Geological and structural set-up of Kiserian-Matathia area and its influence on groundwater distribution and flow.

Abstract

The Kiserian-Matathia area is prone to subsequent tectonic movements which are related to the formation of the Great Rift Valley. These activities have dominated the geological history of the area since Miocene times and have contributed to the Geology and Structures in the area. Ground geophysical and hydrogeological investigations have been carried out in order to evaluate the significance of the rift activities on the geology and hydrogeology of the area. Faults have been delineated and mapped through the analysis and interpretation of aerial photographs. Inversion of ground resistivity soundings have revealed resistivities and thicknesses that are consistent with the geology. The area is underlain by volcanic rocks estimated to be of Tertiary age. The fractured and weathered trachytes and basalts, the sands and sediments comprise the aquifer formations in the area. Tuffs act as aquifers only when fractured. However, their aquifer characteristics are limited because they are generally impervious. The lithological stratigraphy in most of the area where resistivity soundings were carried out is represented, from the surface, by: soils and laterites; trachytes; and basalts with a . varied degree of fracturing and weathering. Sands and sediments occur, in most cases, at the contact zones between any two formations or intercalated within the formations. Fractured and weathered formations are excellent aquifers, and their resistivities range between 3 and 35 Q.m. Compact formations with resistivities ranging between 100 and 170 Q.m are aquifers only when fractured. Faults influence the borehole yield and the flow direction of groundwater. Boreholes with the highest yield are those located in areas immediately recharged by faults. The groundwater flows in lateral (E-W) and longitudinal (N-S) directions. The longitudinal flow is due to the influence of faults.