In situ tracer tests to determine retention properties of a block scale fracture network in granitic rock at the Äspö Hard Rock Laboratory, Sweden

Journal of Contaminant Hydrology 70 (2004) 271 – 297 www.elsevier.com/locate/jconhyd

In situ tracer tests to determine retention properties of a block scale fracture network in granitic rock ¨ spo¨ Hard Rock Laboratory, Sweden at the A Peter Andersson a, Johan Byega˚rd a,*, Eva-Lena Tullborg b, Thomas Doe c, Jan Hermanson d, Anders Winberg e a

GEOSIGMA AB, Uppsala and Kunga¨lv, Sweden b Terralogica AB, Gra˚bo, Sweden c Golder Associates, Seattle, WA, USA d Golder Associates AB, Stockholm, Sweden e Conterra AB, Partille, Sweden

Received 7 May 2002; received in revised form 1 September 2003; accepted 12 September 2003

Abstract ¨ spo¨ Hard Rock Laboratory in order to improve the Experiments were conducted at the A understanding of radionuclide retention properties of fractured crystalline bedrock in the 10 – 100 m scale (TRUE Block Scale Project, jointly funded by ANDRA, ENRESA, Nirex, JNC, Posiva and SKB). A series of tracer experiments were performed using sorbing tracers in three different flow paths. The different flow paths had Euclidian lengths of 14, 17 and 33 m, respectively, and one to three water conducting structures. Four tests were performed using different cocktails made up of radioactive sorbing tracers (22,24Na+, 42K+, 47Ca2 +, 85Sr2 +, 83,86Rb+, 131,133Ba2 + and 134,137 Cs+). For each tracer injection, the breakthrough of sorbing tracers was compared to the breakthrough of a conservative tracer, 82Br
, 131I
, HTO and 186ReO
4 , respectively. In the two longer flow paths, no breakthrough of 83Rb+ and 137Cs+ was observed after 8 months of pumping. Selected tracer tests were subject to basic modelling in which a one-dimensional (1D) advection – dispersion model, including surface sorption, and an unlimited matrix diffusion were used for the interpretation of the results. The results of the modelling indicated that there is a slightly higher mass transfer into a highly porous material in the block-scale experiment compared with in situ experiments performed over shorter distances and significantly higher than

* Corresponding author. Bultgatan 40b, SE-44240 Kunga¨lv, Sweden. Tel.: +46-303-208573; fax: +46-303208579. E-mail address: [email protected] (J. Byega˚rd). 0169-7722/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jconhyd.2003.09.009

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what would have been expected from laboratory data obtained from studies of the interactions in nonaltered intact rock. D 2003 Elsevier B.V. All rights reserved. Keywords: Migration; In situ; Retention; Radionuclide; Sorption; Crystalline rock; Fracture

1. Introduction For a repository of spent nuclear waste, the transport resistance of the geosphere to released radionuclides constitutes an important safety barrier. Performance assessment has so far mainly relied on data obtained from laboratory experiments. A need has therefore been identified to study retention properties under in situ conditions to enable comparison with laboratory data and to assess the ability of laboratory data to describe the in situ retention properties. For this purpose, in situ tracer retention experiments have been performed at several places and described by different authors, e.g., Haderman and Heer (1996), Andersson et al. (1993) and Albelin et al. (1985). For practical reasons, most of the in situ radionuclide migration experiments have been focused on single fracture and transport over relatively short distances. It can be argued that in order to address the variety of natural flow paths existing in a large rock volume, studies of tracer retention over longer distances may be of interest. Effects of transport across fracture intersections, in-plane fracture heterogeneity, and anisotropy may influence the tracer retention in a way that cannot be studied over short distances in a single fracture. It would therefore be of advantage to obtain retention characteristics from tracer experiments performed over longer distances and involving multiple fractures. This would enable a comparison of transport properties obtained in single fracture flow paths with those obtained from fracture network flow paths. For the past several years, SKB, the Swedish Nuclear Fuel and Waste Management has been carrying out a program of studies to understand radionuclide retention in fractured granitic rock (Olsson and Winberg, 1997). The program was named the Tracer Retention Understanding Experiment (TRUE). The field work for the program has been performed in ¨ spo¨ Hard Rock granitic rock at depths around 450 to 500 m below the surface at SKB’s A Laboratory in southeastern Sweden (Fig. 1). An international consortium of radioactive waste management organisations from Finland, France, Great Britain, Japan, Spain, Sweden and Switzerland has supported this effort. A similarly international team of researchers has carried out the work. The goal of the TRUE program is to understand tracer retention in fractured crystalline rock. In this context, retention is defined as the set of physical and chemical processes that retard the transport of radionuclides relative to the advecting groundwater velocity, e.g., diffusion of species from the conducting portion of fractures into the rock matrix or into stagnant porosity within the fractures and interactions (sorption) between the radionuclides and mineral constituents of the fracture walls and the rock matrix. The experimental study of retention in situ involves variables of several disciplines, e.g., mineralogy, hydrogeology and geochemistry.

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Fig. 1. Sketch of the investigated rock block with tunnels and niches drawn in the upper right. The solid grey lines correspond to traces of the interpreted deterministic structures projected on the horizontal plane (Z =
450 masl). The respective numbered structures are given in the circles connected to the grey lines. Injection points for the four tests with sorbing tracers (C1 – C4) are also indicated. The solid black lines correspond to projected traces of the boreholes intersecting the different structures.

The overall goal of the TRUE project has been to use field experiments, laboratory experiments and numerical simulations to develop a tested methodology for predicting radionuclide transport behaviours in fractured granitic rock. This paper reports a portion of the results of this project, specifically, (1) the results of sorbing tracer experiments in fracture networks at a scale of tens of meters, and (2) a series of simple one-dimensional (1D) simulations to evaluate transport parameters at the same scale.

¨ spo¨ Hard Rock Laboratory and an overview of the TRUE experiments 2. The A ¨ spo¨ Hard Rock Laboratory is a research facility for developing technologies for The A radioactive waste disposal in plutonic rocks. The laboratory lies below the island of ¨ spo¨ in an archipelago along the Baltic Sea coast roughly midway between Stockholm A and Sweden’s southern tip. Over 15 years of geophysical exploration, surface drilling and testing, and underground site characterisation have produced a robust conceptual model of the geology and geometry of the major conducting faults and fractures across

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the island. Island-scale studies of geochemistry and groundwater flow have produced a large-scale conceptual understanding of the groundwater flow system of the laboratory. This background of information and experience provides a valuable context for scientific studies of transport and retention at the decimetric and hectometric scales of the TRUE program. The TRUE program has used two study sites in the underground laboratory. The experimental strategy of TRUE was first to describe retention properties of a single fracture in the detailed scale, which we define as less than 10 m. Winberg et al. (2000), Cvetkovic et al. (2000) and Widestrand et al. (2001) have reported on the tracer test results of the single-fracture site which was called TRUE-1. The retention parameters from TRUE-1 then factored into the predictions of the second tracer test site, TRUE Block ¨ spo¨ Scale. We define ‘‘block scale’’ as the region between the major fracture zones of the A laboratory, effectively 10– 100 m. The Block Scale experiments characterised a 150-mscale region and then hosted tracer tests both in single features and in networks of features using pathways of ten to several tens of meters. The TRUE Block Scale project studied tracer retention in an integrated and iterative manner. The integration involved parallel studies of: 

Determining the geometry and hydrogeology of conducting features, Developing detailed geologic descriptions of the mineralogy and microstructure of the conducting features,  Measuring retention properties in laboratory experiments,  Simulating numerically flow and transport using alternative approaches including onedimensional transport, porous continuum and fracture network models. 

