Influence of Land Reclamation on Fresh Groundwater Lenses in Oceanic Islands: Laboratory and Numerical Validation

Fresh groundwater lenses in tropic oceanic islands are important freshwater resources for local inhabitants and ecosystems. These reef-carbonate islands typically have a dual-aquifer structure, with poorly consolidated Holocene sediments deposited unconformably on Pleistocene limestone reef deposits. Land reclamation has been carried out on the reef flats or shallow water areas in coral islands, a very common practice in the Indian Ocean and the South China Sea. However, there is a lack of comprehensive understanding of how the groundwater system will respond to land reclamation. In this study, the impact of land reclamation on the fresh groundwater lens in an island is investigated using multiple methods: sand-tank experiments, sharp-interface analytical solutions, and numerical modeling. This study demonstrates that land reclamation can increase the freshwater storage of the lens and shift the water divide toward the reclamation area. Furthermore, a lower permeability fill material especially with a greater scale or thickness leads to higher freshwater storage. However, the expansion of the lens will be truncated due to the high-permeability of the lower layer. These observations and findings from the laboratory experiments and two-dimensional numerical simulations are further ground-truthed by the three-dimensional modeling of Yongxing Island in the South China Sea. The findings of this study provide a comprehensive understanding of the impact of land reclamation on island groundwater system and the theoretical supports for Small Island Developing States to use the reclamation not only for urban development but also for extra aquifer to enhance the water resource sustainability under the climate change.