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Cultivability of microorganisms introduced into a compacted bentonite clay buffer under high-level radioactive waste repository conditions

Cultivability of microorganisms introduced into a compacted bentonite clay buffer under high-level radioactive waste repository conditions

Engineering Geology 58 (2000) 149–161 www.elsevier.nl/locate/enggeo Cultivability of microorganisms introduced into a compacted bentonite clay buffer...

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Engineering Geology 58 (2000) 149–161 www.elsevier.nl/locate/enggeo

Cultivability of microorganisms introduced into a compacted bentonite clay buffer under high-level radioactive waste repository conditions Karsten Pedersen a, *, Mehrdad Motamedi a, Ola Karnland b, Torbjo¨rn Sande´n b a Go¨teborg University, Department of Cell and Molecular Biology, Microbiology Section; Box 462, S-405 30 Gothenburg, Sweden b Clay Technology AB, Ideon Research Centre, S-223 70 Lund, Sweden Received 17 June 1999; accepted for publication 18 May 2000

Abstract To study the physical properties of a bentonite buffer, long-term test (LOT ) of buffer material were performed in ¨ spo¨ Hard Rock Laboratory (HRL), Oskarshamn, Sweden. The LOT was granitic rock 450 m underground at the A set up under conditions similar to those in a high-level radioactive waste (HLW ) repository, except for the absence of radioactivity and difference in scale, and provided the opportunity of exposing strains of bacteria to conditions realistic to a repository buffer. The main focus was on sulphide-producing bacteria and their ability to survive. Bacteria were chosen for different relevant characteristics. Sulphate-reducing bacteria (SRB) included Desulfovibrio ¨ spo¨ HRL groundwater, the moderately halophilic bacterium Desulfovibrio salexigens aespoeensis isolated from deep A and the thermophilic, spore-forming Desulfotomaculum nigrificans. Aerobic bacteria included Deinococcus radiophilus, a bacterium that can tolerate high doses of radiation and severe desiccation, the chemoheterotrophic bacterium Pseudomonas aeruginosa that frequently occurs in soil, the chemo-organotrophic and chemolithotrophic (hydrogenutilizing) organism Ralstonia eutropha, the chemoheterotrophic, spore-forming bacterium Bacillus subtilis and the thermophilic spore-forming bacterium Bacillus stearothermophilus. Suspensions of the SRB (anaerobic) and aerobic bacteria were mixed with bentonite clay to give a solution of approximately 100 million bacteria per gram dry weight (gdw−1) clay. The clay with bacteria were subsequently formed into cylindrical plugs with 20 mm length and diameter, and installed in bentonite blocks exposed to low (20–30°C ) and high (50–70°C ) temperatures. The blocks were installed in the LOT boreholes immediately after incorporation of the bacteria plugs. The experiment was terminated after 15 months. The major outcome was elimination below the detection limits for all except the spore-forming bacteria. All of the three spore formers survived at the low temperature. The numbers remaining were, however, much lower than the ca. 100 million spore-forming bacteria gdw−1 clay initially introduced, so that only between one-100th and one-10 000th of the original number were left. The cell (spore) death rate could therefore be interpreted as being higher than the growth rate, which may have been zero, or close to zero. At the high temperature, the sporeforming SRB D. nigrificans and some of the introduced B. subtilis were the only surviving bacteria. They most probably survived as spores, which are metabolically inactive and do not produce sulphide. A slow but significant death rate of viable cells and spores would eventually lead to the complete eradication of life in the buffer. It has not yet been clarified, however, whether this would occur within the lifetime of a HLW repository. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Bacteria; Bentonite; Clay; Corrosion; Radioactive; Repository

* Corresponding author. E-mail address: [email protected] ( K. Pedersen) 0013-7952/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 1 3 -7 9 5 2 ( 0 0 ) 0 0 05 6 - 9

