Environmental Geology

, Volume 58, Issue 1, pp 109–118 | Cite as

Hydrogeochemical study in the Main Ethiopian Rift: new insights to the source and enrichment mechanism of fluoride

Original Article
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Abstract

The central Main Ethiopian Rift suffers a severe water quality problem, characterized by an anomalously high fluoride (F) content that causes an endemic fluorosis disease. The current study, conducted in the Ziway–Shala lakes basin, indicates that the F content exceeds the permissible limit for drinking prescribed by the World Health Organization (WHO; 1.5 mg/l) in many important wells (up to 20 mg/l), with even more extreme F concentration in hot springs and alkaline lakes (up to 97 and 384 mg/l respectively). The groundwater and surface water from the highlands, typically characterized by low total dissolved solids (TDS) and Ca (Mg)–HCO3 hydrochemical facies, do not show high F content. The subsequent interaction of these waters with the various rocks of the rift valley induces a general increase of the TDS, and a variation of the chemical signature towards Na–HCO3 compositions, with a parallel enrichment of F. The interacting matrixes are mainly rhyolites consisting of volcanic glass and only rare F-bearing accessory minerals (such as alkali amphibole). Comparing the abundance and the composition of the glassy groundmass with other mineral phases, it appears that the former stores most of the total F budget. This glassy material is extremely reactive, and its weathering products (i.e. fluvio/volcano-lacustrine sediments) further concentrate the fluoride. The interaction of these “weathered/reworked” volcanic products with water and carbon dioxide at high pH causes the release of fluoride into the interacting water. This mainly occurs by a process of base-exchange softening with the neo-formed clay minerals (i.e. Ca–Mg uptake by the aquifer matrix, with release of Na into the groundwater). This is plausibly the main enrichment mechanism that explains the high F content of the local groundwater, as evidenced by positive correlation between F, pH, and Na, and inverse correlation between F and Ca (Mg). Saturation indices (SI) have been calculated (using PHREEQC-2) for the different water groups, highlighting that the studied waters are undersaturated in fluorite. In these conditions, fluoride cannot precipitate as CaF2, and so mobilizes freely without forming other complexes. These results have important implications for the development of new exploitation strategies and accurate planning of new drilling sites.

Keywords

Main Ethiopian Rift Aqueous geochemistry Geoscience and health Leaching tests Water–rock/sediment interaction 
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Notes

Acknowledgments

This paper is based on the Ph.D. studies of the first author at the University of Ferrara, where the chemical analyses have been carried out with the assistance of Renzo Tassinari. The authors also thank Raul Carampin for the microprobe analyses at the IGG-CNR Institute of Padova.

Supplementary material

254_2008_1498_MOESM_ESM.doc (186 kb)
Supplementary tables (DOC 186 kb)

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Copyright information

© Springer-Verlag 2008

Authors and Affiliations

  1. 1.Dipartimento di Scienze della TerraUniversità di FerraraFerraraItaly
  2. 2.School of Earth Sciences and GeographyKingston UniversityKingston-upon-ThamesUK
  3. 3.Department of Earth SciencesAddis Ababa UniversityAddis AbabaEthiopia
  4. 4.Dipartimento di Scienze della TerraUniversità La SapienzaRomeItaly

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