Abstract
In order to evaluate the behavior of a freshwater lens in the uplifted atoll island of Minami-Daito, Japan, groundwater level and electrical conductivity were simultaneously measured in six monitoring wells. The monitoring allowed determination of the position and variability of the top and bottom positions of the freshwater lens. The freshwater lens periodically oscillates with phase lags (delay time) every 3 h following sea tides. Recharge from local rainfall temporarily increases the volume of the freshwater lens but is disturbed by the low-permeability muddy sediments deposited on the central lowland of the island. Changes in the groundwater levels correlate well with rainfall, after first removing semi-diurnal, diurnal, and long-term components of the sea tides from the groundwater level data using a multiple regression analysis. Changes in the water electrical conductivity provide information on the temporal and spatial fluctuation of the freshwater lens. The monitoring scheme for this freshwater lens could be applied to other uplifted atolls, contributing to better evaluation of the potable groundwater resources and to making freshwater use sustainable on other islands.
Résumé
Afin d’évaluer le comportement d’une lentille d’eau douce au niveau de l’île surélevée de l’atoll de Minami-Daito, Japon, le niveau de l’eau souterraine et la conductivité électrique ont été mesurés simultanément dans six piézomètres. Le suivi a permis de déterminer la position et la variabilité des positions supérieure et inférieure de la lentille d’eau douce. La lentille d’eau douce oscille périodiquement avec des retards de phase (décalage dans le temps) chaque trois heures suivant les marées de la mer. La recharge due aux précipitations locales augmente de manière temporaire le volume de la lentille d’eau douce mais est perturbée par les sédiments boueux à faible perméabilité déposés au niveau des parties basses centrales de l’île. Les modifications dans les niveaux piézométriques sont bien corrélées avec les précipitations, une fois qu’on a enlevé les composantes semi-diurnes, diurnes, et de long terme des marées de la mer des données piézométriques en utilisant une analyse de régression multiple. Les modifications dans la conductivité électrique de l’eau fournissent une information sur la fluctuation temporelle et spatiale de la lentille d’eau douce. Le système de surveillance de cette lentille d’eau douce pourrait être appliqué à d’autres atolls surélevés, contribuant à une meilleure évaluation des ressources d’eaux souterraines potables et à rendre l’utilisation de l’eau douce durable sur d’autres îles.
Resumen
Para evaluar el comportamiento de las lentes de agua dulce en la isla del atolón elevado de Minami-Daito, Japón, se midió simultáneamente el nivel de las aguas subterráneas y la conductividad eléctrica en seis pozos de monitoreo. El monitoreo permitió determinar la posición y la variabilidad de las posiciones superior e inferior de la lente de agua dulce. La lente de agua dulce oscila periódicamente con desfases de tiempo (“delay time”) cada 3 horas después de las mareas marinas. La recarga de las precipitaciones locales aumenta temporalmente el volumen de la lente de agua dulce, pero se ve perturbada por los sedimentos limosos de baja permeabilidad depositados en la planicie central de la isla. Los cambios en los niveles de las aguas subterráneas se correlacionan adecuadamente con las precipitaciones, tras eliminar primero los componentes semidiurnos, diurnos y a largo plazo de las mareas marinas de los datos del nivel de las aguas subterráneas mediante un análisis de regresión múltiple. Los cambios en la conductividad eléctrica del agua proporcionan información sobre la fluctuación temporal y espacial de la lente de agua dulce. El plan de monitoreo de esta lente de agua dulce podría aplicarse a otros atolones elevados, lo que contribuiría a una mejor evaluación de los recursos de agua subterránea potable y a hacer sostenible el uso del agua dulce en otras islas.
摘要
为了评估在日本Minami-Daito隆起环礁岛上的淡水透镜体行为,在六个监测井中同时测量了地下水位和电导率。监测考虑了淡水透镜体位置的确定及其顶部和底部位置的可变性。海水潮汐后每3小时,淡水透镜体会周期性地发生相位滞后(延迟时间)振荡。局部降雨补给暂时增加了淡水透镜体的体积,但受到岛中央低地上沉积的低渗透性泥状沉积物的影响。在使用多元回归分析首先从地下水位数据中删除海潮的半日,日和长期组成部分之后,地下水位的变化与降雨有很好的相关性。水的电导率变化提供了有关淡水透镜体时空波动的信息。该淡水透镜体的监测方案可适用于其他隆起的环礁,有助于更好地评估饮用水资源,并使其他岛屿上的淡水利用可持续。
요지
일본 미나미다이토섬 (융기된 환상섬)의 담수렌즈의 거동을 평가하기 위해 6개의 관측정에서 지하수위와 전기전도도를 동시에 측정했다. 이 연구를 통해 담수렌즈의 위치 및 변동성을 결정할 수 있었다. 담수 렌즈는 조석에 대해 매 3시간의 위상 지연(지연 시간)을 보이며 주기적으로 진동한다. 국지성 호우로 인한 지하수 침투는 일시적으로 담수렌즈의 부피를 증가시키지만 섬의 중앙 저지대에 퇴적된 저투수성의 점토 퇴적물로 인해 차단 된다. 다중 회귀분석을 사용하여 조석의 반일주기, 일일주기, 장기변동 성분을 제거한 지하수위 변화는 강수량과 밀접한 상관관계를 보인다. 물의 전기전도도의 변화는 담수 렌즈의 시간적, 공간적 변동에 대한 정보를 제공한다. 이 담수렌즈의 관측방법은 다른 융기된 환성섬에 적용할 수 있으며 음용 가능한 지하수 자원에 대한 더 나은 평가와 담수렌즈의 지속적인 사용을 가능하게 하는데 기여할 수 있다.
Resumo
Afim de avaliar o comportamento de lentes de água doce no atol elevado de Minami-Daito, Japão, o nível e a condutividade elétrica das águas subterrâneas foram simultaneamente medidos em seis poços de monitoramento. O monitoramento permitiu determinar a localização e a variação das posições de topo e base das lentes de água doce. As lentes de água doce oscilam periodicamente com defasagens (tempo de atraso) de 3 horas após as marés. A recarga pelas chuvas locais aumenta temporariamente o volume das lentes de água doce, mas é influenciada pelos sedimentos argilosos de baixa permeabilidade depositados na planície central da ilha. Uma análise de regressão múltipla demonstrou que a variação nos níveis das águas subterrâneas se correlaciona bem com as águas da chuva, após a remoção prévia dos componentes semidiurnos, diurnos e de longo prazo das marés nos dados do nível das águas subterrâneas. As variações na condutividade elétrica da água fornecem informações sobre a flutuação temporal e espacial das lentes de água doce. O esquema de monitoramento para essas lentes de água doce pode ser aplicado a outros atóis elevados, contribuindo para uma melhor avaliação dos recursos hídricos subterrâneos potáveis, contribuindo para o uso sustentável das águas doces em outras ilhas.
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Acknowledgements
Special appreciation is extended to the members of the Hydrology Laboratory in Kumamoto University for their constructive comments and thanks to Mr. Yasuhiro Tawara (Geosphere Environmental Technology Corp.) and Ms. Mika Kashima (Kumamoto University) for their assistance in the field surveys.
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This study was supported financially by CREST in Japan Science and Technology Agency (JST).
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Yang, H., Shimada, J., Shibata, T. et al. Freshwater lens oscillation induced by sea tides and variable rainfall at the uplifted atoll island of Minami-Daito, Japan. Hydrogeol J 28, 2105–2114 (2020). https://doi.org/10.1007/s10040-020-02185-z
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DOI: https://doi.org/10.1007/s10040-020-02185-z