GROUNDWATER POTENTIAL INVESTIGATION USING INTEGRATED GEOPHYSICAL METHODS; THE CASE STUDY OF ZALA GRABEN, GOFFA ZONE, SOUTH ETHIOPIA.

Abstract

Groundwater is the essential resource to support agricultural activities, ensure food security and industrial consumption as surface water is facing challenges with increased and variable climate changes in arid and semi-arid areas like the Zala Graben. This study aims to investigate groundwater potential using the integrated geophysical approach in Zala graben, Goffa zone, south Ethiopia. The methods applied in this study include the integration of geological mapping, hydrogeological investigation technique and geophysical approaches. The local geology of the study area is assessed and mapped to know the effect of surface lithologies and geological structures on groundwater occurrence and movement. The aquifer of the study area was also mapped and highlighted to determine the aquifer productivity and distribution throughout the study area. From geophysical approaches, Magnetic survey of 550 data points collected to identify geological structures, 16 vertical electrical sounding (VES data) to map subsurface lithology for groundwater detection and 2D electrical imaging to identify low resistivity zones were employed to delineate and investigate the groundwater potential of the Zala Graben. The Analysis of geophysical surveys showed that, geological structures and features like fault, circular features, fractures and weak zones (Magnetic survey) were found around Galma area and surroundings which may affect groundwater occurrence as well as its dynamics. VES results showed that, there is a low resistivity zone which shows a saturated zone indicating groundwater potential in different localities where the VES survey took place. Also, 2D electrical resistivity imaging showed the same thing having low resistive zones telling groundwater potential areas. The results of pseudo sections indicating apparent resistivity distribution, geoelectric sections modeled weathered ignimbrite and trachyte with lithologies of 3 to 4 layers having different degrees of weathering and aquifer thickness of 60m to 120m. It is recommended that well sites to be placed in Galma, Daboch and Mella. Further research should be conducted especially hydrogeology of area to understand groundwater flow dynamics and direction for good groundwater management.