Origin and geochemical evolution of groundwater in the Abaya Chamo basin of the Main Ethiopian Rift: application of multi-tracer approaches

Haji, Muhammed; Qin, Dajun; Guo, Yi; Li, Lu; Wang, Dongdong; Karuppannan, Shankar; Shube, Hassen

Origin and geochemical evolution of groundwater in the Abaya Chamo basin of the Main Ethiopian Rift: application of multi-tracer approaches | Origine et évolution géochimique des eaux souterraines dans le bassin d’Abaya Chamo du Grand Rift éthiopien: application d’une approche multi-traceurs Origen y evolución geoquímica de las aguas subterráneas en la cuenca de Abaya Chamo del Main Ethiopian Rift: aplicación de métodos de múltiples trazadores 运用多种示踪方法研究地下水的成因和地球化学演化,以埃塞俄比亚裂谷Abaya Chamo盆地为例 Origem e evolução geoquímica da água subterrânea na bacia de Abaya Chamo no Principal Rifte Etíope: aplicação das soluções de multimarcadores

2021

The fractured volcanic aquifer of the Abaya Chamo basin in the southern Ethiopian Rift represents an important source for water supply. This study investigates the geochemical evolution of groundwater and the groundwater flow system in this volcanic aquifer system using hydrochemistry and environmental tracers. Water types of groundwater were found to transform from Ca-Mg-HCO₃ (western part of Lake Abaya area) to Na-HCO₃ (northwestern part), from the highland down to the Rift Valley. Silicate hydrolysis and Ca/Na ion exchange are the major geochemical processes that control groundwater chemistry along the flow path. Groundwaters are of meteoric origin. The δ¹⁸O and δD content of groundwater ranges from −4.9 to −1.1‰ and –27 to 5‰, respectively. The δ¹⁸O and δD values that lie on the summer local meteoric water line indicate that the groundwater was recharged mainly by summer rainfall. δ¹³CDIC values of cold groundwater range from −12 to −2.7‰, whereas δ¹³CDIC of thermal groundwater ranges from −8.3 to +1.6‰. The calculated δ¹³CCO₂₍g₎ using δ¹³CDIC and DIC species indicates the uptake of soil CO₂ for cold groundwater and the influx of magmatic CO₂ through deep-seated faults for thermal groundwater. In the western part of Lake Abaya area, the shallow and deep groundwater are hydraulically connected, and the uniform water type is consistent with a fast flow of large gradient. In contrast, in the northern part of Lake Abaya area, water underwent deep circulation and slow flow, so the water types—e.g. high F⁻ (up to 5.6 mg/L) and Na⁺—varied laterally and vertically.

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