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Fabric studies on contaminated mineral layers in composite liners
Applied Clay Science 21 (2002) 89 – 98 www.elsevier.com/locate/clay
Fabric studies on contaminated mineral layers in composite liners Wolfgang Berger *, Ute Kalbe*, Ju¨rgen Goebbels Federal Institute for Materials Research and Testing (BAM), Unter den Eichen 87, 12205 Berlin, Germany Received 27 March 2000; received in revised form 5 February 2001; accepted 16 July 2001
Abstract The fabric of mineral liner materials that had been exposed to organic compounds over a 12-year period was investigated as part of a wider research project. Macromorphological and micromorphological changes in the fabric were identified using computed tomography and polarisation microscopy. Special care was taken to ensure artefact free sampling and sample preparation, in particular, the drying method used, has a substantial influence on the quality of the thin sections. Fabric changes due to contaminant permeation over several years are, by and large, relatively small and their intensity is material specific. Silty clay CML1, in particular, contains a somewhat greater number of fissures and voids in comparison with the original material. The superposition of various processes in both test procedures and sample preparation may lead to fabric changes which can impede interpretation of the results. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Mineral liners; Organic compounds; Fabric; Microscopy; X-ray computed tomography (CT)
1. Introduction Contaminant transport processes in the mineral liners on landfills and contaminated sites may be permanently influenced by, among other factors, the material fabric. Significant fabric characteristics in this respect are grain and pore size distribution, the arrangement of clay minerals and other mineral phases
*
Corresponding authors. W. Berger is to be contacted at Tel.: +49-30-8104-1431; fax: +49-30-8104-1437. U. Kalbe, Tel.: +4930-8104-3862; fax: +49-30-8104-1437. E-mail addresses: [email protected] (W. Berger), [email protected] (U. Kalbe).
and heterogeneities, for example, microfissures. Little attention has so far been paid to the influence of organic contaminants on the fabric of mineral liner materials—a compilation can be found in Mattiat (1998). This paper focuses on the results of a research project in which, among other things, the long-term influence of organic contaminants on the fabric of mineral liner materials was investigated. In 1986/1987, a series of test cells (Fig. 1) was assembled to study the permeability of composite liners (geomembrane and different mineral liners) and their long-term behaviour under exposure to a concentrated mixture of organic compounds. The permeation rates of the contaminant mixture components were periodically determined from the simulated groundwater (August et al., 1992; Kalbe et al., 2001).
0169-1317/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 ( 0 1 ) 0 0 0 9 5 - 3
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W. Berger et al. / Applied Clay Science 21 (2002) 89–98
hydrocarbons). In the mixture, all compounds were present in identical weight fractions (11.11 wt.%). The influence of this multi-component mixture on the fabric of three different mineral liner materials was tested. The liner materials were compacted prior to being placed into the test cells; they will be denoted CML1 to CML3 (compacted mineral layer). The materials with thicknesses varying between 7.5 and 30 cm used for the mineral layer were:
a silty clay (CML1) containing 64% kaolinite, 29% illite and 7% smectite in the clay fraction; sporadic presence of sulphide concretions, a silty –clayey sand (CML2) containing 56% kaolinite, 43% illite and < 1% smectite in the clay fraction; amended with 1% Na-bentonite, a granulometric graded material (CML3), gravel content about 50%, containing 51% kaolinite, 47% illite and 2% smectite in the clay fraction; amended with 2% Na-bentonite. Each cell contained only one compacted mineral liner material of one thickness. Fig. 1. Test cell (approximate diameter 30 cm). (1) Mixture of organic compounds, (2) geomembrane, (3) mineral layer.
3. Methods 3.1. Sampling Some of these cells have now been disassembled and the liner materials analysed using a variety of techniques. This contribution concentrates on the influence of the contaminant mixture on the fabric of the mineral liner materials. Computed tomography (CT) and polarisation microscopy were used to undertake macromorphological and micromorphological studies.
