•4 HYDROGEOLOGICAL INVESTIGATIONS in a GRANITE
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University of Plymouth PEARL https://pearl.plymouth.ac.uk 04 University of Plymouth Research Theses 01 Research Theses Main Collection 1983 HYDROGEOLOGICAL INVESTIGATIONS IN A GRANITE CATCHMENT, DARTMOOR, DEVON Alexander, Jean http://hdl.handle.net/10026.1/1787 University of Plymouth All content in PEARL is protected by copyright law. Author manuscripts are made available in accordance with publisher policies. Please cite only the published version using the details provided on the item record or document. In the absence of an open licence (e.g. Creative Commons), permissions for further reuse of content should be sought from the publisher or author. 4 •4 HYDROGEOLOGICAL INVESTIGATIONS in a GRANITE CATCHMENT, DARTOOOR, DEVON. c Jean Alexander, B.Sc, M.Sc, Assoc. M.I.G. A thesis submitted in partial fulfillment of the requirements for the de^ee of Doctor of Philosophy of the Council for National Academic Awards. Dint: 1 Collaborating Est^b-lrshment South V/est Water - September I983, PLYMOUTH POLYTECHNIC Department of Geographical Sciences 6 I • r \ PlYMOur/Y POLyTECllNIC No ^'500143-1 Class. No. Memorandum The material presented In this thesis is the result of my own independent research work carried out in the Department of Geographical Sciences» Plymouth Polytechnic, and in the Narrator Brook Catchment, an experimental small drainage basin, on Dartmoor, Devon, during the period October 1978 - September 1980. The project was supervised by Dr. L. Ternan and Mr. P. Sims, Department of Geographical Sciences, Plymouth Polytechnic, with Mr. C. Tubb as the external advisor from the collaborating establishment. South West Water (S.W.W.). Any reference to previous work, published or unpublished is given full acknowledgement. To my parents "he dwelt from the beginning of Arda In the outer Ocean, and still he dwells there. Thence he governs the flowings of all waters, and the ebbing, the courses of all rivers and the replenishment of springs, the distilling of all dews and rain in every land beneath the sky." From "The Silmarillon" J.R.R. Tolkein "There is no life without water. It is a treasure indispensable to all human activity." European Water Charter, Council of Europe, May 6th 1968. p Abstract: "HYDROGEOLOGICAL INVESTIGATIONS in a GRANITE If CATCHMENT, DAHTMOOR, DEVON". Jean Alexander I A network of 13 observation wells were utilized in the Narrator Brook i Catchment, Dartmoor, Devon, to monitor groundwater level fluctuations, in an attempt to assess the water resource potential of a weathered granite aquifer. Meteorological parameters, soil moisture data and reservoir ^A' levels together with waterlevel observations were used in a stepwise regres- sion model with a view to predict and account for the behaviour of ground• water levels in weathered granite areas. Variations in groundwater and surface water chemistry were used as a basis for the hydrogeological grouping of the wells in the valley, whose individual charateristics were determined by site topography, lithology and slope position. Spring and river discharges were recorded and their recession characteristics determined. In a typical year approximately 7J^ of total discharge in the Narrator Brook channel is derived from groundwater, • ^ and 32% of the effective rainfall on the catchment finds its way into the J stream channel. The potential storage Rapacity of the weathered granite valley aquifer was estimated as 570x lOr m3 and an average value of (A 557 X id?. m3 per year was calculated for potential recharge. Igneous bedrocks are not us\ially considered to make significant groxind- water contributions to riverflow, but Dartmoor in general, and the Narrator Brook Catchment in particular, have extensive deposits of weathered granite in situ overlain by periglacial deposits and river alluvium. These materials enhance the waterbearing properties of a granite based catchment, and combine to provide small isolated but significant valley aquifers. i Acknowledgements: My thanks to geography, geology and ecology technicians for their assistance with fieldwork and laboratory work, particularly Mr, R, Cockerton and Mrs* J. Edwards for their help during the drilling and well completion procedures. To Mr. L. Cook, Mr. J. Harper and assistants in the Polytechnic's workshops, for their advice and assistance in the construction of field equipment used during this investigation. To Mr. J. Kltching of the Hydrographlc Department of the School of Maritime Studies, Polymouth Polytechnic, and the final year undergraduates (1979) for surveying in the well sites in the Narrator Brook Catchment. To Dr. C. Bowler, Geology Department, for the use of the Minute- man-drill, and to Mr. D. Clark for his services as a driller during