Characterization of redox conditions in the excavation disturbed zone of a drift in the Kamaishi Mine, Japan

Engineering Geology 151 (2012) 100–111

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Characterization of redox conditions in the excavation disturbed zone of a drift in the Kamaishi Mine, Japan Hiroshi Sasamoto a,⁎, Katsuhiro Hama b, Toshihiro Seo c a Japan Atomic Energy Agency (JAEA), Tokai Research and Development Center, Nuclear Fuel Cycle Engineering Laboratory, 4-33 Muramatsu, Tokai-mura, Naka-gun, Ibaraki-ken 319-1194, Japan b Japan Atomic Energy Agency, Mizunami Underground Research Laboratory, 1-64 Yamanouchi, Akeyo-cho, Mizunami-shi, Gifu-ken 509-6132, Japan c Japan Atomic Energy Agency, Tokyo Office, 2-2-2 Saiwai-cho, Chiyoda-ku, Tokyo 100-8577, Japan

a r t i c l e

i n f o

Article history: Received 8 March 2012 Received in revised form 10 September 2012 Accepted 16 September 2012 Available online 4 October 2012 Keywords: Redox conditions EDZ Kamaishi mine Crystalline rock Oxygen consumption

a b s t r a c t The excavation of drifts during construction of a geologic repository for high-level nuclear waste (HLW) could affect mechanical and hydraulic properties of the rock within a zone extending a short distance into the rock from drift walls. Related impacts on groundwater chemistry within such an Excavation Disturbed Zone (EDZ) are largely unknown, but the oxygen in air circulating through drifts could conceivably diffuse into groundwater within the EDZ and cause these solutions to become strongly oxidizing. A study was therefore undertaken of redox conditions within the EDZ of crystalline host rocks in the abandoned Kamaishi Fe– Cu mine in Japan, which is believed to be generally representative of conditions that could exist in the EDZ of a HLW repository. The chemical compositions of groundwaters flowing into three boreholes that were drilled various distances into the Kurihashi granodiorite were monitored continuously. The results indicated that dissolved oxygen concentrations tend to decrease with increasing distance into the rock. Oxygen penetrated, at most, a few meters into the rock from a drift (E.L. 250 m drift) that intersected a specific fracture (Fracture No. 99), for example. A conceptual model was developed that takes into account both the reaction rate of dissolved oxygen with ferrous minerals and the diffusion rate of oxygen into the rock matrix. A quantitative evaluation of the model using a numerical solver indicated that the oxygen diffusion depth could vary from 3 to 30 cm from the unsaturated/saturated zone boundary, depending on the reactive surface areas of ferrous minerals. These estimates may overestimate the migration distance of a redox front, however, because ferrous minerals other than biotite were not considered in the model and the partial pressure of O2(g) was fixed at the atmospheric value of 0.21 bar throughout the unsaturated zone. More accurate assessments of redox-front behavior may be possible given better constraints on mineralogy and associated kinetic parameters (i.e., reactive surface areas and rate constants), and on microbial effects on oxygen consumption. Additionally, the use of more realistic boundary conditions on dissolved oxygen concentrations in the unsaturated zone would help reduce uncertainties in model results. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Between 1988 and 1998, the Japan Nuclear Cycle Development Institute (JNC, the predecessor of JAEA) conducted a geoscientific research program at the abandoned Kamaishi Fe–Cu mine in northeastern Honshu, Japan. The objective of the program was to develop technologies that can be used to characterize the geological, hydrogeological, geomechanical and geochemical properties of crystalline (or sedimentary) bedrock (e.g., JNC, 1999; Yoshida et al., 2000). The developed technologies could also be used to characterize similar characteristics at candidate sites chosen for the permanent disposal of high-level nuclear wastes (HLWs) in a deep geologic repository, although such sites have not yet been selected in Japan. ⁎ Corresponding author. Tel.: +81 29 282 1133x67550; fax: +81 29 282 9258. E-mail address: [email protected] (H. Sasamoto). 0013-7952/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.enggeo.2012.09.012

The Kamaishi study included an investigation of conditions within the so-called Excavation Disturbed Zone (EDZ). This is a narrow zone that extends into the rock a short distance from drift walls. Certain properties of the rock within the EDZ may be significantly altered by disturbances caused by drift excavation (Sato et al., 1998). The affected properties include changes in the local stress field, as well as changes in porewater pressure (Figure 1). These impacts can induce fracturing, and associated changes in fracture aperture, and may also cause the rock to become unsaturated, leading to localized changes in hydraulic behavior. The effects of the EDZ on groundwater chemistry are not well understood. Air circulating in drifts during the construction and operation period of a HLW repository, for example, will diffuse some distance through this zone into the surrounding rock. Should the air come into contact with groundwater, these solutions, which normally are chemically reducing, may become oxidizing as O2(g) from the air

H. Sasamoto et al. / Engineering Geology 151 (2012) 100–111

Stress redistribution zone: (2D* to 3D) *D represents diameter of drift

Oxygen diffusion

Unsaturated zone: (~several meters; porewater pressure and groundwater chemistry would change)

Fracture fillings

Intact rock matrix

101

Fracture opening

Excavation damaged zone:

(~1 meter; micro cracks occur)

