Geochemical and statistical analysis of toxic elements in tsunami deposits occurred at March 11, 2011

Huge amount of tsunami deposits remain after the large earthquake and tsunami occurred on March 11, 2011. This event may bring a possibility of environmental pollutions, particularly in the environment of soil and sediments around coastal areas of eastern Japan. Therefor a geochemical survey and investigation for soil contamination risk was carried out, to make clear the risk level caused by tsunami event and its deposits. First more than 200 points of sampling soil and sediment samples were selected on the basis of tsunami event hazard and topography features. Samples were analyzed by means of chemical and physical methods to accumulate the database for evaluating the environmental risk. Various kinds of tsunami deposits were observed at the coastal areas, some of them are sandy sediments and others are muddy with much clay components. The result of chemical analysis showed that some portions of deposits contain a little higher content of arsenic and lead, however, almost are similar component compared with normal subsurface soils. Environmental risk assessment by using our developed GERAS system indicated that tsunami deposits sampled at around north Miyagi and Iwate pref. have relatively higher risk level. In this case some kind of risk management is necessary for their storage and utilization. Other amount of deposits and soils can be safely used for reconstruction activity because of acceptable risk level. In the analysis of physical properties of deposits, a series of database was developed for particle distribution, soil and clay components, and content of organic matters. The behaviors of biological effects and aging trend in terms of components of tsunami deposits with sulfide minerals were clarified by the precise investigation by a long term testing method. The authors also conduct an approach of statistical analysis of elements in tsunami deposits by using an original technic of sparse modeling, in which the discrimination between tsunami deposits and normal sediments has been investigated. The results of survey, analysis and risk assessment as well as various databases will be disclosed in a public after the variation process.