Huge tsunamis have been recorded in many places in the world (Okal 2011;
Satake et al. 2011). The Indian Ocean tsunami of 2004 was a recent huge tsunami,
such huge tsunamis have occurred repeatedly in the region in the past (Monecke
et al. 2008). The evidence of previous tsunamis is recorded as relatively well-sorted
sandy layers.
According to geological records (Minoura et al. 2001), the present study area has
also been struck repeatedly by huge tsunamis. Figure 6.16a shows a soil profile
including an old sandy tsunami deposit. Although the present tsunami deposits are
removed in Fig. 6.16a, the sand fraction of the 2011 tsunami deposit is shown in
Fig. 6.16b. At the depth of 20–30 cm from the surface, the Towada-a (To-a) tephra,
an indicator of the soil age of AD 915, occurs as light-brown patches (Fig. 6.16a).
The underlying sand layer is the tsunami deposit of AD 869 (Minoura et al. 2001).
The sand fraction particles shown in Fig. 6.16b, d have similar subrounded shapes,
which suggests sea sand. The
14 C age of plant fragments collected from the soil
horizon beneath the tsunami deposit layer (AD 869) is consistently old (1450 BP)
(Kanno et al. 2013).
6.3 Radiocesium
Radiocesium is a radioactive element that is an artificial contaminant of the soil–
biota system. Although radiocesium is sorbed strongly by soil inorganic constituents, especially weathered mica (Fig. 2.21), a trace fraction of the radiocesium is
absorbed by plants, fungi, and other organisms. The half-lives of
134 Cs and
137 Cs are
2.1 and 30.2 years, respectively.
Radiocesium was introduced into soils all over the world in the 1950s–60s due to
atmospheric nuclear testing and it remains detectable in soil. Furthermore,
radiocesium was released into the environment by the accidents at the nuclear
power plants in Chernobyl (1986) and Fukushima (2011).
Although Steinhauser et al. (2014) estimated that the total release of radionuclides
from the Fukushima Daiichi Nuclear Power Plant (FNPP) accident (approximately
520 (340–800) PBq) was approximately one order of magnitude lower than the
release from Chernobyl (approximately 5300 PBq), both accidents have been rated
as “major accidents” by the International Atomic Energy Agency. This section
describes the radiocesium contamination of soil from the FNPP accident, which
was caused by the huge tsunami that occurred on March 11, 2011.
6.3.1 Horizontal Distribution of Radiocesium
Highly contaminated areas are distributed in an inverse L-shaped pattern in the
northwestern direction from the FNPP (Fig. 6.17), although more than 80% of the
entire radioactive elements was released to the Pacific Ocean. Factors related to the
152
6 Role of Inorganic Soil Constituents in Selected Topics
Satake et al. 2011). The Indian Ocean tsunami of 2004 was a recent huge tsunami,
such huge tsunamis have occurred repeatedly in the region in the past (Monecke
et al. 2008). The evidence of previous tsunamis is recorded as relatively well-sorted
sandy layers.
According to geological records (Minoura et al. 2001), the present study area has
also been struck repeatedly by huge tsunamis. Figure 6.16a shows a soil profile
including an old sandy tsunami deposit. Although the present tsunami deposits are
removed in Fig. 6.16a, the sand fraction of the 2011 tsunami deposit is shown in
Fig. 6.16b. At the depth of 20–30 cm from the surface, the Towada-a (To-a) tephra,
an indicator of the soil age of AD 915, occurs as light-brown patches (Fig. 6.16a).
The underlying sand layer is the tsunami deposit of AD 869 (Minoura et al. 2001).
The sand fraction particles shown in Fig. 6.16b, d have similar subrounded shapes,
which suggests sea sand. The
14 C age of plant fragments collected from the soil
horizon beneath the tsunami deposit layer (AD 869) is consistently old (1450 BP)
(Kanno et al. 2013).
6.3 Radiocesium
Radiocesium is a radioactive element that is an artificial contaminant of the soil–
biota system. Although radiocesium is sorbed strongly by soil inorganic constituents, especially weathered mica (Fig. 2.21), a trace fraction of the radiocesium is
absorbed by plants, fungi, and other organisms. The half-lives of
134 Cs and
137 Cs are
2.1 and 30.2 years, respectively.
Radiocesium was introduced into soils all over the world in the 1950s–60s due to
atmospheric nuclear testing and it remains detectable in soil. Furthermore,
radiocesium was released into the environment by the accidents at the nuclear
power plants in Chernobyl (1986) and Fukushima (2011).
Although Steinhauser et al. (2014) estimated that the total release of radionuclides
from the Fukushima Daiichi Nuclear Power Plant (FNPP) accident (approximately
520 (340–800) PBq) was approximately one order of magnitude lower than the
release from Chernobyl (approximately 5300 PBq), both accidents have been rated
as “major accidents” by the International Atomic Energy Agency. This section
describes the radiocesium contamination of soil from the FNPP accident, which
was caused by the huge tsunami that occurred on March 11, 2011.
6.3.1 Horizontal Distribution of Radiocesium
Highly contaminated areas are distributed in an inverse L-shaped pattern in the
northwestern direction from the FNPP (Fig. 6.17), although more than 80% of the
entire radioactive elements was released to the Pacific Ocean. Factors related to the
152
6 Role of Inorganic Soil Constituents in Selected Topics