Assessment of Seasonal Groundwater Variability Using Time-Lapse Electrical Resistivity Tomography in the Co To Island Area, Gulf of Tonkin, Vietnam

This research utilized time-lapse Electrical Resistivity Tomography (ERT) to assess the seasonal variability of groundwater resources on Co To Island, Gulf of Tonkin, Vietnam. Repeated ERT surveys were conducted during the dry (April 2024) and rainy (September 2024) seasons to monitor subsurface resistivity changes related to groundwater salinity and saturation. Physicochemical parameters were also measured in 15 wells to understand seasonal hydrogeological variations. The study demonstrated the effectiveness of time-lapse ERT in assessing seasonal groundwater variability on Co To Island, revealing a general increase in subsurface water volume during the rainy season. Notably, the research highlighted localized complexities, including a paradoxical expansion of a saline zone in a paleo-lagoon area during the rainy season (Line T2), contrasting with the typical dilution of seawater intrusion observed elsewhere (Line T23). The research also indicated aquifer resilience in an area with minimal extraction (Line T22). This work provides a valuable baseline for future monitoring and sustainable water resource management in this sensitive coastal environment.