Radionuclides Behavior of Subsurface Water in Small
Catchments, Covered by Different Vegetation in
Kawamata Town, Fukushima Prefecture
Ishwar PUN
201121329
January 2013
A Dissertation Submitted to
The Graduate School of Life and Environmental Sciences,
The University of Tsukuba
in Partial Fulfillment of the Requirements
for the Degree of Master of Environmental Sciences
Abstracts
The study was conducted after the catastrophic earthquake and tsunami triggered in 2011
March 11th and Fukushiam Dai-ichi Nuclear Power Plant (FDNPP) accident in Fukushima
Prefecture. The accident results the huge number of radionuclides deposition in the
environment. The study was performed in three different places covered by Grassland,
Farmland and Forest (Matured and Young conifer trees).
For the study, suction lysimeters in three different depths 10 cm, 30 cm and 50 cm were
installed. The soil water was collected in a conical flask. At the same time, the soil moisture
loggers were also installed in three places to know the soil water movement with different
vegetation. The significant changes with rainfall event show in a Iboishi Yama Watershed
(grassland) in every depth of 10, 30 and 50 cm depths. However, in a forest areas covered by
conifer trees and some litter, the soil water movement are not so effective. The presence of
litters in forest areas acts as the resistance of infiltration process, this results the overland
flow.
The soil water was analysed in Gamma ray detector. Gamma ray emissions at the energies of
604 keV (Cs 134) and 662 keV (Cs 137) were measured. The First samples were measured at
Tsukuba Meteorological Institute and the University of Tsukuba. Rest of the sample was
measured at Hiroshima University. The cesium (Cs-134 and Cs-137) concentration values
varied during the study period is from 0.009 Bq/Kg ~ 2.38 Bq/kg and 0.021
Bq/Kg~2.48Bq/kg respectively. The study shows that Cesium is strong attached with soil in 2
cm depth of soil in Kawamata town (Kato et al., 2012). Only the few radiocesium goes to the
soil water with preferential path of root channel or worm hole. The level of Cesium in soil
water is very low concentration.
i
Contents
Page
Abstracts .................................................................................................................................................... i
Contents.................................................................................................................................................... ii
List of Table ............................................................................................................................................ iii
List of Figures ......................................................................................................................................... iv
Chapter 1 Introduction ............................................................................................................................. 1
1-1 Background ........................................................................................................................................ 1
1-2 Previous studies ................................................................................................................................. 1
1) The radionuclides fallout by Nuclear Weapon testing in mid-19th Century: ............................... 1
2) Radionuclides fallout from Chernobyl ......................................................................................... 1
3) Radionuclides fallout from Fukushima Dai-ichi Nuclear Power plant ........................................ 2
1-3 Research Objectives: .......................................................................................................................... 5
Chapter 2 Study Area and Method ........................................................................................................... 6
2-1 Study Area Description ...................................................................................................................... 6
2-2 Sample Collection .............................................................................................................................. 7
2-3 Measurement of Cesium .................................................................................................................... 7
2-5 Soil Texture Analysis ......................................................................................................................... 8
2-5. Soil Moisture Monitoring ................................................................................................................. 8
Chapter 3 Results ................................................................................................................................... 17
3-1 Temporal and Spatial Variation of Cesium Concentration of Soil Water........................................ 17
3-2 Soil Texture...................................................................................................................................... 23
3-3 Soil Moisture Change ...................................................................................................................... 26
Chapter 4 General Discussion ................................................................................................................ 30
Chapter 5 Conclusion ............................................................................................................................. 34
Acknowledgements ................................................................................................................................ 36
References: ............................................................................................................................................. 37
ii
List of Table
Table 1 Field Survey .............................................................................................................................. 13
iii
List of Figures
