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This paper quantitatively investigates groundwater-seawater interactions and explores the annual variations and spatial distributions in submarine groundwater discharge (SGD) and seawater intrusion (SWI) in the Bohai Sea coastal zone in Tianjin, China. A three-dimensional finite element model, FEMWATER in the GMS environment, is developed to simulate density-dependent flow and transport in coastal groundwater aquifers. A sensitivity analysis is used to explore how the model output varies with the hydrogeological parameters and boundary conditions. The results suggest that both SGD and SWI occur across the sea-aquifer interface. Along the modeled 45 km stretch of coastline, the annual SGD and SWI rates are 4.23 × 10 7 m 3 /yr and 0.86 × 10 7 m 3 /yr, respectively. The results also indicate that SGD is highest in the winter and lowest in the summer, and SWI exhibits the opposite trend. This change in flow direction across the sea-aquifer interface corresponds to seasonal changes in sea level. SGD mainly occurs in the southern and northern parts of the study area, and SWI primarily occurs in the central part. The results of the sensitivity analysis suggest that the SGD and SWI model outputs are most sensitive to sea level and the hydraulic conductivity in the top layer. [ABSTRACT FROM AUTHOR]