|Titre||Modern sediment records of hydroclimatic extremes and associated potential contaminant mobilization in semi-arid environments: lessons learnt from recent flood-drought cycles in southern Botswana|
|Auteur||Franchi, F; Ahad, J M E; Geris, J; Jhowa, G; Petros, A K; Comte, J -C|
|Source||Journal of Soils and Sediments 2019 p. 1-19, https://doi.org/10.1007/s11368-019-02454-9 (Accès ouvert)|
|Séries alt.||Ressources naturelles Canada, Contribution externe 20190579|
|Document||publication en série|
|Media||papier; en ligne; numérique|
|Lat/Long OENS|| 24.8333 26.1667 -24.8000 -25.0833|
|Sujets||inondations; barrages; reservoirs; effets climatiques; qualité de l'eau; substances polluantes; carottes; géochimie du sédiment; repartition granulométrique; contamination des métaux lourds; géochimie du
fer; géochimie du zinc; géochimie du cuivre; géochimie du chrome; géochimie du plomb; eaux de surface; ressources en eau souterraine; aquifères; géochimie de l'eau; météorologie; précipitation; tempêtes; eaux de ruissellement; érosion; milieu
hydrologique; datation radiométrique; datation au radiocarbone; analyse par spectromètre de masse; descriptions de carottes; corrélations stratigraphiques; végétation; géologie du substratum rocheux; changement climatique; effets cumulatifs;
approvisionnement en eau; sécheresse; infrastructure; géologie des dépôts meubles/géomorphologie; géologie de l'environnement; géochimie; hydrogéologie; stratigraphie; géochronologie; Nature et environnement; Sciences et technologie|
|Illustrations||location maps; satellite images; geoscientific sketch maps; tables; photographs; profiles; time series; bar graphs; correlation sections|
|Diffusé||2019 11 18|
|Résumé||(disponible en anglais seulement)|
Purpose: The aim of this work was to identify and analyze the records of flood-drought cycles as preserved in the sediments of the Notwane reservoir, southern
Botswana, in order to better understand how extreme events affect water and sediment quality. This work represents the first attempt to study the reservoir sediments in arid to semi-arid environments and suggests that they could be used as proxies
for the characterization of the effects of flood-drought cycles.
Materials and methods: For the first time in an arid context like Botswana, sediments from artificial reservoirs were explored through correlating sediment records with the presence
and quantity of pollutants in the reservoir's wider arid and semi-arid catchment after the latest extreme flood event of 2017. Sediments from the Notwane reservoir were collected with a push corer to a maximum depth of 80 cm. Sediments were then
analyzed for grain size distribution, organic matter content, and concentrations of heavy metals (Fe, Zn, Cu, Cr, and Pb). Concentrations of heavy metals from surface water and groundwater were compared with the metal profiles from the sediment cores
and with rainfall series from the CHIRPS (Climate Hazards Group InfraRed Precipitation with Stations) database.
Results and discussion: The sediments from Notwane reservoir clearly showed two flood couplets characterized by fining upward beds.
Water quality data from Notwane reservoir and the surrounding aquifer showed peaks of contaminants following rainfall. Although the couplets found in the sediment record were not always clearly coupled with peaks of metals, some correlation was found
between the vertical distribution of metals within the sediments and the most recent sequence and the seasonal metal variation in water. Overall, trace metal contents were very low: < 1 mg/L for Cu and Zn and < 2 mg/L for Cr and Pb, well below the
sediment quality assessment guidelines (SQGs), indicating that the above-average precipitations of the last 10 years did not noticeably contribute to the input of heavy metal contaminants in the reservoir sediments.
Conclusions: The 2016/17 Dineo
cyclone flood was triggered by above-average rainfall, preceded by a 4-year period of severe drought. The deterioration of the basin during the drought has enhanced the effects of the flood, worsening the damages on structures and livelihoods. The
lessons learnt from the Dineo cyclone in Botswana highlight the importance of integrated studies that combine hydrological data, rainfall series, and sediments. It is recommended to extend the research for longer time periods.