Abstract
Groundwater on small coral islands exists in the form of freshwater lenses that serve as an important water resource for local inhabitants and ecosystems. These lenses are vulnerable to salinization due to groundwater abstraction and precipitation variation. Determination of the sustainable yield from freshwater lenses is challenging because the uncertainties of recharge and hydrogeological characteristics make it difficult to predict the lens response to long-term pumping. In this study, nine pumping well layout schemes along a line are designed using the orthogonal experimental design method, and an optimal well layout scheme is determined by multi-index range analysis and comprehensive balance analysis method. The total critical pumping rates of the freshwater lens corresponding to different schemes are calculated by numerical simulation, and the sensitivity of the total critical pumping rates to hydrogeological parameters is analyzed. The results show that the calculation of the total critical pumping rates needs to be combined with the specific well layout scheme with consideration to the length of well screens, the number of wells and the distance between wells. The difference in total critical pumping rates between different schemes can be up to three times. The uncertainty of hydrogeological parameters has a great impact on the total critical pumping rates. Within the range of a 30% reduction in parameters, α and K are the key risk factors of pumping; within the range of a 30% increase in parameters, α, ne and K are the key risk factors; α-ne combined changes had the greatest impact. The management of freshwater lenses and the assessment of sustainable yield will continue to be important tasks for coral islands in the future, and this study can help with the sustainable exploitation of island freshwater lenses.
Highlights
The freshwater lens called the Ghyben–Herzberg lens, was discovered by Ghyben [1] and Herzberg [2] in a freshwater supply study of the coastal areas of Europe in 1889 and 1901, respectively
Fresh groundwater may appear in the form of a “freshwater lens”, which covers the seawater in a high permeability aquifer
Research on freshwater lenses has been conducted using numerical simulation and physical experiments, and these results have enhanced the understanding of groundwater resources management in coral islands
Summary
The freshwater lens called the Ghyben–Herzberg lens, was discovered by Ghyben [1] and Herzberg [2] in a freshwater supply study of the coastal areas of Europe in 1889 and 1901, respectively. Research on freshwater lenses has been conducted using numerical simulation and physical experiments, and these results have enhanced the understanding of groundwater resources management in coral islands. Numerical models have been used to research the formation, evolution, and dynamic process of freshwater lenses in coral islands [7,14,15,16]. While the role of real experiments on coastal aquifers has existed, there is still a study deficit on the impact of pumping on freshwater lenses, especially withoptimal well layouts and the uncertainty of hydrogeological parameters. A numerical model coupled with water flow and solute transport is utilized to simulate the up-coning caused by pumping in a coral island. The objective of this paper is to serve as a resource for the development and utilization of island freshwater lenses, which have important practical applications
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