# Unexpected chemical composition of waters and salts in the Okavango Delta (Botswana)

Valentin Challier, Marc Jolivet, Nashaat Mazrui, Aline Dia, Olivier Dauteuil, Mélanie Davranche

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Valentin Challier, Marc Jolivet, Nashaat Mazrui, Aline Dia, Olivier Dauteuil, et al.. Unexpected chemical composition of waters and salts in the Okavango Delta (Botswana). 29 ème édition de la réunion des sciences de la Terre (RST 2025), Société Géologique de France (SGF), Oct 2025, Montpellier, France. pp.658047. insu-05350453

HAL Id: insu-05350453

https://insu.hal.science/insu-05350453v1

Submitted on 6 Nov 2025

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# Unexpected chemical composition of waters and salts in the Okavango Delta (Botswana)

Valentin Challier*†<fnref n="1" />, Marc Jolivet<fnref n="1" />, Nashaat Mazrui<fnref n="2" />, Aline Dia<fnref n="3" />, Olivier Dauteuil<fnref n="4" />, and Mélanie Davranche<fnref n="5" />

## Résumé

The Okavango Delta wetlands are world-famous for their landscapes and biodiversity richness. But, below the surface of this wildlife paradise is hiding something much less life friendly. Indeed, the Okavango Delta is also known for the presence of alkaline groundwaters with pH values up to 10, total organic carbon values up to 1200 ppm and elevated concentrations of dissolved metals and metalloids, some of which are toxic (arsenic up to 6 ppm, uranium up to 12 ppm, vanadium up to 4 ppm, etc.). A first model explained the formation of the saline groundwater through evapotranspiration of the flood fresh water infiltrating through the tree line ringing the islands. However, our recent trace-elements geochemical studies of groundwater and sediment in Nxaraga Island in the central part of the Delta, showed that groundwater composition could not result through a simple evapotranspiration of surface water. Following this initial result, the present study addresses the nature, composition and origin of salt deposits that have been described on several of the numerous islands of the Delta, especially in its eastern part along the Maunachira and Mboroga rivers. For the first time, we provide a complete major and trace elements geochemical description of these salts and compare them to evaporites from the Makgadikgadi pans. We demonstrate that the composition of the Delta salts (essentially natron) is very different from that of the Makgadikgadi evaporites (mostly halite) but, similar to that of the alkaline groundwater previously described. This result suggests a direct communication between the toxic groundwater and the surface environment, although this remains to be further described.

<footnote n="1">¹Institut de Physique du Globe de Paris – Institut National des Sciences de l'Univers, Université de la Réunion, Institut de Physique du Globe de Paris, Centre National de la Recherche Scientifique, Université Paris Cité – France</footnote>

<footnote n="2">²University of Botswana – Botswana</footnote>

<footnote n="3">³Géosciences Rennes (GR) – Université de Rennes 1, Observatoire des Sciences de l'Univers de Rennes, INSU, CNRS : UMR6118 – Bâtiment 15 – Université de Rennes 1 – Campus de Beaulieu - CS 74205 - 35042 Rennes Cedex - France, France</footnote>

<footnote n="4">⁴Géosciences Rennes – Université de Rennes, Institut National des Sciences de l'Univers, Centre National de la Recherche Scientifique, Observatoire des sciences de l'environnement de Rennes – France</footnote>

<footnote n="5">⁵Géosciences Rennes [Université de Rennes 1] – Université de Rennes 1, Institut National des Sciences de l'Univers, Observatoire des Sciences de l'Univers de Rennes, Centre National de la Recherche Scientifique : UMR6118 – Bâtiment 15 – Université de Rennes 1 – Campus de Beaulieu -263 Av du général Leclerc- CS 74205 - 35042 Rennes Cedex - France, France</footnote>

<footnote>*Intervenant</footnote>

<footnote>†Auteur correspondant: challier@ipgp.fr</footnote>
The concentrations of trace elements in the Delta salts (As: up to 110 ppm, U: up to 12 ppm, V: up to 14 ppm) are again highly toxic for the environment and local populations. The project will investigate further the way plants directly in contact with the salt deposits are concentrating the toxic elements initiating their uptake in the trophic chain. A specific effort will also be made to evaluate the potential threat represented by the groundwater and salt composition to the health of the local communities.

**Mots-Clés:** Okavango, Rare Earth Elements, Arsenic