Figure 1 Radiocesium Deposition in Regional......................................................................................... 3
Figure 2 Regional Dispersion of Radionuclides from Chernobyl Accident ............................................. 4
Figure 3 Study Area ................................................................................................................................. 9
Figure 4 Overview of Grassland Site and Suction Lysimeters Sampling Devices of Soil Water .......... 10
Figure 5 Overview of Young Conifer Forest and Suction Lysimeters Sampling Devices of Soil Water
................................................................................................................................................................ 11
Figure 6 Overview of Matured Conifer Forest and Suction Lysimeters Sampling Devices of Soil Water
................................................................................................................................................................ 11
Figure 7 Outline of Suction Lysimeters Devices for Soil Water Sampling and Hand Pump ................. 12
Figure 8 Soil Core Sampling in winter ................................................................................................... 14
Figure 9 Soil Core Sampling in Winter .................................................................................................. 14
Figure 10 Soil Water Extraction by Centrifugal Process ....................................................................... 14
Figure 11 Soil Core Sample Shipped and Sieving ................................................................................. 15
Figure 12 Particle Size Separation ......................................................................................................... 15
Figure 13 Soil Moisture Monitoring Sensors installed at Multiple Depth (Left) and Data Logging
System (Right) ....................................................................................................................................... 16
Figure 14 Temporal Variation of Cesium Concentration in Iboishi Yama Watershed (Grassland) ...... 18
Figure 15 Vertical Profile of Cesium Concentration in Soil Water ....................................................... 19
Figure 16 Vertical Profile of Cesium Concentration in Kotaishi Watershed (Farmland) ...................... 20
Figure 17 Temporal Variation of Cesium Concentration in Young Conifer Forest ............................... 21
Figure 18 Temporal Variation of Cesium Concentration in Matured Conifer Forest ............................ 22
Figure 19 Vertical Profile of Cesium Concentration in Matured Forest ................................................ 22
Figure 20 Iboishi Yama Watershed (Soil Texture) ................................................................................ 24
Figure 21 Kotaishi Watershed (Soil Texture) ........................................................................................ 24
Figure 22 Young Conifer Forest (Soil Texture) ..................................................................................... 25
Figure 23 Matured Conifer Forest (Soil Texture) .................................................................................. 25
Figure 24 Temporal Variation of Soil Water Movement in Iboishi Yama Watershed .......................... 27
Figure 25 Temporal Variation of Soil Water Movement in Young Conifer Forest ............................... 28
Figure 26 Temporal Variation of Soil Water Movement in Matured Conifer Forest ............................ 29
Figure 27 Vertical Soil Profile of Cesium Concentration in Kawamata Town ...................................... 32
Figure 28 Cesium Concentration in Through Fall Water in Kawamata Riko ........................................ 33
iv
Chapter 1 Introduction
1-1 Background
Soil water movement is one of the hydrological processes of hydrological cycle. In the
hydrological process, water comes in the surface through precipitation. Some portions of the
water run in the surface of the earth. Some portion of the water enters in the soil surface by
the process of infiltration, percolation and in the shallow and deep groundwater. The soil
water is one of the important factors in agriculture activity. It shows the availability of water
for the plant and how much rainfall can affect the groundwater recharge. It is also important to
know the spatiotemporal variation of soil water for the water availability.
There are many studies about the soil water movement by using isotopes of stable and
unstable elements. There are very few studies using the radioisotopes as a tracer to study the
soil water movement. The previous studies using stable isotopic ratio shows that soil water
movement is active between the soil surface and a depth of 100 cm (Tsujimura et al., 1998).
1-2 Previous studies
Radionuclides in the environment occurs either by natural cosmic ray reaction in the
atmosphere or human activities like nuclear weapon testing in 19th century and the
development of nuclear power plant. For the study of radionuclides in the environment, the
three phases of radionuclides fallout in the environment will be studied. First, radionuclides
fallout by nuclear testing in mid of 19th century, Second is radionuclides fallout by Chernobyl
accident in 1986 in Ukraine and third one is Fukushima dai- ichi nuclear power plant accident
in 2011.
1) The radionuclides fallout by Nuclear Weapon testing in mid-19th Century:
There are several practices of nuclear weapon testing. First it was tested in atmosphere and
underground testing. The first atmospheric nuclear weapon test occurred in USA on 16th July
1945. The most powerful countries at that time USA, UK and USSR continued the testing of
Nuclear weapon untill 1962. After, there was a treaty between these countries to ban the
testing in 1963. After the treaty in 1963, there were few number of nuclear testing by France
and China. The last explosion of nuclear weapon testing occurred in 1980. The fallout of
radionuclides in the 19th century was typical as shown in figure 1.
