Objectives: In the coastal areas, which were damaged by the tsunami caused by the Great East Japan Earthquake in 2011, the salinity of the groundwater rose by infiltration of seawater. Currently, fresh water layer has been formed by rainwater during these 8 years after the tsunami on the on top of the salt water layer in unconfined aquifer. The objective of this study is to clarify the possibility of reuse of freshwater in this area by field survey.
Design and methodology: The study area Watari district, located on the east coast of northeastern Japan, was damaged by the tsunami up to 5 km from the coastline. Pumping tests were carried out every three months in 19 monitoring wells between 2017 and 2019 in this area. The monitoring wells were made of PCV and had depths of 10 to 15 m and a diameter of 50 mm. Since saltwater was distributed in the deep part of the monitoring well, a packer was installed at the boundary between saltwater and freshwater in the pumping tests, and pumping was carried out avove the packer. Each pumping test was carried out for 10 minutes and groundwater pumping ratio was 10-15 liter/minute. EC, pH, DO, and ORP of groundwater that was pumped were measured during the pumping.
Results and conclusion: At 9 out of 19 monitoring wells, the average EC of pumped groundwater was less than 70 mS/m, which was suitable for the cultivation of strawberries, a main product of this area. These 9 monitoring wells were located relatively far from the coast. These results show that groundwater quality has recovered from salinization at relatively far from the coast. On the other hand, in the monitoring wells relatively close to the coast, the EC of pumped groundwater was in the range of 70 to 200 mS/m, which was suitable for cultivating crops with high salt tolerance. In these wells, the EC of pumped groundwater tended to decrease as the installation depth of the packer decreased. These results show that the recovery of groundwater quality is slow in the area close to the coast, and it is necessary to reduce depth of well to obtain groundwater with low EC. However, even though they were located at relatively short distances, some wells had large differences in the EC of the groundwater being pumped. It is estimated that there are factors other than the distance from the coast that affect the degree of recovery from groundwater salinization.