Saltwater - freshwater interface is well known a keyword of groundwater development at coastal area. Problems of groundwater salification has been expanded to the deep zone as years go by. The history of saltwater - freshwater interface study is shown from the viewpoints of theoretical research and practical research in this paper. Due to the groundwater flow, the up-end of the interface is squeezed to the offshore and makes a submarine groundwater discharge, in usual. The evidence of groundwater discharge is found even in the deep sea floor, and the groundwater flow is thought to be formed along the interface, in the recent studies. The form of saltwater - freshwater interface and groundwater flow along the intertace have been made clear by the submarine groundwater discharge research and the progress of groundwater flow simulation.
In Awaji Island located near the epicenter of the 1995 Kobe earthquake in Japan, a large amount of groundwater discharged along active faults and geological boundaries after the earthquake. On the other hand, water levels of many wells remarkably dropped in highland of the island. The both hydrological phenomena are thought to be caused by increase of permeability in the groundwater aquifer by the earthquake. To prove the hypothesis, we analyzed oxygen and hydrogen isotopic ratios of the anomalously discharged groundwater, and estimated the mean altitude of the recharge area of the groundwater by using altitude effect of hydrogen isotopic ratio. As the results, the estimated mean altitude of the recharge area is clearly different from the altitude of the discharge area, and corresponds to that of the area where water level dropped after the earthquake. The results support the hypothesis that increase of permeability caused the increase of discharge rate of groundwater in the low land and the drop of groundwater level in the highland. However, as to the groundwater along the Higashiura fault, the estimated mean altitude of the recharge area is consistent with that of the discharge area. The reason may be related to the hydrological disturbance effect of the Kusumoto fault to block the groundwater flow.