Geo-electric Investigation of the Aquifer Characteristics and Ground Water Potential of the Lake Chala Watershed, Taita Taveta County

Abstract

Groundwater systems are composed of complex set of interactions between physical environment such as soils, geology, precipitation, vegetation and topography and social economic activities exerting pressure on the physical environment. The interactions between these elements influence the hydrological cycle directly or indirectly and pose complex uncertainties to occurrence, movement and storage of groundwater. Lack of knowledge about the geographical unit responsible for groundwater occurrence, movement and storage within and around the Lake Chala watershed poses a major uncertainty in the analysis of and thus casts a dilemma in the management and conservation of groundwater within the watershed. This study was therefore carried out to evaluate the hydrogeological characteristics of the aquifers responsible for the recharge of Lake Chala so as to better understand the groundwater system of its watershed. The aim of the study was to identify the locality of groundwater potential zones, determine the hydrogeological characteristics of the groundwater potential zones and to quantify groundwater potential of the Lake Chala watershed. This was achieved by; locating surface groundwater potential zones through integrating factors influencing groundwater occurrence, movement and storage into Geographical Information Systems (GIS) and Remote Sensing platform, determining hydrogeological characteristics using Vertical Electrical Soundings (VES) and groundwater quantification using Darcy’s Law. The results revealed that Lake Chala watershed is composed of relatively high groundwater potential zones with the very high and high potential zones occurring near the Lake. These potential zones contained high yielding water bearing layer that is made up of highly weathered ryholite to moderately weathered basalt and volcanic ash. The results revealed that the water bearing layer had a mean, minimum and maximum resistivities of 93, 43 and 182 ohm, depths of 0.8, 142, and 48.30 m, thicknesses of 0.8, 91, and 32.09 m, aquifer elevation of 507, 1017, and 798 m.asl, transmissivity of 0.00024, 0.0273, and 0.00963m2/day and porosity of 23, 49, and 35% respectively. This layer is located within Lake Chala ground watershed which covers an area of 23.6km2. It has a recharge zone that contains recharge aquifer covering 13.85 km2, the discharge zone which contain discharge aquifer covering 5.55km2 and the lake covering an area of 4.2 km2. The mean groundwater flow within the Lake Chala ground watershed was estimated at 0.46m3/s for recharge and 0.32m3/s for discharge; while the equivalent mean annual recharge and discharge were 14.28Mm3/year and 10.16Mm3/year respectively. In conclusion, the study showed that Lake Chala watershed has a high groundwater potential with a thick and a high water bearing layer. This water bearing layer is a good reservoir as well as a good conduit system. These results are instrumental in providing information on the potential for groundwater resources within Lake Chala watershed which are useful in sustainable exploitation and conservation of the resource.