The tracer testing proceeded in iterative steps of prediction and tracer testing: 

Tracer dilution tests to confirm connectivity predicted in the geometric conceptual models of the fracture network  Short-term nonsorbing tracer tests to confirm the feasibility of testing  Longer term nonsorbing tracer tests to define physical transport properties  Sorbing tracer tests which are reported in this paper. This paper presents the results of the final set of tracer tests in the project, injections C1 –C4, which studied sorbing tracer transport. A full coverage of the other components of the project (such as structural geology, hydrogeological investigations, construction of hydrostructural models, prediction and modelling of different tracer experiments) is currently presented in project reports (Andersson et al., 2002a,b; Poteri et al., 2002; Winberg et al., 2002).

3. Description of conducting features and tracer test flow paths The TRUE Block Scale Project was carried out using a step-wise approach. In order to assure flexibility, the experimental strategy was that of an iterative process involving site

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characterisation with strong interaction between modelling and experimental work. Site characterisation data from each new borehole was used to update the hydrostructural model of the investigated rock. The major goal of the site characterisation program for the TRUE Block Scale volume was to identify and parameterize the conductive features in the context of a joint hydrologic and geologic model. Conducting features were of three categories: 1. Bounding structures of the block that had been identified prior to the TRUE Block Scale program. These features have transmissivities of about 10
5 m2/s. 2. Numbered features within the block that could be identified and treated as deterministic conductors that have transmissivities of about 10
7 m2/s. 3. Background fractures that are treated stochastically and appear in only one hole. These generally have transmissivities of about 10
8 m2/s or less. The characterisation program used a combination of methods to identify the major conducting structures which are identified by number such as Structure #20. The numbered structures met several common criteria that included: 

Flow anomalies in heat-pulse flow logs which could resolve the position inflow points in the boreholes to 0.1 m or less; the numbered features accounted for all of the significant flow log anomalies;  Cross-hole pressure responses during drilling and pressure-interference testing, again, which are entirely accounted for among the numbered features; and  Appearance of geologic features with consistent locations and orientations in multiple holes based on core and borehole image logging using high-resolution borehole imaging. The numbered structures also appear to form networks (Fig. 2). The network behaviour appeared both in the cross-hole interference responses and in variations in water compositions. The major structure of the block, Structure #20, is the core of a network including Structures #13, #21, #22 and #23. This network is also characterised by water with relatively high salinity. In contrast, Structures #10 and #5, at the extreme ends of the block, appear relatively isolated and contain water that is relatively low in salinity. Structure #19 is an extensive feature that is relatively isolated but has connections with Structure #20 network and carries water with a water composition intermediate to Structures #10 and #20. Whereas, a major goal of the TRUE Block Scale Project was to test the networks, the Structure #20 network was the focus of the tracer testing activities. The main rock type of the TRUE Block Scale rock volume is porphyritic quartz – ¨ spo¨ diorite. In addition, a granodioritic – granitic variety called monzo diorite denoted as A ¨ vro¨ granite has been identified in a minor portion of the laboratory. Lenses and dikes of A fine-grained aplitic granites are common. The characterisation stages of the project revealed 24 conductive structures within the investigated rock volume (Fig. 1). Four of these structures, #20 to #23, were subsequently addressed in the in situ tracer experiments using sorbing tracers.

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Fig. 2. Illustration of the 3D structure of the block where the boreholes and the interpreted hydrogeological structures involved in the tracer experiment are given. The background grid for reference is 50 m, and, in order to better illustrate the flow paths, the view (top down) is from underneath. The borehole section that was used was located in the intersection of Structure #21 in borehole KI0023B and is marked with (o). The three different injection sections were located in: (a) the intersection of Structure #20 in borehole KI0025F03 (C1 and C4), (b) the intersection of Structure #23 in KI0025F03 (C2) and (c) the combined intersection of Structure #13 and Structure #21 in KI0025F02.

The conducting structures are not single, isolated fractures. Rather, the structures appear to be more complex features resulting from deformation during tectonism. For the most part, the structures appear to be cataclasites or mylonites that may have one or multiple fractures within a zone that may have an alteration-enhanced porosity as well as zones of partially rehealed crushed rock. A conceptual model of the fractures is given in Fig. 3. Cataclasites have a high frequency of microfractures and variable degrees of chemical alteration and recrystallisation such as quartz recrystallisation, alteration of biotite to chlorite, saussuritisation of plagioclase and oxidation of magnetite to hematite. Mylonites involve more severe deformation characterised by complete grain-size reduction and recrystallisation resulting in the formation of epidote, quartz, sericite/chlorite, albite and Kfeldspar. Almost all of the intercepts studied in the TRUE Block Scale rock volume show alteration and tectonisation of the wall rock, and most intercepts are associated with mylonites. A brittle reactivation of earlier ductile/semiductile deformation zones thus seems to be associated with most structures, and slickenlines indicate fault movements along several of the actual fracture planes. The width of the cataclastic zones varies from about a centimetre to a decimetre, while the mylonites usually vary in width from millimetres up to a few centimetres.

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Fig. 3. Generalised conceptual model of a typical conductive structure involved in the tracer experiments. Details on porosity and porosity estimates are provided in Andersson et al., 2002a.

Fault breccia and fault gouge material have been identified in some of the structures, especially in the larger structures, e.g., #19, #20 and #22. The fault gouge is an unconsolidated material consisting of altered wall rock fragments infilled with calcite and clays. The clay minerals identified in fault gouge from structures within the TRUE Block Scale rock volume are chlorite, illite, mixed layer clays, (in Structure #22, they were estimated as 25% of the < 0.125 mm size fraction) and smectite (in Structure #19, they were estimated as 15% of the < 0.125 mm size fraction). There is, however, a high variability in the geological character not only between the different structures but also between the different intercepts of each individual structure. Therefore, heterogeneity in the transport and retention characteristics is to be expected between and along the different flow paths. The connected porosity is one of the transport parameters that have received considerable attention (Andersson et al., 2002a). Generally, there seems to be a very steep gradient (decrease) in the connected porosity going from the