Micro cracks

EDZ (Excavation Disturbed Zone) Stress redistribution zone

Water inflow

Unsaturated zone

Fracture

Drift

Fracture

Excavation damaged zone

Fig. 1. Schematic diagram illustrating features of the EDZ. This zone is assumed to result from the superimposition of an excavation damaged zone, an unsaturated zone and a stress-redistribution zone. The excavation damaged zone, which is characterized by a complex system of microcracks, extends about 1 m into the rock from the drift wall. The unsaturated zone, characterized by significant changes in porewater pressure and by changes in groundwater chemistry due to the intrusion of air from the drift, extends several meters into the rock from the drift wall. The stress redistribution zone, characterized by changes in rock mechanical properties, extends to be 2D to 3D (where D equals diameter of drift) from the drift wall.

dissolves in the groundwater. This could have an adverse impact on the safety of the repository because the expected life time of the over-pack (i.e., a carbon-steel canister isolating the waste from contact with groundwater) would decrease under oxidizing conditions, and because redox-sensitive elements (e.g., U, Np) would be more mobile under oxidizing conditions. For these reasons, the REX project (Redox EXperiment in Detailed Scale), as well as an earlier project (Redox Project in Block Scale), were carried out within the Äspö Hard Rock Laboratory in Sweden (Banwart et al., 1994; Puigdomenech et al., 2001). The main objectives of these projects were to develop an understanding of biogeochemical mechanisms controlling the reduction of O2(g) dissolved in groundwater and to characterize the redox buffer capacity of a complex rock–groundwater system. The present paper presents the results of a companion study to these projects that was undertaken to characterize the localized oxidation of solutions within the EDZ using drifts in the Kamaishi mine as a test case. A conceptual model of redox-front migration into rocks surrounding a drift, and the results of a technique used to estimate O2(g) intrusion into the rock matrix as a result of excavation, are also presented.

2. Geological setting The Kamaishi mine is located approximately 600 km north of Tokyo (Figure 2). The geology of the study area consists of Paleozoic and Cretaceous sedimentary rocks as well as the Ganidake granodiorite and Kurihashi granodiorite (ca. 120 Ma, Kawano and Ueda, 1969). The in-situ tests were conducted mainly in the northern-most part of the E.L. (Elevation Level) 550 m drift and E.L. 250 m drift, both of which lie within the Kurihashi granodiorite. These locations were selected because they were the least disturbed by mining. Fractures were mapped over a portion of the E.L. 250 m drift. Fracture properties, including orientation, mineralogy of fracture-fillings and widths of alteration haloes, were characterized (Sasamoto et al., 1993). The primary, rock-forming minerals of the Kurihashi granodiorite were identified by X-ray diffraction analysis (Osawa et al., 1995). The rock matrix consisted of quartz, plagioclase, biotite>k-feldspar, hornblende, chlorite>sericite, sphene, and magnetite. Minerals in alteration haloes (e.g., involving the complete alteration of biotite to chlorite) differed from those in fractures. The minerals in fracture fillings also differed from those in the rock matrix, and included calcite, stilbite>quartz,

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Fig. 2. Location and geology of the Kamaishi in-situ test site.

chlorite, laumontite > plagioclase, epidote > hornblende, sericite, and prehnite.

evolution of groundwater in the Kurihashi granodiorite were described by Sasamoto et al. (1999) and are briefly summarized below.

3. Hydrochemical characteristic of groundwaters

3.1. Groundwater origin and age

Groundwater samples were collected at points of seepage from drift walls and from boreholes (Figure 3). The latter included two deep boreholes; KH-1 (drilled to a depth of ca. 500 m from the floor of the E.L. 550 m drift) and KG-1 (drilled to a depth of ca. 800 m from the ground surface). In order to evaluate the evolution of groundwater chemistry, groundwater samples were subjected to a comprehensive set of chemical analyses and isotopic analyses (hydrogen, oxygen and dissolved inorganic carbon). Results concerning the origin, age and chemical

The hydrogen and oxygen stable isotope data suggest that the groundwater is of meteoric origin. The residence time of the groundwater in the granodiorite is generally less than 40 years, but deeper groundwaters are apparently considerably older because tritium concentrations in these solutions are below the detection limit. Radiocarbon dating of a groundwater sample from the KH-1 borehole indicates a residence time between 1450 to 3030 years BP (before present).

Redox Experiment

Fig. 3. Groundwater sampling locations in the Kurihashi granodiorite. The samples were collected as seepage along existing drifts (E.L.550 m and E.L.250 m) and from boreholes KH-1 and KG-1.

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3.2. Variations in groundwater chemistry with increasing depth Vertical variations in groundwater chemistry within the Kurihashi granodiorite are shown in Fig. 4. The results suggest that: • pH varies from weakly acid–neutral to weakly alkaline with increasing depth; • Na + tends to increase with increasing depth; and • Ca 2+ and carbonate (i.e., as HCO3− + CO32−) in groundwaters from the E.L. 550 m drift are greater than in groundwaters from the E.L. 250 m drift. As shown in Fig. 4, the data for samples collected in the drifts show considerable scatter compared to the data obtained for samples from boreholes. This scatter may result from the relatively large number of sampling points in the drifts over a total sampling length of 500 m.