2. Materials The contaminant mixture selected represents different classes of organic compounds with a variety of properties, first of all, as regards water solubility. The hydrophilic compounds used were methanol (alcohol), acetone (ketone) and tetrahydrofurane (ether). The hydrophobic compounds were iso-octane, trichloroethylene, toluene, tetrachloroethylene, chlorobenzene and xylene (hydrocarbons and chlorinated
As the fabric tests require undisturbed samples, a special sampling technique had to be developed. Hinged cylindrical sample cutters (diameter approximately 35 mm, manufactured by UGT Mu¨ncheberg; Figs. 2 and 3) were pressed manually into the test material, aided hydraulically for layer thicknesses z 15 cm. Test material CML3 was frozen whole ( 85 jC) because of its high coarse-grained content (approximately 50% gravel) and then cut up using a diamond saw. Samples for computed tomography were taken directly from the samplers, or in case of CML3 material, special freeze-dried segments were prepared. 3.2. Sample preparation The samples must be dried prior to impregnation because the impregnating resin is immiscible in water.
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Macromorphological features were visualised using 3D or Cone Beam Tomography (the principle is shown in Fig. 4) using undisturbed core samples (e.g. Fig. 6). The result obtained from a CT measurement is a 3D image matrix. Each point represents a single volume element (voxel). CT delivers a measure, the linear attenuation coefficient l, of the absorbed Xray radiation averaged over one voxel (Goebbels et al., 1996). The tomography used was developed at the Federal Institute for Materials Research and Testing (BAM) and uses an X-ray tube (tungsten anode) with a microfocal spot (about 10 Am), together with a combination of an image intensifier and a CCD camera as the detection system. Measurement of the samples is performed at 200 kV and 0.2 mA using a 2-mm Cu pre-filter to reduce the effect of beam hardening. The resolution of the used CT system is about 10 – 100 Am depending on the size of the investigated samples. Fig. 2. Hinged sample cutter.
Using an appropriate drying technique is the most crucial factor in preserving the fabric during sample preparation. Opposing views on the suitability of air-drying for clay samples have been published (Stephan, 1969; Altemu¨ller, 1975). Freeze-drying is considered as particularly appropriate by a number of authors (Junge and Magnus, 1994; Werner, 1966). Other authors have obtained good results by displacing water with a suitable organic solvent (usually alcohol or ketone) (Komodromos and Mattiat, 1989; Gran and Hansen, 1998; Murphy, 1986). Thin sections were prepared from undisturbed samples to analyse micromorphological features using polarisation microscopy. Oriented samples were taken from 2-cm horizons in the sample cores (Fig. 3). They were freeze-dried and epoxy resin impregnated. Parallely, the samples were dehydrated using acetone. The grinding procedure was performed without the use of water-containing suspensions. The thin sections were evaluated by means of polarisation microscopy using digital image analysis.
Fig. 3. Sample core from test cell CML2/15 cm.
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Fig. 4. Principle of 3D or Cone Beam Tomography.
A four-processor parallel computer can be used for the reconstruction, which gives image reconstruction times comparable to the time needed for one measure-
ment. If the size of a sample (e.g. length) exceeds its diameter, several measurements at different heights are performed and the resulting images are combined.
Fig. 5. (a) (Above) CT-slice of a sample core from CML1 material, non-contaminated, compacted. (b) (Bottom) CT-slice of a sample core from test cell CML1/15 cm ((1) iron sulphide concretions, (2) fissures).
