Tracer and reactive transport modelling of the interaction between high-pH fluid and fractured rock: Field and laboratory experiments

Journal of Geochemical Exploration 90 (2006) 95 – 113 www.elsevier.com/locate/jgeoexp

Tracer and reactive transport modelling of the interaction between high-pH fluid and fractured rock: Field and laboratory experiments Wilfried Pfingsten a,*, Benoıˆt Paris b, Josep M. Soler c, Urs K. Ma¨der d a

Paul Scherrer Institut, Villigen PSI, Switzerland b ITASCA Consultants, Ecully, France c CSIC, Barcelona, Spain d University of Bern, Bern, Switzerland

Received 1 December 2003; accepted 12 September 2005 Available online 6 January 2006

Abstract In order to understand the interaction of a hyperalkaline solution with a fractured shear zone in granite and its influence on the migration of radionuclides, laboratory and underground field experiments at the Grimsel Test Site (Switzerland) were analysed by means of numerical modelling. Supporting data came from hydrogeological testing, structural and mineralogic characterisation of boreholes and cores and from dye tracer experiments in different dipole geometries within the shear zone. One-dimensional flow and reactive transport models have been applied to reproduce the breakthrough curves measured in a small-scale laboratory experiment (forced infiltration into a drill core containing a fracture with fault gouge). Major ion concentrations and water flow through the system could be fitted using a kinetic approach for mineral dissolution and precipitation. To reproduce the measured reduction of the flow rate over time, a small value for the effective mineral surface area had to be chosen together with an empirical relationship between the hydraulic conductivity of the fracture and the amount of precipitated calcium silicate hydrate. Tracer dipole experiments have been interpreted using different concepts to reconcile transport processes and radionuclide migration within a shear zone at the Grimsel Test Site altered by high-pH fluid. Parameter fits were possible for a multiple fracturematrix approach, as well as for a two-dimensional heterogeneous medium approach. A discrimination between approaches was not possible, although the extended dipole flow field geometry, the quite dissimilar breakthrough curves measured for experiments with different dipole geometries and the measured lateral spreading of the high-pH plume favoured the heterogeneous porous medium approach. Using a heterogeneous porosity distribution, together with an empirical Kozeny–Carman equation that relates porosity and hydraulic conductivity, a heterogeneous flow field could be calculated for the shear zone. This flow field was used to predict the interaction of the hyperalkaline solution with the shear zone. Calculations were also performed to predict the spreading of Cs, Co and Eu radionuclide tracers within the shear zone altered by high-pH interaction. It has been shown that the calculated Cs, Co and Eu breakthroughs and their concentration distributions depend on the assumptions on sorption behaviour. In addition, the observed decrease in hydraulic conductivity of the system, which is observed both in the field and in small-scale core infiltration experiments, and the related changes in the flow field, which are linked to mineral alteration, will strongly influence the migration of radionuclides. D 2005 Elsevier B.V. All rights reserved. Keywords: Hyperalkaline plume; Transport modelling; Column experiment; Dipole experiment; Reactive transport; Breakthrough curves; Radionuclides; Grimsel; Crystalline rock

* Corresponding author. Tel.: +41 56 310 2418; fax: +41 56 310 2821. E-mail address: [email protected] (W. Pfingsten). 0375-6742/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2005.09.009

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1. Introduction Leachates from degrading cement or concrete are hyperalkaline (pH N 13 for ordinary Portland cement) and chemically reactive with the engineered barrier system (EBS) and any host rock of a deep repository for radioactive waste. The extent of the expected chemical modification is directly relevant for the safety of a repository as it may influence the transport and retardation properties of radionuclides in the near-field and far-field. This situation is of particular concern in fractured rock systems where transport is not diffusion-limited outside the EBS and, hence, the potential formation of preferential fluid pathways, or a decrease in retention capacity for radionuclides (e.g., sealing of rock matrix porosity) could be a limiting factor for the performance. However, mineral reactions and porosity evolution induced by a hyperalkaline plume can also have a beneficial effect on the safety of a repository if secondary mineral precipitation will predominantly occur within the preferential pathways, clog them and isolate herewith the whole repository from its host rock environment. It is for this reason that efforts have been made to understand the chemical alteration processes and predict the effects of such processes over relevant time frames of up to a million years. One such effort is represented by the HPF Project (High-pH Plume in Fractured Rock), an international cooperation funded by ANDRA (F), JNC (J), NAGRA (CH), SKB (S) and the DoE (US), including both field activities at the Grimsel Test Site (Switzerland, http://www.gts.com) and laboratory experiments, paired with numerical modelling interpretations and predictions. Although many laboratory experiments and modelling interpretations have been performed on high-pH interaction with host rock material, the results show that an up-scaling from laboratory scale to field scale is difficult. The same holds true for the up-scaling from simplified model assumptions to complex near-field heterogeneity present in a fractured host rock. The experimental results and the modelling predictions range from increased transport to completely vanishing transport of a high-pH front and related radionuclide migration depending on hydraulic conditions, flow path geometry (open fracture with adjacent matrix or heterogeneous porous medium), and groundwater and mineralogical composition (Berner, 1992; Haworth and Noy, 1993; Neall, 1994, 1996a,b; Steefel and Lichtner, 1994; Lichtner and Eikenberg, 1995; Grindrod and Takase, 1996; Bateman et al., 1998, 1999; Berner, 1998; Lichtner et al., 1998; Pfingsten, 2001; Soler, 2003).

The combination of laboratory and field experiments with the same host rock material allows the identification of relevant transport processes on the small laboratory scale and its transfer to the field scale by field experiments and their adequate modelling at both scales. A direct up-scaling is not straightforward because the experimental conditions in the laboratory and in the field are different. Laboratory experiments have been performed with fractured samples (drill cores) forcing a one-dimensional flow through the sample, whereas field experiments are at least two-dimensional within a shear zone with a potentially unlimited lateral extension of the flow field, which is more realistic compared to a repository situation. In the laboratory, the hydraulic conditions are much easier to control than at the field scale. Therefore, intense hydrogeological testing has been performed in the field prior to the reactive transport experiment. This included several independent tracer dipole experiments between different borehole configurations (differing dipole geometries, pumping rates and orientation with respect to the background water flow) in addition to single-hole injection and recovery tests. Different model approaches have been applied to fit measured breakthrough curves of major components, dyes and radionuclide tracers for the modelling of laboratory and field experiments. The procedure was to take into account different model geometries, transport and geochemical processes in order to discriminate between the dominant processes at the laboratory and field scale, and to compare the different modelling results. Thus, single, dual and heterogeneous porous medium approaches in one and two spatial dimensions (Fig. 1), as well as kinetic and chemical equilibrium reaction approaches partly coupled to hydraulics have been applied to describe the high-pH interaction with granitic fault rocks at both laboratory and field scales. Finally, a prediction for the extension of mineral alteration zones and the spreading of a radiotracer cocktail has been calculated in order to estimate an optimum volume of the shear zone to be excavated for analysis and radioprotection measures after a planned long-term reactive transport experiment in the field using variably sorbing and repository-relevant radionuclides. A treatment of the high-pH plume in a clay host rock environment (Opalinus Clay) is provided by NAGRA (2002), and relevant laboratory experiments and modelling interpretations are summarised by Adler et al. (2001) and Ma¨der and Traber (in press). It could be demonstrated that the effects of a plume were negligible for the safety of a repository in the diffusion-dominant

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Fig. 1. Illustration of the model for a real shear zone and its transformation into different transport models.

