Litho-stratigraphic And Structural Controls On The Groundwater Flow Dynamics And Hydrogeochemical Setting Of The Mekelle Outlier And Surroundings Northern Ethiopia

The study area (Mekelle outlier and surroundings) is located in the north-eastern part of thernTekeze River Basin, northern Ethiopia. It constitutes a wide range of lithologic units with agesrnranging from Neoproterozoic to Quaternary. The older WNW-ESE oriented normal faults,rnyounger NNE-SSW directed lineaments and associated fractures and the N-S trending foliationsrnand lineations associated with the Negash synclinorium are prominent structures in the area. Thernisotopic signatures (δ18O, δ2H and tritium) and patterns of dissolved-ion concentrations in therngroundwater, coupled with understanding of the three-dimensional geological framework, arernused to conceptualize the groundwater flow model and recharge-discharge mechanisms in thisrncomplex hydrogeological environment.rnThe basement rocks, glacial deposits and dolerites in the study area are characterized by shallowrnand localized aquifers mostly associated with faults and shear zones. Boreholes drilled into thesernrock formations have well yields mostly less than 5 L/s. The Agula Shale and the upper part ofrnAntalo Limestone are dominated with shale and marl units although gypsum and thin beds ofrnlimestone are also minor constituents found as intercalation. They are mostly intruded by doleriternsills 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 tornclay dominated lithology. Wells and springs have low discharges hardly exceeding 2 L/s. At therncontact zones with the dolerite intrusions, discharge rates of 12-40 L/s are common. Whereas thernAdigrat Sandstone and the lower part of Antalo Limestone are characterized by extensive/semiregionalrnaquifers with boreholes’ yield of up to 60 L/s.rnA pattern of high discharge springs, higher boreholes’ yield and transmissivity values parallel tornthe major faults and lineaments and at the contacts between the Mesozoic sedimentary rocks andrnthe Mekelle dolerite is observed. Based on the piezometric map, along with the relatively morerndepleted isotopic compositions, higher d-excess, and lower dissolved-ion concentrations of therngroundwater samples, the highland areas (northwest, north, east and south of the study area) arerncharacterized as recharge areas, while the narrow major river valleys of Giba, Illala, Chelekotrnand Fucea Mariam are found to be discharge areas. The groundwater divide between the Tekezernand the Denakil basins coincides with the surface-water divide line of these two basins.rnMost groundwater samples lay close to the Addis Ababa LMWL indicating meteoric origin.rnSlight shifts of some samples from this line are attributed to the altitude effect and the isotopicrnexchanges of rain droplets with the local air mass that have different isotopic composition (morerndepleted and higher d-excess) from that of Addis Ababa. This effect also results to a smallerrnslope of the LMWL of the study area (δ2H = 6×δ18O + 9.8; R2 = 0.8). The equation for thernevaporation line for the area is δ2H = 4.47δ18O + 6.42; R2 = 0.8. The δ18O of shallowrngroundwater at different altitudes indicates a depletion rate of -0.51 ‰/100m towards highlands.rnIn most cases, groundwater feeds the semi-perennial streams and rivers in the area. But isotopicrnsignatures 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 groundwaterrnsamples from wells in the summer and dry seasons indicates a significant amount interflowrn(throughflow) that is pumped in wells during the summer season and disappears shortly after thernrainy season passed.rnThe lithostratigraphic, geomorphologic, isotopic and hydrochemical settings observed in thisrnstudy indicate that three groundwater flow systems (shallow/local, intermediate and deep/semiregional)rncan exist here. Tritium data indicate that the groundwater in the study area hasrngenerally short residence time and is dependent on modern precipitation. The range observed inrnthe study area (1.73 to 3.66 TU) indicates components of pre- and post-1952 recharge.

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