Pumping Well Layout Scheme Design and Sensitivity Analysis of Total Critical Pumping Rates in Coral Island Based on Numerical Model

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.

Hydrogeochemistry and Acidic Property of Submarine Groundwater Discharge Around Two Coral Islands in the Northern South China Sea

Submarine groundwater discharge (SGD) is an important source of nutrients in many coastal regions, yet little information is available on its carbonate chemistry and controlling factors. This study examined the processes and factors controlling the hydrogeochemistry and acidic property of the groundwaters and SGD waters of two isolated coral islands, Liuqiu Island (13 km off southwestern Taiwan) and Dongsha Island (located in the northern South China Sea, 420 km away from Liuqiu Island). Our results showed that the total alkalinity and dissolved inorganic carbon (DIC) of the fresh SGD waters were controlled mainly by the chemical weathering of carbonate minerals. Part of the DIC came from the organic matter decomposition or soil CO2, reducing the pH and CO32− concentration. Distributions of the carbonate chemistry and nutrients of the SGD waters were controlled mainly by physical mixing between the groundwater and the ambient seawater under the seabed, the so-called subterranean estuary. The Ca2+ released through weathering significantly increased the saturation state of aragonite or calcite, reducing the corrosiveness of the SGD waters on the carbonate rocks. This study is likely the first to examine the effects of the acidic property of SGD waters on the biogenic carbonate spine of a sea urchin and a pteropod shell. The spring water with similar carbonate chemistry to that of the freshwater SGD endmember from Liuqiu Island with a saturation state of aragonite of 0.96 caused observable dissolution on the spine of a sea urchin and a pteropod shell, but the spine dissolved more readily. This was because the spine is made of high-Mg calcite, which has higher solubility than that of aragonite or calcite. Such a result implies that some marine organisms with carbonate skeletons or shells containing high Mg:Ca ratios may suffer the impact of ocean acidification earlier. Although the SGD may contribute less than 10% of freshwater discharge by rivers to the coastal area, its impact on coastal biogeochemical cycles and ecosystems due to its acidic property and continual effect on the coast all year round deserves further investigation.

Evapotranspiration on Natural and Reclaimed Coral Islands in the South China Sea

Studies of evapotranspiration on remote tropical coral islands are important to explore and sustain scarce freshwater resources. However, there is a significant knowledge gap between research to evaluate evapotranspiration based on remote sensing methods and the influences of different land use types on water dynamics on reclaimed coral islands. This study applied the remote-sensing-based Vegetation Interfaces Processes (VIP-RS) model to estimate actual evapotranspiration (ETa) on Zhaoshu Island, Yongxing Island, and Yongshu Island in the South China Sea from 2016 to 2019. The results showed that the average annual ETa of Zhaoshu Island, Yongxing Island, and Yongshu Island was 685 mm, 530 mm, and 210 mm, respectively. Annual transpiration (Ec) and soil evaporation (Es) exhibited similar patterns on the natural islands; however, Es controlled the water consumption on the reclaimed islands. Water dynamics exhibited seasonal fluctuations due to the uneven distribution of precipitation (PRP). However, ETa of the natural islands was higher than PRP in the dry season, indicating vegetation has to absorb water from the groundwater to sustain growth. The results also agreed with the analysis of dominant driving factors based on partial correlation analysis, which demonstrated that the Normalized Difference Vegetation Index (NDVI) is the most important factor that influences ETa, while relative humidity (RH) controlled the bare land or sparsely vegetated areas on the reclaimed islands. The setting of different land use types showed that vegetation and built-up or hardened roads took control of evapotranspiration and rainwater collection, respectively, which play important roles in water dynamics on corals islands. The evaluation of ETa based on a remote-sensing-based model overcame the difficulty in fieldwork observation, which improves the certainty and accuracy at a spatial scale. In addition, it gave us a new reference to protect and manage scarce freshwater resources properly.