103

A piper plot for groundwaters in the Kurihashi granodiorite is shown in Fig. 5. As can be seen, these solutions change with increasing depth from Ca-HCO3 type waters (E.L. 550 m) to Na-HCO3 type waters (E.L. 250 m). The chemistry of deeper, and hence older, groundwaters in the Kurihashi granodiorite has been interpreted by assuming local equilibrium for selected mineral–water reactions (Sasamoto et al., 1999).

4. Experimental methods As shown in Fig. 3, there are two main drifts (i.e., E.L. 550 m and 250 m drifts) used for sampling of groundwaters. The redox experiment was carried out in the E.L. 250 m drift within the Kurihashi granodiorite (about 700 m below the ground surface; see Figure 3). The drift was excavated in 1973. At the time of the experiment, a portion of the rock surrounding the drift had been continuously exposed

Fig. 4. Trends with depth in selected chemical parameters for groundwater samples from the Kurihashi granodiorite.

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1 mg/L). The measurements were carried out over a period of seven months. Groundwater samples in each of the packed-off sections were also collected periodically and analyzed for total Na +, Ca 2+, Mg 2+, K +, Al 3+, Total-Fe, Fe 2 +, dissolved Si, inorganic C, Cl −, SO42− and F −. Sodium and K + were determined by AAS (Atomic Absorption Spectroscopy), Mg2+, Ca2+, Al3+, Total-Fe and Si were determined by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopy), Fe 2+ was determined by adsorption photometry, inorganic C was determined by combustion oxidation infrared spectrometry, Cl−, SO42− and F− were determined by ion chromatography. Additional samples of groundwater were collected from the east side of the drift wall where it intersects Fracture No. 99, and were analyzed immediately (in contact with air) for pH, Eh, EC and DO using a portable meter. 4.2. TK-24 borehole

Fig. 5. Piper plot of surface waters and groundwaters in the Kurihashi granodiorite.

to air for about 25 years. The experimental approach entailed continuous monitoring and periodic sampling of groundwaters flowing into three boreholes (KRE-1, TK-24 and KM-2) that were drilled various distances into the Kurihashi granodiorite (Figure 6). Details concerning each of these boreholes are described below. 4.1. KRE-1 This borehole was used to investigate the chemistry of groundwaters near the drift wall. Previous investigations indicated that 400 fractures with trace lengths greater than 3 m intersect the E.L. 250 m drift, and that more than 100 of these are water bearing (Sasamoto et al., 1993). KRE-1 (66 mm in diameter) was drilled 2 m into the Kurihashi granodiorite along with a target water-bearing fracture (Fracture No. 99). Because there was no direct information concerning the extent of unsaturated conditions around the drift prior to drilling, the target drilling length was selected using direct measurements of water potential at the Grimsel test site in Switzerland as a first approximation (Baer et al., 1993). The Grimsel results suggested that the unsaturated zone could extend 1.6 m into the granitic rocks from the drift wall. A multiple-packer system with 4 sections (each 20 cm long) was installed in the borehole. Each section had access to a flow-through cell type monitoring system (Figure 7). These systems enable continuous measurements to be made, in isolation from the atmosphere, of temperature, pH, Eh (using both Pt and Au electrodes), electrical conductivity (EC) and dissolved oxygen (DO). The respective sensors were obtained from DKK (Denki Kagaku Kogyo) Inc. They had the following resolutions; temperature (platinum resistance thermometer type, accurate to within ± 0.5 °C), pH (glass sensor type, accurate to within ± 0.1 pH unit), Eh (platinum and gold wire type, accurate to within ± 10 mV), EC (3-cell conductivity and AC drive type, accurate to within ±10 μS/cm over a range of 0 to 500 μS/cm at 25 °C), DO (polarographic type, accurate to within ± 0.03 mg/L between 0 and

This borehole was drilled from the east side of the E.L. 250 m drift. It was about 300 m long and inclined 30° downward from the horizontal. Cores obtained during drilling indicated that the borehole was entirely within the Kurihashi granodiorite, and suggested that water entered the borehole near its lower end. A core sample from the lower end of the borehole revealed several small mineralized veins containing pyrite, chalcopyrite and pyrrhotite. Flow rates greater than 1000 mL/min were measured in this borehole. A single packer, which was set at the borehole top, and a similar monitoring system as used in the KRE-1 borehole were installed near the drift wall (Figure 8). Measurements of pH, Eh, EC and DO were monitored continuously for about one year. Additional groundwater samples were obtained periodically and analyzed for the constituents noted in Section 4.1. The length of this borehole, and the likelihood that water enters the borehole from near its lower end, suggest that these solutions were probably unaffected by air entering the rock from the drift. 4.3. KM-2 borehole This borehole (76 mm in diameter) was of intermediate length (about 22 m) compared to KRE-1 and TK-24. Borehole video observations indicated that groundwater entered the borehole approximately 20 m from the drift wall. The borehole was entirely within the Kurihashi granodiorite. Although some disking of core samples occurred near the drift wall, most of the other core samples recovered were generally intact. A double-packer system and similar monitoring system as used in KRE-1 and TK-24 were installed near the drift wall (Figure 9). Measurements of pH, Eh, EC and DO were monitored continuously for about two months. Additional groundwater samples were obtained periodically and analyzed for the constituents noted in Section 4.1. 5. Results and discussions 5.1. Redox parameters Groundwater flow into the four packed-off sections of KRE-1 was limited because this borehole encountered only the unsaturated region of the rock. Continuous monitoring of pH, Eh, EC and DO was possible only in the first section (1.8 m into the rock from the drift wall), where flow rates exceeded 100 mL/min. These results are presented in Fig. 10. Temperature, EC and DO were essentially constant throughout the monitoring period at 15 °C, 90 μS/cm and 0.3 mg/L, respectively. The measured values of pH and Eh varied with time, however. The pH was initially around 9.0 to 9.5 and then decreased gradually to about pH 7. These variations may indicate that an excess of KCl solution (pH 7) was released from the glass junction of the pH sensor. A representative range of pH values between about 9.0 and 9.5 was therefore considered to be more realistic of undisturbed in-situ conditions.

