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.

为了评估在日本南大东隆起的环礁岛上的淡水镜的行为，在六个监测井中同时测量了地下水位和电导率。监测允许确定淡水晶状体的顶部和底部位置的位置和可变性。海水潮汐后每3小时，淡水晶状体就会周期性地发生相位滞后（延迟时间）振荡。局部降雨补给会暂时增加淡水晶状体的体积，但会受到岛中央低地上沉积的低渗透性泥泞沉积物的干扰。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。水电导率的变化提供了有关淡水晶状体时空波动的信息。该淡水透镜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。