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1. Introduction Bentonite clay has been proposed as a buffer material in several concepts for high-level radioactive waste (HLW ) repositories. In the Swedish concept, the bentonite buffer would have to serve as a mechanical support for the copper canisters storing the waste, reduce the effects on the canister of possible rock displacement, and minimize water flow over the deposition holes (SKB AB, 1999a). Transport through the buffer is expected to be reduced principally to diffusion, with respect both to corrosive components in the groundwater and to escaping radionuclides in case of a canister failure. Oxygen and sulphide are potential corrosive elements that may threaten the integrity of the copper canisters. Oxygen is introduced during the open phase of a repository but, after closure, is expected to be rapidly reduced by microorganisms in the groundwater (Banwart et al., 1996; Kotelnikova and Pedersen, 1999). By contrast, sulphide is produced by a specific group of microorganisms, the sulphate-reducing bacteria (SRB), which are common in deep groundwater (Haveman et al., 1999). Sulphate-reducing bacteria are active under anaerobic conditions after closure of a repository (Pedersen, 1999). The worst possible case scenario in copper canister corrosion would be if SRB formed biofilms on the canisters or grew intensively in the buffer close to the canister. The sulphide corrosion process would be controlled by the transport of sulphate to the SRB in the vicinity of the canisters, if enough hydrogen or degradable organic carbon was available for their growth. This could lead to considerably accelerated corrosion, since the transport of sulphate in the buffer is expected to be much faster than the transport of sulphide, owing to the fact that the sulphate concentration in the bentonite can be up to tens of mmol dm−3. The water content, availability of nutrient and energy sources, high temperatures, and radiation are factors that influence the development of microbial populations in the buffer around nuclear waste storage canisters, an environment classified as extreme ( Kushner, 1978; West et al., 1985; Pedersen and Karlsson, 1995; Gascoyne, 1996). In

contrast to nutrient, energy, pressure and temperature constraints, few microorganisms can withstand desiccation. A full-scale Canadian field experiment showed that microorganisms in the studied sand/bentonite buffer mix (50/50%) did not survive at a water content below a certain value (Stroes-Gascoyne et al., 1997). In the Canadian experiment, SRB were not detected, possibly because the anaerobic and reduced conditions required by SRB did not develop during the time course of the experiment (approximately 2.5 years). Laboratory experiments with SRB were therefore subsequently started. The results showed that a low water content was lethal to the SRB studied (Motamedi et al., 1996). It may, however, be argued that more desiccation-resistant SRB than those studied could survive and be active in a bentonite buffer and also that the laboratory batch conditions used may be more restrictive for SRB survival than the field conditions. Long-term tests (LOT ) of buffer material for between 1 and 20 years are currently being per¨ spo¨ Hard Rock Laboratory, (HRL), formed at A Oskarshamn, Sweden ( Karnland and Sande´n, 1998). The LOT consists of pre-fabricated test parcels with bentonite blocks that are placed around the centre of a copper tube containing a heater to maintain defined high temperatures (90– 150°C ) at the surface of the central and lower parts of the tube. The main purpose of the tests is to study the stability of the physical properties of the bentonite material under realistic conditions (with differences in scale) and investigate the survival of introduced microorganisms in the clay, focusing on desiccation and high-temperature-tolerant species including SBR. Two of the six LOT experiments were short term and run for 15 months. In one of these, a series of different bacteria, including three species of SRB, were mixed with bentonite clay and introduced into the bentonite blocks to be used. The parcels were then placed into deposition holes in a granite rock structure with water-bearing fractures at a depth of 450 m. The numbers of cultivable microorganisms were analysed when the parcel was removed 15 months later.

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2. Material and methods

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bated for each species at each determination and at its optimum temperature.