H. Sasamoto et al. / Engineering Geology 151 (2012) 100–111

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Fig. 6. Location of redox experiments in the E.L.250 m drift.

Observed variations in Eh (Table 1) determined using both the Pt and Au electrodes may be due to trace-level variations in dissolved oxygen concentrations [Eh measurements are known to be extremely sensitive to this parameter (e.g., Grenthe et al., 1992)]. Groundwaters at the water-inflow point on the drift wall where the KRE-1 borehole was drilled, along with Fracture No. 99, were periodically sampled and analyzed. Results obtained using portable sensors are shown in Fig. 11. The error bars in this figure were determined by reproducing the respective measurements 5 different times. For physico-chemical parameters the errors were determined to be as follows: temperature ± 0.1 °C; EC ± 0.6 μS/cm; pH ± 0.1; and Eh ± 10 mV. An average value for DO was used due to difficulties in obtaining reproducible measurements of this parameter. As shown

in Fig. 11, the temperature, EC and pH were essentially constant at 15 °C, 80 μS/cm and pH 9.5, respectively. The scatter in Eh and DO values were attributed to local variations in oxygen concentrations in the air. The averaged values are summarized in Table 1. The DO value is nearly equal to the equilibrium value for water equilibrated with air at 15 °C (9.8 mg/L; Drever, 1988). In contrast to the KRE-1 borehole and groundwater sampled at the drift wall, stable Eh values were measured in TK-24 after 1.5 months of continuous monitoring (Figure 12). The temperature, EC and DO were essentially fixed throughout the monitoring period at 15 °C, 90 μS/cm and ≈0.1 mg/L. Steady decreases in pH with time were attributed to a measurement artifact, as discussed above, and a range between 9.0 and 9.8 was therefore considered to be representative.

Fig. 7. Schematic diagram of the redox experiment involving borehole KRE-1.

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Fig. 8. Schematic diagram of the redox experiment involving boreholeTK-24.

Eh values determined using the Pt electrode decreased with time and reached a stable value that was consistent with Eh values determined using the Au electrode (about −250 mV). Representative Eh results, obtained after initial drifts in Eh values stabilized, are shown in Table. 1. Low DO values (≈ 0.1 mg/L) are consistent with the Eh data, and both parameters indicate that groundwaters in the Kurihashi granodiorite are reducing when undisturbed by excavation effects. Because groundwater inflow into KM-2 was quite limited (b 100 ml/min), Eh values could only be determined periodically in discrete groundwater samples, in contact with the atmosphere, using a portable meter. Representative results for one sample (September 10, 1997) are shown in Table 1. The Eh value suggests that these groundwaters are weakly reducing, and this is supported by the low DO value measured in this sample. Overall, the DO and Eh data summarized in Table 1 clearly indicate that groundwaters near the drift wall are relatively more oxidizing than those deeper in the rock away from the drift. The data also suggest that oxygen penetrates, at most, a few meters into the host rock from the drift wall. The chemistry of groundwaters sampled from the KRE-1, TK-24 and KM-2 boreholes are shown in Table 2. As can be seen, all of the analyses have an acceptable charge-balance error within ±0.2 meq/L, which is generally regarded as being indicative of a reliable analysis (Friedman and Erdmann, 1982) Large differences in analyzed compositions do not exist. Total anion concentrations in all samples, for example, are less than 3.0 meq/L. Eh values are quite oxidizing compared to those shown Table 1, however. This observation indicates that great care must be taken during sampling and analysis (i.e., using a continuous flow-through cell type monitoring system) in order to isolate the

sample from contact with the atmosphere and thus to obtain reliable Eh measurements. 5.2. Potential redox-controlling reactions Although the concentration of Fe2+ in Kamaishi groundwaters is generally extremely low (b 0.1 mg/L; Sasamoto et al., 1999), the Kurihashi granodiorite contains relatively small amounts of ferrous aluminosilicates and oxides, including biotite, hornblende, chlorite and magnetite (Osawa et al., 1995). According to Ishihara and Suzuki (1974), for example, the content of biotite in the Kurihashi granodiorite is about 10% by volume. Dissolution of these minerals and aqueous oxidation of Fe2+ could consume any oxygen diffusing into the rock matrix. Based on experimental results reported by Banwart (1995); Malmström et al. (1995) found that the dissolution rate of biotite, normalized to the surface area of the reacted mineral [determined by N2(g) adsorption], is between 1.5 × 10−9 to 2.4× 10−9 mol/m2/h in neutral to slightly alkaline solutions (7b pHb 8.2). A rate constant of 1.5 × 10−9 mol/m 2/h for biotite dissolution was adopted in our conceptual model to estimate O2(g) intrusion into the EDZ (see Section 5.3). The mechanisms and kinetics of reactions involving biotite and dissolved oxygen were investigated in experiments carried out by White and Yee (1985). Generally, such reactions were shown to be controlled surface electron-cation transfer reactions of the form; h

