Abstract
The Middle Awash basin is an arid region in Ethiopia where surface waters are scarce and local communities are dependent on groundwater resources for water supply. The complex hydrogeological system of this basin has been conceptualized. Multivariate statistical analysis of hydrochemical variables and water isotopes were used to study the rock–water interaction, geochemical reaction processes and the hydrological link between aquifers. Groundwaters from aquifers of the high-rainfall plateau bounding the rift are slightly mineralized, as well as depleted in δ18O and δD, and contain 3H above 0.8 TU. This suggests a low degree of rock–water interaction and that groundwater is under recharge from heavy rain that falls on surrounding highlands. On the other hand, groundwaters from aquifers of the rift floor are highly mineralized and show slight enrichment in δ18O and δD with positive oxygen shift, but contain 3H below 0.8 TU. The positive oxygen shift in rift floor groundwaters may be caused by the isotopic exchange of oxygen between groundwater and aquifer materials during rock–water interaction, whereas the low 3H content could be due to the decay of tritium along relatively long flow paths. The approach utilized in this study may be applicable to understanding hydrogeochemical processes in other complex volcanic terrains.
Résumé
Le bassin de l’Awash moyen est situé dans une zone aride en Ethiopie où les eaux de surface sont rares; les communautés locales dépendent des ressources en eaux souterraines pour leur alimentation. Le complexe hydrogéologique de ce bassin a été conceptualisé. Des analyses statistiques multi-variées des variables hydrochimiques et des isotopes de l’eau ont été utilisées afin d’étudier les interactions eau roche, les processus de réactions géochimiques, les relations hydrologiques entre les aquifères. Les eaux souterraines des aquifères du plateau bordant le rift, recevant de fortes précipitations, sont légèrement minéralisées et sont caractérisées également par l’absence de δ18O et δD; elles contiennent du tritium avec des valeurs au-dessus de 0.8 TU. Ceci suggère un faible degré d’interaction eau roche ainsi que les eaux souterraines sont alimentées par des infiltrations d’eau de pluie interceptée par les hauts plateaux environnants. D’un autre côté, les eaux souterraines des aquifères situés au cœur du rift sont fortement minéralisées et montrent un léger enrichissement en δ18O et δD avec un déplacement positif pour l’oxygène; elles contiennent des valeurs en tritium inférieurs à 0.8 TU. Le déplacement positif de l’oxygène dans les eaux souterraines des aquifères situés au cœur du rift peut être du à l’échange isotopique de l’oxygène entre l’eau souterraine et les matériaux constitutifs de l’aquifère au cours des réactions eau roche, alors que les faibles teneurs en 3H peuvent être dues à la décroissance du tritium le long des cheminements relativement long des écoulements. L’approche utilisée dans le cadre de cette étude peut être utilisée pour la compréhension des processus hydrochimiques dans d’autres complexes hydrogéologiques en terrains volcaniques.
Resumen
La cuenca media del Awash es una región árida en Etiopía donde las aguas superficiales son escasas y las comunidades locales dependen de los recursos de agua subterránea para su abastecimiento. Se ha conceptualizado el complejo sistema hidrogeológico de esta cuenca. Se usaron análisis estadísticos multivariados de variables hidroquímicas y isótopos del agua para estudiar la interacción agua–roca, los procesos de reacciones geoquímicas, y la relación hidrológica entre acuíferos. Las aguas subterráneas de los acuíferos de la meseta de alta precipitación que bordea el rift están ligeramente mineralizadas, así como empobrecidos en δ18O y δD, y contienen 3H por encima de 0.8 TU. Esto sugiere un bajo nivel de interacción agua – roca y que el agua subterránea está bajo recarga a partir de las intensas lluvias que caen en las tierras circundantes. Por el otra parte, las aguas subterráneas de los acuíferos en el piso del rift están altamente mineralizadas y muestran un leve enriquecimiento en δ18O y δD con un desplazamiento positivo del oxígeno, pero contiene 3H por debajo de 0.8 TU. El desplazamiento positivo del oxígeno en el piso del rift puede ser causado por el intercambio isotópico de oxígeno entre el agua subterránea y los materiales del acuífero durante la interacción agua–roca, mientras que el bajo contenido de H podría deberse a la desintegración del tritio a lo largo de trayectorias de flujo relativamente largas. El enfoque utilizado en este estudio puede ser aplicable para entender los procesos hidrogeoquímicos en otros terrenos volcánicos complejos.
