Freshwater lens oscillation induced by sea tides and variable rainfall at the uplifted atoll island of Minami-Daito, Japan

Yang, Heejun; Shimada, Jun; Shibata, Tomo; Okumura, Azusa; Pinti, Daniele L.

Freshwater lens oscillation induced by sea tides and variable rainfall at the uplifted atoll island of Minami-Daito, Japan | Oscillation d’une lentille d’eau douce induite par les marées de la mer et les précipitations variables au niveau de l’île surélevée de l’atoll de Minami-Daito, Japon Oscilación de las lentes de agua dulce inducida por las mareas y las variaciones de las precipitaciones en la isla del atolón de Minami-Daito, Japón 日本Minami-Daito隆起环礁岛上海潮和降雨变化引起的淡水透镜体振荡日本の隆起環礁南大東島における潮汐と降水による淡水レンズの変動 일본 미나미다이토섬의 조석과 강수량에 의한 담수렌즈의 변동 Oscilação de lentes de água doce induzida pelas marés e variações de chuva no atol elevado de Minami-Daito, Japão

2020

Yang, Heejun | Shimada, Jun | Shibata, Tomo | Okumura, Azusa | Pinti, Daniele L.

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.

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