W. Berger et al. / Applied Clay Science 21 (2002) 89–98
4. Results 4.1. Macromorphology Computed tomography was performed on selected sections (depth ranges) of all the organically contaminated liner materials from the test cells. Furthermore, non-contaminated reference samples were investigated (original material compacted under identical conditions as the materials in the test cells). Observation of CML1 in the test cell (Fig. 5b) showed a large number of both large and small, mainly horizontal fissures. Numerous voids of varying forms and sizes were also found. It was conspicuous that in test cell CML1/15 cm, the majority of fissures occurred in the top third of the sample (Fig. 5b). The non-
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contaminated reference material exhibited a similar fissure distribution but the number of fissures and voids appeared to be somewhat lower than in the materials from the test cells (Fig. 5a). In general, the material CML1 proved itself to be relatively homogeneous, the light areas indicating coarse mineral components with a higher density than the groundmass (e.g. iron sulphide concretions). CML2 (Fig. 6) exhibits a different fabric pattern from liner CML1. First of all, a large number of smaller voids can be observed and fissures occur mainly on the periphery which were probably caused by sampling. The material is overall relatively heterogeneous, shown by the rather frequent change from light to dark areas representing higher and lower densities (Fig. 6). No obvious differences were observed between the
Fig. 6. Three perpendicular CT-slices of the 3D image data set of a sample from the test cell with 15-cm CML2 layer. Additional iso-surface visualisation of the outer shape of the sample is shown.
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Fig. 7. CT-slice of a sample from test cell CML3/20 cm (example for (1) rock fragments, (2) individual mineral particles, e.g. quartz, and (3) fissures).
reference material and the contaminated liners in the test cells. The CML3 liner sample contained a very wide grain-size spectrum (Fig. 7). Medium to coarsegrained components with various degrees of roundness (angular to highly rounded) are embedded in a relatively fine-grained matrix. Rock fragments of various origins and compositions, individual mineral particles and mineral aggregates can be identified (Fig. 7, (1), (2)). Fissures of varying size stand out: they began at the grain boundary of larger components and continued in the finer liner material (Fig. 7, (3)). It was not possible to determine unambiguously whether these fissures were the consequences of changes due to contaminant influence or were caused by sampling or sample preparation (freezing).
certain amount of parallel orientation of the clay domains and a somewhat oriented anisotropy (Fig. 9). Thin sections exhibited a large number of fissures of varying size in the compacted and contaminated material CML1 from the test cells which had been freeze-dried (Fig. 10). These fissures mostly exhibited a clear parallel orientation, which obviously followed the preferred orientation of clay layers in the liner body caused by the compaction. Samples of the test cell material CML1 dehydrated by an ethanol/acetone series (Fig. 11) show considerably fewer microfissures. The method of sample drying plays, therefore, an important role for the quality of thin sections (Kalbe et al., 2000a,b). Unlike the CML1 clay, the CML2 liner material is highly heterogeneous. Using the criteria of Bullock et
4.2. Micromorphology Thin sections were prepared from the (non-contaminated) original material with and without the same compaction as used in the test cells. The mineral liner material CML1 exhibits a ‘‘porphyric’’ micromorphological fabric whose relatively homogeneous groundmass (S-matrix or plasma according to Brewer, 1976) consists mainly of clay minerals. The isolated hypidiomorphic to xenomorphic inclusions consist of quartz, mica and pyrite/ marcasite. A more or less clearly identifiable asepic fabric is a characteristic of the natural non-mechanically compacted initial material (Fig. 8). Proctor compacted original materials, however, exhibit the characteristics of a sepic fabric with a
Fig. 8. CML1 material, non-contaminated, non-compacted, dehydrated by ethanol/acetone; plain light.
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Fig. 9. CML1 material, non-contaminated, compacted, dehydrated by ethanol/acetone; crossed Nicols ((1) Fe-sulphide, (2) quartz, (3) fissure).
al. (1985), it exhibits an intergranular microaggregate structure, i.e. a large number of lenticular clayey aggregates of various sizes can be found among silty – sandy components, in particular, rock fragments and
quartz. Numerous, mainly parallelly oriented microfissures that end at the clayey aggregate boundaries (Fig. 12) can be seen within these fabric elements. Coarse pores and voids which occur frequently in this
Fig. 10. Test cell 7.5-cm CML1 material, contaminated, compacted, freeze-dried; crossed Nicols ((1) fissure).