claystone environment. This is in contrast to a fractured host rock environment portrayed in this contribution. 2. Methods The interaction of a high pH-solution with the fractured Grimsel granodiorite has been investigated in laboratory (column) and field experiments (Ma¨der et al., 2006—this issue). Breakthrough curves of the highpH solution, dye- and radiotracers measured at the injection and extraction side of the column and at the injection and extraction boreholes through a fractured shear zone in the field were used to fit different reactive transport models and/or identify the dominant geochemical and transport processes. 2.1. Description of the laboratory experiment In the small-scale laboratory experiment, a high-pH (K–Na–Ca–OH) solution (ionic strength: 6 0.2 mol/kg, pH = 13.36, T = 15 8C) was continuously injected, under a constant pressure gradient, into a cylindrical core of granodiorite containing a fault zone. The injected hyperalkaline solution was strongly undersaturated with respect to atmospheric CO2. Any leakage of at-

mospheric CO2 into the reservoir solution would have resulted in rapid precipitation of calcite. Therefore, pCO2 had been assumed to be at the equilibrium with calcite defined by the Ca concentration of the solution. The solution was collected at the opposite end of the core (outlet), after having circulated through the core. The length of the core was 7.8 cm and its axis was oriented parallel to the fracture. The fault zone was approximately planar, 5–10 mm wide and composed of fault gouge bound by ultramylonite containing also some brittle fractures. The injection lasted 9 months. Before injecting high-pH solution, a tracer (NaCl) test had been performed with the core in order to estimate the porosity, dispersivity and hydraulic conductivity of the fault zone. The experimental results included the pH and concentrations of Al, Si and Ca at the outlet of the granodiorite core. The pH did not seem to undergo a significant retardation or buffering with respect to the injected hydroxyl concentration. A decrease in fluid flow with time has been observed, which indicates a decrease in hydraulic conductivity and implies an increasing water residence time within the sample. After the experiment, the observed amount of mineral alteration in the core was only minor and difficult to detect.

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The amounts were so small, that it was not possible to fully characterise the secondary phases that precipitated except for a qualitative identification of a Ca–Si– hydrate phase. This column experiment is described by one-dimensional reactive transport codes taking into account constant hydraulic boundary conditions (hydraulic head), advective–dispersive transport in a porous medium (fault gouge), chemical equilibrium reactions in solution and kinetic mineral reactions. Two different models have been applied for sorption and/or secondary mineral precipitation and their coupling to hydraulic conductivity. 2.2. Description of the field experiment At the field site in the Grimsel Underground Laboratory, hydraulic testing using 10 boreholes yielded transmissivities from 1  10 9 m2/s to 5  10 8 m2/s, as well as information on the background flow field within the shear zone (Fisch, 2000). Borehole core samples and borehole image analysis yielded a shear zone varying in thickness from 0.33 m to 0.65 m with up to 10 fractures with open apertures up to 0.01 m, but with an unknown amount of fracture infill due to preferential loss of fault gouge material during drilling (Bossart and Mazurek, 1991; Moeri and Inderbitzen, 2002). Injection and extraction flow rates that define the shape of the applied dipole flow field, have been monitored in 10 boreholes (about 7 m to 17 m long, with distances from 0.77 m to 16.26 m between each other) through the shear zone. Six boreholes intersect the shear zone within a 2 m by 2 m area in which the dipole experiments were performed. Their distance from each other is from 0.77 m to 1.8 m. Results from dipole experiments with dye tracers in different dipole geometries indicated a rather heterogeneous shear zone, because measured breakthrough varied a lot in arrival time and peak height for comparable dipoles (Pfingsten, 2002). A model fit for such a dyetracer dipole experiment served as binitial conditionQ for the long-term high-pH alteration experiment with the same dipole geometry. A measured dye-tracer breakthrough curve has been fitted by adjusting by trail-anderror a heterogeneous two-dimensional porosity and hydraulic conductivity distribution within the shear zone, whereby hydraulic conductivity and porosity are related by a non-linear Kozeny–Carman equation. The injected hyperalkaline solution was the same as used for the core infiltration experiment, i.e., strongly undersaturated with respect to atmospheric CO2, and

the pCO2 had been assumed to be at the equilibrium with calcite defined by the Ca concentration of the solution. For the Grimsel groundwater, it was assumed to be at the equilibrium with calcite defined by its Ca concentration. During the injection of the hyperalkaline solution with forced constant injection rate but increasing observed injection pressure, additional geochemical observations from neighbouring boreholes made some revised fitting of the binitial fitQ necessary. This revised fit has been used to predict the migration of the high-pH solution in the shear zone, the migration of a dye-tracer and a tracer cocktail with variably sorbing radionuclides in the high-pH-altered shear zone. The field experiments have been modelled by a dual porous and a heterogeneous porous medium approach in one and two spatial dimensions, respectively, in order to identify the relevant geochemical and transport processes within a fractured shear zone and a hydraulic dipole geometry. 3. Results and analysis 3.1. Modelling of laboratory experiments 3.1.1. Modelling with a finite difference reactive transport code (GIMRT) Modelling was performed with a modified version of the finite difference GIMRT program (Steefel and Yabusaki, 1996), which is part of a numerical software package for simulating multicomponent reactive transport in porous media. GIMRT (global implicit multicomponent reactive transport) solves numerically the reaction-transport differential equations in either one or two dimensions, making use of a one-step or global implicit approach, i.e., solving the transport and reaction terms simultaneously. Mineral reactions are described in GIMRT by kinetic rate laws. The rate laws implemented in the calculations included the effects of mineral surface area and solution saturation state. The rates for primary minerals were based on experimentally determined values published in the literature (Table 1). Extrapolation of the experimental rates to the high pH conditions, making use of the observed rate dependencies with respect to pH, was usually necessary. Fast rates were used for the secondary minerals such that the results resembled local equilibrium for these phases and that precipitation was not rate-limiting. Mineral reactions cause changes in porosity and hydraulic conductivity that may affect significantly the transport properties of rocks. A flow module that

W. Pfingsten et al. / Journal of Geochemical Exploration 90 (2006) 95–113 Table 1 Reactions rates for primary minerals used for the modelling Primary mineral ma

k m (mol/m2/s)

Quartz Albite Microcline Phogopite Muscovite

3.5 d 10 12 10 11 3.5 d 10 12 10 11 4.5 d 10 12

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Table 2 Secondary minerals used for the modellinga

a Precipitation of albite and microcline was assumed not to be allowed.

allowed the update of the fluid flow field was added to GIMRT. This flow module solves the equation of conservation of fluid mass using the updated porosities and hydraulic conductivities of the medium. Hydraulic conductivities were calculated by means of Kozeny’s equation, which takes into account both porosity and mineral surface area. The results of the NaCl tracer test were analyzed by fitting the analytical solution of the one-dimensional dispersion–advection equation to the observed breakthrough curve (Fig. 2). The results of the tracer test yielded: porosity: / = 0.21, dispersivity: a = 0.014 m and hydraulic conductivity: K = 3.3  10 9 m/s. An attempt was made to reproduce the three types of available experimental observations (composition of the solution at the outlet, change in flow rate and small amount of secondary phases) through the modelling exercise. It has to be noted that the nature of the secondary phases that actually formed in the experiment is not known. The possible secondary phases that were taken into account included phases usually associated with the alteration caused by hyperalkaline solutions. The full list of potential secondary phases

Fig. 2. Comparison of the observed and calculated breakthrough from the tracer test (core infiltration experiment, relative concentration vs. time).

a For the modelling, all rate constants for the secondary minerals are assumed to be equal to 10 9mol/m2/s, except for calcite, for which it is assumed to be 10 8 mol/m2/s. The shaded secondary minerals have been calculated to precipitate.

contained: brucite, ettringite, portlandite, calcite, gibbsite, kaolinite and several zeolites: analcime, laumontite, mesolite, natrolite, scolecite, stilbite; calcium silicate hydrate (CSH): foshagite, gyrolite, hillebrandite, okenite and tobermorite; and calcium aluminum silicate hydrate (CASH): prehnite (Table 2). Figs. 3 and 4 show the evolution of the composition of the outlet solution and the change in fluid

Fig. 3. Evolution of the observed and calculated composition of the solution at the outlet of the core infiltration experiment for two different cases: (a) initial total surface area for primary minerals A p 6 5  104 m2/m3 rock and initial surface area for secondary minerals A s 6 105 m2/m3 rock (top). (b) A p 6 2  105 m2/m3 rock and A s 6 6  104 m2/m3 rock (bottom). Symbols correspond to experimental data and lines to model results.

