Abstract
The 2011 off the Pacific coast of Tohoku Earthquake on March 11, 2011 was followed by massive Tsunami which had a devastating effect on the coastal area of eastern Japan. The impact of the Tsunami on groundwater usage and quality in Asahi-city, Chiba prefecture was investigated.
The maximum height of the Tsunami reached Asahi-city was estimated to be 7.6 m. All wells that covered by the Tsunami higher than human knee level lost their groundwater usage due to electric pump failure. Only 3 out of 67 wells investigated were collapsed by the Tsunami. Many wells and pumps were repaired/replaced within one week after the Tsunami and all wells are in use 2 month after the Tsunami. This suggests that groundwater could be obtained through the wells right after the Tsunami if proper pumping measure would have been implemented.
Most groundwater affected by the Tsunami showed high electric conductivity (EC) of more than 1,000μS/cm, indicating broad area of groundwater contamination by seawater. The seawater entered into the shallow aquifer by infiltrating through the ground surface during the inundation, and also by flowing downward along/through the wells. The large variation in EC was observed among the affected wells. The EC value was, however, correlated with height of the Tsunami reached, topography and also with post-tsunami groundwater pumping. The post-tsunami pumping is effective in reducing salt concentration in the groundwater.
The groundwater quality should recover naturally in the long term. It was, however, suggested that removing salt containing deposits on the ground surface to avoid further salt infiltration, and groundwater pumping with careful monitoring to remove contaminated groundwater from the aquifer would be effective for faster recovery of the groundwater quality.
