Groundwater vulnerability analysis of the tana sub-basin: An application of drastic index method

Abstract

In the Blue Nile River basin and particularly in the Tana sub-basin, Ethiopia, groundwater development and use for various purposes is increasing. This calls for understanding of how the groundwater system is functioning including its vulnerability to contamination. In this chapter, the vulnerability of the groundwater resources in the Tana sub-basin, upper Blue Nile River basin is evaluated with the application of the DRASTIC index method. Geographic Information System (GIS) application is implemented to prepare the different layers of the DRASTIC model and to calculate the DRASTIC index. Accordingly, the high DRASTIC index values indicate the sites highly susceptible to pollution in the presence of contaminant sources. The DRASTIC index in the Tana sub-basin is in the range of 66–120. The highest DRASTIC index is indicated in the southern part of the Tana sub-basin. The southern part, where the higher DRASTIC index is recorded, is known with highly porous vesicular basalt, lower soil thickness, and near-surface water table. The mountainous regions and the floodplain in the eastern part of the sub-basin exhibit lower vulnerability. The thick clay layers, which cover the floodplains of the sub-basin, have a significant role in purifying the water before it reaches the groundwater system.