For better understanding of the interaction of groundwater and seawater in coastal aquifers hydrogeological studies have been carried out. The regional water quality of groundwaters, reservoirs, and stream waters surrounding the study area was determined with a total of 100 samples in pre- and post-monsoon seasons of April to May and September to October in 2018 by analyzing the major cation/anion and radon contents. The main water quality types of groundwater, reservoir, and stream water are Ca-Cl, Ca-HCO3, and Ca-Cl, respectively. Groundwater and seawater interaction has been evaluated for the identification of tidal effect by time series analysis, hydrogeophysical method, and 2-dimensional semi-analytical solution. The time series analysis shows the groundwater level and freshwater/saltwater interface are influenced by 73% and 76%, respectively, mainly governed by tidal effect. In the result of electrical resistivity survey at the surrounding area of monitoring wells the subsurface area of the measuring line of 240~255m reflects the tide effect of the coastal aquifers. The analytical solution, on the other hand, reveals little changes of the interface, resulting that the interface depth is far away from the point in which the upper groundwater level changes occur due to the tidal effect. The preliminary results shown in this study need to be identified by further detailed investigations.