Girmay, Ermias Hagos (2015) Litho–structural Controls on the Groundwater Flow System and Hydrogeochemical Setup of the Mekelle Outlier and Surrounding Areas, Northern Ethiopia. PhD thesis, Addis Ababa University.
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PDF (Litho–structural Controls on the Groundwater Flow System and Hydrogeochemical Setup of the Mekelle Outlier and Surrounding Areas, Northern Ethiopia)
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Abstract
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.
| Item Type: | Thesis (PhD) |
|---|---|
| Subjects: | Q Science > Q Science (General) Q Science > QE Geology |
| Divisions: | Africana |
| Depositing User: | Selom Ghislain |
| Date Deposited: | 28 Jun 2018 14:15 |
| Last Modified: | 28 Jun 2018 14:15 |
| URI: | http://thesisbank.jhia.ac.ke/id/eprint/6208 |
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