2þ

Fe ; 1=zM

Zþ

i silicate

h i þ 3þ þ H þ 1=4O2 → Fe

Fig. 9. Schematic diagram of the redox experiment involving borehole KM-2.

silicate

þ 1=zM

Zþ

þ 1=2H2 O ð1Þ

H. Sasamoto et al. / Engineering Geology 151 (2012) 100–111

Fig. 10. Summary of physico-chemical parameters measured in groundwater samples from borehole KRE-1.

where M is a cation of charge + z and the bracketed silicate refers to an Fe-bearing alumino-silicate such as biotite. In addition to the surface oxidation reaction, concurrent silicate hydrolysis is also expected to occur during silicate–water interactions, for example; h

2þ

2Fe ; 2=zM þ2Fe

2þ

Zþ

i silicate

þ 2=zM

Zþ

þ Fe

3þ

h i þ 3þ þ þ 3 H → Fe ; 3 H

ð2Þ

silicate

:

Fig. 11. Summary of physico-chemical parameters measured in groundwaters sampled at the water-inflow point in Fracture No. 99. Dates have the general format “year/ month/day”; for example, “931019” refers to October 19, 1993.

oxidized to Fe 3+ in the silicate and half is transferred to aqueous solution. Ferrous iron and other cations are then released to solution by protonation of the silicate surface. The released Fe 2+ could be oxidized to ferric hydroxide. A kinetic rate law for oxidation of Fe 2+ according to the reaction: 2þ

Fe Eq. (2) assumes equal reaction rates for surface oxidation and hydrolysis in which half of the initial ferrous iron in the silicate is

107

þ

þ 1=4O2 ðaqÞ þ 5=2H2 O→FeðOHÞ3 ðsÞ þ 2 H :

[Fe(OH)3(s) denotes ferric oxyhydroxide] is given by (Stumm and Morgan, 1996): h i h i−2 h i 2þ þ 2þ Fe ; −d Fe =dt ¼ k½O2 ðaqÞ H

Table 1 Measured values of dissolved oxygen and redox potential at various locations in the rock from the drift wall (“bhl.” refers to borehole). Drift wall

Measurement point Dissolved oxygen (mg/L) Redox potential (Eh, mV) a

1.8 m from drift wall

Deep in 20 m rock from drift wall mass

ð3Þ

¼ 5  10

−14

  ∘ k 20 C

ðmol=L⋅sÞ

ð4Þ

where the brackets refer to concentration (mol/L), t stands for time (y), and k denotes the rate constant. In the present study, the above rate law for the oxidation rate of Fe 2+ and the value of k at 20 °C were used in our conceptual model.

Fracture No. 99 KP.E-1 bhl. KM-2 bhl. TK.-24 bhl. 65–9.5

Pt electrode 218–390 Au electrode –

0.3a

b0.1

b0.1a

106–363a 11–313a

−28 –

−244a −245a

Measured in a flow-through cell type monitoring system.

5.3. Conceptual model to estimate O2(g) intrusion into the EDZ The EDZ around drifts is relatively small when considered in terms of effects on mechanical properties [e.g., about 0.5 m into the Kurihashi granodiorite from the drift wall (Sato et al., 1998)], but the extent and nature of this zone with regard to chemical properties are largely unknown. Moreover, conceptual models of geochemical

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iron released by the dissolution of biotite was assumed to diffuse toward the unsaturated zone from regions of the rock that are reducing. Oxygen diffusing into the rock from the drift wall then reacts with Fe 2+ in the groundwater. The following equations, which fully couple the diffusion rate of O2(g) with the dissolution rate of biotite and the oxidation rate of Fe 2+, were solved using SPADE:

εR

h i ∂ Fe2þ ∂t

¼ εR DFe

h i ∂2 Fe2þ ∂x2

h −εR kR

i Fe2þ ½O2  ½H þ 2

h i 2þ 2 Fe ½O2  ∂½O2  ∂ ½O2  1 εR − ε R kR : ¼ ε R DO 4 ∂t ∂x2 ½H þ 2

Fig. 12. Summary of physico-chemical parameters measured in groundwater samples from borehole TK-24.

