Geological and structural influence on groundwater distribution and flow in Ngong area, Kenya

Groundwater is increasingly becoming an important resource within and around the Ngong area, of Kenya. This is mainly as a result of increase in human population due to rural-urban migration, industrial expansion and farming activities. Ngong area forms a water catchment zone to the west of the city of Nairobi in Kenya. The geology of the area is comprised of volcanic layers of basalts, trachytes, phonolites and tuffs all overlain by thick layers of clay soil. The soil retains water for an extended period of time after the rains and this partly recharges aquifers in the area. Basalts and trachytes form good aquifers whereas tuffs are aquifers only when fractured. Faults delineated from a study of aerial photographs trend in a North-South direction in conformity with the structural pattern of the eastern branch of the Great Rift Valley. The influence of the faults on groundwater is two fold. They act as drainage channels to the flow of groundwater and also as aquifers in the area.