Hydrogeology of the Aynalem Wellfield, Tigray , Northern Ethiopia

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Addis Ababa Universty


The shortage of drinking water is one of the critical social issues concerning the Mekele town. Groundwater is the only source to fulfil the need so far. However, there is no sufficient water supply for the town. Therefore a detailed hydrogeological investigations of the well field supplying the Mekele Town and its reserve assessment with respect to the increasing demand is very important. In light of this and based on the available data of well logs, pumping tests, meteorological data and field observations an attempt has been made to map the area and characterize the aquifer parameters. The study area constitute limestone, shale & marl, and dolerite as main units with variable hydrogeological importance. Limestone is the main aquifer unit with hydraulic conductivity values ranging from 29-74 m/day while the dolerite is the second important aquifer with hydraulic conductivity values 0.02-1 m/day . The role of the shale and marl is confined to that part of the recharge and transition zone. Precipitation is the most important source of recharge. Nine percent (57 mm) of the total rainfall is detained by the soil and ultimately joins the groundwater. Channel loss manifested by the disappearance and reappearance of the Aynalem river the second important component of groundwater recharge. The presence of ponds in the recharge area and their construction on fault zones and cavernous limestones also contribute significant amount of water. Two main fault systems affect the area. The role of these structures is not only confined in controlling drainage patterns but also serve as groundwater conduits into and out of the catchment. The presence of cavernous features play an important role in the groundwater movement. Bedding planes and joints are also important vertical and horizontal pathways. Water quality assessment on a number of samples revealed that the well filed is entirely free from antropogenic pollution. They are basically within the acceptable limits for the use as drinking water and for irrigation. The water type varies form calcium carbonate to calcium sulfate as one goes form east to west indicating groundwater evolution. The results of water balance calculations and rough groundwater reserve estimation were compared with the population growth rate of Mekele and the corresponding estimated water demand. The outcome of this analysis can be stated as that the groundwater recharge by far overweighs the need