Browsing by Author "Hagos, Ermias"

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    Hydrogeology of Mehoni Sub-Basin and Lake Ashange Catchment in the Rayavally (Northenr Ethiopia)
    (Addis Ababa University, 2005-06) Hagos, Ermias; Ayenew, Tenalem (Professor)
    The study area is found in northern Ethiopia at the northern portion of the Raya valley, which forms its own trough at the western shoulder of the Afar Depression. Its total aerial coverage is about 1302 krn2 . It is largely covered by the Mehoni Sub-basin, which is surrounded by volcanic hills with sediment filled depression at the center. The Lake Ashange basin is also located on the south-western highlands of the study area. T~e main objective of this work was to study the overall hydrogeological and hydrochemical setup of the study area. The overall water balance of the study area was computed with an aim of estimating the amount of annual recharge to the groundwater. Accordingly, the mean annual rainfall as the only water input into the study area was estimated to be 722.42 mm1year. On the out put side; the actual annual evapotranspiration was estimated to be 687.24mm1year, the annual surface water out flow was estimated to be 52.3 MCM, the annual lake surface evaporation was estimated to be 16.73 MCM and the annual bareland evaporation was assumed to be half the lake surface evaporation. Finally, a annual recharge to the groundwater of the study area is approximated to be 60.9 MCM. The study area comprises two major hydrolithologic units. They are the alluvial aquifers 'and the basaltic aquifers where the alluvial aquifers especially on the western flank of the valley floor and along large rivers were found to be good groundwater potential zones and wells sunk into these aquifers have fairly good efficiencies and productiVities. Generally, the groundwater has good chemical quality with the exception high EC (TDS) and N03 values in some areas. The Lake Ashange water does not have similar chemistry to that of the springs that emanate from the western highlands and the groundwater in the valley floor of the Mehoni sub-basin. Therefore, it is concluded that the lake may not have significant sub-surface outflow towards the sub basin. Lowering of the static water level by about 2m within 2 years time was detected in a monitoring well located at the groundwater irrigated field around Werabaye. This could be attributed to the large-scale pumping from the groundwater and intensive diversion activities in the upstream areas. Generally, two ground water flow systems were detected. One is in the Mehoni sub-basin where the groundwater flows from all directions towards the central valley floor with probable groundwater out flow at the southeastern part of the study area. And the other is in the lake Ashange basin where groundwater flows from all dirrections towards the lake.
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    Litho-stratigraphic and Structural controls on the Groundwater Flow Dynamics and Hydrogeochemical setting of the Mekelle Outlier and Surroundings, Northern Ethiopia
    (Addis Ababa Universty, 2015-09) Hagos, Ermias; Ayenew, Tenalem (Professor)
    The study area (Mekelle outlier and surroundings) is located in the north-eastern part of the Tekeze River Basin, northern Ethiopia. It constitutes a wide range of lithologic units with ages ranging from Neoproterozoic to Quaternary. The older WNW-ESE oriented normal faults, younger NNE-SSW directed lineaments and associated fractures and the N-S trending foliations and lineations associated with the Negash synclinorium are prominent structures in the area. The isotopic signatures (δ18O, δ2H and tritium) and patterns of dissolved-ion concentrations in the groundwater, coupled with understanding of the three-dimensional geological framework, are used to conceptualize the groundwater flow model and recharge-discharge mechanisms in this complex hydrogeological environment. The basement rocks, glacial deposits and dolerites in the study area are characterized by shallow and localized aquifers mostly associated with faults and shear zones. Boreholes drilled into these rock formations have well yields mostly less than 5 L/s. The Agula Shale and the upper part of Antalo Limestone are dominated with shale and marl units although gypsum and thin beds of limestone are also minor constituents found as intercalation. They are mostly intruded by dolerite sills and to a lesser extent dolerite dykes. Although they are affected by intense joints, the groundwater occurrence and flow in this upper part of the sedimentary section is limited due to clay dominated lithology. Wells and springs have low discharges hardly exceeding 2 L/s. At the contact zones with the dolerite intrusions, discharge rates of 12-40 L/s are common. Whereas the Adigrat Sandstone and the lower part of Antalo Limestone are characterized by extensive/semiregional aquifers with boreholes’ yield of up to 60 L/s. A pattern of high discharge springs, higher boreholes’ yield and transmissivity values parallel to the major faults and lineaments and at the contacts between the Mesozoic sedimentary rocks and the Mekelle dolerite is observed. Based on the piezometric map, along with the relatively more depleted isotopic compositions, higher d-excess, and lower dissolved-ion concentrations of the groundwater samples, the highland areas (northwest, north, east and south of the study area) are characterized as recharge areas, while the narrow major river valleys of Giba, Illala, Chelekot and Fucea Mariam are found to be discharge areas. The groundwater divide between the Tekeze and the Denakil basins coincides with the surface-water divide line of these two basins. Most groundwater samples lay close to the Addis Ababa LMWL indicating meteoric origin. Slight shifts of some samples from this line are attributed to the altitude effect and the isotopic exchanges of rain droplets with the local air mass that have different isotopic composition (more depleted and higher d-excess) from that of Addis Ababa. This effect also results to a smaller slope of the LMWL of the study area (δ2H = 6×δ18O + 9.8; R2 = 0.8). The equation for the evaporation line for the area is δ2H = 4.47δ18O + 6.42; R2 = 0.8. The δ18O of shallow groundwater at different altitudes indicates a depletion rate of -0.51 ‰/100m towards highlands. In most cases, groundwater feeds the semi-perennial streams and rivers in the area. But isotopic signatures in some wells indicate that there are localities where river flow and seepage from micro-dams locally feed the adjacent aquifers. Double isotopic measurement of groundwater samples from wells in the summer and dry seasons indicates a significant amount interflow (throughflow) that is pumped in wells during the summer season and disappears shortly after the rainy season passed. The lithostratigraphic, geomorphologic, isotopic and hydrochemical settings observed in this study indicate that three groundwater flow systems (shallow/local, intermediate and deep/semiregional) can exist here. Tritium data indicate that the groundwater in the study area has generally short residence time and is dependent on modern precipitation. The range observed in the study area (1.73 to 3.66 TU) indicates components of pre- and post-1952 recharge.