南海珊瑚礁吹填造岛淡水赋存演化机制与备需节用策略

The South China Sea acts as the throat for China to India Ocean and South Pacific. It is also the important economic strategic zone for “Chinese 21st century maritime silk road”. The engineering of reclamation islands have significant strategic meaning and economic value for China to promote the economy development and safeguarding maritime rights and interests. However, the gap between the demand and supply of fresh water on these reclaimed islands increasingly become an urgent problem needing to be solved. Previous experiences show that the large area reclamation land could store abundant fresh water, becoming an underground storage warehouse of fresh water on coral reefs. As a result, the dependency of water supply from on-land on remote reefs can be significantly reduced; the survival ability of people living on the remote reefs can be significantly enhanced as well. In this project, we aim to understand to variation characteristics of the formation time, as well as the occurrence of space of the freshwater in new reclamation coral sand islands, which owning special geological condition, adopting field observation tests, laboratory model testing, theoretic analysis and numerical modelling. Based on the obtained results, we would like to reveal the time process of formation and its evolution characteristics of freshwater lens; establish temporal model of formation and evolution of freshwater lens in the new reclamation coral islands. Based on the data observed in laboratory model test and in-site pumping test, the pumping evolution process and its characteristics of pumping inverted cone will be further understood. Finally, some strategic policies on storage and protective development of freshwater will be proposed. The investigation results in this project could provide valuable reference for the utilization of freshwater on similar reclamation coral sand islands in the future. The work in this project has important strategic meaning and application value for smooth implementation of China's economic development strategic planning, construction of powerful marine country, and safeguarding maritime rights and core interests.

南海是我国西出印度洋，南下太平洋的咽喉，是我国“21世纪海上丝绸之路”经济战略的重要区带，吹填造岛工程对促进我国经济发展、维护海洋权益具有重要的战略意义和经济价值。随着吹填造岛的进行淡水和饮用水的需求与供给矛盾愈发凸显。大面积的吹填陆域可以储存丰富的淡水，成为岛礁宝贵的地下淡水储仓，从而减少偏远岛礁对淡水输运的依赖性，提高岛礁生存能力。本项目旨在通过现场观测试验、室内模型试验、理论分析和数值模拟，查明新近吹填珊瑚礁砂特殊地质条件地下淡水形成时间和赋存空间的变化规律，揭示淡水透镜体的形成周期和演化特征，建立岛礁吹填陆域淡水透镜体形成演化的时空模型；基于室内模型试验和现场抽水试验获得抽水倒锥的演化过程和特征，提出战略储备和保护性开发的备需节用策略，为类似岛礁淡水资源利用提供借鉴，对推动我国经济发展战略规划顺利实施、建设海洋强国、维护海洋权益和核心利益具有重要的战略意义和应用价值。

南海是我国西出印度洋，南下太平洋的咽喉要道，也是“海上丝绸之路”的必经之地。星罗棋布于南海的珊瑚岛礁，是海洋开发和国防建设的前哨基地和我国海洋强国战略的重要依托。我国近年来大力开展的南海吹填造陆工程，形成相互支援的战略支撑点，从根本上扭转了我国对南海的管控能力，有效遂行海上维权，捍卫国家主权和海洋权益。在自然岛内部，海平面高度范围内存在着上淡下咸，内淡外咸，形似透镜状的淡水区域，是支撑岛礁动植物生态系统的生命之源。地表水收集、海水淡化等难以替代地下淡水的生态环境效应，不能彻底解决岛礁淡水资源难题。偏远岛礁大陆远距离运水保障模式也不能适应复杂台风气候环境和军事斗争特殊时期的需要。对无胶结、欠固结、饱水含盐的珊瑚礁砂，均匀致密且不整合的覆盖于礁坪之上形成的人工珊瑚岛礁，能不能和多久能形成具有稳定边界，可资利用的淡水资源是目前面临和亟待研究的科学难题。本项目抓住国家大力推进南海岛礁建设的宝贵机会，开展现场钻探和重型动力触探、礁砂密度含水量试验、钻孔抽水试验和双环渗透试验，揭示礁砂地层结构及其水文地质参数；建立岛礁水文气象监测平台和地下水位、潮汐、水质监测网络并开展长期监（观）测；监测发现南海岛礁降雨充沛，淡水涵养来源充足，年有效入渗降雨量1500~2000mm；地下水电阻率、电导率和氯离子浓度测试结果均显示上淡下咸的基本格局，尤其潜水面及以下2米范围变化最为明显，监测期内氯离子浓度从10000mg/L降低到平均2500~3000mg/L范围；随时间增加地下水呈由外向内，由深到浅逐渐淡化的趋势，氯离子浓度和电导率逐渐降低，中心位置浅表区甚至达到1000mg/L；变化趋势和速率与大气降雨、丰枯水期密切相关且具有一定滞后性；尚未实测到达到淡化水水质标准（600 mg/L）的区间。项目获取的大量岛礁水文地质、气象和水质监测本底数据，为机理分析和数值模拟奠定基础，同时为我国岛礁生态建设、地下淡水资源保护和利用提供重要的参考。