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Fig. 4. Observed and calculated changes in the Darcy flux during the core infiltration experiment. The experimental results are plotted as circles. The two lines correspond to the two calculations shown in Fig. 3: (a) initial total surface area for primary minerals A p 6 5  104 m2/m3 rock and initial surface area for secondary minerals A s 6 105 m2/m3 rock. (b) A p 6 2  105 m2/m3 rock and A s 6 6  104 m2/m3 rock.

flow calculated by the model, respectively. The agreement between model calculations and experimental results for the concentrations at the outlet of the experiment is not very good, although the main trends were reproduced. No attempt was made to include sorption in the model (no experimental data available). The model results were very sensitive to the values of the mineral surface areas. Small changes in the surface areas caused large changes in both the concentrations and the evolution of fluid flow with time. Only with a narrow range of values of the surface areas was it possible to reproduce approximately the

evolution of both the composition of the solution and the change of fluid flow with time. It is remarkable, though, that, without knowing the exact nature of the secondary phases in the experiment, it was still possible to reproduce the main trends of the evolution of the solution composition and the changes in hydraulic conductivity (change in flow velocity). The secondary phases that precipitated in the simulations were tobermorite (a CSH phase) and prehnite (a CASH phase). The calculated amounts of secondary minerals were only minor, which is consistent with the experimental observations. Figs. 5 and 6 show mineral reaction rates after (a) t = 0.25 year and (b) t = 0.745 year (end of the experiment). The dissolution rates of the primary minerals albite, quartz and muscovite remain constant in space and time (the solution remains always strongly undersaturated with respect to these phases). Phlogopite, used as a surrogate for biotite, is at equilibrium. Microcline dissolves initially under far-from-equilibrium conditions (constant rate), but during later stages shows a decrease in its rate along the fracture, reflecting an approach to equilibrium. Concerning the secondary minerals, at early stages (Fig. 5), there is a zone near the inlet where tobermorite precipitates and a zone further along the fracture where precipitation is dominated by prehnite, although there is also minor tobermorite precipitation. During later stages, tobermorite starts dissolving from the fracture outlet and is replaced by prehnite. Fig. 6 shows the calculated situation at the end of the experiment; the interface between the tobermorite and pre-

Fig. 5. Mineral reaction rates along the column after 0.25 year. Negative rates indicate dissolution; positive rates indicate precipitation (core infiltration experiment).

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Fig. 6. Mineral reaction rates along the column after 0.75 year. Negative rates indicate dissolution; positive rates indicate precipitation (core infiltration experiment).

hnite zones is located approximately in the middle of the domain and prehnite is replacing tobermorite at the contact between the two mineral zones. In these later stages, the silica supplied by the dissolution of primary minerals near the outlet is not balanced by the precipitation of secondary minerals, causing a marked increase in the concentration of silica in solution. The reduction in flow rate with time has caused the overall displacement towards the inlet of the zone of secondary mineral precipitation (high Si and Al concentrations are reached closer to the inlet under conditions of slower flow rate). Calcium concentration cannot initially be efficiently consumed by the precipitation of secondary phases (small rates); the initial increase in concentration at the outlet (breakthrough) reflects the arrival of the high-Ca front linked to the injected solution. The experimental data show a retardation of the arrival of the Ca front that is not well reproduced by the model (the magnitudes of the reaction rates are not well reproduced). During later stages, the precipitation of CSH and CASH causes the observed decrease in Ca concentration at the outlet. The experimental results indicated a net removal of 70 mg of Ca from the injected solution over the course of the experiment. The results of the simulations reflect a removal of Ca ranging from 14 to 34 mg, caused by the precipitation of prehnite and tobermorite. In order to obtain a reasonable agreement between the model and experimental results, surface areas of the order of 105 m2/m3 rock had to be used. These values are much smaller than the value measured in the fault

gouge filling the fracture, which is in the order of 106– 107 m2/m3 rock. This difference suggests that not all the mineral surface area in the fault gouge is accessible to the solution and available for reaction. An important finding from this experiment is that the interaction between the hyperalkaline solution and the Grimsel granodiorite causes a significant reduction in hydraulic conductivity in the rock, even though the amount of mineral alteration is minor. 3.1.2. Modelling with a finite element reactive transport code (3FLO) Modelling was performed with the ITASCA code 3FLO (Billaux et al., 2003), designed for modelling flow and reactive transport in homogeneous and/or fractured media. 3FLO computes the flow field in steady or in transient state using the finite element method and simulates solute mass transport through the network elements using the Discrete Parcel Random Walk method (DPRW). This technique can be applied to both fractured media (Robinson, 1984; Cacas, 1989) and porous media (Seguin, 1992). Each particle represents a number of moles or a vector of numbers of moles, which allows transporting several chemical elements at the same time. The approach implemented in 3FLO in order to simulate reactive transport is described by Fabriol et al. (1993). First, an extended speciation module computes the solute concentrations of the system basis components from their total concentrations in all of the domain elements where at least one particle is present. Second, the code assigns a vector of mobile

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upstream and downstream boundaries. The initial porosity and hydraulic conductivity were calibrated on the early pH breakthrough curve and yielded: porosity: / = 0.28; dispersivity: a = 3.0  10 3 m and hydraulic conductivity: K = 2.8  10 9 m/s. These values are not identical but remain relatively closed to those used in the GIMRT modelling, which was calibrated on the NaCl tracer test performed before the injection of the hyperalkaline solution. Porosity is adjusted during the simulation according to the evolution of mineral volume fractions due to precipitation and dissolution. The impact of such phenomena is accounted for by linking the hydraulic conductivity of each grid block to its volume fraction of CSH using an empirical relationship deduced from the initially measured hydraulic conductivity of the sample and the volume fraction of CSH calculated for individual grid blocks: Fig. 7. Comparison of the calculated and observed breakthrough of the pH for the core infiltration experiment.

moles to each particle depending on the solute concentrations. The coupling between geochemistry and transport is performed without iterations. Based on both the shape of the tracer breakthrough curves (Figs. 7 and 8) and on the description of the core, it was assumed that flow and reactive transport essentially take place within the fault gouge, represented by a 1D porous medium, initially homogeneous. The domain was discretised in 50 identical pipes (two-node elements) with a cross sectional area of 470 mm2 corresponding to the observed fault gouge area. The measured head values were assigned at the

K ¼ 2:5  1011 þ 2:8  109  expð  volume of CSHT110Þ The flow field is updated for each time step assuming a storage coefficient S = 1.0  10 7. A kinetic approach was used to describe most of the mineral dissolution and precipitation reactions. The rate laws implemented are identical to those used in the GIMRT simulations. The calibrated initial surface area of primary minerals is 7.5  104 m2/m3. This value, used in the computations of kinetic rates, is updated according to the mineral evolution throughout the simulation. The list of secondary minerals was restricted to those commonly associated with the alteration caused by

Fig. 8. Evolution of the observed and calculated composition of the solution at the outlet of the core infiltration experiment (initial total surface area for primary minerals A p 6 7.5  104 m2/m3 rock).

