength (M)                    0.22                 0.23
Water Saturation Degree               0.88                 0.62
Ca (mol/L)                     1.06 x [10.sup.-2]   1.17 x [10.sup.-2]
Mg (mol/L)                     1.65 x [10.sup.-2]   2.22 x [10.sup.-2]
Alkalinity                             89                  153
S[O.sub.4.sup.2-] (mol/L)      8.02 x [10.sup.-2]   8.13 x [10.sup.-2]
Mg/Ca                                 1.55                 1.89
Fe (mol/L)                     1.61 x [10.sup.-6]   1.43 x [10.sup.-6]
Fe (II) (mol/L)                        --                   --
pe Fe(II)/Fe(III)                      --                   --
Zn (mg/L)                             0.4                  0.4
B (mg/L)                              0.8                  1.1
  [Index.sub.dolomite]                0.11                 1.42
  [Index.sub.calcite]                -0.09                 0.52
  [Index.sub.gypsum]                  0.02                 0.05
  [Index.sub.anhydrite]              -0.20                -0.17

Sample                                C205                 C253

S[O.sub.4.sup.2-]/[Cl.sup.-]          2.95                 2.75
pH                                    7.65                 7.17
Calculated log pC[O.sub.2]           -2.70                -2.29
Electrical Neutrality (%)             0.59                -4.25
Humidity (%)                          24.4                 15.5
Ionic Strength (M)                    0.22                 0.24
Water Saturation Degree               0.85                 0.95
Ca (mol/L)                     9.36 x [10.sup.-3]   9.61 x [10.sup.-3]
Mg (mol/L)                     1.60 x [10.sup.-2]   1.09 x [10.sup.-2]
Alkalinity                            123                  110
S[O.sub.4.sup.2-] (mol/L)      7.81 x [10.sup.-2]   8.54 x [10.sup.-2]
Mg/Ca                                 1.71                 1.13
Fe (mol/L)                             --                   --
Fe (II) (mol/L)                        --                   --
pe Fe(II)/Fe(III)                      --                   --
Zn (mg/L)                             0.8                  1.4
B (mg/L)                              0.9                  21.0
  [Index.sub.dolomite]                0.57                -0.74
  [Index.sub.calcite]                 0.11                -0.45
  [Index.sub.gypsum]                 -0.04                -0.01
  [Index.sub.anhydrite]              -0.26                -0.23

Sample                                C257                 C283

S[O.sub.4.sup.2-]/[Cl.sup.-]          3.04                 1.56
pH                                    7.72                 6.90
Calculated log pC[O.sub.2]           -2.46                -1.44
Electrical Neutrality (%)             8.77                 0.98
Humidity (%)                          20.9                 11.4
Ionic Strength (M)                    0.23                 0.30
Water Saturation Degree               0.91                 0.88
Ca (mol/L)                     1.10 x [10.sup.-2]   2.17 x [10.sup.-2]
Mg (mol/L)                     1.42 x [10.sup.-2]   1.97 x [10.sup.-2]
Alkalinity                            252                  410
S[O.sub.4.sup.2-] (mol/L)      7.71 x [10.sup.-2]   9.69 x [10.sup.-2]
Mg/Ca                                 1.29                 0.91
Fe (mol/L)                     3.58 x [10.sup.-6]   6.09 x [10.sup.-6]
Fe (II) (mol/L)                        --                   --
pe Fe(II)/Fe(III)                      --                   --
Zn (mg/L)                             0.9                  3.8
B (mg/L)                              1.4                   --
  [Index.sub.dolomite]                1.35                 0.51
  [Index.sub.calcite]                 0.56                 0.22
  [Index.sub.gypsum]                  0.01                 0.31
  [Index.sub.anhydrite]              -0.20                 0.10

Sample                                C290

S[O.sub.4.sup.2-]/[Cl.sup.-]          2.08
pH                                    8.13
Calculated log pC[O.sub.2]           -3.50
Electrical Neutrality (%)             2.61
Humidity (%)                          14.2
Ionic Strength (M)                    0.27
Water Saturation Degree               0.91
Ca (mol/L)                     1.88 x [10.sup.-2]
Mg (mol/L)                     1.52 x [10.sup.-2]
Alkalinity                             66
S[O.sub.4.sup.2-] (mol/L)      9.38 x [10.sup.-2]
Mg/Ca                                 0.81
Fe (mol/L)                     2.20 x [10.sup.-3]
Fe (II) (mol/L)                5.94 x [10.sup.-4]
pe Fe(II)/Fe(III)                     1.49
Zn (mg/L)                             16.0
B (mg/L)                              0.6
  [Index.sub.dolomite]                1.14
  [Index.sub.calcite]                 0.56
  [Index.sub.gypsum]                  0.27
  [Index.sub.anhydrite]               0.05

Table 6.- Chemical and isotopic results considered to analyze the
evolution of the redox sys- tem in the Oligocene-Miocene Clay.

