Abstract
Failure of borehole sources in weathered and fractured crystalline basement aquifers in Malawi in southern Africa has been linked with poor borehole design, mechanical failure and badly sited boreholes. However, recent work in Malawi indicates that demand may now exceed long-term resource potential in some places and that this is also a cause of water point failure. An 11-year climate cycle (including a wet and dry period) necessitates overdraft from groundwater storage during the dry-cycle years before episodic rainfall events in the wetter part of the cycle again recharge the aquifers. Data, particularly groundwater hydrograph data, are sparse, but sufficient to evaluate the long-term renewable groundwater potential for both fractured and weathered basement-aquifer types in each of the 15 management areas in Malawi. The groundwater potential or long-term renewable resource (recharge) is given by the sum of Darcian throughflow and dry-season depletion of storage. Estimated rural demand exceeds the renewable resource in the fractured-rock aquifer in two management units and in the weathered-rock aquifer in two other units. Although there is inherent uncertainty in the water-balance estimates, the likelihood that rural demand is exceeding long-term average recharge in some areas is cause for concern.
Résumé
L’insuffisance du débit des forages dans les aquifères du socle cristallin altéré et fracturé du Malawi (Afrique australe) est liée à une conception défectueuse, un défaut mécanique et une mauvaise implantation de l’ouvrage. Cependant, un travail récent au Malawi indique que la demande peut maintenant excéder la ressource potentielle de long terme en plusieurs emplacements et que c’est aussi une cause d’échec du point d’eau. Un cycle climatique de 11 ans (comprenant une période humide et une période sèche) oblige à une surexploitation de la réserve en eau souterraine pendant les années du cycle sec, avant que les événements pluvieux épisodiques de la partie plus humide du cycle rechargent de nouveau les aquifères. Les données, particulièrement les données d’hydrologie souterraine, sont rares, mais suffisantes pour évaluer le potentiel à long terme en eau souterraine renouvelable des deux types d’aquifères du socle, fracturé et altéré dans chacune des 15 zones de gestion du Malawi. Le potentiel souterrain ou ressource renouvelable à long terme (recharge) est donné par la somme de l’écoulement de Darcy et de la diminution de l’emmagasinement en saison sèche. La demande rurale estimée dépasse la ressource renouvelable de l’aquifère rocheux fracturé dans deux unités de gestion et de l’aquifère rocheux altéré de deux autres. Bien qu’il y ait une incertitude inhérente aux estimations du bilan hydrique, la probabilité que la demande rurale dépasse la recharge moyenne à long terme dans plusieurs secteurs est une cause d’inquiétude.
Resumen
El fracaso de las fuentes de agua en pozos de acuíferos del basamento cristalino fracturado y meteorizado en Malawi en África del Sur se asocia a un deficiente diseño de los pozos, a fallas mecánicas y a la mala ubicación de los pozos. Sin embargo, trabajos recientes en Malawi indican que la demanda puede exceder los recursos potenciales a largo plazo en algunos lugares y que esta también es una causa del fracaso de las fuentes de agua. Un ciclo climático de 11 años (incluyendo períodos húmedos y secos) requiere la puesta a disposición del almacenamiento de agua subterránea durante los años del ciclo seco antes que eventos de lluvias episódicas en la parte más húmeda del ciclo produzcan una nueva recarga de los acuíferos. Los datos, particularmente los datos de hidrogramas de agua subterránea, son escasos, pero suficientes para evaluar la renovabilidad potencial de agua subterránea a largo plazo para los acuíferos fracturados y meteorizados del basamento en cada una de las 15 áreas de manejo en Malawi. El potencial de agua subterránea o la renovabilidad del recurso (recarga) a largo plazo está dada por la suma del flujo Darciano y la depleción del almacenamiento en la estación seca. La demanda rural estimada excede el recurso renovable en el acuífero de roca fracturada en dos unidades de manejo y en el acuífero de roca meteorizada en otras dos unidades. Aunque hay una incertidumbre inherente a la estimación del balance de agua, la probabilidad que la demanda rural exceda a la recarga promedio a largo plazo en algunas área es causa de preocupación.
Resumo
As falhas no abastecimento de água com base em furos localizados em sistemas alterados e fraturados dos aquíferos cristalinos basais no Malawi, no sul da África, têm sido associadas à conceção deficiente de furos, a falhas mecânicas e à localização desadequada. No entanto, o trabalho recente efetuado no Malawi indica que a procura atual pode ultrapassar o potencial a longo prazo do recurso em alguns lugares, sendo esta também uma causa de falha do recurso nalguns pontos de água. Um ciclo climático de 11 anos (incluindo períodos secos e húmidos) requere a identificação dos principais locais de armazenamento de águas subterrâneas durante os anos de seca, antes de eventos episódicos de precipitação durante a época mais húmida do ciclo recarregarem novamente os aquíferos. Os dados, especialmente de níveis piezométricos, são escassos, mas suficientes para avaliar o potencial de renovação de longo prazo, tanto para as águas subterrâneas em aquíferos basais fraturados, como alterados, em cada uma das 15 áreas de gestão no Malawi. O potencial de recursos de águas subterrâneas ou de recursos renováveis de longo prazo (recarga) é dada pela soma do escoamento de Darcy e pela diminuição do armazenamento na estação seca. A procura de água estimada para abastecimento rural excede o recurso renovável no aquífero em rochas fraturadas em duas unidades de gestão e no aquífero de rochas alteradas noutras duas unidades. Embora haja incertezas inerentes às estimativas de balanços de água, o risco da procura rural de água, a longo prazo, ser superior à recarga média em algumas áreas é já um motivo de preocupação.
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The authors are grateful to the Malawi Government and to the non-governmental agencies for supply of data.
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Robins, N., Davies, J. & Farr, J. Groundwater supply and demand from southern Africa’s crystalline basement aquifer: evidence from Malawi. Hydrogeol J 21, 905–917 (2013). https://doi.org/10.1007/s10040-013-0956-5
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DOI: https://doi.org/10.1007/s10040-013-0956-5