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hyperalkaline solutions: brucite, ettringite, portlandite, calcite, and several CSH and CASH phases, but not zeolites. C1.1SH (Ho¨glund, 2001) was also included in the list. Secondary minerals such as hydroxides or carbonates were assumed to be at thermodynamic equilibrium. Thermodynamic constants were taken from the EQ3/6 database (Wolery, 1992). An option to allow for surface complexation of Ca was also included and is discussed below. In the analysis of the experimental and modelled data, the breakthrough curves can be divided into three parts: (1) During about 10 days, Ca remains more or less constant, while Si and Al increase sharply. The injected high-pH solution enhances primary mineral dissolution and the release of Si and Al, which can potentially form secondary minerals together with Ca (CSH/CASH). Since Ca is involved in the precipitation of secondary phases and also sorption, the arrival of the high-Ca front is retarded (compared to pH, for example). These reactions also cause a decrease in the amount of Ca in solution. Then (2), for about 40 days, the increase of calcium concentration together with the decrease of both Si and Al reflect the arrival of the high-Ca front. Low Si and Al are caused by the precipitation of secondary phases reaching the outlet. Finally (3), an inversion of the trend is observed, i.e., diminishing of calcium and raise of Si and Al concentrations at the outlet, which is explained by the increase of the solution residence time due to the reduced flow rates. The rising excess of released Si and Al results in a precipitation of CSH and CASH closer to the fracture inlet (further upstream from the outlet) and causes the

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decrease in the concentration of Ca in the effluent. These results are a remarkable illustration of the coupled effects between flow and kinetically controlled chemical reactions. In order to reproduce the first stage, adsorption of Ca through a surface complexation model was introduced. This surface complexation model would cause a specific Ca adsorption at high pH. The nature of this potential adsorbent is not known, although biotite or the recently identified chlorite could be good candidates. We included in the database the following species (= Site-OH is the non-identified surface site): = Site-O, = Site-OH2+, = Site-OCa+ and = Site-OCaOH. The equilibrium constants and the site densities (6 0.05 mol/kg of solid) are calibrated parameters. The simulation of the pH breakthrough curve is displayed in Fig. 7. The calculated values are close to the experimental ones, even though we did not account for the observed increase of flow-rates at the early times of the experiment. Fig. 8 compares the modelling results with the observed data for the main parameters (Ca, Al, Si). Trends and orders of magnitude are well reproduced, especially for Ca and Si. Fig. 9 shows a very good agreement between the measured and the simulated flow rates, which is consistent with the assumption that flow might be directly controlled by the volume of CSH precipitating and not by the total porosity of the medium, which remains almost constant throughout all the domain due to the absence of significant dissolution of primary minerals and precipitation of CSH-like phases (Fig. 10). Looking closely at the calculations, it appears that albite and

Fig. 9. Comparison of the calculated and observed change in the Darcy velocity during the core infiltration experiment.

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The double-porosity codes RANCHMD (Jakob, 1990) and PICNIC (Barten and Robinson, 2001) were used to fit measured breakthrough curves for the dye-tracer dipole experiments. Both codes were successfully applied to tracer and radionuclide transport experiments performed in the MI shear zone (Frick et al., 1992; Heer and Hadermann, 1994; Hadermann and Heer, 1996; Heer and Smith, 1998), but for differing geometries of the flow field. In addition, the two-dimensional code MCOTAC (Pfingsten, 1996; Pfingsten, 2001), which describes reactive transport in a heterogeneous porous medium, was used.

Fig. 10. Calculated porosity and volume fraction of primary minerals in the core.

muscovite provide the most significant contribution of silica and alumina. The predicted secondary minerals are displayed in Fig. 11. The code predicts a succession of minerals with a decreasing Ca/Si ratio from the inlet to the outlet.

3.2.1. Double-porosity approach (RANCHMD, PICNIC) RANCHMD assumes that tracer transport in the shear zone (advection and dispersion) takes place in one-dimensional structures (open channels in the fault gauge material or veins) within planar open fractures (see Fig. 1). The water flow velocity in the fracture is a mean value for a stream tube within the dipole flow field. Longitudinal mechanical dispersion reflects the variability of the fracture aperture and fracture connectivity along a stream tube. In addition, diffusion into the accessible rock matrix is considered perpendicular to the fracture. Reversible sorption on the surfaces of the fracture wall and on surfaces within the matrix can be taken into account. In addition, the PICNIC code allows for calculations of transport in a fracture network, including multiple flow paths. None of the codes in-

3.2. Flow and transport modelling of field experiments It is not obvious a priori which approach is appropriate to model the dipole experiments performed in the shear zone, although the double porous medium approach was successful in fitting and predicting the transport and retardation experiments performed in the neighbouring Migration (MI) shear zone (Hadermann and Heer, 1996). The application of both model concepts–double porous or heterogeneous porous–allows for a complementary description of such experiments and an estimate of their applicability to different dipole flow fields. A narrow flow field formed in the experiments carried out in the MI shear zone due to a high extraction rate (ratio extraction to injection rate N 10), whereas a wide flow field from injection to extraction is more likely for the HPF shear zone because of the applied pumping rates (extraction to injection ratio is about 1). For the two different modelling approaches, three different codes have been used to model the field experiments. Their concepts are illustrated in Fig. 1.

Fig. 11. Calculated volume fraction of secondary minerals in the core.

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clude a flow model. The pore velocities in the individual fractures have to be given in advance of the transport calculation based on information regarding pumping rates and the background flow field. Only a few independent parameters can be modified, however, for each single flow path (L = the fracture length, d = the inverse of the fracture aperture b, d = the thickness of the matrix accessible by diffusion, e f = the matrix porosity and D p = the pore diffusion constant). The double porosity approach is characterised by three characteristic times visible in the breakthrough curve (Jakob, 1997). The advection time characterises the water flow velocity, v f, and dominates the early breakthrough. The diffusion time indicates the influence of the diffusion processes from the fracture into the matrix and vice versa, resulting in a retardation of the peak concentration of the breakthrough curve. The tail end perturbation is an indication for limited matrix diffusion and may be seen only in the tailing of the breakthrough. In case of several independent flow paths with individual transport parameters, a superposition of the individual paths will contribute to the calculated breakthrough. Additional parameters can be used to weigh the individual flow paths. It should be noted that a single good fit is not unique because several combinations of parameter sets for L / v f, D p, d, e f and d may lead to equivalent fits. Although knowing that these modelling approaches are a simplification with respect to the expected real flow field of the dipoles, modelling in a manner analogue to the MI experiments allows for parameter comparison with those of the migration experiments in a neighbouring shear zone. The fitting procedure started by considering model parameters from the modelling of the MI experiments. The lower transmissivity of the HPF shear zone, the measured tracer recovery for the fixed pumping rates and information from analysis of borehole samples have also been taken into account. Fracture apertures and matrix porosity have been varied to fit the measured dye-tracer breakthrough curves. It has been shown that the smaller pumping rates for the HPF tracer experiments generally produced slower pore water velocities in the fractures and that a single fracture approach was only able to fit short-term tracer experiments. Only the two-flow-path fracture-matrix approach has allowed the fitting of a long-term tracer experiment with its complex tracer breakthrough (Fig. 12; Pfingsten, 2002; Pfingsten and Soler, 2003). Quite different flow velocities (pore velocities) have to be assumed to fit the early and late breakthrough for the two different flow paths—500 m/year

