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The role of local perched aquifers in regional groundwater recharge in semi-arid environments: evidence from the Cuvelai-Etosha Basin, Namibia

  • J. T. HamutokoEmail author
  • V. E. A. Post
  • H. Wanke
  • M. Beyer
  • G. Houben
  • B. Mapani
Paper

Abstract

The influence of local perched aquifers on groundwater recharge in semi-arid areas at a regional scale remains inadequately studied, especially in southern Africa where data scarcity limits the understanding of recharge and groundwater flow processes. The objective of this study was to understand the interaction between localized perched aquifers and an underlying regional aquifer in the Cuvelai-Etosha Basin in Namibia. The connection between the perched and regional aquifers was assessed based on an extensive dataset of hydrochemical and isotope (δ18O and δ2H) measurements for groundwater from both shallow and deep hand-dug wells as well as boreholes. Ephemeral rivers and pans/depression landforms were analyzed separately. Recharge was calculated using the chloride mass balance method. It was found that groundwater originates from precipitation that undergoes strong evapotranspiration and water–rock interaction, especially carbonate dissolution. Overlapping chemical and isotopic compositions suggest a common or similar origin of groundwater in the perched and regional aquifers in the pan/depression hydrotope. No such similarity was established for the ephemeral river hydrotope, suggesting that ephemeral rivers contribute negligibly to regional groundwater recharge. This has important implications for groundwater management and helps to understand recharge processes in similar semi-arid environments.

Keywords

Groundwater recharge Hydrochemistry Stable isotopes Perched aquifers Namibia 

Der Einfluss lokaler, schwebender Grundwasserleiter auf die regionale Grundwasserneubildung in semiariden Gebieten: ein Beispiel aus dem Cuvelai-Etosha Basin, Namibia

Kurzfassung

Der Einfluss von lokalen, schwebenden Grundwasserleitern auf die Grundwasserneubildung in semiariden Gebieten ist auf regionaler Skala weitgehend unerforscht. Besonders im südlichen Afrika sind oberflächennahe Grundwasserressourcen essentiell und Grundwasserneubildungs- und -fließprozesse kaum untersucht. Das Ziel dieser Studie war es, die Interaktion zwischen lokalen, schwebenden Aquiferen und dem regionalen Grundwasserleiter im Cuvelai-Etosha Becken (CEB), Namibia, zu untersuchen. Hierfür wurde ein umfangreicher Datensatz hydrochemischer Messungen und stabiler Wasserisotopen (δ18O und δ2H) des Grundwassers aus flachen Bohrlöchern sowie handgegrabenen Brunnen verwendet. Auch der Einfluss von Landformen wie ephemeren Flussbetten und Pfannensystemen sowie lokalen Depressionen, die Einfluss auf Neubildungsprozesse und -raten haben, wurden untersucht. Grundwasserneubildungsraten wurden mit der Chlorid-Bilanzmethode bestimmt. Das Grundwasser in den untersuchten Landformen hat seinen Ursprung in Niederschlagswasser, das bei der Infiltration stark durch Evapotranspiration beeinflusst ist und einer Interaktion von infiltrierendem Wasser und Gestein ausgesetzt ist (besonders Karbonat-Lösung). Sich überschneidende chemische und Werte für stabile Wasserisotope lassen auf einen gemeinsamen Ursprung von Grundwasser aus dem hängendem und regionalen Aquifer im Hydrotop Pfannensystem/Depressionen schließen. Für die ephemeren Flussläufe ist dies nicht der Fall. Daraus kann geschlossen werden, dass dieses Hydrotop nicht zur Grundwasserneubildung des regionalen Aquifers beiträgt. Die hier vorgestellten Ergebnisse tragen zu besserem Prozessverständnis bei und sollten somit auch praktische Auswirkungen für das Management von oberflächennahen Grundwasserressourcen in semiariden Gebieten haben.

Le rôle des aquifères perchés pour la recharge des eaux souterraines à l’échelle régionale dans des environnements semi-arides : observations du bassin Cuvelai-Etosha, Namibie

Résumé

L’influence des aquifères perchés sur la recharge des eaux souterraines à l’échelle régionale dans les régions semi-arides demeure insuffisamment étudiée, en particulier en Afrique australe où la rareté des données limite la compréhension des processus de recharge et des écoulements souterrains. L’objectif de cette étude est de comprendre les interactions entre les aquifères perchés localisés et l’aquifère régional sous-jacent du bassin de Cuvelai-Etosha en Namibie. La connexion entre les aquifères perchés et régional est évaluée sur la base d’un large jeu de mesures hydrochimiques et isotopiques (δ18O et δ2H) des eaux souterraines à partir de puits peu profonds et profonds. Les morphologies des cours d’eau temporaires et de creux/dépressions ont été analysées séparément. La recharge est calculée en utilisant la méthode du bilan de masse en chlorures. L’étude montre que l’eau souterraine provient de précipitations ayant subi une forte évapotranspiration et des interactions eau-roche, en particulier la dissolution des carbonates. Pour les hydrotopes en creux/dépressions, le recoupement des compositions chimiques et isotopiques suggère une origine commune et similaire des eaux souterraines dans les aquifères perchés et régional. Cette similitude n’est pas observée pour les hydrotopes de type cours d’eau temporaire. Cela suggère que les cours d’eau temporaires apportent une contribution négligeable à la recharge de l’aquifère régional. Ce résultat a d’importantes implications pour la gestion des eaux souterraines et aide à comprendre les processus de recharge dans des environnements semi-arides similaires.