2.1. Bacterial species Bacteria with varying relevant characteristics were chosen for the test and ordered from international culture collections as follows: aerobic bacteria included Deinococcus radiophilus [Deutsche Sammlung von Mikroorganismen (DSM ) No. 20551], a bacterium that can tolerate high doses of radiation and is very desiccation-resistant, the chemoheterotrophic bacterium Pseudomonas aeruginosa [Culture Collection of the University of Go¨teborg (CCUG) No. 241], which frequently occurs in soil, the chemo-organotrophic and chemolithotrophic (hydrogen-utilizing) organism Ralstonia eutropha (CCUG No. 1776), the chemoheterotrophic endospore-forming bacterium Bacillus subtilis (CCUG No. 163), and the thermophilic endospore-forming bacterium Bacillus stearothermophilus (CCUG No. 6241). Anaerobic bacteria included the SRB species Desulfovibrio aespoeensis (DSM No. 10631) isolated from deep ¨ spo¨ HRL groundwater (Motamedi and Pedersen, A 1998), the moderately halophilic SRB Desulfovibrio salexigens (DSM No. 2638), and the thermophilic, endospore-forming SRB Desulfotomaculum nigrificans (DSM No. 574). Detailed characteristics of the investigated species can be found in Bergey’s Manual of Systematic Bacteriology (Holt, 1989). 2.2. Determination of cultivable numbers of aerobic bacteria The aerobic bacteria were pre-cultured in nutrient broth (NB) medium (Merck KgaA, Darmstadt, Germany) and in Petri dishes on a solid nutrient agar (NA) medium, prepared by adding 1% w/w agar (Oxoid, Basingstoke, UK ) to the NB medium. The numbers of cultivable aerobic bacteria were determined by the plate count (PC ) technique on nutrient agar medium, as described elsewhere ( Koch, 1994). Briefly, the samples were serially diluted in a phosphate buffer to the inverse of the total number of cells ml−1 and spread in triplicate on the solid nutrient agar plates. One complete dilution series was prepared and incu-

2.3. Determination of cultivable numbers of sulphate-reducing bacteria Desulfovibrio aespoeensis and D. salexigens were cultivated in the anoxic mineral medium described by Widdel and Bak (1992) containing 0.13 and 0.86 M NaCl, respectively. The medium was added, with 6 mM lactate as the carbon source and 14 mM sodium sulphate as the electron acceptor. Medium E (Postgate, 1984) was used for cultivation of D. nigrificans. The numbers of cultivable SRB was estimated by the most probable number (MPN ) method ( Koch, 1994). This involves the mathematical inference of the most probable cultivable count from the fraction of multiple cultures that fail to show growth in a series of dilution tubes containing SRB medium. For dilution of SRB, a brackish SRB medium without energy and carbon sources and electron acceptors was used. The samples were serially diluted to the inverse of the total number of cells ml−1, and five repetitions of each dilution were inoculated. Tubes were graded ‘positive’ or ‘negative’ in comparison with negative controls, and the MPN was calculated using a computer program from Yamanashi University/ Ishikawajima-Harima Heavy Industries Ltd., Japan ( Kohno and Fukunaga 1998). 2.4. Estimation of the number of spores The number of spores of spore-forming bacteria was analysed using a heat shock technique. A portion of a ca. 10 ml sample was heated for 10 min to 80°C for B. subtilis, and to 100°C for B. stearothermophilus and D. nigrificans, and counted with the PC technique. 2.5. Bentonite composition and characteristics Bentonite is a natural clay containing smectite and several other common minerals. In this study, commercial MX-80 Wyoming bentonite clay from the American Colloid Co., WY was used. The MX-80 bentonite consists of approximately 75%

