Hydrogeological framework and occurrence of groundwater in the Ethiopian aquifers

Abstract

Comprehensive hydrogeological investigations have been carried out in the Ethiopian volcanic terrain and associated quaternary sediments. The occurrence and distribution of groundwater are systematically analysed in relation to the geomorphological and geological setting within one of the world’s thickest volcanic provinces. The study starts from regional conceptualization to a more focussed analysis of four selected areas with distinct hydrogeological features. The chosen areas represent highland trap volcanic sequence characterized by multi-layer aquifers, intermountain sedimentary graben, fractured rift floor volcanics boarded by steep high-altitude plateau, and vast rift floor flood plain covered with alluvial sediment with adjoining gently-sloping pediments. The result revealed extreme variations in aquifer hydraulic characteristics and recharge rates. The flow and occurrence of groundwater is strongly controlled by the structure and geomorphological setup of the volcanic rocks and associated sediments. Groundwater flow in the rift and escarpment areas is largely controlled by faults. The recharge varies in a wide range between a few mm to 400 mm/yr. Despite the occurrence of permeable rocks and high recharge rates in some highlands adjacent to steep escarpments, the groundwater reserve is low due to the fast release of the recharged water to the rift plains through large open faults. Wide basic and acidic volcanics, alluvio-colluvial and lacustrine deposits form shallow unconfined aquifers, with transmissivity varying between 27 and 135 m 2/d. The thick trap series volcanics interbedded with river gravels and sands form deep confined and semi-confined aquifers locally with artesian conditions. Intermountain grabens and rift floor sediments associated with fractured volcanics form the largest aquifers under water table and semi-confined conditions. High well yields in deeper volcanic aquifers are often associated with regional fault lines and permeable sediments interbedded with the volcanics. Limited water quality analysis indicates that the groundwaters are Ca–MgHCO 3 type in the highlands, Na–HCO 3 type in the rift with local SO 4 and Cl enrichment. Hard waters are dominant in the basic volcanics. The highland waters have relatively lower ionic concentration (TDS ranging from 50 to 1200 mg/l). The rift waters have high salinity and fluoride, which is the major national water quality problem.