Water pollution and salt damage are caused by seawater intrusion into coast area. Salt and fresh water boundary fluctuation is a one of an index to detect seawater intrusion. In this research, electrical survey was applied to grasp the salt and fresh water boundary fluctuation. The measurement line was set at southeastern J-PARK in Tokai village, Ibaraki prefecture where is on coast area. The survey was carried out every one to three months during one year. Electrical survey machine was used in this survey so that enough current can flow coast area where is covered by sand layer. As a result, high resistivity zone fluctuates at the 35 m point of east to south in the depth of 12 to 15 m. This fluctuation is clear on dry and rainy seasons. Hence, seasonal resistivity fluctuation is clarified in the coast area. Comparing this fluctuation with precipitation, underground temperature and conductivity, salt and fresh water boundary is extracted from resistivity distributions. From this, underground structure and salt and water boundary model in rainy and dry seasons can be estimated in this site.
