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Hydrogeology Journal

, Volume 21, Issue 3, pp 673–686 | Cite as

Residence times of shallow groundwater in West Africa: implications for hydrogeology and resilience to future changes in climate

  • D. J. Lapworth
  • A. M. MacDonald
  • M. N. Tijani
  • W. G. Darling
  • D. C. Gooddy
  • H. C. Bonsor
  • L. J. Araguás-Araguás
Report

Abstract

Although shallow groundwater (<50 mbgl) sustains the vast majority of improved drinking-water supplies in rural Africa, there is little information on how resilient this resource may be to future changes in climate. This study presents results of a groundwater survey using stable isotopes, CFCs, SF6, and 3H across different climatic zones (annual rainfall 400–2,000 mm/year) in West Africa. The purpose was to quantify the residence times of shallow groundwaters in sedimentary and basement aquifers, and investigate the relationship between groundwater resources and climate. Stable-isotope results indicate that most shallow groundwaters are recharged rapidly following rainfall, showing little evidence of evaporation prior to recharge. Chloride mass-balance results indicate that within the arid areas (<400 mm annual rainfall) there is recharge of up to 20 mm/year. Age tracers show that most groundwaters have mean residence times (MRTs) of 32–65 years, with comparable MRTs in the different climate zones. Similar MRTs measured in both the sedimentary and basement aquifers suggest similar hydraulic diffusivity and significant groundwater storage within the shallow basement. This suggests there is considerable resilience to short-term inter-annual variation in rainfall and recharge, and rural groundwater resources are likely to sustain diffuse, low volume abstraction.

Keywords

Groundwater recharge Groundwater age Climate variability Environmental tracers Africa 

Temps de séjour de ll’eau superficielle en Afrique de l’Ouest : implications hydrogéologiques et résilience aux futurs changements climatiques

Résumé

Bien que la nappe phréatique (<50 mbgl) supporte la plus grande partie d’une consommation croissante d’eau potable en Afrique rurale, peu d’informations indiquent quelle peut être la résilience de cette ressource aux futurs changements climatiques. Cette étude présente les résultats d’une étude de nappe utilisant des isotopes stables, CFCs, SF6, and 3H couvrant différentes zones climatiques (précipitations annuelles 400–2,000 mm) dans l’Ouest africain. L’objectif était de quantifier les temps de séjour des eaux superficielles dans l’aquifère sédimentaires et dans celui du socle et d’examiner la relation entre les ressources en eau et le climat. Les résultats basés sur les isotopes stables indiquent que les aquifères les plus superficiels échantillonnées se rechargent rapidement suite à la précipitation, mettant en évidence une petite évaporation précédant la recharge. Le bilan des chlorures indique qu’à l’intérieur des zones arides (précipitations annuelles <400 mm), il existe une recharge atteignant 20 mm/an. Les traceurs montrent que la plupart des eaux de nappe ont un temps de résidence moyen (MRTs) de 32–65 an, avec des MRTs comparables dans les différentes zones climatiques. Des MRTs identiques mesurées dans les sédiments d’une part et dans les aquifères du socle d’autre part suggèrent une diffusivité hydraulique similaire et un stockage important dans le substrat proche. Ceci suggère qu’il existe une résilience considérable aux variations interannuelles court terme de la précipitation-recharge, et que les ressources souterraines en milieu rural peuvent vraisemblablement supporter un prélèvement diffus, de petit volume.

Tiempos de residencia del agua subterránea somera en el África occidental: implicancias para la hidrogeología y la resiliencia a futuros cambios en el clima

Resumen

Aunque el agua subterránea somera (<50 mbgl) sostiene a la gran mayoría de las fuentes de abastecimiento mejoradas de agua potable en el África rural, hay poca información sobre como cuan resilientes pueden ser estos recursos frente a futuros cambios en el clima. Este estudio presenta los resultados de un relevamiento de agua subterránea usando isótopos estables, CFCs, SF6, y 3H a través de diferentes zonas climáticas (precipitación 400–2,000 mm/año) en el África Occidental. El propósito fue cuantificar los tiempos de residencia de aguas subterráneas someras en acuíferos sedimentarios y de basamento, e investigar la relación entre el recurso de agua subterránea y el clima. Los resultados de los isótopos estables indican que la mayoría de las aguas subterráneas someras muestreadas son recargadas rápidamente después de la lluvia, mostrando pocas evidencias de evaporación previa a la recarga. Los resultados del balance de masa de cloruro indican que dentro de las áreas áridas (<400 mm anual de precipitación) existe una recarga de hasta 20 mm/año. Los trazadores de edad mostraron que la mayoría de las aguas subterráneas tienen tiempos de residencia media (MRTs) de 32–65 años, con MRTs comparables en las diferentes zonas climáticas. Medidas de MRTs similares en los acuíferos sedimentarios y de basamento sugieren una difusividad hidráulica similar y un almacenamiento de agua subterránea significativo dentro del basamento somero. Esto sugiere que hay una considerable resiliencia para variaciones interanuales de corto plazo en la precipitación y la recarga, y que los recursos de agua subterránea rural están en condiciones probablemente de sostener una extracción distribuida y de bajo volumen.

