3D modeling of tsunami-induced seawater intrusion and aquifer recovery in Niijima Island, Japan, under the future tsunami scenario

A 3D numerical model was developed to simulate seawater intrusion and aquifer recovery in Niijima Island under the future Nankai earthquake and tsunami scenario. The FEFLOW code was used to solve density-dependent groundwater flow and mass transport in unsaturated-saturated porous media. The simulations indicated that the maximum amount of seawater intrusion during the tsunami was controlled by the total unsaturated void space of the soil beneath the inundation area. After the tsunami, directions of seawater movement and flushing time depended on the pre-tsunami groundwater flow conditions and bedrock structures. Some groundwater was found to be survived from salinization, and showed the potential to provide water supply in an equivalent amount of the pre-tsunami level without worsening the recovery process. The simulated attempt to remove the intruded seawater from a polluted well could accelerate aquifer recovery but might not be practical due to the cost of maintaining intensive pumping over years.