摘要
埃塞俄比亚Awash中部盆地是一个干旱区,地表水稀少,当地群落用水主要依靠地下水资源。盆地复杂的水文地质系统已经被概念化。本文使用水化学变量的多元统计分析以及水同位素方法研究水岩相互作用、地球化学反应过程以及不同含水层之间的水力联系。该高原降雨量较高,以裂隙为边界,含水层中的地下水轻微矿化,δ18O和 δD贫化,氚含量高于0.8 TU。这表明,本区水岩相互作用程度很低,暴雨落到周围高地并补给地下水。另一方面,裂隙地面含水层中的地下水矿化度高,δ18O和 δD轻微富集,且氧正漂移,氚含量低于0.8 TU。裂隙地面的地下水正向氧漂移可能由于水岩相互作用时地下水及含水层介质之间氧同位素交换引起,低的氚含量是由于沿着相对长的流线上氚衰减引起的。在本研究中使用的方法可适用于其他复杂火山地形,了解其水文地球化学过程。
Riassunto
Il bacino del Awash centrale è situato nel regione arida in Etiopia in cui le acque di superficie sono limitate e le comunità locali dipendono dalle risorse idiriche freatica. Il sistema idrogeologico complesso di questo bacino è stato concettualizzato. L'analisi statistica sui variabili idrochimici e degli isotopi dell'acqua è stata usata per studiare l'interazione tra roccia e acqua, includendo i processi geochimici di reazione ed il collegamento idrologico fra gli strati acquiferi. Le acque freatiche dagli strati acquiferi dell’ altopiano sono pocco mineralizzate, come pure l’ impoverito del δ18O e δD e contengono 3H al di sopra di 0.8 TU. Ciò suggerisce che un basso grado di interazione tra roccia e acqua, e che le acque freatiche sono sotto ricarica da pioggia persistente che cade sulle alture circostanti. Da un lato, le acque freatiche dagli strati acquiferi del Rift sono altamente mineralizzate e mostrano un lieve arricchimento sul δ18O e δD con lo spostamento positivo dell'ossigeno, ma contengono 3H al di sotto di 0.8 TU. La variazione positiva dell’ossigeno nelle acque freatiche del Rift può essere causata tramite uno scambio isotopico di ossigeno fra acqua freatica ed i materiali dello strato acquifero durante l’interazione tra roccia e acqua, mentre il basso contenuto di 3H potrebbe essere dovuto decadimento del trizio con percorsi di flusso relativamente lunghi. I metodi utilizzati in questo studio possono essere usati per la comprensione dei processi di idrogeochimica in altri terreni vulcanici complessi.
Resumo
A bacia do Médio Awash é uma região árida na Etiópia onde as águas superficiais são escassas, pelo que as comunidades locais estão dependentes de recursos de águas subterrâneas para o seu abastecimento. Foi conceptualizado o complexo sistema hidrogeológico da bacia. Utilizou-se a análise estatística multivariada de variáveis hidroquímicas e de isótopos na água para estudar a interacção água-rocha, os processos de reacções geoquímicas e a ligação hidrológica entre aquíferos. As águas subterrâneas dos aquíferos dos planaltos com elevada precipitação que delimitam o rift são ligeiramente mineralizadas e empobrecidas em δ18O e δD, e contêm 3H acima de 0.8 UT. Isso sugere que há um baixo grau de interacção água–rocha e, ainda, que as águas subterrâneas sofrem recarga a partir da elevada precipitação nos planaltos vizinhos. Por outro lado, as águas subterrâneas de aquíferos da base do rift são altamente mineralizadas e mostram enriquecimento ligeiro em δ18O e δD, com deslocamento positivo do oxigénio, mas contém 3H abaixo de 0.8 UT. O deslocamento positivo de oxigénio nas águas subterrâneas da zona da base do rift pode ser causada pela troca isotópica de oxigénio entre as águas subterrâneas e o material litológico do aquífero durante a interacção água–rocha, enquanto que o teor baixo de 3H poderia dever-se à desintegração do trítio ao longo de caminhos de fluxo relativamente longos. A abordagem utilizada neste estudo pode ser aplicável para a compreensão dos processos hidrogeoquímicos de outros terrenos vulcânicos complexos.
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Acknowledgements
This study was undertaken in the framework of the MAWARI research project (Sustainable Management of Water Resource in the East African Rift system) funded by the French Ministry of Foreign and European Affairs. CIFEG (Centre International pour la Formation et les Echanges en Géosciences) is highly acknowledged for managing the grant and facilitating field work in Ethiopia. The Oromia Regional Water Bureau, Department of Earth Science at Addis Ababa University and Water Works Design and Supervision Enterprise (WWDSE) are warmly thanked for their valuable contributions in providing data and unreserved cooperation during the field work.
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Furi, W., Razack, M., Abiye, T.A. et al. Hydrochemical characterization of complex volcanic aquifers in a continental rifted zone: the Middle Awash basin, Ethiopia. Hydrogeol J 20, 385–400 (2012). https://doi.org/10.1007/s10040-011-0807-1
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DOI: https://doi.org/10.1007/s10040-011-0807-1