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Fig. 12. Field experiment: measured and calculated uranine breakthrough of the long-term dye-tracer dipole experiment. The bearly 2D fitQ (MCOTAC) and the two-flow paths fitQ (PICNIC) take only into account this dye-tracer dipole experiment. The bnew 2D fitQ (MCOTAC) takes into account additional information from the high-pH breakthrough in neighbouring boreholes.

and 53 m/year, respectively. The volumetric flow fractions were assumed to be 5% and 95%. In addition to the fractionation of the tracer between different flow paths, a further fit parameter took into account that not all injected tracer would reach the extraction borehole for such a dipole set-up. From the measured recovery up to the end of the experiment, a value of 60% was extrapolated for the total potential recovery. The two-flow-path model had to be discarded when additional information from the spreading of the hyperalkaline plume in the shear zone became available,. The occurrence of the high-pH fluid at three further sampling locations in the shear zone located in different directions from the injection borehole could not be explained. Additional flow paths had to be introduced, yielding a more complex fracture network. 3.2.2. Two-dimensional heterogeneous porous medium approach (MCOTAC) The two-dimensional flow and reactive transport code MCOTAC (Pfingsten, 1996, 2001) describes groundwater flow in a two-dimensional heterogeneous shear zone by assuming Darcy flow for stepwise steady state hydraulic dipole conditions. Transmissivity, thickness of the shear zone and recharge rates are considered by assuming distributions of parameter values in space, representing the spatially varying transmissivity, thickness and flow rates. The pore velocity used for solute transport modelling is calculated from the Darcy flux

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and the flow porosity, which is also non-uniform within the shear zone. Transport is calculated by a multi-species random walk method taking into account advection, dispersion, diffusion, decay, sorption and further chemical reactions, and external sources and sinks for species in solution and in the sorbed phases. Matrix diffusion is not explicitly taken into account. Diffusion is incorporated in the hydrodynamic dispersion coefficient, which is a tensor for two-dimensional systems. This yields grid-specific dispersion coefficients. Longitudinal dispersion length, a L, transversal dispersion length, a T = a L / 10, and the diffusion coefficient, D p, were arbitrarily kept constant in the whole model domain: a L = Dx / 10 (where Dx is the grid spacing) and D p = 2.5 d 10 11 m2/s. The code takes into account chemical reactions and sorption at equilibrium. The latter can be described by a retardation factor R, which is a function of space, as are porosity and mineral abundance, or by more complex sorption mechanisms that have to be formulated by a chemical equilibrium equation. To describe a diffuse dipole flow field, which was expected from a pumping ratio of 1, a two-dimensional representation of the shear zone was used. Fig. 13 shows the two-dimensional model domain in the plane of the shear zone together with the locations of the boreholes drilled perpendicularly through the shear zone. A threedimensional representation of the access tunnel, shear zone and boreholes is included in the paper of Ma¨der et

al. (2006—this issue). The model boundary, a domain of 4 m by 4 m, has been chosen to include all performed dipole experiments and not to be influenced by the applied dipole. The hydraulic head boundary conditions could be interpolated from the measured hydraulic background field. They correspond to a hydraulic gradient of about 0.1 m/m in the direction of the applied dipole (from borehole BOHP98.003 to BOHP98.001; see Fig. 13). This dipole configuration was used for several dyetracer experiments and the injection of the high-pH solution. The thickness of the shear zone has been interpolated from 10 measurements in the boreholes contained in the model grid, representing the thickness of the two-dimensional heterogeneous water conducting shear zone. Pumping rates were given by the experimental conditions. The array of local porosities (initial hydraulic conductivities) was used as a parameter array to fit the measured breakthrough curves (Figs. 12 and 14). Modelling was started with a constant porosity of 0.5% in the whole model domain, except at the locations of the boreholes, where 10% porosity was assumed corresponding to the dead volume of the packer system within the shear zone. The value for the hydraulic conductivity was justified by the transmissivity measurements in the six boreholes within the model domain. The estimation of porosity was much more problematic because available borehole images indicated several open fractures across the shear zone. However, the drilling

Fig. 13. Field experiment: calculated tracer concentration in the model domain for a long-term tracer (high-pH solution) injection with the same dipole geometry as for the long-term dye-tracer experiment after 42 days. Increased tracer concentration is calculated for all boreholes except BOHP00.008. The injection borehole is BOHP 98.003 and the extraction borehole BOHP98.001. Boreholes BOHP98.002, BOHP00.008, BOHP00.009 and BOHP00.010 have been used for taking samples during the injection of the hyperalkaline solution.

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Depending on the discretisation of the model domain and the distribution of local values, several porosity distributions could fit the measured breakthrough. These parameter values are not easy to deduce from single experiments, however. 3.3. Reactive transport calculations for high-pH alteration of the shear zone

Fig. 14. Field experiment: surface plot of the porosity distribution used to fit the long-term dye-tracer dipole experiment by MCOTAC calculation on an 80  80 grid (4 m by 4 m) including the information on the spreading of the high-pH plume within the shear zone.

procedure did likely cause erosion of fracture infill material so that the visible open-fracture space was only an upper limit for the open porosity. This interpretation is compatible with evidence from outcrops in the access gallery and a few boreholes drilled with liner techniques that preserved fault gouge material. Hydraulic modelling by a finite difference flow model produced a hydraulic head distribution and a related steady state dipole flow field within the background flow field. From this initial fit for the breakthrough at the extraction, local porosities were varied to improve the fit. Finally, these values for the local porosities ranged from less than 0.1% to more than 10% at the locations of the packer intervals in the boreholes. Local pore velocities varied over several orders of magnitude in the model domain, up to more than 100 m/year for the fit shown in Fig. 12. During later stages of the experiment, a revision of the porosity distribution and related hydraulic conductivity distribution has been necessary because the bearly 2D fitQ (Fig. 12) was not compatible with measured high-pH breakthrough in all boreholes around the applied dipole after about 35 days, except for BOHP00.008 where no pH increase was observed (Pfingsten, 2002). Since pH is not strongly buffered in the shear zone, pH breakthrough is comparable with a tracer breakthrough. As shown in Fig. 13, the calculated tracer distribution reached all neighbouring boreholes of the installed dipole after 42 days, except BOHP00.008, when using the revised fit for the porosity distribution. It is stressed that the fit parameter set is not unique because several porosity distributions may possibly yield a similar goodness of fit. The discretisation of the model domain and the introduction of local gridspecific fit parameters for the hydraulic and transport modelling allows for a huge number of variables.

