/ GEOPHYSICAL AND HYDROGEOLOGICAL INVESTIGATIONS FOR GROUNDWATER IN THE LAKE KENYATTA SETTLEMENT SCHEME, LAMU DISTRICT, COAST PROVINCE, KENYA. ■* • v ­ i BY CHRYSANTHUS M. N. GICHERUH ACCEPTED PCk L’BIS THESIS w ■ ?» %■ ••• tn k r>f.gft’ e ■ 3F PLACED TBUS \ND a Ci>V\ ONrVfcttSl'JL'V iilBKAHV. A thesis submitted in partial fulfilment for the degree of Master of Science (Geology) in the University of Nairobi NAIROBI 1993 0 $ \\lob , 4 -^ ££ G ' 3 GEOPHYSICAL AND HYDROGEOLOGICAL INVESTIGATIONS FOR GROUNDWATER IN THE LAKE KENYATTA SETTLEMENT SCHEME, LAMU DISTRICT, COAST PROVINCE, KENYA. " BY CHRYSANTHUS M. N. GICHERUH A thesis submitted in partial fulfilment for the degree of Master of Science (Geology) in the. University of Nairobi NAIROBI 1003 DECLARATION This is my original work and has not been submitted for a degree in any other University / A* The thesis has been submitted for examination with our knowledge as Uni­ versity supervisors Signed Ol c a j U PROF. S. J. GACIRI ABSTRACT This thesis presents results of geophysical and hydrogeological investigations for groundwater in the Lake Kenyatta settlement scheme, Lamu District, Coast Province, Kenya. During the Pleistocene, the area was a beach and had sand dunes which have since been eroded and filled up with sand giving the present undulating relief. Inversion of ground resistivity sounding data yields resistivi­ ties and thicknesses that are consistent with geology. The area consists mostly of highly fissured coral limestone 5-30 m thick underlying a low resistivity sub­ stratum comprising sands, clays, sandy clays and clayey sands. The deeper coral limestone layers show consistent low resistivities, perhaps due to hard water in the solution caverns. The geoelectric sections reveal a pinch and swell structure of the coral limestone underlain by marine sediments. The freshwater/saltwater interface consistently displays an undulating wavy structure at a dey 'h of 20 to 40 m below the ground surface and extends over the entire area. The freshwater aquifer is mainly found in the highly cavernous fossil coral reef present under most of the area underlain by freshwater-bearing marine sands and silty clays. The aquifer extends over an area approximately 10 km2, the freshwater below the lake inclusive; and has a thickness approximately 20 m. A hydrogeological basin trending NW-SE and one which coincides with the area of highest groundwater potential has been delineated in terms of geometry and extent. This trend coincides with the structural trend, thus suggesting that the groundwater flow is structurally controlled. From the piezometric maps and hydrographs, it is concluded that the lake level approximately corresponds with the groundwater table through seeps in the lake bed. Groundwater generally replenishes the lake from the north and discharges l towards the south. During storm rainfall and high lake levels, groundwater storage is partly recharged by inflow from the lake and deep percolation. Discharge from the lake follows the shallow channel running from the lake to the southern shore stops and only recharge of groundwater into the lake occurs mainly on the northern shore of the lake. The groundwater at the northern and north-eastern shore of the lake has low and stable salinity of electrical conductivity of 400 to 500 S/cm, increasing only relatively slowly during prolonged dry spells. Due to recharge by the lake water, groundwater on the south-eastern shore of the lake has a highly fluctuating salinity depending on the seasonal climatic conditions. Water from the existing boreholes smells of hydrogen sulphide. For further groundwater development in the Lake Kenyatta settlement scheme, it is recommended that boreholes for groundwater abstraction be sited within the Lake Kenyatta aquifer preferably along the struc- tual lines. A well field designed above the flood level of the area on the north coast would serve as a constant source of groundwater supply where the water quality is stable and the possibility of communication remote, besides a greater coral lime­ stone thickness. However, any abstraction should be subject to a meticulous safe yield calculation from a long duration testpumping. 