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partly gouge-filled and brecciated centre of water conducting structures into the altered wall rock and further into the fresh unaltered bedrock. A description is given below of the four interpreted deterministic conductive structures involved in the in situ tracer experiment. All four structures are oriented NNW to NW (cf. Fig. 1): Structure #20: This is the main conductive structure of the block and it has five of the six boreholes used to characterise the block. Structure #20 consists of a core fault with brecciation between minor fault planes. It is a reactivated mylonite with extensive cataclastic deformation and hydrothermal alteration around the structure. Fault gouge (containing ~25% calcite) and fault breccia appear. The conductive part of the structure is likely to involve splay fractures. Structure #21: This structure has only been observed in three boreholes. It is hosted in ¨ spo¨ diorite and fine-grained granite. The geological composition and the orientation of A its intercepts are variable, and it is suggested that it could be several hydraulically connected geological structures rather than a single, well-defined geological structure. The intercept in borehole KI0025F02 contains a reactivated cataclasite/mylonite, with fault breccia and fault gouge trending approximately N –S. The remaining two intercepts are single fractures and look more like splay fractures to larger structures, as in the case of KI0023B (the pumping section) probably to Structure #20 as this intercept is only 1 m away from the Structure #20 intercept in the same borehole. Structure #22: This structure has been observed in three boreholes. It has a variable ¨ spo¨ diorite as well as sections along small geological character involving fractures in A mylonites, or alternatively heavily brecciations, hydrothermal alteration and very clay- and chlorite-rich fault gouge material. Structure #23: This structure (found in two boreholes) is interpreted from the geometry of the fractures, the inflow points identified by the flow logging and the results of interference tests between the two boreholes. One intercept contains mylonite, whereas ¨ spo¨ the other corresponds to a section with three conductive fractures in chemically altered A diorite. One of these fractures has been selected as the main candidate for Structure #23. 3.1. Flow paths The three different flow paths, or source – sink pairs, used in the present tracer experiment were all quite different in character according to the structural model (Andersson et al., 2002a; cf. Table 1). The pump (sink) section used for all injections, C1 through C4, was the intersection of Structure #21 in borehole KI0023B. This 1-m packer interval has one conductive fracture which is coated with chlorite and calcite and is hosted in slightly altered and ¨ spo¨ diorite. At a distance of about 1 m from Structure #21, the borehole tectonised A intercepts Structure #20. It is known from hydraulic tests that the fracture in section P6 of Structure #21 is hydraulically well connected to the Structure #20 fracture system. Consequently, flow path I (injections C1 and C4; injection section KI0025F03: P5, Structure #20) can be regarded as a fast and, more or less, single-structure flow path along Structure #20, probably involving both central parts and splay fractures of the structure.

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Table 1 Details about the injections, flow paths and tracers used in the experiments Injection #/flow path, details

Tracer

Half-life

Injection activity (MBq)

Elemental concentration in the stock solution

140 16 230

29 mg/l 1930 mg/l 7.5 mg/l

Pumping rate at the withdrawal borehole, Qwith: 2.0 l/min Br
Na+ 42 + K

1.47 days 15.0 h 12.4 h

47

Ca2 + Rb+ 134 Cs+

4.54 days 18.6 days 2.06 years

ReO
4 Ca2
131 Ba2 +

3.78 days 4.54 days 11.5 days

170 56 26

137

30.2 years

24

HTO Na+ 85 2 + Sr

12.3 years 2.60 years 64.9 days

240 22 22

83

86.2 days

C1/I, (monitored for 5780 ha) Euclidian distance: 14 m Distance according to the hydrostructural model: 16 m Geol. structures involved: #20, #21 Qinj: 45 ml/min Qdil: 25 ml/min

82

C2/II, (monitored for 5640 ha) Euclidian distance: 17 m Distance according to the hydrostructural model: 97 m Geol. structures involved: #20, #22, #23 +/
additional single fracture; Qinj: 10 ml/min; Qdil: 8.5 ml/min

186

C3/III, (monitored for 5660 ha) Distance (m): 33 Distance according to the hydrostructural model: >33 m Geol. structures involved: #21 (#13, #20) Qinj:
Qdil: 1.8 ml/min C4/I (same geometry as C1, monitored for 3740 ha) Euclidian distance: 14 m Distance according to the hydrostructural model: 16 m Geol. structures involved: #20, #21 Qinj: 45 ml/min Qdil: 25 ml/min

24

86

47

Cs+

22

Rb+

133

Ba2 +

Br


10.5 years

82

1.47 days

131


8.02 days 4.54 days

I Ca2 +

47

131

Ba2 + Mn2 + 57 Co2 + 65 Zn2 + 54

11.5 days 312 days 271 days 244 days

11 13 7.8

46 0.55

11 9.8 1.9 65 71 29 20

1289 mg/l 130 Ag/l 5 Ag/l 64 Ag/l 1289 mg/l 150 Ag/l ( < 80 Ag/l) 24 Ag/l (5 Ag/l)

Aqueous solution 1930 mg/l 17 mg/l 55 Ag/l 2400 Ag/l (80 Ag/l)

29 mg/l 0 (not measured) 1289 mg/l 300 Ag/l (80 Ag/l) 0 (0.5 mg/l) 0 (0.1 Ag/l) 0 (10 Ag/l)

The transport lengths of the different flow paths used are presented both as Euclidian distances and distances calculated along the structures using the hydrostructural model (Andersson et al., 2002a). The elemental concentrations in the stock solution used for injection were aimed to correspond to the natural elemental concentrations. In the cases where this option could not be fulfilled, the natural elemental concentrations are given within brackets for comparison to the injected concentration. a The short-lived tracers included in the injections could in several cases not be monitored for the whole time, cf Table 2 for details for each tracer.

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Within this paper, we describe path lengths as either the Euclidian length, which is the straight-line distance between the midpoints of piezometer intervals, or the network length, which is the shortest path along the interpreted structure planes. There is a greater chance that background fractures are involved in transport when the difference between the Euclidian path length and the structural path length is large. On the other hand, when the structural path length and the Euclidean path length are nearly the same, the flow is almost certainly in the main structure. The Euclidian length of the flow path I, which is predominantly within Structure #20, is 14 m, but structural path length is slightly longer at 16 m long on account of the short section of travel in Structure #21. Flow path II (injection C2; injection section KI0025F03:P7, Structure #23), in contrast, involves several structures, probably #23, #22 and #20, and there may also be short-circuits between these structures through splay fractures and minor single background fractures. The injection section involves, in addition to the main candidate for Structure #23, two additional single conductive fractures. This pathway favors the clay gouge-filled parts of Structure #22. The Euclidian length (17 m) of the presumed network pathway deviates significantly from the distances interpreted on the basis of the hydrostructural model (97 m); hence, this pathway is also likely to involve background fractures. (Andersson et al., 2002a). Flow path III (injection C3; injection section KI0025F02:P3, Structures #13 and #21). This flow path was originally chosen to represent transport over long distances in an interpreted single structure. The injection section is almost 6 m long and exhibits a more complex geology than the other injection sections, with several dikes of fine-grained ¨ spo¨ diorite host rock. Flow logging indicated two inflow points granite intruding the A corresponding to Structures #13 and #21. The latter is most prominent of the three available intercepts with Structure #21 and has an orientation that is almost N – S, suggesting that Structure #21 may have a more N –S orientation than indicated in Fig. 1. The pathway between KI0025F02:P3 and KI0023B:P6 involves Structure #21, probably Structure #20 and, also possibly, parts of other structures (#13?) and/or additional minor nonidentified background fractures. The Euclidian and structural distances are essentially the same at 33 m due to the dominance of the Structure #21 pathway.