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Fig. 11. Test cell 15-cm CML1 material, contaminated, compacted, dehydrated by ethanol/acetone; crossed Nicols ((1) fissure).
liner material are mainly filled with fine-grained material (pore cement) aided by the addition of 1% Nabentonite. The CML3 liner material has a granular fabric (Fig. 13), characterised by a sandy – gravelly grain skeleton
with fine-grained mineral components in the intergranular pore spaces. Besides this, numerous voids of various sizes can be observed. The grain skeleton consists of both more or less well-formed individual minerals, in the main quartz and subordinate feld-
Fig. 12. Test cell 15-cm CML2 material, contaminated, compacted, freeze-dried; plain light ((1) lenticular clayey aggregate with fissures).
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Fig. 13. Test cell 20-cm CML3 material, contaminated, compacted; freeze-dried; plain light ((1) fine-grained mineral components in the intergranular pore spaces, (2) fissure).
spars, as well as fragments of sedimentary and magmatic rocks. Occasionally, lenticular aggregates rich in clay minerals can be found, which may have formed due to the incomplete mixing of bentonite additive into the liner material prior to installation in the test cells. Fissures starting at the grain boundaries of larger grains, identified in the macromorphological fabric (CT), can also be observed in the sections (Figs. 7 and 13).
5. Conclusions It is possible to determine changes in the fabric of mineral layers under the influence of organic contaminants by means of macromorphological and micromorphological studies. These changes depend on the type of mineral layer. Appropriate sample preparation plays a highly significant role in this. It can be stated that the long-term influence of organic compounds has caused only relatively slight macromorphological and micromorphological changes in the fabric of mineral liner materials. Only the CML1 mineral liner (silty clay) appears to exhibit a
somewhat higher number of microfissures in comparison with the original non-contaminated material indicated especially by X-ray tomography images (Fig. 5a,b). Low contaminant concentrations in the mineral materials due to the efficient performance of the geomembrane (Kalbe et al., 2001, this issue) are likely to be the reason why only relatively insignificant changes took place in the fabric. It should be noted that various influences may lead to changes in the fabric during testing (compaction) and thin section preparation (sampling, drying). The superposition of possible influences of these processes hampers the interpretation of the test results.
Acknowledgements The morphological investigations presented in this paper were performed within a research project supported financially by the Bundesministerium fu¨r Bildung und Forschung (Federal Ministry for Education and Research); this support is gratefully acknowledged.
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References Altemu¨ller, H.-J., 1975. Mikroskopie der Bo¨den mit Hilfe von Du¨nnschliffen. In: Freund, H. (Ed.), Handbuch der Mikroskopie in der Technik, Band IV, Teil, 2. Umschau Verlag, Frankfurt/ Main, pp. 309 – 367. August, H., Tatzky-Gerth, R., Preuschmann, R., Jacob, I., 1992. Permeationsverhalten von Kombinationsdichtungen bei Deponien und Altlasten gegenu¨ber wassergefa¨hrdenden Stoffen. UBA-Forschungsber. 102 (3), 412. Brewer, R., 1976. Fabric and Mineral Analysis of Soils. Robert E. Krieger Publisher, Huntington, NY. Bullock, P., Fedoroff, N., Jongerius, A., Stoops, G., Tursina, T., Babel, U., 1985. Handbook for Soil Thin Section Description. Waine Research, Wolverhampton, 152 pp. Goebbels, J., Illerhaus, B., Staiger, H., Riesemeier, H., 1996. Demands on scintillators for Cone Beam Tomography. In: Dorenbros, P., van Eijk, C.W.E. (Eds.), Proc. Int. Conf. on Inorganic Scintillators and Their Applications, Delft, The Netherlands, August 28 – September 1. Delft University Press, The Netherlands, pp. 40 – 47. Gran, H.C., Hansen, E.W., 1998. Exchange rates of ethanol with water in water-saturated cement pastes probed by NMR. Adv. Cem. Based Mater. 8, 108 – 117. Junge, F.W., Magnus, M., 1994. Du¨nnschliffherstellung an gewarvten tonigen Sedimenten am Beispiel der Ba¨ndertone aus der Leipziger Tieflandsbucht/BR Deutschland. Der Pra¨parator, 40/ H.1, Verband Deutscher Pra¨paratoren e.V., Bochum, pp. 33 – 41.
