Hydrological and Hydrogeological Characterization of Dawa River Basin, South Central Ethiopia
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Date
2016-11-11
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Addis Ababa University
Abstract
In the Dawa river basin of southern Ethiopia there is a problem of locating good quantity and quality groundwater particularly in the semi-arid region. Integrated geological, hydrogeological, hydrochemical, and isotopic study together with the preliminary analysis of the river hydrograph were employed to conceptualize hydrological and hydrogeological behavior of the basin giving emphasis to understand the nature of groundwater flow system. Based on litho-structural and hydrogeological data analysis, it is identified that extensive semi-confined multilayer basaltic aquifer, unconfined discrete basement aquifer, and extensive limestone aquifer occur, respectively, in the northern, central, southeastern parts of the study area and the three aquifers do not have strong lateral hydraulic connection. Stable isotope data analysis indicates in the humid to sub-humid region in the north direct recharge occurs from modern local rainfall prior to evaporation, while in the semi-arid region in the south, southwest, and southeast indirect recharge occurs from flush flood which follows the high intensity local rainfall. In the southwest recharge occurs rapidly prior to evaporation; however, in the south and southeast evaporation prior to recharge is significant. The presence of tritium in the groundwater of the study area at amount comparable to modern local rainfall and the dominance of mixed cation-HCO3 type waters in the volcanic and basement terrain and the sulfate type water marked with high Ca2+ and Mg2+ in the sedimentary terrain supports groundwater of local, modern recharge, and/or short residence time, implying the dominance of local flow system throughout the river basin. Low recession index of the tributary streams supports rapid groundwater circulation at least in the headwater region. There is no clear signature of geochemical evolution that would indicate the presence of regional flow system along N-S and/or NW-SE transects. This is supported by stable isotope data that in the absence of favorable physical condition for isotopic fractionation and given the general groundwater flow direction towards southeast, isotopic depletion of the groundwater in the south and southeast relative to the groundwater in the north indicates the absence of regional flow system. Measurement of electrical conductivity and preliminary analysis of river hydrograph reveal the dry period river flow is dominantly supplied by the groundwater reservoir in the volcanic terrain. Graphical plots and statistical analysis show water chemistry is highly variable in the basin, signifying the effect of various factors. It is revealed that silicate hydrolysis is the dominant geochemical process in the volcanic and in the most parts of the basement terrain. Sulfide oxidation and sulfate dissolution has an effect on the groundwater chemistry, respectively, in the sulfide rich areas and along the dry riverbeds in the basement terrain. In the sedimentary terrain, gypsum dissolution is the dominant process. Observation of high nitrate concentration in the groundwater reveals the effect of surface contamination sources on water chemistry of the study area. This study highlights the dominant effect of water-rock interaction on the groundwater chemistry and vulnerability of groundwater resources of the basin to pollution sources related to human activity. Overall, converging evidences from different techniques indicate the dominance of local groundwater flow pattern in most parts of the river basin. Due to structural and lithologic factors the traditional regional groundwater flow model which is common in most large river basins of Ethiopia does not hold true in the Dawa river basin. In the upper part of the basin the flow is dominantly topographic-driven. In the middle part structure plays a vital role. The existence of Precambrian basement complex in the middle makes the flow system discontinuous. Hence, both groundwater flow and hydrochemical signatures display extreme variability. This makes the Dawa river basin in a number of ways unique.
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Keywords
Dawa River Basin, Ethiopia, Groundwater Recharge, Groundwater Flow System, Hydrochemistry, Stable Isotope