Geophysical investigations of Suyien Earthdam in Maralal, Samburu County, Kenya

Abstract

The Current economic and population growth has increased the water supply gap in Maralal, Samburu County. Water sources include boreholes, pans and the Suyien dam. Despite the high rainfall in the area, Suyien dam has never filled since its construction in March 2010. This study investigated the geological factors hindering the dam’s impoundment of water. The aim was to determine the characteristics of the subsurface compromising this process. A desk survey of existing geological and hydrological reports as well as topographic and geological maps was undertaken. Analysis of laboratory tests, aerial photographs, electrical resistivity and trial pits was done. Results showed that the topography, dotted with several sinusoidal hills comprises Mozambiquan rocks, igneous intrusives, and Paleogene-Quaternary volcanics and sediments. Neo-Proterozoic and volcanic formations had surface joints and fractures implying secondary porosity and permeability. Hydrogeological information showed boreholes sited near rivers or laggas, yield about 1.4-2.5 m3/hour and about 4 m3/hour in volcanic rocks. Geophysical investigations showed subsurface rock sequences were fractured allowing groundwater leakage. Sediment deposition and vegetation growth at the dam's entrance also block water drainage into the dam. Suggested mitigation include, unblocking the dam’s entrance to allow in surface runoff and the sealing of fractures with waterproof cement.