Abstract: To analyze the spatiotemporal desalinization of groundwater in a reclaimed coral reef island, the long-term monitoring and regular sampling data of groundwater conductivity were collected from various locations and depths. The data were then analyzed in conjunction with rainfall monitoring data to examine the temporal and spatial changes in groundwater conductivity and explore the trend of groundwater desalination. The random forest model was used to predict the variation of groundwater conductivity in borehole. The findings indicate that the general trend of groundwater conductivity in the coral reef island is characterized by oscillation decline, with the relationship between groundwater conductivity and time following an exponential function. As depth increases, groundwater conductivity also increases until it reaches a level equivalent to seawater, after which it stabilizes. Moreover, the analysis reveals that at the same depth within the reef island, there is a pattern of low conductivity in the middle and higher conductivity around the periphery. This suggests that desalination starts from the central part of the island and progresses towards the periphery, with multiple initial desalination centers present in the central area. The groundwater conductivity is affected by many factors such as rainfall infiltration, tidal action and hydrogeological parameters. Based on the predictions made by the random forest model, it is anticipated that the conductivity at 8 m below the water surface will gradually stabilize at 10 000 μS/cm after 5 years, accompanied by a significant decrease in the desalination rate of groundwater.

Key words: reclaimed coral reef island, groundwater, electrical conductivity, desalinization