THE HYDROGEOLOGY OF WONJI WETLAND AND GROUNDWATER LEVEL CHANGE, MAIN ETHIOPIAN RIFT: RESULTS FROM 2D TOMOGRAPHY AND ELECTRICAL SOUNDING METHODS

Rise of groundwater level has recently becomes a serious concern at Wonji irrigation field, main Ethiopian Rift. An integrated study based on geophysical resistivity methods is conducted at Wonji irrigation field aimed at understanding the link between irrigation water and the shallow aquifer system, a research intended to validate the current concern of groundwater rise and improve the uncertainty of understanding the extent and direction of groundwater-surface water interaction. The vertical and horizontal contacts between the different geological series of the Wonji area are resolved with two-dimensional high-resolution geophysical imaging.

Result from both 1D sounding and 2D tomography show low in resistivity of layers with narrow ranges in resistivity variation (1-5Ωm) which corresponds to a homogeneous layer saturated with saline water, salinity due to groundwater evapotranspiration. The geoelectric sections reveal two fault systems running NW-SE and N-S directions which impede lateral groundwater flow. Furthermore, groundwater is converged toward the Wonji irrigation site strained by these fault systems. The results of geophysical survey show the strong hydraulic connection between irrigation water and the shallow unconfined aquifer as well as hydraulic link among the local and regional flow systems.