Two-dimensional reactive transport simulations have been performed making use of the heterogeneous flow field previously calculated with MCOTAC. The domain of the simulations is a square region of the fracture plane including the injection and extraction boreholes (Fig. 15). The version of GIMRT described above has been used for the reactive transport calculations. The interaction between the injected hyperalkaline solution and the rock (fault gouge filling the fracture) caused a series of mineral reactions. The main processes were the dissolution of albite and precipitation of tobermorite (CSH phase), prehnite (CASH phase) and mesolite (Na–Ca zeolite) close to the injection borehole, which caused a decrease in porosity, and the precipitation of natrolite and analcime (Na zeolites) farther down gradient. Fig. 15 shows the pH distribution in the fracture after 1 year since the start of injection of the high-pH solution (the initially planned duration of the experiment) for the hydraulic dipole set-up calculated previously. The two plots correspond to two different values of the initial surface areas of the primary minerals. The flow was not updated during the calculations (Darcy velocities were assumed to be constant with time). Even with these simplifications, the results show that a large degree of spatial heterogeneity is expected, given the heterogeneous nature of the flow field. Also, the amount of reaction (reflected in the figure by changes in pH) is greatly dependent on the available reactive surface area. If the value of the reactive surface area was about 105 m2/m3rock, as suggested by modelling of the small-scale laboratory experiment, only a small amount of secondary mineral precipitation would be expected. However, its effect on hydraulic conductivity could be significant, as was shown in the laboratory experiment. 3.4. Predictions for radionuclide tracer migration in the altered shear zone Model predictions for Cs, Co and Eu have been performed for a pulse injection in order to estimate the migration of radionuclides within the shear zone altered by high-pH solutions in advance of a planned final

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Fig. 15. Field experiment: calculated pH distribution in the fracture after 1 year since the start of injection of the high-pH solution. The circles show the location of the injection (left) and extraction (right) boreholes. The arrows show the flow field in the fracture. The two figures correspond to initial surface areas for primary minerals of the order of 106 m2/m3 rock (left) and 107 m2/m3 rock (right).

experiment. Also, the planning of the foreseen excavation after the injection of sorbing tracers and issues of radioprotection can be supported by predictive modelling. The spreading and breakthrough of Cs, Co and Eu have been calculated for an injection pulse of 30 days using transport parameters from the fit described in Section 3.2.2. Estimations for the retardation of Cs, Co and Eu have been deduced from literature k d values for a high-pH altered marl (see Table 3); no other data for such a high pH range were available. These estimations are highly uncertain because the actual state of alteration in the shear zone, or the distribution of alteration, which will strongly influence the retardation of the nuclides, is not known. Alteration by the high-pH solution will not be homogeneous in the shear zone, because local flow velocities in the heterogeneous dipole flow field vary over a wide range and cause a comparable range of alteration (primary mineral dissolution and secondary mineral precipitation). Furthermore, the high-pH alteration of the shear zone caused changes in the dipole flow field, which could be seen in the changing tracer breakTable 3 Estimated sorption distribution coefficients R d (equal to model k d values for linear sorption) for Cs, Co and Eu, which are assumed similar to those for a high-pH-altered Marl (Tits et al., 2002) Nuclide

R d (m3 kg 1)

Cs Co Eu

4 d 10 4 2 d 10 2 2 d 10 1

through times for the different intermittent short-term tracer dipole experiments performed about every 3 months during the experiment (Ma¨der et al., 2006— this issue). Therefore, sorbing tracers will bseeQ a spatially altered shear zone with locally different sorption properties. A spatial distribution of sorption coefficients was used for the modelling to account for the heterogeneous alteration. Furthermore, it was estimated that 100% of the fault gauge material contributed to sorption sites for the radionuclides. For the calculated prediction, the pulse injection of the radiotracer cocktail had to be increased from 3 h (used in the dye-tracer dipole experiment) to 30 days, which is about the maximum allowed injection pulse for the foreseen radiotracer cocktail limited by the total dose. Fig. 16 shows the model fit to the previous tracer test (3 h injection) and the calculations for a dye- and radiotracer pulse injection of 30 days. Compared to the former dye-tracer breakthrough, the dye-tracer breakthrough is now calculated to reach much higher concentrations at the extraction borehole. However, it still does not reach the input concentration (only about 30%) due to mixing processes in the extraction borehole with water from elsewhere in the shear zone. The maximum concentration did not reach a steady state level after 30 days, which would be necessary to deduce the mixing ratio at the extraction borehole. The transient nature of the high-pH alteration could also influence this ratio. The breakthrough of Cs, Co and Eu and their distribution within the shear zone differ significantly (see Fig. 16). Cs breakthrough is calculated to be retarded by a factor of 4 compared to the non-sorbing dye-tracer,

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Fig. 16. Field experiment: calculated dye-tracer, Cs, Co and Eu breakthrough for a high-pH altered shear zone using the parameter set of the long-term dye-tracer dipole experiment (short injection pulse), K d values estimated from those for a high-pH-altered Marl and an injection time of 30 days.

Fig. 17. Field experiment: calculated dye-tracer, Cs, Co and Eu distribution within a high-pH altered shear zone after approximately 30 days of injection using the parameter set of the long-term dye-tracer dipole experiment (short injection pulse) and K d values estimated from those for a highpH-altered Marl.

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whereas Co breakthrough is retarded by one order of magnitude. The Eu breakthrough was not calculated to occur at the extraction borehole within the planned experimental time. The Eu plume has been calculated to be very narrow contained in between the injection and extraction boreholes (see Fig. 17). Most of the Eu is predicted to be excavated around the injection borehole during the excavation of the shear zone. However, Cs and possibly Co will be much more spread out across the shear zone, which will necessitate an excavation of a larger volume to reduce the amount of activity left behind in the shear zone. One should keep in mind the large uncertainty of these predictions, first with respect to the fit parameter set, which is not unique, and second with respect to the assumptions related to the sorption coefficients. Sorption may be reduced due to the fact that the high-pH alteration in the shear zone is still in a transient stage such that the accessible altered rock volume for the radiotracer might be much smaller. 3.5. Predictions for dye-tracer breakthrough in the altered shear zone Reactive transport calculations shown in Section 3.3 indicate a porosity decrease near the injection location of the high-pH solution. When trying to consider these calculated porosity changes for the prediction of the intermittent short-term dye-tracer experiments performed in parallel to the injection of the high-pH solution, the modelling did not succeed in reproducing the observed earlier pulse breakthrough accompanied by an increased peak concentration. The observed focusing effect (earlier and higher tracer breakthrough) was accompanied by an increased injection pressure as a result of maintaining a constant injection rate, while the overall transmissivity of the fault zone was decreasing. Independent from the results of the reactive transport calculation with respect to calculated porosity reduction, the porosity and hydraulic conductivity have been reduced much stronger in order to fit a measured earlier dye-tracer breakthrough of experiment brun #10Q (Ma¨der et al., 2006—this issue) shown in Fig. 18. It appears that either the assumed Kozeny–Carman equation or the model discretisation was not applicable. Similar indications could be derived from the laboratory experiments, where the mineralogical analysis of the drill core showed no major pore space reduction, despite that the water flow through the column decreased steadily with time. Therefore, quite small amounts of secondary mineral precipitation may cause huge changes of transport properties. Either, an adequate model discreti-

Fig. 18. Field experiment: measured and calculated uranine breakthrough of dipole experiment run #10, performed during the injection of the high-pH solution. For the calculated breakthrough, quite low porosities in the dipole area, compared to the fit of long-term dyetracer dipole experiment, have to be assumed in order to get some agreement with the measured breakthrough of run #10.