processes that occur within the EDZ are unavailable. We therefore developed a conceptual model to estimate the amount of oxygen introduced into the rock matrix by excavation, and to evaluate associated effects on redox conditions. The model considers two rate-dependent processes (Figure 13): 1) the diffusion rate of O2(g) from the drift into unsaturated regions of the host rock, and 2) the reaction rate of dissolved oxygen with Fe 2+. The latter rate incorporates both the dissolution rate of ferrous minerals and the aqueous oxidation rate of Fe2+. The reaction, O2(g) = O2(aq), is assumed to take place instantaneously. This conceptual model was incorporated into a modeling code, SPADE, which is a general differential equation solver (Williams and Woods, 1994). The code couples the reaction rate of dissolved oxygen with ferrous minerals to the diffusion rate of oxygen into the rock matrix (Chiba et al., 1999). 5.4. Model results Initial conditions considered in the model are shown in Table 3. As shown in Fig. 13, a boundary was assumed to exist in the EDZ separating unsaturated and saturated regions of the rock. The exact location of this boundary could not be determined by direct measurement of water potential, and it was therefore estimated to be located about 1.8 m from the drift wall based on the results of in-situ measurements in the KRE-1 borehole. Oxygen was assumed to diffuse into the saturated zone from the unsaturated zone and the drift. Ferrous

h i3 Fe2þ þ ks Si 41−  2þ  5 ð5Þ Fe  2

ð6Þ

In these equations, εR stands for the porosity of the rock's matrix, t (y) denotes the time elapsed since excavation, x (m) refers to distance into the rock from the drift wall, [Fe2+] represents the Fe2+ concentration in groundwater (mol/m3), [O2(aq)] stands for dissolved oxygen concentration (mol/m3), [H+] refers to the aqueous concentration of the hydrogen ion (mol/m3), [Fe2+]* denotes the saturation concentration of Fe2+ [the measured maximum Fe2+ concentration in the groundwater was used to represent the saturation concentration] (mol/m3), kR stands for the rate constant for Fe2+ oxidation (mol/m3/y), kS represents the rate constant for biotite dissolution (mol/m2/y), Si stands for the surface area of biotite in the rock matrix (m2/m3), DO denotes the diffusion coefficient of dissolved oxygen (m2/y), and DFe refers to the diffusion coefficient of Fe2+ (m2/y). Note that the oxygen diffusion rate was only considered in the saturated zone because oxygen saturation in the unsaturated zone was assumed as an initial boundary condition. This assumption may be reasonable as a first approximation, but in reality a concentration gradient of dissolved oxygen could exist in the unsaturated zone. Van Geet et al. (2006) noted that the extent of oxidation around galleries in the Boom Clay was about 1 m from the gallery wall. In this case, fractures up to 1 m in length were created during excavation of the gallery, and this resulted in rapid oxidation of the rock over that length. Such oxidation resulted in an increase in sulfate (and thiosulphate) concentrations, and this was attributed to the effects of pyrite oxidation. Significant increases in these concentrations were observed between 0.7 and 1.2 m from the gallery. Although these observations pertain to a clay formation, it seems reasonable to expect that similar oxidation effects could occur in crystalline rocks as a result fracturing in the EDZ during drift excavation. The reactive surface area of biotite in the Kurihashi granodiorite is unknown. According to Rimstidt and Barnes (1980), however, the reactive surface area of minerals in rocks typically ranges from 10 to 10 4 m 2/kg, which corresponds to 10 4 to 10 7 m 2/m 3. Noting that an increase in the reactive surface area of biotite would increase the amount of oxygen consumed, we assumed as a first approximation that Si for biotite equals 100, 1000, and 10,000 m 2/m 3 in separate calculations in order to bound the possible transient effects of oxidation of the host rock by O2(g) on the long-term stability of the overpack in a HLW repository (see also Table 3). Fig. 14 shows the calculated profiles for dissolved oxygen and Fe 2+ concentrations in groundwater as a function of distance from the drift wall. The boundary between the unsaturated and saturated zones was assumed to be located 1.8 m from the drift wall. Oxygen that diffuses into the saturated zone reacts with Fe 2+ and a redox front (separating oxidizing conditions from reducing conditions) forms after steady-state conditions are achieved. The results indicate that such redox fronts develop 2.1 m (Si = 100 m 2/m 3) to 1.83 m (Si = 10,000 m 2/m 3) from the drift wall. The oxygen diffusion depth is therefore 3 cm (Si = 10,000 m 2/m 3) to 30 cm (Si = 100 m 2/m 3) into the rock from the unsaturated/saturated boundary.

Table 2 Chemical compositions of groundwaters collected in the redox experiments. No.a

Dateb

Tc (°C)

pHc

Eh (Pt)c (mV vs. SHE)

ECc (μS/cm)

Na+ (mg/L)

K+ (mg/L)

Ca2+ (mg/L)

Mg2+ (mg/L)

Al3+ (mg/L)

∑Fe (mg/L)

Fe2+ (mg/L)

TCd (mg/L)

TICd (mg/L)

HCO3− (mg/L)

CO32− (mg/L)

SO42− (mg/L)

CI− (mg/L)

F− (mg/L)

SiO2(aq) (mg/L)

c.b.d (meq/L)

KRE-1

Fracture No. 99

TK-24

W1

KM-2

W5

930901 931018 931209 931215 940105 940126 940203 940208 940309 940413 930901 931018 931209 931215 940105 940126 940208 940309 940413 941013 941116 941215 950111 950214 950315 930901 931018 931209 931215 940105 940126 940208 940309 940413