Kalbe, U., Berger, W., Helm, R., 2000a. Zur Beprobung kontaminierter mineralischer Dichtungen aus Versuchsmeßzellen und Herstellung von Du¨nnschliffen. Der Pra¨parator, 46/H.1. Verband Deutscher Pra¨paratoren e.V., Hannover, pp. 35 – 45. Kalbe, U., Berger, W., Mu¨ller, W.W., 2000b. Mineralogische und chemisch-physikalische Auswirkungen der Permeation von Kohlenwasserstoffen in Kombinationsdichtungen und -dichtwa¨nden. Research report, Federal Ministry for Education Science, Research and Technology (BMBF), Reference No. 1461027. Kalbe, U., Mu¨ller, W.W., Berger, W., Eckardt, J., 2002. Transport of organic contaminants within composite liner systems. Appl. Clay Sci. 21, 67 – 76 (this issue). Komodromos, A., Mattiat, B., 1989. Einsatz von licht- und rasterelektronenmikroskopischen Untersuchungsmethoden bei der Eignungspru¨fung von mineralischen Deponieabdichtungen. Mu¨ll und Abfall 5/89, 253 – 263. Mattiat, B., 1998. Gefu¨geanalyse. In: Hiltmann, W., Stribny, B. (Eds.), Handbuch zur Erkundung des Untergrundes von Deponien und Altlasten, Band 5, Tonmineralogie und Bodenphysik. Springer-Verlag, Berlin, pp. 189 – 199. Murphy, C.P., 1986. Thin Section Preparation of Soils and Sediments. Academic Publishers, Berkhamsted, UK. Stephan, S., 1969. Gefriertrocknung u. a. bei der Herstellung von Bodendu¨nnschliffen benutzbare Trocknungsverfahren. Pflanzenernaehr., Dueng., Bodenkd. 123 (H.2), 131 – 140. Werner, F., 1966. Herstellung von ungesto¨rten Du¨nnschliffen aus wassergesa¨ttigten, pelitischen Lockersedimenten mittels Gefriertrocknung. Meyniana 16, 107 – 112.
Fabric studies on contaminated mineral layers in composite liners Wolfgang Berger *, Ute Kalbe*, Ju¨rgen Goebbels Federal Institute for Materials Research and Testing (BAM), Unter den Eichen 87, 12205 Berlin, Germany Received 27 March 2000; received in revised form 5 February 2001; accepted 16 July 2001
Abstract The fabric of mineral liner materials that had been exposed to organic compounds over a 12-year period was investigated as part of a wider research project. Macromorphological and micromorphological changes in the fabric were identified using computed tomography and polarisation microscopy. Special care was taken to ensure artefact free sampling and sample preparation, in particular, the drying method used, has a substantial influence on the quality of the thin sections. Fabric changes due to contaminant permeation over several years are, by and large, relatively small and their intensity is material specific. Silty clay CML1, in particular, contains a somewhat greater number of fissures and voids in comparison with the original material. The superposition of various processes in both test procedures and sample preparation may lead to fabric changes which can impede interpretation of the results. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Mineral liners; Organic compounds; Fabric; Microscopy; X-ray computed tomography (CT)
1. Introduction Contaminant transport processes in the mineral liners on landfills and contaminated sites may be permanently influenced by, among other factors, the material fabric. Significant fabric characteristics in this respect are grain and pore size distribution, the arrangement of clay minerals and other mineral phases
*
Corresponding authors. W. Berger is to be contacted at Tel.: +49-30-8104-1431; fax: +49-30-8104-1437. U. Kalbe, Tel.: +4930-8104-3862; fax: +49-30-8104-1437. E-mail addresses: [email protected] (W. Berger), [email protected] (U. Kalbe).
and heterogeneities, for example, microfissures. Little attention has so far been paid to the influence of organic contaminants on the fabric of mineral liner materials—a compilation can be found in Mattiat (1998). This paper focuses on the results of a research project in which, among other things, the long-term influence of organic contaminants on the fabric of mineral liner materials was investigated. In 1986/1987, a series of test cells (Fig. 1) was assembled to study the permeability of composite liners (geomembrane and different mineral liners) and their long-term behaviour under exposure to a concentrated mixture of organic compounds. The permeation rates of the contaminant mixture components were periodically determined from the simulated groundwater (August et al., 1992; Kalbe et al., 2001).