sation would be necessary to account for such processes occurring at the micrometer scale or even smaller, or a better understanding of the porosity–hydraulic conductivity relationship is compulsory for treating such a shear zone. 4. Discussion and conclusions 4.1. Conclusions from modelling the core infiltration experiment An important finding from this experiment is that the interaction between the hyperalkaline solution and the fault zone in Grimsel granodiorite caused a significant reduction in the hydraulic conductivity of the rock core, even though the amount of mineral alteration was minor. This reduction in hydraulic conductivity would be beneficial for the performance of a repository. The same trend is still being observed in the in situ experiment at the Grimsel Test Site after 2.5 years of continuous injection of high-pH fluid. The arrival of the high-Ca front associated with the injected hyperalkaline solution is retarded (compared for instance with pH) due to its consumption by chemical reactions (precipitation, sorption). The dissolution rates of primary minerals remain approximately constant during the experiment. However, the decrease in flow rate caused an overall displacement of the zone of secondary mineral precipitation (CSH/CASH) towards the inlet,

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which explained the increase in Si concentration at the outlet during the later stages of the experiment. Ca concentrations decreased after the arrival of the high-Ca front due to increased net precipitation of CSH/CASH. The modelling results appear to confirm that the dissolution of primary minerals is kinetically controlled. In order to obtain a reasonable agreement between models and experimental results, reactive surface areas of the order of 105 m2/m3 rock had to be used for both GIMRT and 3FLO in combination with published rate constants. These values are much smaller than the value measured for the fault gouge filling the fracture, which is in the order of 106–107 m2/m3 rock. This difference suggests that not all of the mineral surface area of the fault gouge was accessible to the high-pH solution. Nevertheless, except for the calibration of the mineral surface areas and for some minor adaptations, like the use of a surface complexation model for 3FLO and the calibration of the mineral surface areas, most of the chemical data base has been obtained from the literature, including the rate laws for primary minerals. 4.2. Conclusions from modelling the field experiments Although numerous tracer dipole experiments have been performed in the HPF shear zone and a considerable modelling effort was invested to fit measured breakthrough curves, it was not possible to discriminate between the dominant transport processes (fracture-matrix or heterogeneous porous medium dominated transport) in the shear zone by applying different model concepts or codes, respectively. The one-dimensional dual porous medium modelling approach (RANCHMD) succeeded to fit some short-term dipole experiments using single flow paths. Assuming two flow paths for the dual porous medium approach (PICNIC), or assuming a rather heterogeneous porous medium in two dimensions yielded acceptable fits for the measured breakthrough curves in longer-term dipole experiments where the tailing of the breakthrough curve was partly measured. The predictions for the breakthrough of a sorbing tracer would allow to discriminate between the dominant transport processes or to verify one of the two applied models when using a sorbing tracer in an experiment (Pfingsten and Soler, 2003). Most recently, two dipole tracer tests were carried out in the field including weakly sorbing 85Sr, but the data have not yet been interpreted. The breakthrough of pH observed in three of four additional observation boreholes in the vicinity of the dipole within the shear zone but not across the planar structure indicates that the two-dimensional model con-

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cept seems appropriate. The high-pH plume is envisaged to be confined to a pancake-like broad dipole flow field. At least multiple flow paths are necessary to describe the migration of the high-pH solution from the injection borehole to five neighbouring boreholes. The parameter fit to a long-term dipole dye-tracer experiment yielded quite good agreement with the measured breakthrough and the measured arrival of the highpH solution in neighbouring boreholes. However, the general problem of characterising such spatially extended dipole flow fields by information from single borehole tests remains a limiting factor. New experiments would yield new constraints on the parameter set, which is large for the two-dimensional heterogeneous porous medium approach. But new experiments, particularly in combination with variably sorbing tracers, would decrease the uncertainty in predictive modelling. The effect of the observed accelerated dye-tracer breakthrough in the high-pH altered shear zone over the last 2 years was not reproducible by assuming a porosity–hydraulic conductivity relationship based on changes in total porosity. Either dissolution and precipitation are localised on a submodelling scale, which generated the observed focusing of flow, or the chosen Kozeny–Carman equation is not applicable for the porosity range (porosity distribution) within such a shear zone. Excavation of the shear zone by large-diameter overcoring and analysis of core samples will finally decide on the validity of the model assumptions, such as the geometry of the flow paths and the assumed k d’s for mineral reactions for sorbing radionuclides. It became clear that such complex reactive transport experiments in a field setting and related modelling are far from standard and have to be improved in order to understand the behaviour of a cementitious repository in its host rock environment. Site-specific experiments and related modelling have to be performed at laboratory and field scales because reactions and their rates are host rock-specific. On the other hand, it also became apparent–particularly from the experience with the laboratory experiments– that some rather generic approaches already are capable of explaining many of the characteristic features. The ultimate fate of the evolution of the flow field (and associated radionuclide transport) in a repository situation cannot be predicted based on the observed trend to reduce the overall transmissivity in the HPF experiment. The boundary conditions imposed by the constantrate injection selected for technical reasons are too different from settings in a near-natural flow field. This argues for approaches that have been pioneered with the HPF project but that even further expand the potential of

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carefully designed and constrained field experiments supported by laboratory work and modelling. The capabilities of the models themselves are relatively advanced and the major limitations appear to be rooted in the inability to distinguish between alternate concepts and uncertainty due to poorly constrained parameters. Acknowledgments The HPF Project was financially supported by ANDRA (France), JNC (Japan), NAGRA (Switzerland), SKB (Sweden) and the DoE (US). We are grateful for support from Solexperts AG (Schwerzenbach, CH), Geotec AG (Wolfwil, CH), Geotechnical Institute (Bern, CH), Laboratory for Radiation Safety and Security, Paul Scherrer Institut (Villigen, CH), University of Bern (CH), University of Munich (D), University of Neuchaˆtel (CH), Chalmers University (S), CNRS Strasbourg (F), ERM Poitier (F), NAGRA and the GTS staff who have all been involved in the laboratory and field work. Finally, we appreciate the reviews of D. Noy and E. Gaucher whose comments and suggestions helped improve the paper. References Adler, M., Ma¨der, U.K., Waber, H.N., 2001. Core infiltration experiment investigating high-pH alteration of low-permeability argillaceous rock at 30 8C. Proceedings of the 10th International Symposium on Water–Rock Interaction, 1299–1302. Balkema. Barten, W., Robinson, P.C., 2001. Contaminant transport in fracture networks with heterogeneous rock matrices: the PICNIC code. PSI Bericht, vol. 01–02. Paul Scherrer Institut, Villigen, Switzerland. Bateman, K., Coombs, P., Noy, D.J., Pearce, J.M., Wetton, P.D., 1998. Numerical modelling and column experiments to simulate the alkaline disturbed zone around a cementitious radioactive waste repository. In: McKinley, I.G., McCombie, C. (Eds.), 21st Int. Symp. on the Scientific Basis for Nuclear Waste Management, Materials Research Society, vol. 506, pp. 605 – 611. Davos, Switzerland. Bateman, K., Coombs, P., Noy, D.J., Pearce, J.M., Wetton, P.D., Haworth, A., Linklater, C.M., 1999. Experimental simulation of alkaline disturbed zone around a cementitious radioactive waste repository; numerical modelling and column experiments. In: Metcalfe, R., Rochelle, C.A. (Eds.), Chemical Containment of Waste in the Geosphere, London Geological Society, vol. 157, pp. 183 – 194. London, UK. Berner, U.R., 1992. Evolution of pore water chemistry during degradation of cement in a radioactive waste repository environment. Waste Management 12, 201 – 219. Berner, U.R., 1998. Geochemical modelling of repository systems: limitations of the thermodynamic approach. Radiochimica Acta 82, 423 – 428. Billaux, D., Paris, B., Darcel, C., 2003. 3FLO Version 2.1—Calculs d’e´coulements et de transport tridimensionnels. ITASCA Consultants, Internal Report, Volumes 1–4.