4. Groundwater composition The dissolved ionic composition of the groundwater at the investigated site has four major components, Na+, Ca2 +, Cl
and SO42
, with concentrations of 1900, 1300, 5400 and 330 mg/l, respectively. Although the groundwater composition varies over the TRUE Block Scale volume, the composition does not vary across the part of the block traversed by the tracer test pathways used in this experiment. A full coverage of the water compositions and the variety within the different structures is given in Andersson et al. (2002a).

5. Laboratory investigations Within the TRUE-1 programme, laboratory investigations including batch sorption ¨ spo¨ and through-diffusion experiments were performed primarily using unaltered A

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diorite bedrock samples (e.g., Johansson et al., 1997, 1998; Byega˚rd et al., 1998). Sorption experiments were performed on different size fractions of crushed material, and through-diffusion measurements were made on drill core cylinders of variable length (1 –4 cm). Corresponding site-specific data on fracture wall rock samples were also obtained, primarily by using material from the TRUE-1 site. Because no fault breccia/fault gouge material was isolated during the drilling at the TRUE-1 site, no experimental sorption or diffusivity data were obtained for that material. A selection of parameters were derived for TRUE Block Scale usage from the laboratory data (Byega˚rd et al., 1998): 

Rock surface sorption distribution coefficients, Ka (m), intended for the estimation of contact time independent surface retention coefficient Ra (Ra = 1 + 2Ka/d, where d is the fracture aperture). The calculation of Ka was based on sorption experiments with comparatively short contact time (1 –14 days).  Mass-based sorption distribution coefficients, Kd (m3/kg), combined with effective diffusivities, De (m2/s), and rock porosities e. These parameters were intended for the estimation of time-dependent matrix diffusion processes. The diffusion experiments (data from through-diffusion and/or penetration profile studies) were the basis for the numerical values of these parameters. Some fault breccia was collected from the conductive structures such as #19, #20 and #22. Attempts were made to estimate Kd values using the mineralogical analysis of the clay fraction of the fault breccia material and the corresponding groundwater composition data. These data were combined with literature values of the cation exchange capacities and selectivity coefficients for the different minerals, and Kd values could be calculated (see Andersson et al., 2002a for further details).

6. In situ tracer tests 6.1. Tracer selection strategy The main set of sorbing tracers was selected from elements of the alkali and alkaline earth metal groups previously used in the TRUE-1 experiments (Winberg et al., 2000). For these tracers, cation exchange should be the main sorption mechanism. The TRUE-1 program observed a variation of sorptive retention among the tracers. A general trend in sorption among the tracers could be observed in the in situ experiments of the TRUE-1 programme. Na+, Ca2 + and Sr2 + were only very slightly retarded compared to the nonsorbing tracers (uranine and HTO). Rb+ and Ba2 + were moderately retarded, and Cs+ was strongly retarded. Such a trend is also consistent with the results of the laboratory experiments (Byega˚rd et al., 1998). Assuming that the relative retardation of the different tracers would be similar in a block scale experiment, it would be necessary to use tracers like Rb+, Ba2 + and Cs+ in order to obtain a clearly observable retardation compared with conservative tracers. However, it is possible that transport over longer distances would result in an increased retardation, such that breakthrough would only be obtained for

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slightly sorbing tracers like Na+, Ca2 + and Sr2 +. It was therefore decided that at least one slightly sorbing tracer and one strongly sorbing tracer should be included in each injection. Besides the sorbing tracers, radioactive nonsorbing tracers were also used, such as 82Br
in injection C1, 186ReO4
in injection C2, HTO in injection C3 and a combination of 82Br
and 131I
in injection C4. An attempt to address the impact of hydrolysis on sorption was made by using three different di-valent transition metal cations as tracers. Injection C4 was performed using the tracers 54Mn2 +, 57Co2 + and 65Zn2 + for which the first hydrolysis constants, log b1*, have been reported to be
10.6,
9.7 and
9.0, respectively (Smith and Martell, 1989). An in situ retardation in the order 54Mn2 + < 57Co2 + < 65Zn2 + was predicted based on the generally observed dependence of sorption strength on the hydrolysis constant (e.g., James and Healy, 1972). The use of radioactive isotopes for each sorbing tracer made it possible to avoid significant increases in the natural concentration in the groundwater of the elements used as tracers. Effects of oversaturation and nonlinear sorption effects caused by concentration increase could therefore be avoided. Isotopes with decay associated with g-radiation were preferred because they could be measured simultaneously with good selectivity using g-spectrometry. Tritiated water (HTO) was the only tracer without any measurable g-radiation and was thus measured using a liquid scintillation technique that allows only limited possibilities to discriminate the h-particles from other radionuclides. However, the injection of a comparably high activity of HTO combined with the low h-energy in the decay of tritium made it possible to measure HTO over a large dynamic range of concentrations without having to invoke separation procedures. 6.2. Equipment and procedures The Phase C tracer tests used three of the six boreholes that cross the TRUE Block Scale volume. These holes contain multiple packers to isolate the conducting structures. Furthermore, the packer systems are designed for tracer tests by allowing for circulation in the test interval and minimisation of the borehole volume to mitigate problems of tracer storage in the borehole itself. Prior to tracer injection, a controlled water withdrawal of 2 l/min ( Qwith) from Structure #21 in borehole KI0023B established a steady flow field in the block. Further details about the instrumentation of the boreholes and the injection/sampling equipment methodology can be found in Andersson et al. (2002b). Previous tracer tests phases using conservative tracers had already established the sustainable pumping rates and approximate flow velocities through the fracture network (Phase B tests in Andersson et al., 2002b). The tracer source sections had flow lines to allow tracer recirculation through the injection interval to the surface. This recirculation enabled sampling and on-line gspectrometric measurements outside the boreholes, i.e., monitoring of the injection rate of the tracer as a function of time. Knowledge of the evolution of concentration over time in the source section is essential for separating in the breakthrough the effects of retention in the flow path from the storage effects of the source borehole.

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For three of the four tracer injections, C1, C2 and C4, a small injection overpressure in the tracer source section provided assurance of acceptable tracer recovery. This use of this overpressure is consistent with tracer test experience reviewed by Becker and Shapiro (2003) who showed that strictly passive tracer sources frequently produce no breakthrough at the pumping sinks while tracer tests with small-injection rates or overpressures are more consistently successful. The injection flow rates ( Qinj) were 45 ml/min for injections C1/C4 and 10 ml/min for injection C2. For injection C3, no injection overpressure was used.

