The Geochemistry of Pyritic Shale Weathering Within an Active Landslide
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THE GEOCHEMISTRY OF PYRITIC SHALE WEATHERING WITHIN AN ACTIVE LANDSLIDE by Alwyn Vear Submitted in fulfilment of the requirements for the Degree of Doctor of Philosophy Departments of Geography and Geology, University of Sheffield. November, 1981. SUMMARY Intense chemical weathering accompanies physical instability at the site of a repeatedly active landslide in North Derbyshire. In order to describe and quantify the chemical weathering, a programme of water sampling and analysis was devised. A sequence of reactions are proposed, based on theresults of this work, to account for the observed concentra- tions of chemical species in the drainage waters. It is thought that pyrite oxidation, accelerated by the presence of catalytic bacteria, is responsible for the considerable acidity of these waters. Additional reactions involving carbonates and silicates occur at strictly comparable rates and consume over 99% of the acidity prior to the water's emergence in a number of ochre-precipitating springs. Analysis of the solid reactants and products confirms the suggested sequence of events and suggests a number of ways in which chemical weathering might be related to slope stability. Clay minerals appear to be little affected by weathering, and the growth of precipitate minerals such as gypsum in joints and on bedding planes might be a more important mechanism in shale breakdown. After this initial rapid physical disintegration, chemical weathering, at the surface, proceeds relatively slowly. Chemical processes build up stresses within the rocks and possibly help to maintain any inherent planes of weakness. Ultimately landslide movement is triggered by increases in porewater pressures brought about by fluctuations in local ground water levels. ACKNOWLEDGEMENTS I wish to thank Dr. Charles Curtis for his continued encouragement, enthusiasm and invaluable criticism throughout. Professor R.S. Waters gave considerable support, and the Natural Environment Research Council provided the necessary funds. Special thanks also to Margaret, Rod, Villy and Pat - and many others too numerous to mention - of the Geography and Geology Departments, University of Sheffield. CONTENTS 1. INTRODUCTION 1 1.1 Objectives 1.1.1 Physical Background 1 1.1.2 Chemical Background 2 1.1.3 Initial Objectives 3 1.2 Literature Review 3 1.2.1 Landslides 3 1.2.2 Chemical Weathering: General 5 1.2.3 Chemical Weathering: Sediments 8 1.2.4 Weathering and Slope Stability 11 1.3 Experimental Design 12 1.3.1 Geochemistry 12 1.3.2 Engineering Properties 15 2. THE STUDY AREA 18 2.1 Location 18 2.2 Geology 18 2.2.1 Edale Shales 18 2.2.2 Mam Tor Beds 22 2.3 History (Mining) 24 3. GEOMORPHOLOGY OF THE LANDSLIDE 27 3.1 Landslide Classification 27 3.2 Description of the Mam Tor Landslide 28 3.3 Landslide Management 32 4. WATER SAMPLING AND ANALYSIS 35 4.1 Sampling Design and Procedure 35 4.2 Sample Storage 37 4.3 Sample Analysis 38 4.3.1 Temperature, pH and Conductivity 38 4.3.2 Cations 40 4.3.3 Anions 40 4.3.4 Trace Metals 40 4.4 Results 41 4.4.1 Temperature, pH and Conductivity 41 4.4.2 Cations 43 4.4.3 Anions 44 4.4.4 Trace Metals 46 5. INTERPRETATION OF WATER CHEMISTRY DATA 47 5.1 Environmental Responses 47 5.1.1 Rainfall 47 5.1.2 Temperature 49 5.2 Cluster Analysis 50 5.3 Charge Balance 53 5.4 Origin of Drainage Waters 55 5.4.1 Deep and Surface Components 55 5.4.2 Dilution Gauging 57 5.4.3 Additional Inputs 59 6. DEVELOPMENT OF A WEATHERING MODEL 62 6.1 Weathering Reactions 62 6.1.1 pyrite Oxidation 62 6.1.2 Carbonate Dissolution 66 6.1.3 Clay Mineral Transformations 67 6.1.4 A Quantitative Weathering Model 68 6.1.5 The Sulphate Problem 71 6.2 Wider Implications of the Model 72 6.2.1 Catalysts 72 6.2.2 Acid Mine Drainage 75 6.2.2.1 Formation 74 6.2.2.2 Abatement 75 6.2.3 Comparison with Mam Tor Drainage Waters 77 6.3 Environmental Considerations 79 6.3.1 Phytosociological Responses 79 6.3.2 Biological-Chemical Interactions 82 6.4 Conclusions 83 7. MINERAL ANALYSIS 85 7.1 Samp lirig 85 7.1.1 "Little Mam Tor" 86 7.1.2 Retrogressive Units 86 7.1. 