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Addis Ababa University (2012)

Hydrogeochemical and Isotope Hydrology in investigating Groundwater recharge and flow processes :Borena Lowlands ,Southern Ethiopia

Likissa, Fanta

Titre : Hydrogeochemical and Isotope Hydrology in investigating Groundwater recharge and flow processes :Borena Lowlands ,Southern Ethiopia.

Auteur : Likissa, Fanta

Université de soutenance : Addis Ababa University

Grade : Master Science in Hydrogeology 2012

Hydrogeochemistry and isotope hydrology were used to investigate groundwater recharge and flow processes in Borena lowlands. The study area is characterized by semi-arid and arid climate. Surface water flows are intermittent. The available water resource, therefore, restricted to groundwater. Precambrian basement, Tertiary and Quaternary basalt, and recent alluvial and elluvial sediments are major geologic units. Fractured and weathered basalt is the principal aquifer. The hydrochemical analysis shows that Borena groundwater varies in type over a wide range even within few kilometer intervals and broadly distributed rather than forming distinct clusters. However, three major water types can be distinguished depending on their position on a Piper diagram. These are Ca-Mg-HCO3, Na-HCO3 and Ca-Mg-SO4- Cl/Na-Cl/SO4 water types. In most cases Ca-Mg-HCO3 type waters occur in areas of recharge and Na-Cl/Na-SO4 type waters in rift/lower lying section. The intermediate water types are mixed and lay between these two end members. Ten subgroups were examined from the hierarchical cluster analysis in which distinct hydrgeochemical processes were clearly observed. The dissolution of minerals like olivine, pyroxenes, plagioclase, anhydrite, halite and CO2 gases are deriving the observed natural groundwater chemistry, which are in most cases, balanced by the precipitation of calcite minerals and weathering/formation of clay minerals like Ca-montmorilonite, chalcedony, illite, and K-micas. Cation exchange reactions, particularly in Sarite lower plains, were also controlling chemical composition of the groundwater. The isotopic composition of _18O and _2H in groundwater varies from -5.080/00 to 0.080/00 and from -290/00 to -2.40/00 respectively. In this work the altitude effect on _18O composition shows a depletion of -0.10/00 per 100m elevation rise. Three recharge mechanisms ; direct precipitation, regional groundwater and/or fast selective and flash floods recharge mechanisms were known from the groundwater isotope interpretation. The hydrogeochemical and environmental isotope analysis result clearly indicates regional groundwater flows from elevated sections for instance ; Elwaya, Yabello, Utalo, Gololcha via the middle Gelchet plains to main Ririba fault. A flow from Mega and Megado escarpments to megado/Biloko lowlands is also identified. For the sub basin recharge rate was estimated using chloride mass balance method and is 28mm/yr. The recharge rate shows variations mainly with topography, groundwater chloride content and annual rainfall amounts. The northwestern (Mermero), northeastern (Yabello) and eastern (Mega) parts receives highest recharge where as the southwestern and southern sectors get the lowest groundwater recharge rates.

Mots clés : Borena Loelands Environmental Isotopes Groundwater recharge Hydrogeochemistry


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