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of the smectite mineral montmorillonite. Other mineral compounds are 15% quartz, 7% feldspars, 1.4% carbonates, 0.3% sulphides, 0.4% organic carbon, and around 2% residual minerals (Mu¨llerVonmoos and Kahr, 1983). Bentonite has a strong tendency to swell at water uptake, and if swelling is restricted due to mechanical hindrance from the surrounding rock, the bentonite will give rise to a swelling pressure. At a density of 2 g/cm3, the swelling pressure of water-saturated bentonite is about 5 MPa ( Karnland, 1997). 2.6. Experimental configuration The testing philosophy for the LOT tests was to place pre-fabricated units of clay blocks surrounding heated copper tubes into vertical boreholes. The tests were performed under realistic HLW repository conditions, with differences in scale and several different controlled adverse conditions (SKB AB, 1999b). The central, heated copper tube would simulate the decay heat from the spent fuel. Test parcel S1 contained a heater, a central tube surrounded by a bentonite buffer of piled cylindrical blocks and thermocouples, pressure and moisture gauges in the bentonite blocks and bacterial plugs (Fig. 1). It was placed in a borehole with a diameter of 300 mm and a depth of around 4 m. The borehole for the test was located at a depth of 450 m in a side vault in the ¨ spo¨ HRL tunnel (SKB AB, 1999b). A 2.7. Preparation and installation of bacterial bentonite plugs Bentonite clay was used in the preparation of the plugs containing the bacteria, to be installed in the test parcel S1. The clay was heat-dried in a laboratory oven at 100°C overnight before use. The number and types of naturally occurring bacteria in the bentonite clay were analysed before and after the heat treatment. The number of cultivable cells was investigated by the PC and MPN methods described above, at culturing temperatures of 30 and 65°C. Growing bacteria were isolated and partially identified. The acridine orange direct count (AODC ) technique (Hobbie et al., 1977) was used to determine

the total number of bacteria in the cultures used for the plug preparations. Nuclepore filters (0.2 mm pore size, 13 mm in diameter, Nuclepore, costarcooperation, Cambridge, MA, USA) were prestained with a Sudan Black solution. Before use, the filters were thoroughly rinsed with de-ionized water. A portion of the sample was filtered on to a nuclepore filter at −20 kPa and stained for 7 min with Acridine Orange (AO). The total number of bacteria was then counted using blue light with an excitation wavelength of 390–490 nm, and a barrier filter at 515 nm under an epifluorescence microscope (Zeiss, Carl Zeiss, Oberkochen, Germany). For the plug preparations, 25.0 ml of the aerobic bacterial culture mix were transferred to a sterilized spray glass bottle. The same volume of SRB culture mix was transferred to a second sterilized spray glass bottle. A tray (25×35 cm) with a sterilized, ethanol-washed glass Petri dish (15 cm in diameter) was placed on to a balance, and 11.86 g heattreated clay were sieved on to the Petri dish. As the clay was heat-treated, it contained no water at the outset. A bacterial suspension (1.19 g) was sprayed on to the clay. The tray was subsequently wiped with sterilized tissue, and the edge of the Petri dish was wiped with a piece of sterilized cotton. Any bacterial suspension lost to the tray or removed by wiping was replaced with more bacterial culture sprayed on, until the required 1.19 g volume of bacterial suspension per plug was filled. The bacterial clay sample was thoroughly mixed. The clay at this stage contained 10% water, all of which came from the bacterial suspension sprayed. There were 34 preparations in total, 17 of aerobic bacteria and 17 of SRB. Before compacting the bacterial clay mixtures, samples were taken from four different preparations (two containing aerobic bacteria and two with SRB) to estimate the number of cultivable cells and spores present in the mixtures immediately after mixing the clay with the respective bacterial suspensions. Using a laboratory compaction device, the samples were compacted to form plugs to a density corresponding to the test block density of 2 kg m−3 after full water saturation, as described by Karnland and Sande´n (1998). The compaction was accordingly controlled by the final sample

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Fig. 1. Diagram of the test parcel S1. Pre-fabricated bentonite blocks are placed around a copper tube (4700 mm×108 mm). An electric heater in the lower part of the copper tube supplies the lower and central parts of the clay column with a maximum temperature of 90°C. The parcel is covered with sand and concrete after deposition in the deposition vault. Each of the 10 cm high blocks is given a number, starting at the bottom, with 1. In this study, plugs with bacteria were introduced into blocks 17 and 29.