0169-1317/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 ( 0 1 ) 0 0 0 9 5 - 3
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W. Berger et al. / Applied Clay Science 21 (2002) 89–98
hydrocarbons). In the mixture, all compounds were present in identical weight fractions (11.11 wt.%). The influence of this multi-component mixture on the fabric of three different mineral liner materials was tested. The liner materials were compacted prior to being placed into the test cells; they will be denoted CML1 to CML3 (compacted mineral layer). The materials with thicknesses varying between 7.5 and 30 cm used for the mineral layer were:
a silty clay (CML1) containing 64% kaolinite, 29% illite and 7% smectite in the clay fraction; sporadic presence of sulphide concretions, a silty –clayey sand (CML2) containing 56% kaolinite, 43% illite and < 1% smectite in the clay fraction; amended with 1% Na-bentonite, a granulometric graded material (CML3), gravel content about 50%, containing 51% kaolinite, 47% illite and 2% smectite in the clay fraction; amended with 2% Na-bentonite. Each cell contained only one compacted mineral liner material of one thickness. Fig. 1. Test cell (approximate diameter 30 cm). (1) Mixture of organic compounds, (2) geomembrane, (3) mineral layer.
3. Methods 3.1. Sampling Some of these cells have now been disassembled and the liner materials analysed using a variety of techniques. This contribution concentrates on the influence of the contaminant mixture on the fabric of the mineral liner materials. Computed tomography (CT) and polarisation microscopy were used to undertake macromorphological and micromorphological studies.
2. Materials The contaminant mixture selected represents different classes of organic compounds with a variety of properties, first of all, as regards water solubility. The hydrophilic compounds used were methanol (alcohol), acetone (ketone) and tetrahydrofurane (ether). The hydrophobic compounds were iso-octane, trichloroethylene, toluene, tetrachloroethylene, chlorobenzene and xylene (hydrocarbons and chlorinated
As the fabric tests require undisturbed samples, a special sampling technique had to be developed. Hinged cylindrical sample cutters (diameter approximately 35 mm, manufactured by UGT Mu¨ncheberg; Figs. 2 and 3) were pressed manually into the test material, aided hydraulically for layer thicknesses z 15 cm. Test material CML3 was frozen whole ( 85 jC) because of its high coarse-grained content (approximately 50% gravel) and then cut up using a diamond saw. Samples for computed tomography were taken directly from the samplers, or in case of CML3 material, special freeze-dried segments were prepared. 3.2. Sample preparation The samples must be dried prior to impregnation because the impregnating resin is immiscible in water.
W. Berger et al. / Applied Clay Science 21 (2002) 89–98
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Macromorphological features were visualised using 3D or Cone Beam Tomography (the principle is shown in Fig. 4) using undisturbed core samples (e.g. Fig. 6). The result obtained from a CT measurement is a 3D image matrix. Each point represents a single volume element (voxel). CT delivers a measure, the linear attenuation coefficient l, of the absorbed Xray radiation averaged over one voxel (Goebbels et al., 1996). The tomography used was developed at the Federal Institute for Materials Research and Testing (BAM) and uses an X-ray tube (tungsten anode) with a microfocal spot (about 10 Am), together with a combination of an image intensifier and a CCD camera as the detection system. Measurement of the samples is performed at 200 kV and 0.2 mA using a 2-mm Cu pre-filter to reduce the effect of beam hardening. The resolution of the used CT system is about 10 – 100 Am depending on the size of the investigated samples. Fig. 2. Hinged sample cutter.