7. Tracer test results 7.1. Basic evaluation Examples of breakthrough curves and the corresponding injection curves for the different injections are given in Figs. 4 and 5. Travel times for 5%, 50% and 95% mass recovery (t5, t50 and t95) and a total tracer recovery at the finite sampling time (tt) were calculated and are given in Table 2. For the tracers for which 50% recovery was not obtained, retardation factors are calculated for travel times for lower percentage recovery (specified in Table 2). The relative order of retardation between the sorbing species, Na+ < Ca2 + < Ba2 + < Rb+ < Cs+, was consistent with the experience of earlier tracer tests in TRUE-1 (Winberg et al., 2000) and with laboratory data (Byega˚rd et al., 1998). As a comparative measure, recovery-based retardation factors, R50%, were calculated from the ratio of the t50 of a given sorbing tracer divided by the t50 of the simultaneously

Fig. 4. Injection and breakthrough curves for the tracers used in the injections C1/C4, all in flow path I. C/Atot refers to the concentration of the tracer (Bq/kg) divided by the total injected activity (Bq); all activities are decaycorrected to the time of injection.

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Fig. 5. Injection and breakthrough curves for the tracers used in the injection C2 for flow path II (186ReO
4 and 47 Ca2 +; o, ) and for the tracers used in the injection C3 for flow path III (HTO, 22Na+ and 85Sr2 +; 5, n, V). C/ Atot refers to the concentration of the tracer (Bq/kg) divided by the total injected activity (Bq); all activities are decay-corrected to the time of injection.

injected conservative tracer; values are given in Table 3. These retardation factors are compared with the corresponding values obtained in the TRUE-1 experiments (Winberg et al., 2000). However, it should be emphasised that this rather rough concept of comparisons is performed without taking the flow field conditions into account. An increased recoverybased retardation factor is therefore not in any way a definite proof for an increased influence of diffusion or sorption processes on tracer transport. The flow rates through the different injection sections ( Qdil) were evaluated using the dilution data of the conservative tracers used in the different injection sections (see Table 1). For the C1/C4 and C2 injections, comparison between Qdil and the corresponding injection overpressure flow rate ( Qinj) shows, somewhat unexpectedly, that Qinj is higher than Qdil. A possible explanation to this mismatch is that the mixing in the injection section is not complete, that is, the unlabelled water added by the injection flow rate is not completely mixed with the entire volume before it is transferred into the fracture. 7.2. Flow path I (injections C1 and C4) Pathway I is the shortest and fastest pathway of those tested. Hence, this pathway was selected to obtain complete breakthrough curves for a set of tracers with a wide range of sorption strengths. The comparatively short travel time made it possible to perform two injections (C1 and C4) with a 3-month separation and using partly different tracers. Fig. 6 shows tracer breakthrough from both injections, and Table 3 illustrates the large span of travel times (t50) for the different species. The travel times range from 20 h for the conservative tracers to more than 1000 h for the most sorbing ones. Calculations of the recovery-based retardation factors, cf. Table 3, indicate that the retention characteristics are

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285

Table 2 Travel times for 5%, 50% and 95% arrival times of the tracer mass, t5, t50 and t95, and mass recovery at the time for the last measurement (tt) Tracer C1 82
Br 24 Na+ 47 Ca2 + 42 + K 86 Rb+ 134 Cs+

t5 (h) 9 10 14 21 66 530

C2 186 ReO
4 47 Ca2 + 131 Ba2 + 137 Cs+

94 300 >1900 >5640

C3 HTO 22 Na+ 85 2 + Sr 83 Rb+ 133 Ba2 + C4 82
Br 131
I 47 Ca2 + 131 Ba2 + 54 Mn2 + 57 Co2 + 65 Zn2 +

t50 (h)

t95 (h)

tt (h)

Mass recovery (%)

20 27 46 100 400 5000

49 110 250

160 110 300 110 730 5780

100 96 98 53 67 53

a a b

260

a

500 800

80 29 Nothing recovered Nothing recovered

230 340 640 >5300 >5700

820 1500 3000

c

3050 5660 3100

73 79 52 Nothing recovered Nothing recovered

9 9 16 28 150 >3740 >3740

23 22

a

290 1100

a

38 220 69 890 3740 3740

69 90 49 67 71 3 Nothing recovered

a

b a

a

a

b

b

a

Specified recovery not reached because of losses due to the radioactive decay (i.e. short-lived tracers). Last successful measurement specified by tt (h). b Specified recovery not reached, but the recovery was still rising at the end of the monitoring time, tt (h). c Specified recovery not reached; concentrations in the sampled water became below the detection limit after the specified tt (h).

very similar to the results obtained for the TRUE-1 experiments. Because these experiments were performed with pathway lengths and breakthrough times of the same order, the observed similarities are mutually consistent. 7.3. Flow path II (injection C2) The tracers used to investigate the retention properties of flow path II were selected to facilitate a comparison between flow paths I (transport mainly in a singular structure) and II (transport presumed to occur in a fracture network over a longer distance). After 5 months of sampling, no activities from the tracers 131Ba2 + or 137Cs+ had been detected, whereas 80% of 186ReO4
and 30% of 47Ca2 + had been recovered, respectively. The

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Table 3 Retardation factors based on recovery, R50% (unless other specification), for the different TRUE-1 tests (Winberg et al., 2000) and the TRUE Block Scale tests C1/C4 Tracer

Uranine Br
I
ReO
4 HTO Na+ K+ Ca2 + Sr2 + Rb+ Ba2 + Cs+ Mn2 + Co2 + Zn2 +

TRUE-1 detailed scale

TRUE block scale

STT1

STT1b

STT2

C1/C4

C2

C3

r = 4.7 m

R = 5.1 m

r = 4.7 m

r = 14 (16) m

r = 17 (97) m

r = 33 (>33) m

t50ref = 36 h

t50ref = 11 h

t50ref = 79 h

t50ref = 20 h

t50ref = 260 h

t50ref = 820 h

a

Ref.

a

Ref. 1.03b 1.03b

a

Ref. 1.1

Ref.a Ref.a

1.1 1.4 1.2b 1.8 9 5 230c

c

1.04 1.5 1.9b 3.2 20

300c

1.2 1.5 1.9 2.4 15 12 130d

1.4 5.1 2.3 20 13 250 50 >390d >390d n.b.

Ref.a 1.8 4.1c

>20d n.b. >56d n.b.

3.6 >23d n.b. >25d n.b.

R50% is defined as t50 of a sorbing tracer divided by the t50 of the simultaneously injected conservative tracer, that is, how much longer time it takes to reach 50% recovery for a sorbing tracer compared to a conservative tracer. The notation t50ref refers to the elapsed time when 50% recovery was obtained for the conservative tracers. In cases when the recovery of the sorbing tracer was less than 5%, a minimum mass recovery is estimated from the last sampling time. The notation n.b. means no breakthrough, that is, no amount of that tracer has been observed in the withdrawal borehole section. a Used as reference tracer, that is, the tracer in relation to which the retardation factors have been calculated. b Rpeak. c R25%. d R5%.

