Mapping of groundwater through the integration of remote sensing and vertical electrical sounding in ASALs: a case study of Turkana south sub-county, Kenya

Abstract

Turkana South Sub-County falls in the arid and semi-arid lands of Kenya, which are characterized temperatures of 20°C to 41°C with an average of 30.5°C and precipitation in the range of 52 mm to 480 mm per year. The area has limited availability of water resources. The area has a land surface of 18,000 km2 and lies between Longitudes 35°10'00" and 36°45'00" East and between Latitudes 1°0'00" and 3°0'00" North. The study area faces immense difficulties in trying to meet its water requirements for her rapidly increasing populace, livestock needs and other developmental programmes. Surface water in the area is becoming unobtainable, thus the choice of groundwater exploitation. Indeed with the snowballing demand necessitated by energy, agricultural and livestock production needs in this area, there is an appeal into investigation for groundwater in this greatly remote and extensive area. Thus, the success in the exploration, development and management of groundwater in such a large area calls for such methods that can easily be used to zero down to exploitable targets. Given the expansiveness of the study area, remote sensing (RS) has been used to map lineaments which in turn have been used to deduce faulting affecting the geology of the area. The mapping by RS helped in identifying weathered zones, the fracture systems and fault zones sign of deeper weathered zones which are interpreted to be potential areas since mainly water is stored within fractures and the weathered zones in subterranean and in hard rocks. In the field topographic expressions of faults and fractures which include; joints, fractures, scarps, river channels and slope breaks were mapped and it was observed that there was a comparison of the features to the coincidence raster and aspect interpreted lineaments. The earmarked points were later investigated using vertical electrical sounding (VES) to establish their feasibility for groundwater availability and extraction. The investigated VES points in areas of high lineament density revealed decreasing resistivity with increasing depth of investigation, a confirmation of faulting. The data presented curves, type KH, type KK, and type Q. Further investigation was done in points of low lineament density, considered to be least affected by faulting and deep weathering, which turned out to be having increasing resistivity with increasing depth of investigation, an indication of layered lithology. The zones of low lineament density are represented by a type A curve, a type H curve, type HH. There is a good agreement in the results of remote sensing data and VES data models generated where the high lineaments density points concur with areas of good groundwater potential.