Using an appropriate drying technique is the most crucial factor in preserving the fabric during sample preparation. Opposing views on the suitability of air-drying for clay samples have been published (Stephan, 1969; Altemu¨ller, 1975). Freeze-drying is considered as particularly appropriate by a number of authors (Junge and Magnus, 1994; Werner, 1966). Other authors have obtained good results by displacing water with a suitable organic solvent (usually alcohol or ketone) (Komodromos and Mattiat, 1989; Gran and Hansen, 1998; Murphy, 1986). Thin sections were prepared from undisturbed samples to analyse micromorphological features using polarisation microscopy. Oriented samples were taken from 2-cm horizons in the sample cores (Fig. 3). They were freeze-dried and epoxy resin impregnated. Parallely, the samples were dehydrated using acetone. The grinding procedure was performed without the use of water-containing suspensions. The thin sections were evaluated by means of polarisation microscopy using digital image analysis.
Fig. 3. Sample core from test cell CML2/15 cm.
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Fig. 4. Principle of 3D or Cone Beam Tomography.
A four-processor parallel computer can be used for the reconstruction, which gives image reconstruction times comparable to the time needed for one measure-
ment. If the size of a sample (e.g. length) exceeds its diameter, several measurements at different heights are performed and the resulting images are combined.
Fig. 5. (a) (Above) CT-slice of a sample core from CML1 material, non-contaminated, compacted. (b) (Bottom) CT-slice of a sample core from test cell CML1/15 cm ((1) iron sulphide concretions, (2) fissures).
W. Berger et al. / Applied Clay Science 21 (2002) 89–98
4. Results 4.1. Macromorphology Computed tomography was performed on selected sections (depth ranges) of all the organically contaminated liner materials from the test cells. Furthermore, non-contaminated reference samples were investigated (original material compacted under identical conditions as the materials in the test cells). Observation of CML1 in the test cell (Fig. 5b) showed a large number of both large and small, mainly horizontal fissures. Numerous voids of varying forms and sizes were also found. It was conspicuous that in test cell CML1/15 cm, the majority of fissures occurred in the top third of the sample (Fig. 5b). The non-
93
contaminated reference material exhibited a similar fissure distribution but the number of fissures and voids appeared to be somewhat lower than in the materials from the test cells (Fig. 5a). In general, the material CML1 proved itself to be relatively homogeneous, the light areas indicating coarse mineral components with a higher density than the groundmass (e.g. iron sulphide concretions). CML2 (Fig. 6) exhibits a different fabric pattern from liner CML1. First of all, a large number of smaller voids can be observed and fissures occur mainly on the periphery which were probably caused by sampling. The material is overall relatively heterogeneous, shown by the rather frequent change from light to dark areas representing higher and lower densities (Fig. 6). No obvious differences were observed between the
Fig. 6. Three perpendicular CT-slices of the 3D image data set of a sample from the test cell with 15-cm CML2 layer. Additional iso-surface visualisation of the outer shape of the sample is shown.
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Fig. 7. CT-slice of a sample from test cell CML3/20 cm (example for (1) rock fragments, (2) individual mineral particles, e.g. quartz, and (3) fissures).
reference material and the contaminated liners in the test cells. The CML3 liner sample contained a very wide grain-size spectrum (Fig. 7). Medium to coarsegrained components with various degrees of roundness (angular to highly rounded) are embedded in a relatively fine-grained matrix. Rock fragments of various origins and compositions, individual mineral particles and mineral aggregates can be identified (Fig. 7, (1), (2)). Fissures of varying size stand out: they began at the grain boundary of larger components and continued in the finer liner material (Fig. 7, (3)). It was not possible to determine unambiguously whether these fissures were the consequences of changes due to contaminant influence or were caused by sampling or sample preparation (freezing).
