Hydrogeological System Analysis of Allydegi Plain and its Surroundings Using Geochemical Modeling and Isotope Techniques

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Addis Ababa University


Allydegi plain and its surrounding, which is located in the middle Awash river basin of the Main Ethiopian Rift has lack of water supply problems. Hydrogeochemistry and isotope techniques were used to study the groundwater system of the area. Tertiary and Quaternary volcanic rocks with recent alluvial and elluvial sediments are major geologic units. Fractured and weathered volcanic rocks and unconsolidated sediment aquifers are the key aquifers. Hydrochemical study result reveals that the area is characterized by four major water types. Water group of Ca-Mg-HCO3 and Na-Ca-HCO3 types are characterized by recharge area waters. Na-HCO3 type waters are an evolved water types from highland and escarpments of Ca-Mg-HCO3 and Na-Ca-HCO3 water types located in the center of the study area whereas highly concentrated Na-HCO3-Cl and/or Na-HCO3-SO4 type waters are characteristics of discharge area water types. The result of inverse geochemical modeling depicts that dissolution of silicate minerals, calcite, anhydrite and halite minerals, cation exchange, precipitation and/or weathering of clay minerals with a consumption of CO2(g) as a heat from deeper source through fractures control the groundwater chemistry of the area. Beside these, from the correlation of δ18O as a function of chloride content of the groundwater implies that rigorous evaporation and dissolution can be discriminated as salinity sources without ignoring the impact of Beseka Lake after draining in to Awash river. The estimated groundwater recharge of the area using CMB method is 98.11mm/yr and the roughly estimated ground water flow velocity using stable isotope monitoring results is about 5.6x10-3 m/sec. Stable isotopes study result illustrates that the ground water system is recharged from the combination of direct recharge from precipitation, from highlands that bounds the study area through fractures and from Awash river. Isotope monitoring result of Boreholes in Allydegi plain and Awash river also testifies that Awash river recharges the Allydegi plain. The hydrochemical and water level monitoring of reservoir and borehole at dam axis shows that the source of leakage at the left abutment is from reservoir rather than the groundwater system.



Hydrogeological System Analysis, Allydegi Plain, Surroundings Using Geochemical Modeling, Isotope Techniques