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volume and not by the maximum compaction pressure. The plugs were cylindrical, with a 20 mm length and diameter. They were placed into sterilized plastic tubes, and caps were screwed on tightly to maintain the humidity until the placement (ca. 72 h later) in the designated bentonite blocks at the test site. One plug containing aerobic bacteria and another containing SRB were used to estimate the number of cultivable cells after 72 h. Cylindrical holes were drilled from the four cardinal points into the mantle surface in blocks 17 and 29 (Fig. 1). The holes were 21 mm in diameter and 80 mm deep, which meant that there was approximately 3 mm bentonite left to the inner radius of the cylindrical block. Before inserting the plugs into the test holes, the outer plugs were sprayed with a small amount of sterilized, distilled water thus causing the bentonite to start swelling and thus seal up the slots between the wall of the holes and the plugs. The positions of the plugs were marked with 1 mm thick titanium wires inserted into drilled holes placed 20 mm above the centre of the plugs. The northern position was marked with two titanium wires and the remaining positions with one wire. The plugs were assembled into the bentonite blocks approximately half an hour before the S1 parcel was submerged into the rock. In total, 16 plugs (eight plugs inoculated with aerobic bacteria and eight inoculated with SRB) were inserted perpendicularly into each block. Block No. 17 was placed close to the centre of the parcel where the heater was located, while block No. 29 was placed in the upper part of the parcel, which was not exposed to a high temperature (Fig. 1). The parcel S1 was covered with sand and concrete, secured with two beams and left for 15 months.

the humidity of the clay until partition. The test parcel was divided into the starting blocks using a saw, spatula and hammer. Blocks 17 and 29 were placed in plastic bags and delivered to a mobile field laboratory on ground equipped with an anaerobic box (Coy Laboratory Products Inc. Grass Lake, MI, USA), with a mixture of 5% H , 5% 2 CO and 90% N . The blocks were placed in the 2 2 box within 10 min after removal from the test parcel. Inside the anaerobic box, the blocks were cut into pieces to extract the installed plugs (see Fig. 2). Using sterilized spoons, clay samples of the bacterial plugs were taken, and those containing aerobic bacteria were transferred to a phosphate buffer. Plugs containing SRB were transferred to a brackish SRB medium without an electron acceptor or energy and carbon sources. Cultivation of aerobic bacteria was performed outside the anaerobic box. The clay was homogenized in the aforementioned dilution media, and the number of cultivable bacteria was subsequently estimated, as described above. Two uninoculated blocks were sampled to estimate the remaining number of naturally occurring microorganisms. They were block No. 5, which had been exposed to high temperature, and block No. 35, which had not been exposed to a high temperature.

2.8. Sampling of bacterial plugs at termination of the experiment The S1 parcel was drilled out of the rock, lifted slowly in one piece and placed on to the ground of the tunnel using a lorry with a hydraulic crane. This procedure resulted in a test parcel that was covered by approximately 10 cm of the surrounding rock. The rock was removed, and the parcel was immediately covered with plastic to maintain

Fig. 2. Photograph showing block No. 29 from one of the LOT experiments. The block has been ruptured, and a set of three plugs that were inoculated with bacteria are visible. The plugs were removed and analysed after 15 months’ exposure (photograph: M. Motamedi).

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3. Results 3.1. Number of naturally occurring, cultivable bacteria The number of cultivable aerobic bacteria in the heat-treated clay that was used in the plug preparation was determined at 30 and at 65°C and was 1.1×102, and <100 cells gdw−1 of clay, respectively. In the non-heat-treated clay, the number of cultivable cells was determined to be 3.4×104, and <100 cells gdw−1, isolated at 30 and 65°C, respectively. The bacteria isolated from the clay included a new species of Bacillus sp. (CCUG 36961), Bacillus cereus (CCUG 36963), Pseudomonas stutzeri (CCUG 36965), Bacillus subtilis (CCUG 36967) and Breviabacillus brevis (CCUG 36969). No SRB were found, either in the heat-treated or in the non-heat-treated clay. After termination of the experiment, the number of cultivable aerobic bacteria at 30 and 65°C was below the detection limit, of 100 cells gdw−1, in both blocks No. 5 (50–70°C ) and No. 35 (exposed to a temperature of 15–20°C ). However, the blocks

were not sterile because some bacteria could be enriched from them; there were bacteria present at numbers below the detection limit. Block No. 35 carried the same new species, Bacillus sp. (CCUG 39164), as was isolated from the clay at the start of the experiment, B. cereus (CCUG 39161), and a species of the genus Thermoactinomyces (CCUG 39165). Sulphate-reducing bacteria could not be isolated from either block No. 5 or block No. 35.