El papel de los acuíferos locales colgados en la recarga regional de aguas subterráneas en ambientes semiáridos: evidencia de la cuenca Cuvelai-Etosha, Namibia

Resumen

La influencia de los acuíferos locales colgados en la recarga de aguas subterráneas en zonas semiáridas a escala regional sigue sin estudiarse adecuadamente, especialmente en África meridional, donde la escasez de datos limita la comprensión de los procesos de recarga y de los flujos de las aguas subterráneas. El objetivo de este estudio fue comprender la interacción entre los acuíferos locales colgados y un acuífero regional subyacente en la cuenca del Cuvelai-Etosha en Namibia. La conexión entre los acuíferos colgados y los regionales se evaluó sobre la base de un amplio conjunto de datos de mediciones hidroquímicas e isotópicas (δ18O and δ2H) para las aguas subterráneas de pozos excavados a mano, tanto superficiales como profundos. Los ríos efímeros y las depresiones del paisaje se analizaron por separado. La recarga se calculó utilizando el método de balance de masa de cloruro. Se encontró que el agua subterránea se origina en la precipitación que sufre una fuerte evapotranspiración e interacción agua-roca, especialmente la disolución de carbonatos. Las composiciones químicas e isotópicas superpuestas sugieren un origen común o similar de las aguas subterráneas en los acuíferos en posición vertical y regionales en el hidrótopo de la depresión. No se estableció tal similitud para el hidrótopo del río efímero. Esto sugiere que los ríos efímeros contribuyen de manera insignificante a la recarga regional de aguas subterráneas. Esto tiene importantes implicancias para el manejo de las aguas subterráneas y ayuda a entender los procesos de recarga en ambientes semiáridos similares.

半干旱环境局部上层滞水含水层对区域地下水的补给作用:来自纳米比亚Cuvelai-Etosha盆地的证据

摘要

区域尺度半干旱区局部上层滞水含水层对地下水补给的影响仍未得到充分研究, 特别是在南部非洲, 数据缺乏影响了地下水补给和流动过程的认识。本研究的目的是了解纳米比亚Cuvelai-Etosha盆地局部上层滞水含水层与下伏区域含水层之间的关系。根据来自浅层和深层手挖井的地下水水化学和同位素(δ18O和δ2H)测量的大数据集, 评估了上层滞水和区域含水层之间的关系。分析对季节性河流和盘状/凹陷地貌进行了分析。使用氯质量平衡法计算了补给。研究发现, 地下水来源于经历强烈蒸散和像碳酸盐溶解的水-岩相互作用的降水。重叠的化学和同位素组成表明在盘状/凹陷同位素中的上层滞水区和区域含水层中有相同或相似的地下水来源。对于季节性河流同位素中没有这种相似性。这表明季节性河流对区域地下水补给的贡献很少。这对地下水管理具有重要意义, 有助于认识类似半干旱区域地下水的补给过程。

O papel dos aquíferos suspensos locais na recarga regional de águas subterrâneas em ambientes semiáridos: evidências da Bacia de Cuvelai-Etosha, Namíbia

Resumo

A influência de aquíferos suspensos locais na recarga de águas subterrâneas em escala regional nas áreas semiáridas permanece insuficientemente estudada, especialmente no sul da África, onde a escassez de dados limita a compreensão dos processos de recarga e o fluxo das águas subterrâneas. O objetivo deste estudo foi entender a interação entre aquíferos suspensos locais e um aquífero regional subjacente na bacia de Cuvelai-Etosha na Namíbia. A conexão entre os aquíferos suspensos e regionais foi avaliada com base em um extenso conjunto de dados hidroquímicos e isotópicos (δ18O e δ2H) de águas subterrâneas provenientes de poços rasos e profundos, manualmente escavados. Rios efêmeros e formas de relevo em depressões/panelas foram analisados separadamente. A recarga foi calculada utilizando o método de balanço de massas de cloreto. Verificou-se que a água subterrânea é originada da precipitação, que é submetida a forte evapotranspiração, e da interação água-rocha, principalmente a dissolução de carbonatos. A sobreposição das composições químicas e isotópicas sugere uma origem comum ou similar das águas subterrâneas nos aqüíferos suspensos e regionais no hidrótopo panela/depressão. Nenhuma similaridade foi estabelecida para o hidrótopo rio efêmero , sugerindo que sua contribuição na recarga regional das águas subterrâneas é insignificante. Isso agrega implicações importantes para o gerenciamento de águas subterrâneas e auxilia no entendimento sobre os processos de recarga em ambientes semiáridos similares.

Notes

Acknowledgements

This work forms part of a PhD study that was undertaken at the University of Namibia. We thank the local communities for access to sample their wells. We thank UNAM staff and local authorities who assisted in fieldwork. We kindly thank the staff at the BGR hydrochemical laboratory under Dr. Paul Koeniger’s guidance for analysing the samples. Finally, the comments of reviewers who helped to significantly improve this manuscript are highly appreciated.

Funding information

The work is funded by SASSCAL project under Task 007 and Task 010 and the Namibian National Commission of Research Science and Technology.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Geology DepartmentUniversity of NamibiaWindhoekNamibia
  2. 2.Federal Institute for Geosciences and Natural Resources (BGR)HanoverGermany
  3. 3.Department of Geography and Environmental ManagementUniversity of the West of EnglandBristolUK

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