recovery-based retardation factor for 47Ca2 + in flow path II was calculated to be 4.1 which is significantly higher than what was calculated for flow path I, and also higher than for the TRUE-1 experiments. After an elapsed time>5000 h, the relative concentration of 137Cs+ was still below 8  10
9 kg
1, according to an estimate based on the detection limit. This can be compared to the peak concentration of 186ReO4
which reached 3  10
5 kg
1 after an elapsed time of 150 h. The value for the recovery-based retardation factor of 137 Cs+ has been estimated to be>56. This value can be compared to the retardation factor of 134Cs+ in flow path I, which is 250, and the retardation factors 130– 250 for the different Cs+ tracers determined by analysing the TRUE-1 experiments. Based on a comparison of the retardation factors, the absence of breakthrough of Cs in the slow flow path II is thus consistent with the presence of breakthrough in the shorter/faster flow paths. 131 Ba2 + was also used in flow path II. No recovery was observed for this tracer. Because the half-life of this tracer is rather short (11 days), the estimated minimum

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287

Fig. 6. Example of results from the preliminary modelling using a 1D advection – dispersion model with matrix diffusion, matrix sorption and fracture surface sorption. Breakthrough curves are given for the C1 injection; see text for details. C/Atot refers to the concentration of the tracer (Bq/kg) divided by the total injected activity (Bq); all activities are decay-corrected to the time of injection.

recovery-based retardation factor, >20, is based on an elapsed time of 79 days. This value is significantly higher than recovery-based retardation factors observed in the TRUE-1 experiments and in flow path I, indicating that the longer transport distance in the fracture network in combination with a lower flow rate may result in an increased retardation. Such a conclusion is also supported by the results for the 47Ca2 + tracer. 7.4. Flow path III (injection C3) No breakthrough of 133Ba2 + and 83Rb+ was observed after about 8 months of sampling, whereas mass recoveries>73% of HTO, >79% of 22Na+ and >52% of 85Sr2 + were obtained. Calculations of a lowest possible retardation factor for 133Ba2 + and 83Rb+ yield values >25 and >23, respectively. A comparison to the corresponding values obtained from TRUE-1 and flow path I indicates that the retention is somewhat more pronounced in this longer flow path compared to the interpreted short single-fracture flow paths (cf. Table 3). 7.5. Surface complexation sorption The results of the cationic tracers in the transition metal series (i.e., Mn2 +, Co2 + and Zn ) provided support for the hypothesis that retention characteristics depend of the magnitude of the hydrolysis constant. Zn2 + is the cation with the strongest hydrolysis constant (logb1* =
9.0) and strong sorption of this tracer was observed already in the injection borehole. Actually, 65Zn2 + was the only tracer injected in flow path I for which no breakthrough could be detected in the sampling borehole. Furthermore, 57Co2 + (log b1* =
9.7) was sorbed in the injection borehole, but a delayed breakthrough could be 2+

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monitored (retardation factor>390, the most retarded of the tracers in flow path I for which breakthrough was obtained). 54Mn2 + (log b1* =
10.6) is clearly retarded—not as strongly as 57Co2 + but stronger than the very weakly hydrolysed divalent cations of the alkaline earth metal group.

8. Modelling 8.1. Theory and concepts The TRUE Block Scale Project tracer tests have been the subject of an elaborate modelling effort (Poteri et al., 2002) which encompassed a broad range of alternative modelling concepts such as equivalent continuum models, fracture network models and channel network models. A special effort will also be made to study whether effects of fracture intersection zones (FIZ) can be detected in the block scale experiments. However, in this paper, an evaluation is presented using a simple one-dimensional advection – dispersion model that allows for fracture surface retardation and unlimited matrix diffusion and bulk sorption. The rationale for this modelling effort is to obtain an indication of the extent to which a simple model can be used for the interpretation of tracer transport in a complex heterogeneous fracture system. In a radially converging tracer experiment, it is not unreasonable to assume that tracer transport takes place in a single-flow path, i.e., a path connecting the packed-off injection and pumping sections of two adjacent boreholes. To the extent that tracer can be expected to follow a one-dimensional path, albeit through a three-dimensional (3D) network, a simple one-dimensional transport model should have some validity. The consistency of model results over different path lengths provides an indication of the limitations that scale effects might impose on the simple one-dimensional approach. Furthermore, a comparison of model-derived parameters from the field tracer tests with laboratory data address the up-scaling of laboratory data to field experimental conditions at different scales. The governing equations may be written according to Moreno et al. (1983): Ra

BCp BCf B2 Cf BCf 2
DL
De A ¼0 þv y Bt Bx2 Bx Bz z¼0

ð1Þ

for the fracture, and: BCp B2 Cp De
¼0 Bt e þ Kd q Bz2

ð2Þ

for the matrix. Cf and Cp are the solute concentrations (all concentrations are in Bq/kg) in the fracture and rock matrix, respectively; DL is the longitudinal dispersion coefficient (m2/s); v is the average water velocity (m/s); x is the distance (m) in the direction of the flow field; z is the distance (m) in the direction perpendicular to the fracture; and q is the dry bulk density (kg/m3) of the rock matrix.

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289

For a zero initial concentration, constant concentration at the inlet and a zero concentration downstream boundary, a solution to the above equations for two-layer semiinfinite fractured porous medium is (Tang et al., 1981): 

Cf 2 Pe ¼ pffiffiffi exp 2 C0 p

Z

l

l



exp
f2


2

Pe 16f2



0

1 2

B Pet V0 =8Af C erfc@ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Adf 2 t
ðPet V0 =4f Þ

ð3Þ

where rffiffiffiffiffiffiffiffiffiffiffiffi Pet V0 l¼ 4t Pe ¼

vx DL

ð4Þ

ð5Þ

dRa A ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 De ðe þ Kd qÞ

ð6Þ

t V0 ¼ Ra t0

ð7Þ

where tV0 is the tracer residence time (s); t0 is the residence time of a conservative tracer (s); C0 is the constant tracer injection concentration; l and Pe (Peclet number) are dimensionless parameters; and A (s1/2) is a lumped matrix diffusion/sorption parameter. For a finite rectangular tracer pulse injection with duration Dt (s), the solution is: CðtÞ ¼ Cf ðtÞ
Cf ðt
DtÞ Cf ðt
DtÞ ¼ 0 for t
Dt < 0

ð8Þ

For the case of a pathway in a radially converging flow field where the injection is sufficiently weak that dipole effects are negligible, the tracer concentration in the water in the withdrawal borehole (Cbh) is given by: Cbh ¼

Qdil REC CðtÞ Qwith

ð9Þ

where Qdil is determined from dilution measurements and represents the effective flow rate (m3/s) with which the tracer is injected from the injection borehole into the flow path; Qwith is the pumping flow rate (m3/s) from the withdrawal borehole section; and REC (
) is the part of the mass from the injection borehole that is recovered in the withdrawal borehole section (should be equal to unity under ideal conditions). Because all these parameters are used as multipliers applied to the analytical solution for the tracer concentration in the fracture (Eq. 8), they have been lumped together to form a proportionality factor (pf), i.e.; Cbh ¼ pf CðtÞ

