Saltwater Intrusion Modeling on Small Islands: Insights Into Submarine Groundwater Discharge as a Major Nutrient Pathway for Phosphate Accumulation Affecting Coral Reefs
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The study examines saltwater intrusion and freshwater lens dynamics on Kuroshima Island, a small cattle-farming community of roughly 200 people, located in the Yaeyama Island group in southern Okinawa, Japan. The island’s Ryukyu limestone geology, along with its known elevation and precipitation data, provided the foundation for developing a single-layer SWI model using MODFLOW 6 and FloPy. The objective was to estimate freshwater lens thickness, track saltwater intrusion under tidal influence, and identify coastal zones of significant submarine groundwater discharge that potentially could be responsible for high phosphate levels deposited in the surrounding coral reefs. Parameter optimization, incorporating combinations of hydraulic conductivity, precipitation, and tidally varying well groundwater levels, helped refine the model. Resistivity surveys and continuous well groundwater level measurements were employed to verify freshwater thickness and validate model predictions. The results suggest that simple models informed by basic elevation and precipitation data can effectively simulate island hydrogeology, even in the absence of complex geological survey information. This is limited to islands with neither extraction nor injection of groundwater. Our findings pinpointed concentrated submarine groundwater discharge along the island’s northeastern margin, likely contributing to the high phosphate levels accumulating in nearby coral reefs. Moving forward, we aim to investigate model limitations across varying island scales and determine optimal data requirements for reliable simulation.