2) Radionuclides fallout from Chernobyl
After the Nuclear weapon testing, another catastrophic Nuclear power plant accident occurred
in Ukraine in 1986.According to UNSCEAR report (2000), from 1945 to 2000, the number of
atmospheric test was 543 and underground test was 1876. Comparing to Chernobyl nuclear
accident and Nuclear weapon test, almost the radiocesium deposition in Russia and Finland
was half. The contaminated area by radiation is higher in the region of Chernobyl nuclear
1
power plant and lower in far regional part of the world as shown in figure 2. From the
Chernobyl Nuclear accident total amount of radiation was releases approximately 5.2×1018
Bq.
3) Radionuclides fallout from Fukushima Dai-ichi Nuclear Power plant
After the Chernobyl Nuclear accident, another catastrophic nuclear accident was occurred in
Japan ion March 11, 2011. The total amount of radiation was released from the Fukushima
Dai-ichi nuclear power plant was estimated approximately 1.5×1017 Bq (Chino et al., 2011).
Which is the just 7% radiation released from Chernobyl Nuclear accident in 1986.
The atmospheric released radiation dose settled on the ground either through dry or wet
deposition. The primarily deposition of Cesium in the ground surface is associated with
precipitation (Tamura, 1964). The total amount of the cesium concentration is strongly
attached with surface soil particle. It is seems to be moved in the subsurface by heavy rainfall
input causing the infiltration and percolation. So, hypothetically, the movement of
radiocesium will not be uniform because it is considered that infiltration and percolation
process depend on compact of soil, soil texture, litters and vegetation types. After the rainfall,
with the movement of soil water, cesium seems to be undergo subsurface water. Very few
studies have been done on subsurface water contamination by radiocesium.
2
Figure 1 Radiocesium Deposition in Regional
Source: (S.M. Wright et al., 1999)
3
Figure 2 Regional Dispersion of Radionuclides from Chernobyl Accident
Source: UNSCEAR Report, 1998
4
1-3 Research Objectives:
There are many studies about the radiocesium in soil but very few studies have been
conducted about the radiocesium in the water. Specially, there is rare study about the
subsurface water contaminated with radiocesium. Only soil erosion and vertical profile of soil
study was done by using cesium as a tracer (Takenaka et al., 1998, Kato el at., 2011). The
main objective of the study is how the radionuclides move with the water environment. The
objectives of the investigation are as follows.
1) To make clear of the soil water movement comparing with the input of rainfall event in
different layer of the soil surfaces with the temporal changes of soil moisture
2) To explain the behavior of Radionuclides (Cs-137 and Cs-134) in a soil surface of different
layer and it movement in different vegetation.
5
Chapter 2 Study Area and Method
2-1 Study Area Description
To make the clear movement of radionuclides (137 Cs and 134 Cs) in Soil water in the four
places of Kawamata town of Fukushima covered by grassland, Farmland and Forest (cedar).
The Field study area is located 35 km north-west from the Fukushima Daichi Nuclear Power
plant. An observation points in three vertical depth of soil surface. Soil water was collected
from 10 cm, 30 cm and 50 cm depth in Grassland and Matured and Young Forest. In the case
of Farmland, the soil texture is dominated by granite and it became necessary to bore the holes
only 10 cm, 20 cm and 30 cm depths of soil surface, to enable water collection.
The study site is located in Kawamata Town, Yamakiya District, Fukushima Prefecture. The
mean annual precipitation is 1249 mm from the year of 2003-2009 (Yamakiya weather
station, JMA) and the mean annual temperature 12degree centigrade (Nihonmatsu weather
station) as referred to the Kato et al., 2012. For the study, four places were selected with
different vegetation types.
Iboishi Yama Watershed (Grassland):
The grassland was dominated by small vegetations shrubs and herbs rather than the trees. The
upper part of the grassland was dominated by Trees and down part of the grassland is covered
by small vegetation as shown in figure 3.
Kotaishi Watershed (Farmland):
The farmland was mostly covered by small weeds. The surface of soil layer was presence of
gravel. When it was boring, the area of farmland was quite hard because of presence of stone.
The upper part of farmland landscape was also dominated by trees. The landscape of
grassland and farmland are different in vegetation types as shown in fig 4.
Young Conifer Forest:
The landscape of Young forest is steeper. The forest area is dominated by cedar trees. The
small vegetation is not present. Only the litter of the cedar leafs were present in all the season.
Inside the field area, the densities of the trees are more as shown in figure 5.
Matured Conifer Forest:
The landscape of Matured Forest is more flat comparing to Young conifer forest. The area is
dominated by cedar trees. In the surface of area, large volume of litters is present. Comparing
to Young forest, the density of tree is rare as shown in figure 6.
