Addisu, Deressa Geleta (2012) Hydrogeochemical and Isotope Hydrology in Investigating Groundwater Recharge and Flow Processes, South Afar Eastern Ethiopia. Masters thesis, Addis Ababa University.
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Abstract
The research area includes the eastern part of the lower Awash Basin and Aysha Basin located in the eastern part of Ethiopia (South Afar). The areas have two physiographic regions the Rift and the Eastern Escarpment. Stable environmental isotopes (δ18O and δ 2H) and hydrogeochemistry of precipitation and groundwater are used for the eastern lower Awash basin. However, only hydrogeochemistry is used for Aysha basin due to unreliability of isotopic laboratory results. The main objective of this research is to investigate groundwater recharge and flow processes in the area. The integrated application of hydrogeochemistry and stable isotope of water (2H and 18O) for investigating the recharge and flow processes is a new approach for the area under consideration compared to the previous works in the area. The methodologies applied to achieve the objectives set are sampling and analysis, hierarchical cluster analysis (HCA), inverse geochemical modeling, the plot of δ18O vs δ 2H and chloride mass balance (CMB). Analyses of the water samples collected for physico-chemical and isotope were conducted at the laboratory of Water Works Design and Supervision Enterprise (WWDSE) and Addis Ababa University (AAU) respectively. Prior to using the water chemistry results for the intended application, charge balance evaluation is done and those with an error greater than 10% are rejected from the data set. Then hydrochemical facies analysis was conducted to identify the major water types. Accordingly, four major water types in the eastern lower Awash Basin: Ca-Mg-HCO3, NaHCO3-, Na-Cl-SO42-, or Na- SO42-Cl and Ca-Mg-Cl-SO4 or Ca-Mg-SO42-Cl and two major water types in the Aysha Basin: Ca- (HCO3)2, and Na-Ca- SO4 or Cl were identified. Bicarbonate type water represent recharge area groundwater in topographically higher areas and fast moving discharge area groundwater in the rift. However, as groundwater moves toward rift it evolves to chloride or sulfate water type. HCA also classified the groundwaters of eastern lower Awash basin in to two major groups and eight subgroups and that of Aysha basin in to two major groups and five subgroups. The spatial distribution map of HCA in the basins clearly indicated the groundwater flow directions. Based on the clustering results inverse geochemical modelings are carried out along five selected flow paths in the Awash Basin and along one path in the Aysha Basin. Accordingly, the results indicate dissolution of halite, anhydrite, silicate minerals, consumption of CO2 during silicate hydrolysis, cationexchange, and precipitation of calcite and clay minerals along the flow path toward the rift. For the eastern part of lower Awash Basin, the plot of δ18O vs δ2H for groundwater identified three recharge mechanisms using local meteoric water line (LMWL) derived from Addis Ababa rainfall: recharge from local rainfalls, recharge from evaporated flood waters or mixing, and regional groundwater which are most likely recharged by lateral infiltration of meteoric water. The integrated analysis of salinity (TDS) and δ18 Keywords: Groundwater, recharge, flow processes, hydrogeochemistry, environmental isotopes O also identified two major processes for the origin of salinity: evaporation and dissolution. The annual recharge rates computed using CMB method for the eastern lower Awash and Aysha Basins are 29.15 and 58.24 mm/year respectively. However, the figure is extremely over estimated for the Aysha Basin due to fast selective recharge. The average annual rainfall amount and average Cl concentration in rainfall accounted for the CMB method are 662.81mm/year and 0.91 mg/l respectively. Ninety nine (99) groundwater Cl concentrations (93 from Awash and 6 from Aysha basin) are used.
| Item Type: | Thesis (Masters) |
|---|---|
| Uncontrolled Keywords: | Groundwater, recharge, flow processes, hydrogeochemistry, environmental isotopes |
| Subjects: | Q Science > Q Science (General) Q Science > QD Chemistry Q Science > QE Geology |
| Divisions: | Africana |
| Depositing User: | Selom Ghislain |
| Date Deposited: | 18 Jun 2018 14:14 |
| Last Modified: | 18 Jun 2018 14:14 |
| URI: | http://thesisbank.jhia.ac.ke/id/eprint/4484 |
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