Gordon Mitchell
Total Page：16
File Type：pdf, Size：1020Kb
ASPECTS OF SOLUTE MOVEMENT IN THE BRITISH UPLANDS by GORDON MITCHELL Submitted in accordance with the requirements for the degree of doctor of Philosophy. School of Geography, April, 1991 University of Leeds. ABSTRACT The aim of this research is to determine the process by which runoff draining parts of the UK uplands becomes discoloured. Recent increases in colour are perceived as a problem due to the frequency with which EC surface water directives are breached, to the increased cost of meeting standards, to an increase in consumer complaints, and due to the possible deleterious health effects of consuming coloured potable water. The work was conducted over the last four years, largely in the Upper Burn and Nidd valleys, north Yorkshire, and at the University of Leeds. Existing knowledge on the chemical nature and environmental genesis of coloured water is reviewed, and related to the chemical nature of coloured waters examined in this study. Analysis of water quality archives and direct monitoring of selected catchments was conducted enabling the spatial and temporal distribution of coloured waters to be determined. A tentative process theory was produced, and tested by controlled laboratory investigations. Coloured runoff is derived from Winter Hill soil, raw acid oligo-fibrous peat, and is most intense from areas with 0 slopes 5 5 and high drainage densities. The seasonal pattern of colour is highly regular, and is a product of rainfall and soil moisture deficit. Colour is strongly associated with discharge, and demonstrates the operation of a catchment flushing mechanism. Runoff is discoloured in a three phase process; colour production, solubilisation and removal. Colour is a product of organic decomposition, dependent upon microbiological activity, moisture deficit, temperature and peat structure. Solubilisation of colour is dependent upon solution pH, temperature, the chemical structure of organic molecules and the availability of iron. The removal of decomposition products l as colour I is strongly dependent on the moisture status of peat, and its ability to rewet after drying. The implications of these findings for future colour levels and for catchment and catchwater management are discussed. "When water chokes you, what are you to drink to wash it down?" Nicomachean ethics, Aristotle (384-322 BC). ACKNOWLEDGEMENTS I would like to express my thanks to those people who have contributed to this research. The work was funded by the Yorkshire Water authority, without whom it would not have been possible. Numerous people within the authority have offered constructive advice and comment and many have provided archival material. Particular thanks are due to Tony Edwards, David Lyons, Janet Warne, Ian Stevens, Graham Dixon, Geoff Roberts and Matt Brown. Access to the Upper Burn valley was permitted by the Swinton Estate, who are gratefully acknowledged. Within the University I am grateful to Dawn Martin who assisted in the field and who always complained less about the weather than I did. Discussions with Pam Naden were of particular value. Mandy Kelly was an enthusiastic assistant in the laboratory, while Tim Hadwin and John Dixon produced some of the diagrams. My greatest thanks go to my supervisor, Adrian McDonald, who was generous with both his time and expertise and remained good humoured throughout, and to John and Shirley Mitchell, a source of constant encouragement and support. March 1991 CONTENTS Page CHAPTER 1. AIMS AND INTRODUCTION 1.1 Aims 1 1.2 Introduction 2 1.3 The problems arising from colour 10 1.3.1 Customer complaints 10 1.3.2 Increased operating costs 13 1.3.3 Capital expenditure 15 1.3.4 Problems with chlorination 16 1.3.5 Acidification 18 1.3.6 Metal mobilisation 19 1.3.7 Conclusion 20 CHAPTER 2. THE NATURE OF DISCOLOURED WATER 2.1 The chemical composition and environmental genesis of coloured water: literature review 21 2.1.1 Introduction 21 2.1.2 The origins of natural colour in water 23 2.1.3 The chemical nature of organic colour 25 2.1.4 The contribution of iron to discolouration 33 2.1.5 Water discolouration and catchment hydrology 35 2.1.6 Water discolouration and reservoir processes 41 2.1.7 Conclusion 45 2.2 Chemical and physical characterisation of locally coloured waters 48 2.2.1 Colour changes during storage 48 2.2.2 Dilution behaviour 49 2.2.3 Titration of coloured water with strong acids and bases 52 2.2.4 Particle size analysis 2.2.4.1 Centrifuge studies 55 2.2.4.2 Gravimetric settlement 56 2.2.4.3 Ultrafiltration 57 2.2.5 Typical ionic composition 61 2.2.6 FT-IR analysis 65 2.2.7 Conclusion 