3.2. Number of cultivable bacteria in the plugs at the start of the experiment Up to 10% of the introduced aerobic bacteria could be cultured from the clay immediately after preparation ( Table 1). The cultivability of SRB differed markedly, depending on the species. The best cultivability was observed for D. nigrificans. D. salexigens could not be cultivated from the clay, and the number of D. aespoeensis was down to 7.5% of the initial population. Sampling after 72 h showed that only D. radiophilus and the sporeforming species B. subtilis, B. stearothermophilus

Table 1 Total number of cells, number of cultivable cells and number of spores of the tested species in the starting culture, and number of cultivable cells and spores in the plugs immediately and 72 h after plug preparation Species

Aerobic bacteria D. radiophilus P. aeruginosa R. eutropha B. subtilis B. subtilis (spores) B. stearothermophilus B. stearothermophilus (spores) Anaerobic bacteria D. salexigens D. aespoeensis D. nigrificans D. nigrificans (spores) a Counted by AOCD. b S.D.=0.02–0.31. c S.D.=0.21–0.25. d Not performed.

Cells ml−1 culture

Cultivable cells gdw−1 clay

Total number of bacteriaa

Cultivable cells (plate countsb or MPNc)

At preparation of the plugs

In the plugs after 72 h

4.1×108 3.0×109 1.3×109 4.6×108 3.1×108 1.8×108 3.8×107

3.1×108 8.1×108 2.6×108 4.3×108 5.7×107 5.7×107 2.1×104

5.2×107 5.1×107 1.6×107 3.0×107 5.6×106 3.9×107 4.2×104

3.0×107 <100 <100 2.8×107 npd 1.2×103 np

1.3×108 1.5×108 4.5×107 np

1.1×107 1.1×105 np np

<100 8.2×103 3.3×106 4.9×105

<100 <100 1.7×106 np

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Fig. 3.

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Fig. 3. (A–C ) Three different spore-forming bacteria survived after 15 months’ exposure to different temperatures in the LOT experiment. They were exposed to gradients of the temperatures indicated, with the highest temperature closest to the heater. The numbers given are gdw−1 bentonite clay (nf: not found).

and D. nigrificans were cultivable at the time of emplacement of the test.

tured ( Fig. 3c). Plug 12 was not found, owing to extensive fracturing of block No. 17.

3.3. Number of cultivable bacteria in the plugs at termination of the experiment

4. Discussion

All bacteria except for the spore-forming species were eliminated below the detection limits after 15 months’ exposure of the plugs to the test conditions. The numbers of cultivable aerobic bacteria from the plugs of block No. 17 (with a temperature of 50–70°C ) were below the detection limit (<100 cells gdw−1) for all investigated species. B. subtilis and B. stearothermophilus could be isolated from the plugs containing aerobic bacteria in block No. 29 (with a temperature of 20–30°), and B. subtilis was also cultivated from one of the plugs in block No. 17 [Fig. 3(A) and (B)]. In the plugs containing SRB, only D. nigrificans could be cul-

Results of laboratory experiments performed previously with SRB (Motamedi et al., 1996) showed that the environmental conditions in compacted bentonite clay reduce the number of cultivable SRB by many orders of magnitude over a 60 day test period. A high reduction rate of cultivable microorganisms was, therefore, expected in the studied LOT. The preparation of the LOT plugs was performed in the laboratory under exposure to oxygen in air, which mimicked realistic conditions of buffer block production. The tolerance of SRB to oxygen varies but is generally low ( Widdel and Hansen, 1992), and it was expected that the