11 ACKNOWLEDGEMENTS The idea of doing an MSc. research at Lake Kenyatta Settlement Scheme first occurred to me during a vocational field study with “Groundwater Survey (K) Ltd” in September 1989 where I came across a report on a short geophysical fieldwork previously carried out in the area. I therefore take this opportunity to thank the University of Nairobi for awarding me a scholarship to take up an MSc. course through which I was able to carry out this research. During the preparation of successive drafts of my MSc. thesis, the most gener­ ous helper was my supervisor, Dr. J. 0. Barongo. The treatment of the inversion technique is taken from his PhD thesis, by some stimulating graduate supervision. It is he who introduced me to the inversion theory, the singular value decomposi­ tion (SVD) algorithm and tensorial representation of resistivity and conductivity as applied to ground resistivity modelling. He also introduced me to computer modelling in the interpretation phase. I would like to thank him for his individual kindness he showed me and, with the exception of one or two convivial cases, for concealing any dismay at my dilating progress. He provided me with a great deal of patient encouragement while suggesting valuable alterations to certain sections of the text. He read all parts of my original manuscript at different stages and, from very fruitful discussions, made a number of very helpful criticisms. He made many other valuable contributions in the absorbing hours of discussion. I gratefully acknowledge the formal and informal assistance accorded to me in writing this thesis by my other supervisor, Prof. S. J. Gaciri. Besides the legacy from past work and lectures and experience in groundwater hydraulics, I had the good fortune to receive a constant flow of ideas and encouragement from him. Specific thanks are due to him for numerous improvements in the final manuscript. iii As my University supervisors, I find joy and significance in giving both Dr. J. 0. Barongo and Prof. S. J. Gaciri compliments from the bottom of my heart for such a segaceous supervision. Some very special thanks go to Mr. Pieter G. Van Dougen, Managing Direc­ tor of “Groundwater Survey (K) Ltd” for his broad interest and encouragement throughout my undergraduate studies and research period. His learned comments and positive criticisms of my manuscript helped improve its content and quality. I am sure his immense eminence derives from his vast cascade of scientific studies coupled with a long productive professional experience. To me, he is singled out as the man who introduced me to the geophysical methods applied to ground- water exploration and ottered me numerous opportunities to work in his company “Groundwater Survey (K) Ltd” during vacations during my undergraduate studies. He provided me with all the geophysical equipment 1 needed during my fieldwork together with an appreciable financial assistance, and offered me computer facili­ ties for the forward modelling. Moreover, he allowed me access to his rich library facilities. For all these, l express a heartfelt gratitude and wish him many more fruitful and satisfying years of professional practice. Further acknowledgements go to the GTZ-GASP Team Leader, Dr. Michael Van BoguslaWski for allowing me access to the monitoring data for the well and lake. Countless colleagues, friends and relatives have contributed positively to the accomplishing of my research study. It is impossible to mention all who directly or indirectly helped and encouraged me in this work. But I could mention George Nyangweso, Wilson Ngecu, Phillip Visser, Edwin Mukira, Kamau Gachie, Kagwe Githu, Mrs. Mary Gichinga, George Masila, John Gichinga, Peter Mbono and Joseph Mucheri, to all of whom I say a hearty thank you. In the course of my IV I Contents •* \i- TITLE * y / PAGE Abstract......................................................................................................... i Acknowledgements....................................................................................... iii Table of Contents.......................................................................................... vi List of Figures - ............................................................................................. xv List of Tables................................................................................................ xix CHAPTER 1 INTRODUCTION......................................................... 1 1.1 Statement of the problem...................................................... 1 1.2 Purpose and scope of the stu d y ............................................ 4 1.2.1 Introduction........................................................................... 4 1.2.2 Aims and objectives............................................................ 5 1.3 Area of study ........................................................................ 5 1.4 Communication..................................................................... 8 1.5 Physiography ........................................................................ 8 1.6 Climate.................................................................................... 8 1.7 Literature review .................................................................. 9 vi 2 GEOLOGY AND HYDROGEOLOGY 21 Geology...................................................................................