13.9 14.9 15.6 15.6 15.3 – – 15.4 15.3 15.1 14.1 14.7 15.5 15.3 14.7 – 15.4 15.0 15.0 14.9 15.0 15.4 15.2 15.4 15.4 14.1 15.0 15.6 15.2 14.7 – 15.7 15.3 15.6

9.6 9.6 9.5 9.5 9.5 – – 9.5 9.4 9.6 9.5 9.9 9.6 9.5 9.4 – 9.6 9.5 9.6 9.6 9.6 9.6 9.6 9.5 9.6 8.6 9.7 8.7 8.4 8.7 – 8.7 8.0 9.1

205 331 338 309 292 – – 350 291 328 11 233 246 223 131 – 100 3 23 −100 −42 40 64 107 180 332 338 400 390 395 – 396 372 329

77.1 75.3 74.6 76.1 77.2 – – 76.1 76.2 77.1 81.6 78.0 77.0 78.4 78.6 – 78.1 78.2 78.1 79.1 79.3 77.9 70.4 75.6 78.2 76.2 76.1 73.2 77.6 79.9 – 77.6 77.4 77.4

9.6 5.7 6.0 8.9 9.0 9.7 8.9 8.6 8.8 9.3 6.7 4.9 5.6 5.7 5.8 6.2 5.3 5.8 5.8 5.2 5.1 5.3 4.9 5.5 5.3 9.9 7.9 8.8 8.6 8.9 9.5 8.6 8.8 9.2

0.2 0.2 0.3 0.2 0.2 0.2 0.3 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.3 0.2 0.2 0.3 0.2 0.2 0.2

5.6 5.8 5.2 6.0 6.0 5.8 5.8 6.1 5.9 5.6 8.1 8.5 7.6 8.4 8.7 8.3 8.7 8.5 8.3 7.2 8.3 8.2 5.9 8.4 8.2 6.2 6.3 6.1 6.6 6.5 6.7 6.8 6.4 6.2

b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.02 b0.02 b0.02 b0.02 b0.02 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01

b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1

b0.02 b0.01 b0.02 b0.01 b0.01 b0.02 b0.02 b0.01 b0.01 b0.01 b0.02 b0.01 b0.02 b0.01 b0.01 b0.02 b0.01 b0.01 b0.01 b0.3 b0.3 b0.3 b0.3 b0.3 b0.3 b0.02 b0.01 b0.02 b0.01 b0.01 b0.02 b0.01 b0.01 b0.01

– b0.01 – b0.01 b0.01 – – b0.01 b0.01 b0.01 – b0.01 – b0.01 b0.01 – b0.01 b0.01 b0.01 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 – b0.01 – b0.01 b0.01 – b0.01 b0.01 b0.01

5.9 5.1 4.1 4.8 4.4 4.0 5.1 5.9 5.1 6.1 4.4 3.5 3.9 3.7 2.8 3.3 3.1 3.0 3.1 5.2 5.3 5.4 3.2 6.2 5.6 6.5 5.2 5.5 5.9 5.8 5.7 6.6 6.3 5.6

5.0 4.4 3.8 4.3 3.9 3.5 4.0 4.0 4.4 3.7 3.4 2.8 2.4 3.2 2.1 2.6 2.3 2.4 2.3 4.2 4.4 4.9 2.8 5.8 5.2 5.4 4.5 4.2 5.4 4.8 4.9 5.4 5.7 4.7

22.3 19.5 17.2 19.5 17.7 – – 18.1 20.4 16.4 15.4 11.4 10.6 14.5 9.8 – 10.2 10.9 10.2 18.7 19.3 21.5 12.3 26.2 23.1 26.7 19.3 20.9 27.0 23.9 – 26.9 28.5 22.8

3.2 2.8 2.0 2.3 2.1 – – 2.1 1.9 2.3 1.8 2.8 1.5 1.7 0.9 – 1.5 1.3 1.5 2.7 2.8 3.1 1.8 3.1 3.3 0.4 3.5 0.4 0.4 0.4 – 0.5 0.4 1.1

9.1 8.4 9.0 8.2 8.3 10.8 8.8 8.2 8.4 8.7 7.3 12.7 13.5 12.9 12.8 13.7 12.7 12.7 12.8 11.8 11.6 11.8 12.8 13.3 13.4 4.6 8.9 9.3 8.7 8.6 9.5 8.7 9.0 9.2

2.0 1.6 2.0 1.8 1.8 2.0 1.9 1.9 1.8 1.8 2.0 1.8 2.2 2.0 2.0 2.2 2.4 2.1 2.0 1.8 1.8 1.6 2.0 2.0 2.1 1.8 1.7 1.9 1.8 1.7 2.0 1.9 1.9 1.9

0.1 b0.05 0.1 b0.05 b0.05 0.1 0.1 0.1 0.1 0.1 0.1 b0.05 0.1 b0.05 b0.05 0.1 0.1 0.1 0.1 0.3 0.2 0.3 b0.1 0.3 b0.1 0.1 b0.05 0.1 b0.05 b0.05 0.1 0.1 0.1 0.1

12.3 10.9 12.2 13.0 13.3 12.3 11.7 13.5 12.8 12.2 16.1 14.5 15.6 16.3 16.9 15.2 16.9 16.7 15.4 16.5 16.5 14.8 15.5 16.0 15.1 12.7 11.1 11.6 12.4 12.2 11.4 12.6 12.4 11.8

0.0 −0.1 −0.1 0.1 0.1 – – 0.1 0.1 0.1 0.2 0.0 0.1 0.1 0.2 – 0.1 0.1 0.1 −0.1 −0.1 −0.1 −0.1 −0.2 −0.2 0.1 0.0 0.1 0.0 0.1 – 0.0 0.0 0.1

a b c d

H. Sasamoto et al. / Engineering Geology 151 (2012) 100–111

Bhl.