Tabla 6.- Resultados quimicos e isotopicos considerados para
analizar la evolucion del siste- ma redox en la arcilla del

Sample   Unit    Depth   Sulphate   [[delta].sup.34]S
                  (m)     (mg/L)       gypsum (%o)

C201     Upper    197      7700           12.9
C203     Upper    199      7800            --
C205     Upper    202      7500           12.9
C253     Upper    249      8200            --
C257     Upper    253      7400           12.8
C283     Lower    278      9300            --
C290     Lower    286      9000            8.5

Sample    FeO     C[O.sub.2org]
         (wt.%)      (wt.%)

C201      0.80        4.12
C203      0.94        0.90
C205      0.93        2.20
C253      0.98        0.80
C257      0.94        0.71
C283      0.74        0.90
C290      0.66        1.82

Table 7.- Physical parameters to calculate the humidity for the
Oligocene-Miocene Clay assuming that samples are completely

Table 7.- Parametros fisicos para calcular la humedad de las
muestras de la arcilla del Oligoceno-Mioceno suponiendo que estan
completamente saturadas.

Sample   Depth (m)   w (%)   [[rho].sub.d]    [S.sub.r]

C201        197      20.4         1.69          0.88
C203        199      11.8         1.81          0.62
C205        202      19.4         1.70          0.85
C253        249      14.9         1.93          0.95
C257        253      18.3         1.78          0.91
C283        278      12.6         1.98          0.88
C290        286      13.3         1.97          0.91

Sample    n     [w.sub.Sr]=1   [W.sub.Sr=1]/W

C201     0.39      23.08            1.13
C203     0.35      19.17            1.62
C205     0.39      22.71            1.17
C253     0.30      15.70            1.05
C257     0.36      20.06            1.10
C283     0.29      14.39            1.14
C290     0.29      14.67            1.10

Table 8.- Modelling output with the reconstructed pore water
chemistry (mol/L), including pH, pe and />C[O.sub.2] data, for
pore waters from Oligocene-Miocene Clay.

Tabla 8.- Resultados de la composicion quimica (mol/L) del agua
intersticial para la arcilla del Oligoceno-Mioceno, reconstruida
mediante modelizacion. Se incluyen datos de pH, pe y />C[O.sub.2]

Sample    pH     pe            Cl                   S

201      7.64   -3.34   2.4 x [10.sup.-2]   6.7 x [10.sup.-2]
203      7.52   -3.22   1.8 x [10.sup.-2]   6.2 x [10.sup.-2]
205      7.54   -3.16   2.3 x [10.sup.-2]   7.0 x [10.sup.-2]
253      7.60   -4.02   3.0 x [10.sup.-2]   8.2 x [10.sup.-2]
257      7.12   -3.55   2.3 x [10.sup.-2]   6.9 x [10.sup.-2]
283      6.76   -3.04   5.5 x [10.sup.-2]   8.3 x [10.sup.-2]
290      7.44   -3.92   4.2 x [10.sup.-2]   8.8 x [10.sup.-2]

Sample           C                  Ca                  Mg

201      1.3 x [10.sup.-3]   1.1 x [10.sup.-2]   8.3 x [10.sup.-3]
203      1.7 x [10.sup.-3]   1.1 x [10.sup.-2]   8.4 x [10.sup.-3]
205      1.8 x [10.sup.-3]   1.1 x [10.sup.-2]   8.2 x [10.sup.-3]
253      1.7 x [10.sup.-3]   1.0 x [10.sup.-2]   7.8 x [10.sup.-3]
257      4.6 x [10.sup.-3]   1.1 x [10.sup.-2]   8.5 x [10.sup.-3]
283      8.0 x [10.sup.-3]   1.8 x [10.sup.-2]   1.4 x [10.sup.-2]
290      1.5 x [10.sup.-3]   1.7 x [10.sup.-2]   1.3 x [10.sup.-2]

Sample          Na                   K                  Fe

201      1.2 x [10.sup.-1]   1.5 x [10.sup.-3]   1.5 x [10.sup.-6]
203      1.1 x [10.sup.-1]           -           9.9 x [10.sup.-7]
205      1.3 x [10.sup.-1]   8.2 x [10.sup.-4]   1.1 x [10.sup.-5]
253      1.5 x [10.sup.-1]   8.3 x [10.sup.-4]   1.5 x [10.sup.-7]
257      1.5 x [10.sup.-1]   4.6 x [10.sup.-4]   3.0 x [10.sup.-6]
283      1.7 x [10.sup.-1]   8.1 x [10.sup.-4]   4.2 x [10.sup.-6]
290      1.6 x [10.sup.-1]   7.9 x [10.sup.-4]   1.5 x [10.sup.-6]

Sample          Si           C[O.sub.2](g)]

201      2.7 x [10.sup.-4]       -2.87
203      2.7 x [10.sup.-4]       -2.66
205      2.7 x [10.sup.-4]       -2.65
253      2.7 x [10.sup.-4]       -2.72
257      2.7 x [10.sup.-4]       -1.85
283      2.6 x [10.sup.-4]       -1.31
290      2.6 x [10.sup.-4]       -2.64
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Author:Turrero, M.J.; Fernandez, A.M.; Pena, J.; Sanchez, M.D.; Wersin, P.; Bossart, P.; Sanchez, M.; Melon
Publication:Journal of Iberian Geology
Date:Jan 1, 2006
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