certain amount of parallel orientation of the clay domains and a somewhat oriented anisotropy (Fig. 9). Thin sections exhibited a large number of fissures of varying size in the compacted and contaminated material CML1 from the test cells which had been freeze-dried (Fig. 10). These fissures mostly exhibited a clear parallel orientation, which obviously followed the preferred orientation of clay layers in the liner body caused by the compaction. Samples of the test cell material CML1 dehydrated by an ethanol/acetone series (Fig. 11) show considerably fewer microfissures. The method of sample drying plays, therefore, an important role for the quality of thin sections (Kalbe et al., 2000a,b). Unlike the CML1 clay, the CML2 liner material is highly heterogeneous. Using the criteria of Bullock et
4.2. Micromorphology Thin sections were prepared from the (non-contaminated) original material with and without the same compaction as used in the test cells. The mineral liner material CML1 exhibits a ‘‘porphyric’’ micromorphological fabric whose relatively homogeneous groundmass (S-matrix or plasma according to Brewer, 1976) consists mainly of clay minerals. The isolated hypidiomorphic to xenomorphic inclusions consist of quartz, mica and pyrite/ marcasite. A more or less clearly identifiable asepic fabric is a characteristic of the natural non-mechanically compacted initial material (Fig. 8). Proctor compacted original materials, however, exhibit the characteristics of a sepic fabric with a
Fig. 8. CML1 material, non-contaminated, non-compacted, dehydrated by ethanol/acetone; plain light.
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Fig. 9. CML1 material, non-contaminated, compacted, dehydrated by ethanol/acetone; crossed Nicols ((1) Fe-sulphide, (2) quartz, (3) fissure).
al. (1985), it exhibits an intergranular microaggregate structure, i.e. a large number of lenticular clayey aggregates of various sizes can be found among silty – sandy components, in particular, rock fragments and
quartz. Numerous, mainly parallelly oriented microfissures that end at the clayey aggregate boundaries (Fig. 12) can be seen within these fabric elements. Coarse pores and voids which occur frequently in this
Fig. 10. Test cell 7.5-cm CML1 material, contaminated, compacted, freeze-dried; crossed Nicols ((1) fissure).
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Fig. 11. Test cell 15-cm CML1 material, contaminated, compacted, dehydrated by ethanol/acetone; crossed Nicols ((1) fissure).
liner material are mainly filled with fine-grained material (pore cement) aided by the addition of 1% Nabentonite. The CML3 liner material has a granular fabric (Fig. 13), characterised by a sandy – gravelly grain skeleton
with fine-grained mineral components in the intergranular pore spaces. Besides this, numerous voids of various sizes can be observed. The grain skeleton consists of both more or less well-formed individual minerals, in the main quartz and subordinate feld-
Fig. 12. Test cell 15-cm CML2 material, contaminated, compacted, freeze-dried; plain light ((1) lenticular clayey aggregate with fissures).
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Fig. 13. Test cell 20-cm CML3 material, contaminated, compacted; freeze-dried; plain light ((1) fine-grained mineral components in the intergranular pore spaces, (2) fissure).
spars, as well as fragments of sedimentary and magmatic rocks. Occasionally, lenticular aggregates rich in clay minerals can be found, which may have formed due to the incomplete mixing of bentonite additive into the liner material prior to installation in the test cells. Fissures starting at the grain boundaries of larger grains, identified in the macromorphological fabric (CT), can also be observed in the sections (Figs. 7 and 13).
5. Conclusions It is possible to determine changes in the fabric of mineral layers under the influence of organic contaminants by means of macromorphological and micromorphological studies. These changes depend on the type of mineral layer. Appropriate sample preparation plays a highly significant role in this. It can be stated that the long-term influence of organic compounds has caused only relatively slight macromorphological and micromorphological changes in the fabric of mineral liner materials. Only the CML1 mineral liner (silty clay) appears to exhibit a
somewhat higher number of microfissures in comparison with the original non-contaminated material indicated especially by X-ray tomography images (Fig. 5a,b). Low contaminant concentrations in the mineral materials due to the efficient performance of the geomembrane (Kalbe et al., 2001, this issue) are likely to be the reason why only relatively insignificant changes took place in the fabric. It should be noted that various influences may lead to changes in the fabric during testing (compaction) and thin section preparation (sampling, drying). The superposition of possible influences of these processes hampers the interpretation of the test results.
Acknowledgements The morphological investigations presented in this paper were performed within a research project supported financially by the Bundesministerium fu¨r Bildung und Forschung (Federal Ministry for Education and Research); this support is gratefully acknowledged.
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