3 Other Surface Materials 87 7.1.4 Borehole Cores 88 7.2 Particle Size Separation 89 7.2.1 Disaggregation 89 7.2.2 Sedimentation 90 7.2.2.1 Theory 90 7.2.2.2 Practice 92 7.3 X-Ray Diffraction 93 7.3.1 Principles 93 7.3.2 Mineral Identification in Whole Rock Powder Samples 95 7.3.3 Identification and Estimation of Clay Minerals 96 7.3.4 Other Materials 99 7.4 XRD Results 100 7.4.1 Whole Rock 100 7.4.2 Clay Minerals 102 7.4.3 Other Materials 106 7.4.3.1 Carbonate Coatings 106 7.4.3.2 "Allophane" 106 7.4.3.3 Ochre 107 7.5 Thin Sections 108 7.6 The Mechamisms of Formation of Three Precipitate Minerals 109 7.6.1 Gypsum 109 7.6.2 Jarosite 111 7.6.3 Iron Hydroxide 113 8. CHEMICAL THERMODYNAMICS 116 8.1 Temperature Considerations 117 8.2 Mineral Stability and Eh-pH Diagrams 120 8.2.1 The Nernst and Debye-Huckel Equations 120 8.2.2 Stability of Water 122 8.2.3 The Iron Oxides 123 8.2.4 Iron Hydroxides 125 8.2.5 Relationships Among Ions 126 8.2.6 Dissolved Sulphur Species 128 8.2.7 The Composite Diagram 129 8.3 Further Consideration of the Eh-pH Diagrams 130 8.4 Gypsum Solubility 133 9. CHEMICAL WEATHERING AND SLOPE STABILITY 136 9.1 Dissolution and Precipitation 136 9.2 The Mechanism of Shale Breakdown 138 9.3 Porewater Pressure and Failure Surfaces 140 9.4 Future Management 141 10. FINAL CONCLUSIONS 144 11. REFERENCES 148 APPENDICES A. WET CHEMISTRY ANALYTICAL PROCEDURES 167 Ai Determination of Calci~m 167 A ii Determination of Magnesium 168 A iii Determination of Sodium 169 A iv Determination of Potassium 170 A v Determination of Total Iron 171 A vi Determination of Ferrous Iron 172 A vii Determination of Aluminium 173 Aviii Determination of Chloride 174 A ix Determination of Sulphate 175 A x Determination of Trace Metals 177 B. RESULTS OF THE WATER ANALYSES 0 B i Temperature ( C) 180 B ii pH and Rainfall (- log [Hj and mm respectively) 181 B iii Conductivity (VS cm-I) 182 2+ -1 B iv Calcium, Ca (Vg m1 ) 183 . 2+ -1 B v Magnes1um, Mg (Vg m1 ) 184 + -1 H vi Sodium, Na (~g ml ) 185 + -1 B vii Potassium K (~g ml ) 180 -1 Bviii Total Iron, Fe (~g m1 ) 187 3 l B xi Aluminium, A1 + (~g m1- ) 188 - -1 B x Chloride, Cl (Vg ml ) 189 2- -1 B xi Sulphate, S04 {Vg m1 ) 190 -1 B xii Trace Metals (~g ml ) 191 C. MINERAL ANALYSIS C i Sample Description and Treatment 192 C ii Mineralogy of the Samples 197 C iii Comparison of Pure Mineral Samples, After Brown (1961), with Selected Samples From Mam Tor 198 LIST OF FIGURES Preceeding Page Figure 1 Geology and Landslides of the Edale-Mam Tor Area. 19 Figure 2 Schematic Section Through Shafts and Soughs of 26 Odin Mine. Figure 3 A Landslide Classification Scheme. 28 Figure 4 The Major Features of a Rotational Slump 28 Figure 5 Location and Major Geomorphological Features of the Landslide at Mam Tor, Derbyshire. 28 Figure 6 Longitudinal Section through the Landslide and Retrogressive Units. 28 Figure 7 Total Dissolved Solids - Conductivity Relationship. 43 Figure 8 Cluster Analysis Hierarchy. 51 Figure 9 Precipitation Totals for the Study Period. 53 Figure 10 Variations in Temperature. 53 Figure 11 Variations in pH. 53 Figure 12 Variations in Conductivity. 53 Figure 13 Variations in Calcium Content. 53 Figure 14 Variations in Magnesium Content. 53 Figure 15 Variations in Sodium Content. 53 Figure 16 Variations in Potassium Content. 53 Figure 17 Variations in Total Iron Content. 53 Figure 18 Variations in Chloride Content. 53 Figure 19 Variations in Sulphate Content. 53 Figure 20 The Derivation of the Seven Groups of Waters. 57 Figure 21, Principles of Dilution Gauging. 58 Figure 22 Sequence of Chemical Reactions Affecting the 71 Composition of Water at Site 7. Figure 23 Soil Profile Above Retrogressive Units. 87 Figure 24 Location of Samples in Borehole 7 and 8 89 Figure 25 A Method for Separating <2~m clays. 92 Figure 26 Examples of Powder Diffraction Traces. 100 Preceeding Page Figure 27 Effects of Glycolating and Heating a < 0.5 11mSample (BH89) 102 Figure 28 Effects of Glycolating and Heating a < 0.5 11mSample (SP2) 102 Figure 29 Comparison of Four Goethite Samples 108 Figure 30 Eh-pH Diagram for Water and Iron Oxides 123 Figure 31 Eh-pH Diagram for Iron Hydroxides 125 Figure 32 Composite Eh-pH Diagram Illustrating the 'Solubility' of Ferric and Ferrous Ions. Total Activity of Iron = 10-3 molar 128 Figure 33 Eh-pH Diagram for Dissolved Sulphur Species. Total Activity of Sulphur = 10-1 molar 129 Figure 34 Composite Eh-pH diagram for the system S-Fe-H20 [S] = 10-1 m, .!.Fe] = 10-3 m 129 Figure 35 Method of Construction of Field Boundaries and the Plotting of Eh-pH Values of Mam Tor Waters. See Table 22 for Description of Samples 1-4 129 Figure 36 Eh-pH Diagram for Iron Hydroxides and Sulphides, Including Jarosite. After Van Breemen, 1972 130 Figure 37 Eh-pH Diagram Showing the Field of Natural Waters.