[A-HW27] Biodiversity, nutrients and other materials in ecosystems from headwaters to coasts

[AHW27-23] Analyzing Groundwater Flow Dynamics on Kuroshima Island: Impacts of Submarine Groundwater Discharge on Coastal Ecosystems

*Chris Leong¹, Jun Yasumoto^1,2, Ryusei Minami⁶, Makoto Taniguchi¹, Song Ke-Han ⁵, Mariko Iijima⁴, Ko Yasumoto³, Ryuichi Shinjo^1,5 (1.Research Institute for Humanity and Nature, 2.Faculty of Agriculture, The University of the Ryukyu, 3.Kitasato University, 4.National Institute of Advanced Industrial Science and Technology, 5.Faculty of Science, The University of the Ryukyu, 6.Graduate School of Agriculture, The University of the Ryukyu)

Keywords:submarine groundwater, salt water intrusion, Modflow6, freshwater lens, integrated water resources management, coastal

This research investigates the dynamics of saltwater intrusion and freshwater lens formation on Kuroshima Island, a small landmass within Sekisei Lagoon in the southern Okinawan Islands. The study was prompted by previous findings of significant phosphate accumulation that negatively impacted the surrounding coral reefs. With intensive pastoral agriculture practiced on the island, the primary goal was to understand groundwater and submarine groundwater flow dynamics to nearby coastal areas.
Using MODFLOW 6 and FloPy developed by the United States Geological Survey, a single-layer saltwater intrusion model was constructed and calibrated with tidal data and continuously observed water levels from non-extraction, non-injection wells. The results indicated a strong likelihood that the island’s groundwater flow dynamics are contributing to submarine groundwater substantially discharged in areas impacted by nutrient deposition in nearby coastal waters. This information is important for the integrated water resources management related to the protection of marine ecosystems.