南海珊瑚礁人工岛淡水透镜体形成过程及影响因素

Formation processes and influencing factors of freshwater lens in artificial island of coral reef in South China Sea

中国目前在珊瑚礁上建设人工岛的目标是建成生态宜居岛屿,人工岛地下海水在降雨入渗驱替作用下逐渐淡化,形成的淡水透镜体是支撑岛屿生态系统的重要水源。本文针对南海珊瑚礁普遍存在的上土下岩的二元地质结构,考虑不同测量方法得到的南海人工岛填筑所用珊瑚砂渗透系数存在较大差异,建立了珊瑚礁人工岛淡水驱替海水形成淡水透镜体的动态模拟模型,对南海某珊瑚礁人工岛开展了系列数值模拟研究。结果表明,人工岛礁二元地质结构不同的渗透性对人工岛淡水开始形成时间无明显影响,填岛完成2年左右的时间可观测到淡水,估计(或理论计算)在10~20年内可形成相对稳定的淡水透镜体;在人工岛面积达到一定规模的前提下,全新世珊瑚砂沉积物渗透性是控制人工岛礁淡水透镜体的主要因素,能够形成的淡水透镜体体积随渗透系数增大而呈对数减小,并且淡水透镜体体积随全新世沉积物渗透性增大而减小的速率较淡水透镜体厚度更加显著;淡水透镜体越过不整合面进入更新世礁灰岩后,由于含水层渗透系数突然增大,淡水透镜体向水平方向扩展,在礁灰岩顶界形成"髭"状淡水体。

The goal of constructing artificial islands on coral reefs in China is to build ecological habitable islands.The seawater in the artificial islands is gradually replaced by rainfall infiltration,and the freshwater lens formed is an important water source supporting the island ecosystem.The double-layer geological structure is widespread in coral reefs in the South China Sea.Considering the fact that there exists a great large difference in the permeability coefficients obtained using different measurement methods,this study established numerical simulation models for a coral reef island in the South China Sea to simulate the seawater refreshing process.The results show that different permeabilities of the double-layer geological structure of the artificial islands have no obvious influence on the formation time of fresh water on the artificial islands.The permeability of the Holocene coral sand sediment is the main factor controlling the freshwater lens of the artificial reef.The volume of the forming freshwater lens decreases logarithmically with the increase of the permeability coefficient,and the volume of the freshwater lens decreases more significantly with the increase of the permeability of the Holocene sediment than the thickness of the freshwater lens.After the freshwater lens crossed the unconformity surface and entered the Pleistocene reef limestone,due to the sudden increase of the permeability coefficient of the aquifer,the freshwater lens expanded to the horizontal direction and formed the mustached-shape freshwater body in the reef limestone.