Study of groundwater distribution in Nairobi urban using geoelectrical method

Abstract

The Nairobi area is covered by a succession of lavas and pyroclastics of Cainozoic age, These lavas are genetically associated with the rift tectonic movements. Generally, these rocks form good aquifers where they are weathered, fractured and in contact with the old land surface. Thus, the distribution and flow of the ground water is governed by the underground structures which have been identified in this study. A total of 35 sounding stations were analyzed to determine resistivities and thicknesses that could give a geologically meaningful subsurface structure. To meet this goal, analytical results from the sounding stations along selected directions were used to construct two-dimensional vertical sections of the subsurface geo-electric structure. It was then possible to get from this structure the possible geological information, Resistivity sounding measurements were interpreted both qualitatively and quantitatively using curve marching this techniques and a computer program based on the linear inverse theory, to determine the physical parameters of the subsurface layers. Four geoelectrical units with contrasting thicknesses and resistivity were identified. The bottom unit which underlies the whole of the study area is a very conductive layer forming the main aquifer in the area. In addition, geoelectrical profiles have identified horst and graben structures that can effect the flow of the water. Thus hort structures normally effect lateral flow of water, while the grabe structures support flow from out side into the basin.