68 CHAPTER 3. THE SPATIAL DISTRIBUTION OF DISCOLOURED RUNOFF 3.1 Aims and introduction 71 3.2 Regional distribution of discoloured water 73 3.2.1 Methodology 73 3.2.2 Results and discussion 80 3.2.3 Conclusion 91 3.3 The spatial distribution of coloured water within a single catchwater system 93 3.3.1 Initial catchwater studies 93 3.3.2 Characterisation of non-point colour sources in the upper Burn valley, North Yorkshire 95 3.3.2.1 Introduction and methods 95 3.3.2.2 Results and discussion 101 3.3.2.3 Regression model 105 3.3.2.4 Relationship between colour and other water quality variables 107 3.3.2.5 Conclusion 111 3.4 Predicting the spatial distribution of coloured runoff 113 3.5 Sub-catchment studies 120 3.5.1 Micro-snapshot exercise 120 3.5.2 Colour investigations on severely eroded moorland 123 3.5.3 Additional sub-catchment studies 125 3.6 The spatial distribution of discoloured water : Conclusion 126 CHAPTER 4. THE TEMPORAL DISTRIBUTION OF COLOUR 4.1 Aims and Introduction 129 4.2 Analysis of long term colour records 131 4.3 Analysis of scar outflow record 143 4.4 Response to short duration storm events 144 4.5 The temporal distribution of colour at the field plot scale 155 4.6 Seasonal complexation of colour and iron 160 4.7 The impact of catchment characteristics on the temporal distribution of colour 165 4.8 Conclusion 168 CHAPTER 5. CONTROLLED INVESTIGATIONS ON THE GENERATION AND RELEASE OF COLOUR FROM WINTER HILL PEAT 5.1 Introduction and objective 171 5.2 Colour extraction from peats 172 5.2.1 Colour extraction at varied peat-water ratios 172 5.2.2 Colour extraction activity rates 176 5.3 Colour output from prolonged peat leaching 182 5.3.1 System calibration 183 5.3.2 Response to varied 'rainfall' rates 185 5.3.3 Response to change in 'rainfall' rate 190 5.3.4 Variation of peat soil moisture 192 5.3.5 Variation in peat storage temperature 197 5.4 Discolouration of water by peat following induced drought and rainfall simulation 199 5.4.1 Methodology 200 5.4.2 Results 202 5.4.3 Discussion 207 5.4.4 Implications for moorland management 214 5.5 The effect of temperature on colour release during rainfall simulation 215 5.6 The effect of methods of peat drying on colour generation 216 5.7 Peat rewetting potential 219 5.7.1 Methodology 221 5.7.2 Results 222 5.8 Comparison of colour and pH from surface and deep peats following naturally induced drying 229 5.9 The impact of soil and water pH on drainage water colour 232 5.9.1 The effect of drought on drainage water pH 232 5.9.2 Prolonged leaching with near neutral and acidic rainfall 235 5.10 The generation and removal of colour from peat under controlled conditions: Conclusion 238 CHAPTER 6. THE ROLE OF SOIL MICROORGANISMS IN COLOUR PRODUCTION 6.1 Aims and introduction 241 6.2 Temperature dependent colour extraction 244 6.3 Comparative microbiological activity in air dried peats 247 6.4 Colour output following microorganism control 6.4.1 Wet-heat sterilisation 254 6.4.2 Bacteria control using sodium hypochlorite 255 6.4.3 Bacteria control with copper sulphate 256 6.4.4 Antibiotics 260 6.5 Respirometer studies of peat decomposition 266 6.5.1 Methodology 267 6.5.2 Comparative decomposition rates of peat in an aerobic and anaerobic environment 269 6.5.3 The influence of moisture on decomposition 272 6.5.4 The effect of rewetting on decomposition 274 6.5.5 The influence of antibiotics on decomposition 277 6.6 Conclusion 279 CHAPTER 7. CONCLUSIONS AND IMPLICATIONS 7.1 Summary of conclusions 282 7.1.1 Character, sources and timing 282 7.1.2 The process of discolouration 284 7.2 Implications for management 288 7.3 Implications for the long term trend in colour 289 7.4 Implications for future research 291 REFERENCES 293 APPENDICES 316 LIST OF FIGURES Page Figure 1.1 Principal sources of water within the 6 Yorkshire supply region. Figure 1.2 Raw water colour at Thrum Hall water 9 treatment works, 1975-85. Figure 1.3 Mean annual coagulant cost at Chellow 9 heights water treatment works, 1975-85. Figure 2.1 Effect of storage temperature on colour. 50 Figure 2.2 Effect of light intensity on water 50 colour during storage. Figure 2.3 Dilution of low and high colour waters 51 with water of no colour. Figure 2.4 Dilution of low colour water with water 51 of high colour. Figure 2.5 Colour change during acid-base titration 54 Figure 2.6 Change in pH of low and high colour 54 water during base titration. Figure 2.7 Colour decay within a water column. 58 Figure 2.8 Colour change during ultrafiltration. 60 Figure 2.9 Water pH change during ultrafiltration 60 Figure 3.1 Reservoir locations in regional analysis 74 of the spatial distribution of colour.