We measured seepage rates of coastal groundwater from a volcanic island, standing in the South Sea of Korea. The seepage rates measured along the sandy coast were in the range 50--300 m/yr, which are much higher than those reported from typical continental coast. The 18O tracer in potential seeping waters signifies that they are mixtures of basal groundwater and pristine seawaters. On the eastern coast of Jeju, almost all groundwater discharge is attributed to recirculating seawater, while fresh groundwater contributes about 20% of the total submarine groundwater discharge (SGD) on the western coast of Jeju. Thus, SGD appears to be important for the management of water resources in western Jeju. The measured radionuclides (228Ra, 226Ra, and 222Rn) and nutrients in the groundwater suggest that the discharge of both fresh and recirculated seawater will have a significant influence on the budget of coastal nutrients and other chemical constituents in this region.