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Hydrogeology Journal

, Volume 15, Issue 6, pp 1169–1181 | Cite as

Groundwater recharge, flow and hydrogeochemical evolution in a complex volcanic aquifer system, central Ethiopia

  • Molla Demlie
  • Stefan Wohnlich
  • Frank Wisotzky
  • Birhanu Gizaw
Report

Abstract

Hydrochemical, multivariate statistical and inverse hydrogeochemical modeling techniques were used to investigate groundwater recharge, flow and the hydrochemical evolution within the Akaki volcanic aquifer system, central Ethiopia. The hydrochemical and multivariate statistical techniques are mutually supportive and the extracted information was analyzed together with environmental isotope data. Results reveal five spatial groundwater zones with defined hydrochemical facies, residence times, stable isotopic signals and hydrochemical evolution. These zones are designated as the (1) Intoto, (2) central, (3) Filwuha fault, (4) south zones and (5) a highly polluted sub-sector identified within the central zone. Both the hydrochemical and multivariate statistical analyses have shown the central sub-sector as being spite of differentially polluted by \({\text{NO}}^{ - }_{3}\), Cl and \({\text{SO}}^{{2 - }}_{4}\) and its tritium content shows recent recharge. Due to the fact that the main recharge source is precipitation, the hydrochemical and environmental isotope data clearly indicated that the central and southern sectors are also recharged from domestic waste water and leakage from water mains and reservoirs. Inverse hydrogeochemical modeling demonstrated reactions of silicate minerals in a CO2 open system and precipitation of kaolinite, chalcedony, and rare calcite satisfy the observed change in water chemistry from north to south following the regional flow direction.

Keywords

Hydrochemistry Inverse modeling Ethiopia Silicate weathering Groundwater pollution 

Résumé

Des techniques relevant de l’hydrochimie, des statistiques multivariées et de la modélisation inverse hydrogéochimique, ont été utilisées dans le cadre de l’étude de la recharge des eaux souterraines, de l’écoulement et de l’évolution hydrochimique dans le système volcanique aquifère d’Akaki au centre de l’Ethiopie. Les techniques hydrochimiques et multivariées se supportent mutuellement et l’information extraite a été analysée avec les données isotopiques environnementales, des temps de résidence, des signaux isotopiques stables et une évolution hydrochimique. Ces zones ont été désignées comme le (1) Intoto, (2) le centre, (3) la faille de Filwuha, (4) les zones sud et (5) un sous-secteur fortement pollué identifié dans la zone centrale. Les analyses statistiques hydrochimiques et multivariées ont montré que le sous-secteur central a été différemment pollué par \({\text{NO}}^{ - }_{3}\), Cl et \({\text{SO}}^{{2 - }}_{4}\), tandis que la teneur en tritium montre une recharge récente. Malgré le fait que la principale source de recharge soit les précipitations, les données hydrochimiques et isotopiques indiquent clairement que les secteurs centres et sud sont également rechargés par les eaux usées domestiques et les fuites de réservoirs et canalisations d’eau. La modélisation hydrogéochimique inverse a démontré les réactions des minéraux silicatés dans un système ouvert au CO2, et la précipitation de kaolinite, de calcédoine, et la rareté de la calcite satisfont les changements observés dans la chimie de l’eau du nord vers le sud en suivant la direction régionale de l’écoulement.

Resumen

Se utilizaron la hidroquímica y técnicas de modelación hidrogeoquímica inversa y estadística multivariada, para investigar la recarga del agua subterránea, el flujo y la evolución hidroquímica, dentro del sistema acuífero volcánico Akaki, Etiopía Central. La hidroquímica y las técnicas estadísticas multivariadas se complementan entre si y la información así extraída se analizó junto con los datos de isótopos ambientales. Los resultados revelan cinco zonas diferentes de agua subterránea, con facies hidroquímicas, tiempos de residencia, improntas isotópicas estables y una evolución hidroquímica definidas. Estas zonas se designan como (1) Intoto, (2) Central, (3) Falla de Filwuha, (4) las Zonas del sur y (5) un sub-sector altamente contaminado identificado dentro de la zona central. Tanto los análisis estadísticos multivariados como la hidroquímica, han mostrado al sub-sector central como contaminado diferencialmente por \({\text{NO}}^{ - }_{3}\), Cl y \({\text{SO}}^{{2 - }}_{4}\) y su contenido de tritio muestra una recarga reciente. A pesar del hecho que la fuente principal de recarga es la precipitación, los datos de hidroquímica y de isótopos ambientales indican que los sectores central y del sur, también se recargan a partir de agua doméstica usada y del goteo de las conducciones del acueducto y de sus reservorios. El modelamiento hidrogeoquímico inverso demostró reacciones de minerales silicatados en un sistema de CO2 abierto, y la precipitación de caolinita, calcedonia, y rara vez de calcita, satisfacen el cambio observado en la química de agua del norte a sur, siguiendo la dirección del flujo regional.

Notes

Acknowledgments

The authors are grateful to the German Academic Exchange Service (DAAD) for the partial financial support of the field research work and a PhD grant to the first author. Thanks to Dr. W. Stichler (Laboratory of GSF, Forschungszentrum fuer Umwelt und Gesundheit, Institute for Groundwater Ecology, Munich) for the analysis of δ18O, δD and 3H. Thanks to Mr. N. Richard and Mr. D. Barnke (Department of Applied Geology, Ruhr University of Bochum) for the assistance received in analyzing the hydrochemical samples in the laboratory and Dr. T. Fockenberg (Institute of Geology, Mineralogy and Geophysics, Ruhr University of Bochum) for XRF analysis of the rock samples. We thank the anonymous reviewers for constructive comments which have improved the quality of the manuscript.

Supplementary material

10040_2007_163_MOESM1_ESM.doc (681 kb)
ESM 1 Hydrochemical and stable isotope data of groundwater in the Akaki catchment (DOC 697 kb)

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Copyright information

© Springer-Verlag 2007

Authors and Affiliations

  • Molla Demlie
    • 1
    • 2
  • Stefan Wohnlich
    • 1
  • Frank Wisotzky
    • 1
  • Birhanu Gizaw
    • 3
  1. 1.Department of Applied GeologyRuhr University of BochumBochumGermany
  2. 2.Department of Applied GeologyMekelle UniversityMekelleEthiopia
  3. 3.Geological Survey of EthiopiaAddis AbabaEthiopia

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