inclement weather conditions in the catchment. To Mr. E. Durrance, Geology Department, Exeter University for the loan of the hammer seismograph. To Mr. C. Tubb and Mr. T, Jones for their advice and practical guidance during slug-testing procedures in the catchment, and to S.W.W. for the loan of the pressure transducer and recording equipment. My thanks also to Mr. G. Taylor (S.W.W.) manager at Burrator Reservoir, for the access to reservoir level data, co-operation during the drilling of the observation boreholes, and for his additional valuable local knowledge of the catchment area. To the staff of the Computer Centre, Plymouth Polytechnic for their willing assistance, and to Dr. M. Kent for his guidance in the use of the Geog-Stats Package. Special thanks are extended to Mr. M. Coard for the use of, and subsequent adaptation of his stream discharge plotting computer program, and to Mr. A. Williams for many helpful discussions on the Narrator Brook Catchment. I am especially grateful for the skills and patience of Miss. S. Webber and Mrs. J. Jones in the preparation of some of the diagrams, and to Paula Barnes and Jane Moore who typed this thesis. My everlasting gratitude to Howard for his practical assistance and support at each stage of this investigation, and to his acceptance of the Narrator Brook Catchment as an exclusive topic of conversation. My thanks to colleagues in the Fluid Processes Unit of the Institute of Geological Sciences, for their continual encouragement and support. Lastly, I would like to thank two supervisors. Dr. L. Ternan and Mr. P. Sims, Department of Geographical Sciences, Plymouth Polytechnic, for their attention, discussions, recommendations and perserverance throughout the duration of this research project. / Drilling in the Narrator Brook Catchment Chapter 1 INTRODUCTION TO GROUNDWATER STUDIES IN GRANITE AREAS Introduction 1-5 1 Properties of Aquifers 5-8 2 A review of Aquifer Materials in the U.K. 8- 9 3 Unconsolidated Aquifers 9- 13 4 Characteristics of Alluvial Aquifers 13- 14 5 Development of Alluvial Aquifers 14- 17 2 Water Supply from Granite Areas 17 2:1 Sound Granite 17-20 2:2 Decomposed Granite 20-22 2:3 Decomposed Granite Areas in the U.K. 22- 23 2:4 Water in Granites of South West England 23- 26 2:5 Water in Weathered and Solid Granites 26-28 2:6 Recent Granite Investigations 28-33 1:3 AtBg"aiia~ObTe'cCrve"s 33- 34 1:3:1 Broader terms of reference for hydrogeological Investigations in weathered granites 34- 35 1:3:2 Groundwater supply in the South West 35- 36 1:3:3 Surface water supply in the South West 36 1:3:4 Dam Construction 36- 37 1:3:5 Waste Disposal Interests 37- 38 1:3:6 Mining and Geothermal Interests 38- 39 1:4 Conclusions 39-42 Chapter 2 THE HYDROGEOLOGICAL BASIS 2J 1 Introduction 43-44 2: 1 1 Solid Geology 45-49 2 1 :2 Alteration of the Granite 49-50 2 1 3 Weathering Processes 50-52 2 1 :4 Nature of Decomposed Granite 52-54 2 1 :5 Classification of Weathered Granite 54-55 2 :1 :6 Other Superficial Deposits 55-56 2:2 Hydrogeological Properties of Valley Deposits on 56 Granites in South West England 2:2:1 Burrator and Sheepstor 57-61 2:2:2 Hart Tor Valley 62 2:2:3 Cowsic Valley 62 2:2:4 Swlncombe 62 2:2:5 Huccaby 62 2:2:6 Taw Marsh 62 2:2:7 Fernworthy Dam 67 2:2:8 Sibleyback 67-68 2:2:9 Drift 69 2:2:10 Stithians 69 2:2:11 Summary of Site Conditions 69-70 2:2:12 Main Characteristics of Solid and Weathered Granites 70 (i) Fissures and joints 70-71 (11) Kaollnised sections 71 (ill) Veins, stringers and intrusions 71-72 (iv) The junction of weathered and solid sections 72 2:2:13 Water Movement in Weathered Granite 72-74 2:3 Factors Related to Hydrogeological Responses in 74-76 Weathered Granite Areas as Typified" by the NarratorBrook Catchment 2:3:1 Topography 76 2:3:2 Climate 76-82 2:3:3 Soils 82-86 2:3:4 Peat Deposits 86-88 2:3:5 Vegetation 88-89 2:3:6 Nature of the Valley Infill in the Narrator Brook 89 2:3:7 Borehole evidence 89-91 2:3:8 Particle Size Analysis 91-93 2:3:9 Mining Activities 93-95 2:4 Groundwater and Surface Water Relationships 95-96 2:4:1 Surface Water 96-98 2:4:2 Groundwater 98 2:4:3 Laboratory Derived Values of Porosity and 98-100 Hydraulic Conductivity of Granitic Material in the Narrator Brook Catchment 2:4:4 Gross Hydraulic Conductivity Determinations 100-101 2:4:5 Groundwater Transit Time 101-102 2:4:6 Watertable Fluctuations 102 2:5 Conclusions 104-105 Chapter 3 LABORATORY TECHNIQUES 3:1 Introduction 106 3:1:1 Water Level Data 106- 107 3:1:2 Well Design Considerations 107- 109 3:2 The Observation Network In the Narrator Catchment 109- 112 3:2:1 Installation of Observation Wells 112- 113 3:2:2 Well Dimensions 113- 114 3:2:3 Well Casing Materials 114- •116 3:2:4 Well Screens 116- •118 3:2:5 Slot Size Determination 118 3:2:6 Screen Length, Position and Effective Open Area 118- 121 3:2:7 Formation