ð10Þ

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Under nonideal conditions, pf (
) may thus be used to account for both tracer dilution and incomplete recovery. In addition, pf may also account for uncertainties in other parameters such as flow rates and tracer injection concentration (i.e.,injected tracer mass). The solution given in Eq. (3) was used to estimate values for the transport parameters using a nonlinear least square regression procedure (Marquardt, 1963). Estimation parameters were t0, Pe and pf (flow path-specific parameters) and Ra and A (tracerspecific parameters). For each tracer injection, all breakthrough curves were used simultaneously in the regression. For example, tracer injection C1 breakthrough data for Br
(conservative tracer), Na+, Rb+ and Cs+ were fitted simultaneously to the solute transport model. Thus, for injection C1, all four breakthrough curves were fitted to the model using the same values for t0, Pe and pf . In addition, a value of A was estimated for each tracer, and a value of Ra was estimated for all of the sorbing tracers (Ra is assumed to be unity for Br
). This results in a total of 10 estimation parameters. Similarly, HTO data from injection C3 (conservative tracer) were used simultaneously with data for Na+ and Sr2 +, resulting in a total of eight estimation parameters. An alternative procedure to estimate the various parameters would be a sequential approach where the values for t0, Pe, pf and A are estimated using only the breakthrough curve for the nonsorbing tracer. Then, the estimated values for t0, Pe and pf would be used as fixed values when estimating A and Ra for each of the sorbing tracers. Although not presented here, the model fitting generally resulted in low standard errors and moderate correlation between parameters. Despite the seemingly large number of estimation parameters, the regression problem is not overdetermined because several breakthrough curves from each injection are used simultaneously. Under the assumption that transport takes place in a radially convergent flow field in a single parallel plate fracture, the fracture aperture, d, can be calculated directly from the residence time of a conservative tracer; according to: d¼

Qwith t0 pðr2
rw2 Þ

ð11Þ

where r is the travel distance (m); and rw is the borehole radius (m). For the travel distance, the Euclidean distances have been used consistently in this modelling. In the model, it is thus assumed that equilibrium surface retardation is occurring for the transport in the fracture and that the surface retardation is dependent on the fracture aperture according to: Ra ¼ 1 þ

2Ka d

ð12Þ

By inserting Eq. (11) into Eq. (12), Ka (m) can be calculated using t0 and Ra obtained as estimation parameters in the modelling according to: Ka ¼

ðRa
1ÞQwith t0 2pðr2
rw2 Þ

ð13Þ

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291

Furthermore, by inserting Eq. (11) in Eq. (6), a lumped parameter characterising matrix diffusion and matrix sorption can be identified according to:  De ðe þ Kd qÞ ¼

Qwith t0 Ra 2Apðr2
rw2 Þ

2 ð14Þ

The values of Ka and De(e + Kdq) can be directly compared with the laboratory data. 8.2. Interpretation of the modelling results The modelling results are presented in Figs. 6 and 7 together with the experimental data. These results indicate that a 1D advection – dispersion matrix diffusion model is consistent with the results from flow path I for both the conservative and the sorbing tracers. Contrary to the above finding, systematic deviations can be observed for the longer and much slower flow path III. This could be an indication that the internal structure of this flow path is more complex and may be influenced by a number of individual flow paths. In addition, by inserting the estimated mean travel time and the Euclidian distance in Eq. (11), a fracture aperture of 1.5 cm is obtained. This result seems somewhat unrealistic, and it is therefore considered indicative of the inappropriate application of the Euclidian distance in a single-flow path 1D transport model for flow path III. The values of the proportionality factor (pf) obtained from the modelling are presented in Table 4 together with the ratios of the measured flow rate in the injection section to the total withdrawal flow rate Qdil/Qwith. In an ideal radially converging flow field with 100% recovery, pf should be equal to Qdil/Qwith. The comparison shows that the estimated pf factors deviate only to a minor degree from the measured flow ratios.

Fig. 7. Example of results of preliminary modelling using a 1D advection – dispersion model with matrix diffusion, matrix sorption and fracture surface sorption. Breakthrough curves are given for the C3 injection; see text for details. C/Atot refers to the concentration of the tracer (Bq/kg) divided by the total injected activity (Bq); all activities are decay-corrected to the time of injection.

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Table 4 Estimated parameters obtained from the modelling, i.e., t0, Pe and pf (unique values for each flow path), combined with Ra and A (different value for each tracer, Ra, fixed to unity for the conservative tracers) Injection/flow path

Tracer

C1/I t0: 14.6 F 0.1 h ! d: (2.77 F 0.02)  10
3 m Pe: 8.5 F 0.1 pf: 1.6  10
2 (experimental: Qdil /Qwith = 1.3  10
2) C3/III t0: 430 F 50 h ! d: (1.5 F 0.2)  10
2 m Pe: 4.1 F 0.1 pf: 1.1  10
3 (experimental: Qdil /Qwith = 0.92  10
3)

82




Br Na+ 86 Rb+ 134 Cs+ HTO 22 Na+ 85 2 + Sr 24

Ra (
)

A (s1/2)

1 (fixed) 1.08 F 0.02 3.8 F 0.8 6.4 F 1.4 1 (fixed) 1.76 F 0.04 3.5 F 0.2

740 F 30 350 F 20 140 F 40 64 F 15 2400 F 900 2300 F 700 2100 F 500

The equivalent fracture apertures (d) were calculated from Eq. (11) under the assumption that 82Br
and HTO act as ideal conservative tracers. The 1r standard deviation errors of the parameter result from the regression procedure.

However, a preliminary attempt to exclude the pf as an estimation parameter and instead use the measured flow ratio resulted in worse fits and, sometimes, also difficulties in obtaining convergence in the regression calculations. Our interpretation in relation to this finding is that a slight adjustment of pf from the measured Qdil /Qwith is necessary in order to compensate for the uncertainty in the measured injection concentrations and the measured dilution rates. An additional interesting fact is that only one value for pf is needed for the interpretation of all tracers used in each individual injection. Contrary to some interpretations of the results of the TRUE-1 experiment (e.g., Heer and Jakob, 1998; Johansson et al., 2000), there was no need for a separate scaling factor to compensate for an irreversible loss of Cs+ within the time frame of the experiment. A possible explanation for this difference is that the contact time for the tracers within the borehole section was much longer in the TRUE-1 experiment compared to the TRUE Block scale experiment. Laboratory experiments studying sorption of Cs+ on crushed ¨ spo¨ diorite (Byega˚rd et al., 1998) indicated that a substantial part of the tracer was A adsorbed with a non- or slowly reversible mechanism. Similar sorption experiments ¨ spo¨ (Carbol et al., 2002) have indicated performed using fracture filling materials from A a reversible isotope exchange mechanism for Cs+. It is possible that sorption sites capable of non- or slowly reversible uptake of Cs+ have been saturated in materials that have been in a long-time contact with the groundwater (e.g., fracture filling material) but are available for sorption in materials which have been recently exposed to water contact (e.g., crushed rock material and borehole walls). However, further investigations are required to verify this hypothesis. The retention parameters, Ka and De(e + Kdq), calculated from the in situ modelling results (Eqs. (13) and (14)) are given in Table 5. Comparisons are made with the ¨ spo¨ diorite corresponding parameters obtained from laboratory experiments on the A material (surface sorption experiments and matrix diffusion experiments). The comparisons show that the mass transfer processes are much more pronounced in the in situ experiments than can be explained by applying the laboratory data for diffusion and ¨ spo¨ diorite rock. The only exception is the value of surface sorption obtained for intact A Ka for Cs+ where the laboratory and in situ results are quite similar.

