Sustainable Geoenvironmental Engineering Practice - Sustainable Practices in Geoenvironmental Engineering - page 493

of this section's discussion to introduce the concepts of soil attributes and their ability to
respond to cleanup techniques as integral to the practice of sustainable geoenvironment
management.
The soils in Tohoku area have been contaminated by the radioactive substances dis-
charged from the Fukushima Daiichi Nuclear Power Plant accident. The power plant
buildings, which were damaged by the March 11, 2011, east Japan great earthquake and
accompanying tsunami, suffered hydrogen explosions on March 12, 14, and 15, which
destroyed the upper portions of the nuclear power plant buildings. This resulted in the
release of radioactive nuclides consisting of Kr, Xe, I, Cs, Ce, Sr, Ru, etc. (Nakano and Yong,
2013). The resultant fallout onto the surrounding regions is extensive and presents the
geoenvironmental science and engineering professions with the problem of how to reme-
diate the soils contaminated with radioactive cesium (Ebihara et al., 2012; Akai et al., 2013;
Nakano and Yong, 2013). The fallout remains mainly in the surface soil layer, sediments,
and surface water, with some penetration into the subsurface soil (Vray et al., 2003; Lusa
et al., 2009). Of great concern are 134 Cs and 137 Cs since they possess strong photon energies
and long half-lives. The radioactive cesium shows relatively high sorption properties in
soils and sediments (Akai et al., 2013). The sorption characteristics of radioactive cesium
depend on pH (Poinssot et al., 1999; Koarashi et al., 2012), types of minerals, and organic
matter content (Nakano et al., 2003).
According to the Japanese tentative regulation, the soils with radioactive cesium concen-
trations higher than 8000 Bq/kg have to be removed and strictly controlled. Lower concen-
trations are also detrimental to human health and other living things. This situation holds
not only for the Fukushima accident, but to other places under similar circumstances. It is
a signiicant challenge and responsibility for geoenvironmental engineers to undertake to
decontaminate the affected regions—taking note of the fact that more than 20% of the 137 Cs
radioactive nuclides released were deposited on land surfaces in areas between 60 and 400
km from the power plant site (Nakano and Yong, 2013). Reduction of the volume of moder-
ately contaminated soils over a widely spread area may be an approach. The present target
is to produce remediated soils with radioactive concentrations lower than 8000 Bq/kg. By
rehabilitating these soils, a wide area around the center of the accident can be readied for
reconstruction and many victims can return. To achieve this, it may be necessary to reme-
diate the contaminated soils and to reuse some of the remediated soils.
13.6.2 Rehabilitation Schemes
There have been several schemes proposed for remediation of contaminated soils. However,
when it comes to large areas of land surface soils, many of these schemes are not practical—
to a large degree because of the vast expanse of land surface and also because the tech-
niques proposed are not economically practical and/or suitable for thin surface layers.
Such a case is the problem posed by the cesium-contaminated farmlands in the Tohoku
region. Nakano and Yong (2013) have estimated that the total volume of surface soil to be
removed for treatment in the 80 km affected zone is about 34.23 × 10 6 m 3 —meaning that if
the farmlands are to be returned to their original site and regional functionality, eficient,
and robust treatment techniques for handling such large quantities of contaminated mate-
rial are required. The following discussion details the underlying rationale for the applica-
tion of a set of procedures that has been applied to treatment-remediation of vast areas
of contaminated sediments. The basic elements of the technique show that this technique
is applicable for treatment and rehabilitation of vast areas of contaminated surface layer
soils, and is a prime example of sustainable geoenvironmental engineering practice.