Impact of the 2011 Tohoku-oki earthquake tsunami on cultivated soil in Miyagi Prefecture, Northeastern Japan: An overview

To investigate damage resulting from the 2011 Tohoku-oki earthquake tsunami on cultivated soil, a regional soil survey covering 344 sites of coastal farmland in Miyagi Prefecture was conducted in May 2011 as an initiative of the Agriculture Promotion Division of Miyagi Prefecture. Samples numbering 390, 344, and 340 were carefully collected from the tsunami deposit layer (TD), the surface 10-cm layer (SL1), and the next 10-cm layer (SL2) of the cultivated soil, respectively, and were used for laboratory analyses. Tsunami deposits covered the surfaces of 275 field sites of the total 344 field sites. The cumulative thickness of TD at each site represented a log-normal distribution with a median of 4.8 cm (maximum, 40.3 cm) and decreased with increasing inland distance from the shoreline. An electrical conductivity of 1:5 water leachate (EC_1:5) of TD showed a median of 5.9 dS m⁻¹, whereas that of the cultivated soil layers SL1 and SL2 was 2.1 and 1.2 dS m⁻¹, respectively. TD with high EC_1:5 mostly comprised muddy TD and that with a distribution biased to 2–4 km inland. Due to the direct impact of seawater, the EC_1:5 values of SL1 and SL2 were relatively high at the inland sites with a thin layer or no sediment. The ion exchange reaction between sodium in seawater and calcium held by soil particles caused an increase in water-soluble calcium and exchangeable sodium within the tsunami-affected area. The similarity in the exchangeable components in TD and SL1 strongly suggests that the deposits of the inland tsunami are derived from the eroded soil surface of neighboring farmland. As a result of 5 years of government-sponsored reconstruction and salt removal, the restoration of farmland has been completed in 88% (11,411 ha) of the target area in Miyagi Prefecture.