Tracer

Surface retention parameter In situa Ka (m)

Br
Na+ 86 Rb+ 134 Cs+ HTO 22 Na+ 85 2 + Sr 82

24

– (1.1 F 0.1)  10
4 (4 F 1)  10
3 (7 F 2)  10
3 – (6 F 2)  10
3 (1.8 F 0.5)  10
3

¨ spo¨ diorite, A lab Ka (m)

– 7  10
7 5  10
4 8  10
3 – 7  10
7 8  10
5

Matrix diffusion retention parameter In situb

¨ spo¨ diorite, lab; e = 0.004; A De = F  Dw; F = 5  10
5; q = 2700 kg/m3

Fault gouge material ( < 125 Am), estimated e = 0.2; De = F  Dw; F = 0.71e 1.58 = 6  10
2; q = 2400 kg/m3

De(e + Kdq) (m2/s)

Kd (m3/kg)

De(e + Kdq) (m2/s)

Kd (m3/kg)

De(e + Kdq) (m2/s)

(3.5 F 0.2)  10
12 (1.8 F 0.2)  10
11 (1.4 F 0.6)  10
9 (1.9 F 0.8)  10
8 (9 F 5)  10
12 (3.2 F 1.5)  10
11 (1.6 F 0.6)  10
11

– 1.4  10
6 4  10
4 8  10
4 – 1.4  10
6 4.7  10
6

4  10
16 5  10
16 1  10
13 2  10
13 4  10
16 5  10
16 7  10
16

– 7  10
5 1  10
2 1  10
1 – 7  10
5 4  10
4

2  10
11 3  10
11 4  10
9 4  10
8 3  10
11 3  10
11 8  10
11

Values estimated based on the analysis of the mineralogy of the clay fraction of the fault gouge material (Andersson et al., 2002b) are also presented for comparison. a From Eq. (13). b From Eq. (14).

P. Andersson et al. / Journal of Contaminant Hydrology 70 (2004) 271–297

Table 5 ¨ spo¨ diorite Retention parameter values obtained from preliminary modelling compared to corresponding values determined in laboratory experiment using unaltered A (Byega˚rd et al., 1998)

293

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For the conservative tracers, Ra = 1 and Kdq = 0 which means that the parameter A will ¨ spo¨ diorite, De can be only depend on d, De and e. Using the laboratory determined e for A
10 2 82
calculated (Eq. (6)) to 9  10 m /s for Br in flow path I and 2.5  10
9 m2/s for HTO in flow path III. De can be expressed as: De ¼ Dw F

ð15Þ

where Dw is the diffusivity (m2/s) for the tracers in a pure electrolyte; and F is the formation factor defined as: F¼

et dD s2

ð16Þ

where et is the transport porosity (
); dD is the constrictivity (
); and s is the tortuosity (
). One can identify the largest possible F for the case when et = e and dD /s2 = 1 which ¨ spo¨ diorite; implies that F would be equal to the total porosity (determined to 0.004 for A Byega˚rd et al., 1998). Applying this concept to the calculated De above, the minimum Dw can be calculated to 2  10
7 m2/s for 82Br
and 6  10
7 m2/s for HTO by using Eq. (15) which is greater than two orders of magnitudes higher than the values tabulated by Mills and Lobo (1989). It is therefore obvious that the mass transfer observed in the in situ experiment cannot be explained solely by diffusion into the surrounding crystalline rock matrix. A possible explanation for the observed discrepancies is that the in situ transport actually takes place in fractures where the dominant rock material in contact with the groundwater is the fine-grained fault gouge material. In order to investigate the inference of large mass transfers obtained from the modelling results, an attempt was made to estimate the highest possible retention parameters that could be obtained from a fault gouge material. The following assumptions were used: 

Kd values were estimated using the mineralogical data for the clay fraction ( < 125 Am) of the fault breccia material sampled in the fractures, the in situ groundwater composition data and literature values of the cation exchange capacities, and selectivity coefficients for the different identified mineral fractions (cf. Andersson et al., 2002a)  A porosity (e) of 20% was assumed for the fine-grained fault gouge based on the inferences of Mazurek et al. (1996).  The effective diffusivities (De) was estimated from Archie’s law (Parkhomenko, 1967), i.e.; De ¼ Dw 0:71 e1:58

ð17Þ

where tabulated values for Dw were applied. The calculated sorption and matrix diffusion parameters for fault gouge material using this concept are presented in Table 5. As can be seen, these results are similar to the in situ values. This is an indication that the retention characteristics obtained within the time frames (days – months) of this type of in situ experiments may be dominated by small amounts of fracture filling materials. However, for the nonsorbing tracers

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295

(HTO and Br
), the De(e + Kdq) calculated for fault gouge is somewhat higher than what could be determined for the in situ experiment. A possible explanation for this is that these tracers have penetrated the thin layers of fault gouge material, and that these tracers are also influenced by diffusion in rock materials with lower porosity and lower diffusivity.

9. Conclusions 



 





Three scales of tracer investigation—laboratory experiments, the TRUE-1 singlefracture experiments and TRUE Block Scale experiments—produce a consistent sequential order of breakthrough times for sorbing tracers: Na+ < Ca2 + c Sr2 + < Rb+ c Ba2 + < Cs+. Hydrolysis influences the tracer retention in the in situ experiments as indicated by the results of the transition metal cation tracers (Mn2 +, Co2 + and Zn2 +), for which the relative retention properties could be correlated to their respective hydrolysis constant. The recovery-based retardation data indicates a slightly increased retardation in the block scale when compared to data from the detailed, single-fracture scale. A simple 1D advection – dispersion-matrix diffusion model approach has indicated that a significantly higher retention is occurring in the block scale experiment than ¨ spo¨ diorite. what would be expected based on laboratory data using nonaltered A Estimated data for retention in fault gouge is more consistent with the field results. The simple 1D advection –dispersion-matrix diffusion model works better for the interpretation of the relatively short flow path I compared to the longer flow path III. This could be an indication of influences of heterogeneity and multiple flow paths on larger scales. The different flow paths I, II and III tested by injections C1, C2 and C3, respectively, were initially interpreted to involve 1 – 3 different deterministic structures with flow path lengths varying from 16 to 97 m, where I < III < II. However, results from the tracer tests, cf. Table 2, indicated a different relative travel time distribution with IbII < III. These inconsistencies indicate remaining uncertainties in the exact geometry of pathways (background fractures, unidentified splays of structures) or pathway properties (thickness, effective aperture or porosity).

Acknowledgements The TRUE Block Scale Project is funded jointly by ANDRA (France), Nirex (UK), JNC (Japan), ENRESA (Spain), Posiva (Finland) and SKB (Sweden). We are indebted to the colleagues of the TRUE Block Scale Technical Committee, field and analysis teams for their contributions to this work. The constructive criticism and proposals extended by anonymous reviewers are also gratefully acknowledged.

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