This study evaluates the effectiveness of Managed Aquifer Recharge (MAR) in preventing seawater intrusion near rivers in Gimpo, South Korea. MAR is a technique used to enhance groundwater storage by artificially recharging aquifers through infiltration or injection methods. It plays a crucial role in mitigating groundwater depletion, improving water quality, and preventing seawater intrusion in coastal areas. To assess the impact of MAR on seawater intrusion, a field experiment was conducted using an injection well and three observation wells located 5m, 10m, and 15m away. Long-term monitoring was carried out to measure groundwater level changes, electrical conductivity profiles, and freshwater-saltwater interface movement. The study employed the “Interface-Egg” method, which tracks the position of the interface using buoyancy-driven neutral density devices. The results showed that after MAR implementation, groundwater levels increased by 0.8 to 1.2 meters at all observation wells. The most significant seawater intrusion mitigation was observed at wells closest to the injection site, demonstrating the localized impact of MAR. Additionally, the effect persisted for several days after the injection ceased, indicating potential long-term benefits. However, variations in response time and duration were noted due to differences in geological conditions, highlighting the need for site-specific assessments. This study confirms the effectiveness of MAR in controlling seawater intrusion and demonstrates the application of the “Interface-Egg” method in monitoring interface dynamics. The findings suggest that MAR can be a viable solution for sustainable groundwater management in coastal regions. Continuous monitoring is essential to assess the long-term effectiveness and potential environmental impacts of MAR systems.