Groundwater Resource Assessment In Hard Rock Terrain Using Conventional, Remote Sensing And GIS approach: A Case Study In Southern Peripheral Part Of Ethiopia

Khalid Adem
Lecturer, Hydrogeologist, Remote sensing and GIS Expert
Addis Ababa University, Ethiopia
adem@geol.aau.edu.et

Abstract :
Integrated studies were implemented for studying the groundwater resource in the drought prone peripheral part of Ethiopia, Negelle Borena. The main objective of this study is to assess groundwater resource using the results from multi-source data (satellite images, field measurements, hydrogeological and geochemical surveys).
Geological studies have revealed that basement rocks consisting of gneissic and schist composition characterize half of the catchment. The basement is dissected by numerous fractures systems (NE-SW, NNE, SSW and E-W) and granitic intrusive which facilitate movement of groundwater and contribute towards spring development. On the eastern portion of the study area, the Phanerozoic cover outcrops showing an increase in thickness up to 500m deep further towards the east. It consists of calcite rocks, clay, sand, and loam with intercalating shale units. The Phanerozoic unit constitutes aquifers, which facilitate deep groundwater systems. Analysis of hydrometeorological components shows that mean precipitation for the catchment is about 647mm per annum. The area is characterized by bimodal type of rainfall, and the high proportion of the total amount of the annual rainfall comes from the first rainy season that lasts from March to May. Temperature in the study area varies from 13.9 o C low to 29.7 o C high. Actual evapotranspiration is evaluated using the simple “book keeping” Thornwait and Mather method and is found to be about 93.6% of the annual input from the precipitation. The mean annual recharge to the catchment is 31.4mm and about 9.2mm of water leaves the catchment as surface runoff. Qualitative analysis has been conducted to characterize the different hydrolithologic units. From the analysis, five hydrostratigraphic units have been characterized based on porosity, permeability and storage capacity. Recharge area in the catchment is mainly confined to the Phanerozoic cover and the discharge areas are in the basement terrain. Application of the GIS and remote sensing shows that movement of groundwater in the study area is mainly controlled by the structures. Overlay of the groundwater table data, structures and vegetation pattern from NDVI (Normalized Difference Vegetation Index) image shows that shallow water table conditions and potential points for groundwater extraction is along the NE-SW structures in the basement terrain which are mostly concentrated in the southwestern parts of the catchment. This orientation coincides with the regional spreading vector of the Main Ethiopian Rift system. These structures collect groundwater from other lineaments and transport the water along their margins towards the topographic depressions.
A series of water types was identified in the study area, hydrochemical studies reveal that the hand dug wells mainly on the basement show variety of water types calcium-bicarbonate, calcium-chloride-bicarbonate and calcium-sodium-chloride-bicarbonate attributed to the arid condition of the area. The boreholes show calcium-bicarbonate and calcium-sulphate type of waters leaching the limestone unit. Although most of the water points have fresh waters the computed TDS and measured EC values show high values with most of them near transition the range (800-1000mg/l) from fresh to the brackish type of water. Most of the waters have acceptable limits in major ion concentration for drinking purposes. Agricultural and domestic wastes are potential pollutants for the groundwater system. Results from hydrochemical analysis further reveal the influence of structures on groundwater movements in the basin.
The intersection on other lineament orientations and the NE-SW structures are considered to be the potential sources due to the effect of double porosity, the NE-SW structures existing at topographically depressed (south western) parts of the study area are also considered to be potential sites, which is in agreement with the available ground data.