Estimation of Sedimentary Area of Tsunami Deposits and Evaluation of Heavy Metals Related to the Great East Japan Earthquake

The physical and chemical properties of tsunami deposits of the Great East Japan Earthquake on March 11, 2011, are investigated. In addition, environmental impact assessments are conducted based on estimations of tsunami deposit areas. The contents of heavy metals in tsunami deposits are similar to those in common soils. The leaching values of arsenic and lead are slightly higher than those of general soils. Large volumes of silt and clayey tsunami deposits were deposited from Kamaishi City in Iwate Prefecture to the area south of Minamisoma City in Fukushima Prefecture. Ignition losses are high in these areas, and a positive correlation is observed between ignition loss and silt/clay content. In addition, arsenic content in tsunami deposits is positively correlated with silt/clay contents, and the fraction of silt/clay is more than 30% at many sites where Japanese environmental criteria for leaching values of arsenic are exceeded. These results indicate that the environmental impacts of arsenic might need to be evaluated in areas where fine-grained tsunami deposits are deposited, because humans might ingest arsenic in tsunami deposits, or it might be leached and transferred to the groundwater and agricultural products.