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The freshwater lens of Benjamín Aceval, Chaco, Paraguay: a terrestrial analogue of an oceanic island lens

La lentille d’eau douce de Benjamin Aceval, Chaco, Paraguay: un analogue continental des lentilles d’eau douce d’île océanique

Lentes de agua dulce de Benjamín Aceval, Chaco, Paraguay: una analogía terrestre de lentes de islas oceánicas

巴拉圭查科地区Benjamín Aceval的淡水透镜体:海洋岛透镜体的陆地类似体

A lente de águas doces de Benjamín Aceval, Chaco, Paraguai: uma analogia terrestre da lente de uma ilha oceânica

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Abstract

The occurrence of a freshwater lens in the Paraguayan Chaco, 900 km away from the ocean, is reported. It is located underneath sandstone hills, surrounded by lowlands with predominantly saline groundwater. Its geometry was delineated using geoelectrical and electromagnetic investigations. The unusual height of the fresh groundwater level can be attributed to the presence of a confining layer at depth. The lens receives its recharge exclusively from rainfall during the hot and humid summer months. It predominantly contains water predating the atmospheric atomic bomb tests, some of it probably up to a thousand or more years old. The water balance shows that extraction currently does not exceed recharge in normal years. However, the available volume of groundwater leaves little room for a further increase of extraction in the future. Recharge is augmented by return flow from thousands of latrines and cess pits, and this has lead to widespread contamination of the groundwater by faecal bacteria.

Résumé

La présence d’une lentille d’eau douce dans le Chaco Paraguayen, à 900 km de l’océan est rapportée. Elle est située sous des collines gréseuses, entourées de basses terres à prédominance d’eaux souterraines salines. Sa géométrie a été déterminée grâce à des investigations géophysiques électriques et électromagnétiques. L’altitude inhabituelle du niveau d’eau souterraine peut être attribuée à la présence d’une couche imperméable en profondeur. La lentille est rechargée exclusivement par les précipitions durant les mois chauds et humides de l’été. Elle contient principalement de l’eau antérieure aux essais atomiques dans l’atmosphère, probablement d’un âge de 1,000 ans et plus. Le bilan hydrique montre que les prélèvements n’excèdent pas actuellement la recharge pour des années normales. Cependant, le volume d’eau souterraine disponible ne laisse que peu de possibilité pour une augmentation des prélèvements futurs. La recharge est augmentée par les infiltrations provenant de milliers de latrines et de fosses à excréments, ce qui conduit à une large contamination de l’eau souterraine par des bactéries fécales.

Resumen

Se informa la ocurrencia de lentes de agua dulce en el Chaco Paraguayo, situado a 900 km del océano. Están localizadas debajo de lomas de areniscas, rodeadas por terrenos bajos con agua subterránea predominantemente salina. Su geometría fue delineada usando investigaciones geoeléctricas y electromagnéticas. La altura inusual del nivel de agua subterránea dulce puede ser atribuida a la presencia de una capa confinante en profundidad. Las lentes reciben su recarga exclusivamente de la lluvia durante los cálidos y húmedos meses de verano. Contienen mayormente agua anterior a las pruebas atmosféricas de las bombas atómicas, alguna de ellas probablemente hasta mil años o más de antigüedad. El balance de agua muestra que la extracción actualmente no excede la recarga en años normales. Sin embargo, el volumen disponible de agua subterránea deja poco espacio para un aumento adicional de la extracción en el futuro. La recarga se ve aumentada por el flujo de retorno de miles de letrinas y pozos negros, y esto ha dado lugar a una contaminación generalizada de las aguas subterráneas por bacterias fecales.

摘要

距离海洋900千米的巴拉圭查科地区发现有淡水透镜体。它位于砂岩丘陵之下,周围是主要含咸水的低地。通过地电和地磁调查描述了其几何形状。地下淡水水位与众不同的高度可归因于在深部存在着承压层。透镜体的补给仅仅依靠高温潮湿的夏季降雨。透镜体主要含原子弹爆炸实验之前的水,有些水的年龄可能达1000年或者更长。水平衡显示,目前的抽水量没有超过正常年份的补给量。然而,地下水的可利用量给将来增加抽水量留的余地很小。数千个厕所、粪坑的污水回流增加了对地下水的补给,这导致了地下水受到粪便细菌的广泛污染。

Resumo

É reportada a ocorrência de uma lente de água doce no Chaco paraguaio, a 900 km do oceano. Situa-se por baixo de elevações areníticas, rodeada por terras baixas com águas subterrâneas predominantemente salgadas. A sua geometria foi delineada utilizando investigações geoelétricas e geomagnéticas. A altura pouco usual do nível piezométrico da água doce pode ser atribuída à presença de uma camada confinante em profundidade. A lente recebe a sua recarga exclusivamente da água de precipitação durante os meses quentes e húmidos do verão. Contém predominantemente água atmosférica anterior aos ensaios com bombas atómicas, alguma dela provavelmente com mais de mil anos de idade. O balanço hídrico mostra que, em anos normais, as extrações atuais não excedem a recarga. Contudo, o volume de águas subterrâneas disponível não permite muita margem para um aumento futuro das extrações. A recarga é aumentada pelo fluxo de retorno proveniente de milhares de latrinas e fossas, o que conduziu a uma contaminação alargada das águas subterrâneas por bactérias fecais.

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Acknowledgements

This study was funded by the German Federal Ministry of Economic Cooperation and Development (BMZ) within the Paraguayan-German Project 2004.2189, carried out jointly by the Secretaria del Medio Ambiente (SEAM), Paraguay, and the Federal Institute of Geosciences and Natural Resources (BGR), Germany.

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Authors and Affiliations

  1. Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, 30655, Hannover, Germany

    Georg Houben, Ursula Noell, Sara Vassolo, Christoph Grissemann & Susanne Stadler

  2. Rübeland 12 - OT Bannetze, 29308, Winsen, Aller, Germany

    Mebus Geyh

  3. Secretaria de Medio Ambiente (SEAM), Avenida Madame Lynch 3500, Asuncion, Paraguay

    Eduardo J. Dose & Sofia Vera

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  1. Georg Houben
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  2. Ursula Noell
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  3. Sara Vassolo
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  5. Mebus Geyh
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Correspondence to Georg Houben.

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Houben, G., Noell, U., Vassolo, S. et al. The freshwater lens of Benjamín Aceval, Chaco, Paraguay: a terrestrial analogue of an oceanic island lens. Hydrogeol J 22, 1935–1952 (2014). https://doi.org/10.1007/s10040-014-1169-2

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