Geophysical, geological and hydrogeological studies of groundwater distribution and flow in Nairobi urban area

Abstract

The study area is covered by a succession of lavas and pyroclastics of Cainozoic age. These volcanic lavas are associated with tectonic movements related to the rift system. Generally, these rocks do form groundwater potential, when they are weathered, fractured and where they form contact zones between two successive lavas and or Old land surfaces. Thus, the distribution and flow of the ground water are governed by the rock structures, which are the subject of this study. The Resistivity survey was carried out in the research area and curve marching with computer program based on the linear inverse theory were used to determine the physical parameters of the subsurface layers. Geophysical and geological data analyses have identified three geoelectrical units with contrasting thicknesses and depths. The upper and the lower units are conductive forming underground aquifers. The lower conductive layer which underlies the whole of the study area forms the main aquifer in the area. Geoelectrical profiles have also identified horst and graben structures which affect the flow of the water. The horst structures normally control the lateral flow of ground water, while the grabens determine the drainage pattern.