No. refers to the sampling point at the drift wall in the E.L. 250 m drift. The general format is “year/month/day”; for example, “930901” indicates September 1, 1993. These parameters were measured using a portable meter. Note that the groundwater samples were contacted with air in the drift. TC — total dissolved C; TIC — total dissolved inorganic C; c.b. — charge balance.

109

110

H. Sasamoto et al. / Engineering Geology 151 (2012) 100–111

Fig. 13. Schematic diagram illustrating key features of a conceptual model of chemical changes in the EDZ. The unsaturated zone contains gaseous oxygen, which diffuses from the drift wall through newly developed microfractures. The oxygen dissolves in water of the saturated zone, and then reacts with Fe2+ released by the dissolution of ferrous alumino-silicates such as biotite. The embedded figure shows a schematic profile of dissolved oxygen in the rock matrix and fracture. Note that within the fracture, advective flow of water toward the drift suppresses oxygen diffusion, and that the flux of oxygen is therefore greater in the rock matrix. A static condition for groundwater flow was assumed.

These results may overestimate the oxygen penetration depth because they are based on the assumption that the partial pressure of O2(g) is fixed at the atmospheric value of 0.21 bar throughout the unsaturated zone. The model also does not take into account the presence of other reductant minerals, such as biotite, or the presence of microorganisms that could also buffer against the development of oxidizing conditions [such as were observed in the REX project (Puigdomenech et al., 2001) and in Kamaishi groundwaters (Sasamoto et al., 1999)]. The results are probably conservative from a performance-assessment perspective, however, because the extent to which the rock is oxidized may be overestimated, and this may tend to decrease the life-time of the iron overpack in a HLW repository.

6. Conclusions The main results of the redox experiments carried out in the Kamaishi mine are as follows:

Table 3 Parameters used in the calculations. Parameter

Value

Porosity of rock mass, εR Fe2+ saturation concentrations in groundwater, [Fe2]* Rate constant of ferrous iron, kR Dissolution rate of biotite, kS Surface area of biotite in rock mass, Si Diffusion coefficient of ferrous iron in groundwater, D Fe Diffusion coefficient of oxygen in groundwater, DO Groundwater flow rate, Rf Elapsed time after excavation, t

0.01 1 × 10−3 (mol/m3)a 1.58 × 10−3 (mol/m3/y)b 1.50 × 10−9 (mol/m2/h)c 100, 1000, 10,000 (m2/m3) 9.46 × 10−5 (m2/y)d 9.46 × 10−5 (m2/y)d 0 (m/y) 25 (y)

a b c d

Sasamoto et al. (1999). Stumm and Morgan (1996). Banwart (1995). Sato et al. (1992).

• Groundwaters that are unaffected by air entering the Kurihashi granodiorite from drifts are reducing. • The chemical and isotopic compositions of groundwaters flowing into three boreholes that were drilled in various distances from the drift wall were monitored continuously. The results indicate that dissolved oxygen concentrations decrease with increasing distance into the rock. Oxygen penetrates, at most, a few meters into the rock from the drift wall. • A conceptual model of redox-front development and propagation within the EDZ of a crystalline-type host rock was developed. The model couples the reaction rate of dissolved oxygen with ferrous minerals to the diffusion rate of oxygen into the rock matrix. • Estimates of oxygen diffusion depth from the unsaturated/saturated boundary is between 3 and 30 cm, depending on the reactive surface area of biotite. The model developed in this study may overestimate the migration distance of a redox front into the rock, however. More accurate assessments of redox-front behavior may be achievable if data characterizing the mineralogy, associated kinetic parameters (i.e., reactive surface areas and rate constants), and microbial effects on oxygen consumption can be better constrained. Additionally, the use of more realistic boundary conditions on dissolved oxygen concentrations in the unsaturated zone would help reduce uncertainties in model results. Acknowledgments The authors would like to thank co-researchers [Mikazu Yui of JAEA, Tamotsu Chiba of JGC (Japan Gasoline Corporation)] of Kamaishi redox experiments for their helpful suggestions and contributions. Also, we are grateful to Dr. Randolph C. Arthur of INTERA Inc. for his assistance in editing the English version of the manuscript. We acknowledge the editor and anonymous reviewers for their constructive comments to improve the quality of manuscript. H.S. also expresses his thanks to Dr. Tsutomu Sato of Hokkaido University for his encouragement to prepare the manuscript.

H. Sasamoto et al. / Engineering Geology 151 (2012) 100–111

111

Fig. 14. Calculated concentration profiles for dissolved oxygen and Fe2+ as a function of distance from the drift wall. “Si” indicates the surface area of biotite.

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