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preparation procedure of the plugs would reduce the number of SRB. Large numbers of microorganisms, in the range of 107–109 cells gdw−1 clay ( Table 1), were chosen as starting concentrations to counteract the harsh conditions in the clay and hostile conditions during plug preparation. The method used to measure the presence of viable cells in the clay was cultivation, a conclusive method in the sense that cultivable microorganisms in the sample were viable. However, the method does not guarantee that all the non-cultivable bacteria would be dead or would have disappeared. The possibility that some of the investigated microorganisms had been inactivated and were unable to grow, though still potentially viable, cannot be fully excluded. Cultivable laboratory strains with cultivation media protocols that have been demonstrated to be suitable for successful cultivation of respective species were used to reduce this potential problem. There would, however, have been much a higher degree of uncertainty if only naturally occurring microorganisms had been used, since it is well known that currently available culturing methods commonly fail to cultivate up to 99% or more of environmental microorganisms (e.g. see Amann et al., 1995). A suitable measure of availability of water, or of water content, is the thermodynamic water activity (a ) of a system in equilibrium (Potts, w 1994). The water activity of a solution is related to the relative humidity of air in equilibrium with a solution or a water-containing bentonite clay. In other words, at a given temperature, a is the ratio w between the vapour pressure of the solution (clay) (P ) and that of water (P ). s 0 a =P /P . (1) w s 0 Thus, water activity is a measure of the relative tendency of water to escape from the system, compared with pure water, and can adequately be described by the relative humidity that the system can maintain in equilibrium. Microorganisms can grow over a large range of a s (0.999–0.75), but w most favour an a of seawater (0.98) or above. w Bentonite that has been compacted to 2 kg m−3, and which is water-saturated, has an a of 0.96 w (Motamedi et al., 1996). This number illustrates that the water content will be low in a HLW

bentonite buffer compared with most aquatic systems where microorganisms proliferate, such as lakes, the sea, and groundwater. The water content of the LOT plugs directly after preparation was, however, much lower, only 10% corresponding to an a of 0.76. The number of cultivable cells of w aerobic bacteria decreased dramatically after preparation of the plugs ( Table 1), most probably owing to the very desiccated and therefore harsh environmental conditions in the plugs. With the exception of D. radiophilus, only spore-forming bacteria could be detected after 72 h, showing the intolerance of non-spore-forming bacteria to a low a . Table 1 shows that part of the bacterial populaw tions mixed with the clay were spores, as indicated by cultivability after heat treatment. The heat treatment should have killed all vegetative cells. Spore formation is a life process that requires energy for spore growth and the formation of spore components; this process takes several hours. Most probably, therefore, it was not possible for those spore-forming bacteria not already in a spore, to form spores once sprayed on the clay, since the desiccation effect would have inactivated them rapidly. This conclusion is supported by the data in Table 1, showing that the number of spores in the cultures exceeded, or was equal to, the number of cultivable cells after 72 h in the plugs. It can therefore be anticipated that the plugs only carried spores and some vegetative cells of D. radiophilus at emplacement of the S1 test parcel. D. radiophilus could be isolated from the plugs after 72 h. This can be explained by the unique characteristics of D. radiophilus, which, in addition to tolerance of very high radiation doses, include survival in desiccated environments such as soil and in culturing media containing up to 5% NaCl (Murray, 1991). Therefore, this species stayed cultivable in the dry clay for a longer time than the other, less desiccation-resistant, non-sporeforming bacteria studied. The desiccation effect is most probably also an important reason for the reduction of cultivable cells of the non-sporeforming species D. salexigens and D. aespoeensis to <100 and 8200 cells gdw−1 clay, respectively, immediately following plug preparation ( Table 1). This effect was found in addition to the oxygen exposure effect discussed above. After 72 h, only