Bossart, P., Mazurek, M., 1991. Structural Geology and Water FlowPaths in the Migration Shear Zone. NTB, vol. 91-12. NAGRA, Wettingen. Cacas, M.-C., 1989. De´veloppement d’un mode`le stochastique discret pour la simulation de l’e´coulement et des transferts de masse et de chaleur en milieu fracture´. PhD dissertation, Ecole Nationale Supe´rieure des Mines de Paris. Fabriol, R., Sauty, J.-P., Ouzounian, G., 1993. Coupling geochemistry with a particle tracking transport model. Journal of Contaminant Hydrology 13, 117 – 129. Fisch, H.R., 2000. GTS/HPF: Hydraulic Characterisation of the Test Site (March 2000). Interner Bericht, NAGRA, Wettingen. Frick, U., Alexander, W.R., Baeyens, B., Bossart, P., Bradbury, M.H., Buehler, C., Eikenberg, J., Fierz, T., Heer, W., Hoehn, E., McKinley, I.G., Smith, P.A., 1992. The Radionuclide Migration Experiment—Overview of Investigations 1985–1990. NTB, vol. 91-04. NAGRA, Wettingen, Switzerland. Grindrod, P., Takase, H., 1996. Reactive chemical transport within engineered barriers. Journal of Contaminant Hydrology 21 (1–4), 283 – 296. Hadermann, J., Heer, W., 1996. The Grimsel (Switzerland) migration experiment: integrating field experiments, laboratory investigations and modelling. Journal of Contaminant Hydrology 21, 87 – 100. Haworth, A., Noy, D.J., 1993. Migration of cement leachate in a fractured crystalline rock. Report NSS/R342 AEA D&R/0466. BGS, United Kingdom. Heer, W., Hadermann, J., 1994. Grimsel Test Site: Modelling Radionuclide Migration Field Experiments. PSI Bericht vol. 94-13. Paul Scherrer Institut, Villigen, Switzerland, pp. 137. Heer, W., Smith, P.A., 1998. Modeling the radionuclide migration experiments at Grimsel. What have we learned? In: McKinley, I.G., McCombie, C. (Eds.), 21st Int. Symp. on the Scientific Basis for Nuclear Waste Management, Mat. Res. Soc., vol. 506, pp. 663 – 670. Davos, Switzerland. Ho¨glund, L.O., 2001. Project SAFE—modelling of long-term concrete degradation processes in the Swedish SFR repository. SKB Report R-01-08. SKB, Stockholm, Sweden. Jakob, A., 1990. RANCHMD—RAdio Nuclide CHain transport with Matrix Diffusion. Manual. Paul Scherrer Institut, Villigen, Switzerland. Jakob, A., 1997. Modelling solute transport using the double porous medium approach. In: Grenthe, I., Puigdome`nech, I. (Eds.), Modelling in Aquatic Chemistry. OECD Nuclear Energy Agency, Paris, pp. 525 – 576. Lichtner, P.C., Eikenberg, J., 1995. Propagation of a Hyperalkaline Plume into a Geological Barrier Surrounding a Radioactive Waste Repository. PSI Bericht, vol. 95-01. Paul Scherrer Institut, Villigen, Switzerland. Lichtner, P.C., Pabalan, R.T., Steefel, C.I., 1998. Model calculations of porosity reduction resulting from cement–tuff diffusive interaction. In: McKinley, I.G., McCombie, C. (Eds.), 21st Int. Symp. on the Scientific Basis for Nuclear Waste Management, Mat. Res. Soc., vol. 506, pp. 709 – 718. Davos, Switzerland. Ma¨der, U.K., Traber, D., in press. Reactive transport model of cement–clay stone interaction with application to a HLW repository in Opalinus Clay. In: Final Report of the ECOCLAY-II Project, the European Commission. Mðder, U., et al., 2006. Interaction of hyperalkaline fluid with fractured rock: field and laboratory experiments of the HPF project (Grimsel Test Site, Switzerland). Journal of Geochemical Exploration 90, 68 – 94. doi:10.1016/j.gexplo.2005.09.006 (this issue).

W. Pfingsten et al. / Journal of Geochemical Exploration 90 (2006) 95–113 Moeri, A., Inderbitzen, L., 2002. GTS V/HPF-Experiment: structural geological model of the HPF shear zone. Internal Report. NAGRA, Wettingen. NAGRA, 2002. Projekt Opalinuston-Synthese der geowissenschaftlichen Untersuchungsergebnisse. NAGRA Technischer Bericht NTB vol. 02-03. Wettingen, Switzerland. Neall, F., 1994. Modeling of the Near-Field Chemistry of the SMA Repository at the Wellenberg Site. PSI Bericht vol. 94-18. Paul Scherrer Institut, Villigen, Switzerland. Neall, F., 1996. The pH and composition of fluids arising from a cementitious repository—implications for a high pH plume. Internal Report TM-44-95-07. Paul Scherrer Institut, Villigen, Switzerland. Neall, F., 1996. Modelling the release of alkalis from cement in a diffusion-dominated system. Internal Report AN-44-96-07. Paul Scherrer Institut, Villigen, Switzerland. Pfingsten, W., 1996. Efficient modeling of reactive transport phenomena by a multispecies random walk coupled to chemical equilibrium. Nuclear Technology 116 (2), 208 – 221. Pfingsten, W., 2001. Indications for self-sealing of a cementitious L&ILW repository. PSI Bericht vol. 01-09. Paul Scherrer Institut, Villigen, Switzerland, pp. 96. Pfingsten, W., 2002. Modelling of dipole tracer experiments at the Grimsel Test Site to investigate the HPF shear zone AU126 properties with respect of a long-term injection of a high pH fluid. Internal Report TM-44-02-08. Paul Scherrer Institut, Villigen, pp. 90. Pfingsten, W., Soler, J.M., 2003. Modelling of non-reactive tracer dipole tests in a shear zone at the Grimsel Test Site. Journal of Contaminant Hydrology 61 (1–4), 387 – 403.

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Robinson, P.C., 1984. Connectivity, flow and transport in network models of fracture media. PhD dissertation, St Catherine’s College, Oxford, United Kingdom. Seguin, J.-J., 1992. TRICTRAC—Logiciel de simulation tridimensionnel du transport hydrodispersif dans les eaux souterraines. BRGM Report n8620RP BRG 92002. Soler, J.M., 2003. Reactive transport modeling of the interaction between a high-pH plume and a fractured marl: the case of Wellenberg. Applied Geochemistry 18, 1555 – 1571. Steefel, C.I., Lichtner, P.C., 1994. Diffusion and reaction in rock matrix bordering a hyperalkaline fluid-filled fracture. Geochimica et Cosmochimica Acta 58 (17), 3595 – 3612. Steefel, C.I., Yabusaki, S.B., 1996. OS3D/GIMRT, software for multicomponent-multidimensional reactive transport: user’s manual and programmer’s guide. PNL-11166. Pacific Northwest National Laboratory, Richland, Washington. Tits, J., Bradbury, M.H., Eckert, P., Schaible, A., Wieland, E., 2002. The uptake of Eu(III) and Th(IV) by calcite under hyperalkaline conditions: The influence of gluconic acid and a-isosaccharinic acid. PSI Bericht vol. 02-03. Paul Scherrer Institut, Villigen, Switzerland. Wolery, T.J., 1992. EQ3NR, A Computer Program for Geochemical Aqueous Speciation-Solubility Calculations: Theoretical Manual, User’s Guide, and Related Documentation (Version 7.0). LLNL, pp. 262.