非洲西部浅层地下水的滞留时间:对水文地质学和应对未来气候变化的意义

摘要

虽然浅层地下水(<50mbgl)维持着非洲农村地区大部分改善饮用水的供应,这种资源对未来气候变化的应对能力大小是我们所不知道的。本文展示了一项对非洲西部跨越不同气候区(年降水量为400 ~ 2,000 mm/y)的地下水中稳定同位素、CFCs、SF63H的调查结果。这项调查的目的是对沉积含水层与基底含水层中的浅层地下水的滞留时间进行定量化,并调查地下水资源与气候的关系。稳定同位素的数据表明,所采的大部分浅层地下水在降雨事件过后迅速获得补给,并未显示补给前发生蒸发的证据。氯的质量平衡结果表明,干旱区(年降水量 < 400 mm)的补给量最大为20 mm/y。年龄示踪剂显示,大部分地下水的平均滞留时间(MRTs)为32 ~ 65年,不同的气候区的MRTs具有可比性。在沉积含水层和基底含水层所测定的相似的MRTs说明了水力扩散系数是相似的,而且在浅部的基底含水层中具有相当的地下水储量。这表明,地下水资源对于短期的年内降水量、补给量的变化具有相当的应对能力,农村地区的地下水资源很可能将保证较低水平的开采。

Tempos de residência da água subterrânea superficial na África Ocidental: implicações na hidrogeologia e na resiliência às alterações climáticas futuras

Resumo

Embora as águas subterrâneas de aquíferos superficiais (<50 m sob a superfície do terreno) sustentem a grande maioria dos abastecimentos de água potável nas zonas rurais de África, há ainda pouca informação sobre o seu grau de resiliência face às alterações climáticas no futuro. Este estudo apresenta os resultados de uma investigação de águas subterrâneas usando isótopos estáveis, CFC, SF6 e 3H em diferentes zonas climáticas (precipitação anual 400–2,000 mm) na África Ocidental. O objetivo foi quantificar os tempos de residência das águas subterrâneas em aquíferos pouco profundos, sedimentares e de soco, e investigar a relação entre os recursos de água subterrânea e o clima. Os resultados da aplicação de isótopos estáveis indicam que as águas subterrâneas superficiais são recarregadas rapidamente a seguir a episódios pluviosos, revelando pouca evidência de evaporação antes do fenómeno de recarga. Os resultados obtidos utilizando o método de balanço de cloretos mostram que nas zonas áridas (precipitação anual <400 mm) ocorre recarga que pode atingir os 20 mm/ano. A aplicação de traçadores para cálculo da idade da água mostrou que a maioria das águas subterrâneas tem tempos de residência médios (TRM) de 32–65 anos, com valores de TRM comparáveis em diferentes zonas climáticas. Valores de TRM semelhantes medidos em aquíferos sedimentares e aquíferos de soco sugerem difusividade hidráulica semelhante e um significativo armazenamento de águas subterrâneas no soco pouco profundo. Estes resultados sugerem que há uma considerável resiliência da precipitação e da recarga à variação inter-anual de curto prazo, e que os recursos em água subterrânea em zonas rurais serão provavelmente capazes de manter pequenos volumes de exploração difusa.

Notes

Acknowledgements

This work was funded by the UK Department for International Development. The authors are grateful for the support of the following: field staff from the Nigerian Geological Survey Agency and for logistical support in Nigeria; Vinny Casey and Richard Carter of WaterAid London; Adama Sanogo and colleagues in WaterAid Mali; and local partners GAAS (Bandigara), AFRAD (Koro), and ADDA in Mali. The research was carried out under guidance from a steering group headed by Guy Howard (DFID) and Stephen Foster (IAH). David M.J. Macdonald (BGS) is thanked for reviewing the draft manuscript. The authors thank J Barth and A Herczeg for their constructive comments during the review process. BGS authors publish with the permission of the Executive Director of the British Geological Survey (NERC).

Supplementary material

10040_2012_925_MOESM1_ESM.pdf (354 kb)
ESM 1 (PDF 353 kb)

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Copyright information

© British Geological Survey (NERC) 2012

Authors and Affiliations

  • D. J. Lapworth
    • 1
  • A. M. MacDonald
    • 2
  • M. N. Tijani
    • 3
  • W. G. Darling
    • 1
  • D. C. Gooddy
    • 1
  • H. C. Bonsor
    • 2
  • L. J. Araguás-Araguás
    • 4
  1. 1.British Geological SurveyWallingfordUK
  2. 2.British Geological SurveyEdinburghUK
  3. 3.Department of GeologyUniversity of IbadanIbadanNigeria
  4. 4.International Atomic Energy AgencyViennaAustria

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