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the spore-forming SRB D. nigrificans could be isolated, while the other two SRB were below detection. Several species could be isolated directly from the clay before and after the heat treatment. Generally, with the exception of P. stutzeri, all other bacteria found were spore-forming bacteria. The ability of some non-spore-forming bacteria to withstand desiccating conditions over extensive periods, days or even years has been documented (McEldowney and Fletcher, 1988). P. stutzeri is a non-spore-forming bacterium that is widely distributed (Rossello et al., 1991). It is a non-fluorescent, denitrifying pseudomonad that can grow in a medium containing up to 7.5% NaCl, and it utilizes a wide variety of sugars and aromatic carbohydrates. P. stutzeri was frequently observed in different locations in the buffer/container experiment at the Atomic Energy of Canada Limited’s (AECL) underground research laboratory (StroesGascoyne et al., 1997) and it was also found in this investigation. It appears that this bacterium is tolerant to the extreme conditions that occur in compacted bentonite. Though it is not expected to be able to threaten the integrity of the HLW canisters, a focused investigation of this particular species may reveal new information about survival strategies of non-spore-forming bacteria in desiccating environments. After emplacement, it took more than 2 months for the bentonite to become fully water-saturated in the borehole. The period of very desiccating conditions consequently continued for at least 60 more days, in addition to the first 3 days between plug preparation and emplacement. In addition, some of the plugs experienced a significant increase in temperature. One basic requirement for the LOT was to keep a defined high temperature in the central and lower part of the clay column, which would give a temperature gradient in the bentonite. Block No. 29 was not close to the heater, and the temperature gradient in this block was between 30°C (inner part) and 20°C (outer part). B. subtilis, B. stearothermophilus and D. nigrificans could all be isolated from all the plugs of block 29 (Fig. 3). Block 17 was located at the centre of parcel S1 and exposed to the highest temperature. The temperature gradient in this

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block was between 70 and 50°C. D. nigrificans was cultivated from four, B. subtilis from one, and B. stearothermophilus from none of the plugs in this block. The high temperature of block 17 during 15 months obviously added a significant constraint to the cultivability of the introduced microorganisms, that is, the spores. Bacterial spores generally exhibit a high resistance to adverse physical conditions (e.g. temperature and desiccation) and to chemical agents. The survival of spores of different spore-forming species under similar conditions varies and depends on factors such as species, environmental constraints, and the time period that the spores are exposed to the environment (Briggs, 1960; Bruch and Smith, 1968; Molin, 1976; Donnelly and Busta, 1980; Acea et al., 1988). The heat resistance of bacterial spores depends on the species, as well as the spores’ initial water content, the rate of spore desiccation during heating, the water retention capacity of the material in or on which spores are located, and the relative humidity of the system at the test temperature (Angelotti et al., 1968). In other words, the viability of bacterial spores will show a specific declination rate that may be large or negligible, as a function of a range of spore and environmental characteristics, as was found in this investigation. The importance, for a HLW repository, of the results obtained on the survival and activity of microorganisms in compacted bentonite can be summarized as follows: at the start of the deposition, there will be a canister, bentonite blocks and a hole in the rock. Microorganisms indigenous to the bentonite, and possibly introduced during bentonite block production, will be present inside the bentonite. The results obtained for survival of non-spore-forming microorganisms in bentonite, reported above, suggest that the number of viable microorganisms will decrease rapidly and that very few viable cells will be present at full compaction. The only survivors will be microorganisms that have formed spores. Although, in general, spores are very resistant to difficult environmental conditions, they do still die off. The experiments presented here indicate a decrease in the number of viable spores at full compaction. A slow but significant death rate of spores would eventually lead to a complete eradication of life in the bentonite

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buffer. It has not yet been clarified whether this will occur within the lifetime of a HLW repository. Once the bentonite becomes sterile, it will probably not be reinfected. The theoretical pore size of the clay is 100–1000 times smaller than the averagesized microbe, meaning that no new microorganisms can enter into the buffer once the old population there have died out. This conceptual model is based on current data, obtained with laboratory cultures. It can be argued that naturally occurring microorganisms will be more tolerant, although the working hypothesis remains to be a total eradication of all life in the buffer. Currently ongoing and planned experiments will continue to test this hypothesis under increasingly relevant field conditions.

Acknowledgements We would like to thank Farideh Taherinejad, Agneta Welin, Nadi Jahromi and Berit Ertman Ericsson for their laboratory assistance and Proper English AB for correcting the language. The work was financially supported by the Swedish Nuclear Fuel and Waste Management Company (SKB AB).

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