6
2-2 Sample Collection
To collect the soil water, every four place were chosen covered by different vegetation
covered by grass, forest. The soil water sampling was performed from the beginning of the
June 2011 to end of December 2011. The soil water was sampled at the depths of 10cm, 30cm
and 50 cm in grassland and Forest areas. In the Farmland, 10 cm, 20 cm and 30 cm depths
were chosen. The soil water was collected by suing suction lysimeter consisting of a ceramic
cup with a diameter of 18mm. The water was collected by using the flasks of 500 ml volume.
To make the low pressure into the flask, hand pump (DK-8390, Daiki Rika Kogyo Co., Ltd)
were used until -70 to -80 kPa as shown in Fig 11.
In comparison of four sites, larger volumes of water were collected in Grassland than
farmland. In the forest area, the larger volumes of soil water were extracted in Matured forest
than Young Forest as shown in Table 1.
In the winter season, from mid-December to end of the March, soil water sampling by suction
lysimeter and flask, were not possible. In the winter, most of the glass were broken and frozen
of low temperature. So, during that period, soil core sample were collected. The collected soil
core samples were process in Centrifugal and extracted the soil water as shown in the Fig 8
and 9. The volume of water collected by centrifugal is very low. The average volume of soil
water was 10 ml.
2-3 Measurement of Cesium
The soil water was filtered by using 0.45 m membrane, and the Cesium 134 and 137
concentrations were determined by a gamma ray spectrometry using a germanium
semiconductor detector counting for 30,000 seconds per sample in Meteorological Research
Institute, Japan Meteorological Agency. The sample taken in end of June and July, 2011 were
measured in Tsukuba Meteorological institute. The soil core samples taken in winter were
measured at Onda laboratory in the University of Tsukuba.
The method used in Hiroshima University for the cesium measurement was “cesium
enrichment method by Ammonium phosphomolybdate”. For this process, HNO3 adjusted to
pH1, Cs-133 carrier solution, ammonium phosphomolybdate were mixed. Then agitating for
one hour and then left for 12 hours. After that the solution was filtered using 0.45 µm
membrane filter. Hence, filtrate residue (sediment) was dried under the temperature (90ºC for
12 hours). After weighing, the residue sample was measured in in Gamma Ge semiconductor
detector.
7
2-5 Soil Texture Analysis
In order to study the soil physical property, soil core sample were taken from every sites;
Iboishi Yama Watershed (grassland) , Kotaishi Watershed (Farmland) and Forest sites (Young
and Matured Conifer). Each core sample was extracted from 10 cm, 30 cm and 50 cm in
Iboishi Yama Watershed and Conifer Forest sites and 10cm, 20 cm and 30 cm depth in
Kotaishi Watershed (Farmland). In order to avoid the falling of soil from the vessel, the lower
part of vessel was fixed with cloth by using rubber band. The sample soil was then shipped in
laboratory by keeping it in a plastic container.
The soil core samples were dried for 12 hours at 105 degree centigrade. The sample was then
crushed to pass through a 2mm sieve (gravel) , 2mm>> 850µm(Sand), 850 µm>>425µm (Silt)
and 425µm< (Clay). Then the weight of soil percentage and water content were calculated.
Also the particle sizes of soil distribution were analyzed using sieve method as shown in
figure 10 and 11.
2-5. Soil Moisture Monitoring
To measure the soil water movement in every sites covered by different vegetation, soil
moisture logger and sensors (HOBO micro station, ONSET Computer Corporation) were
installed. The soil moisture loggers were installed in Iboishi Yama Watershed (Grassland) and
Conifer Forest sites (Young and Matured) in each depth of 10cm, 30 cm and 50 cm depths as
shown in figure 12. However in Kotaishi Watershed (Farmland), the presence of gravel made
it difficult to install the soil moisture sensors.
8
Figure 3 Study Area
9
Figure 4 Overview of Grassland Site and Suction Lysimeters Sampling Devices of Soil
Water
10
Figure 5 Overview of Young Conifer Forest and Suction Lysimeters Sampling Devices of
Soil Water
Figure 6 Overview of Matured Conifer Forest and Suction Lysimeters Sampling Devices
of Soil Water
11
Figure 7 Outline of Suction Lysimeters Devices for Soil Water Sampling and Hand
Pump
12
Table 1 List of soil water samples and soil cores for moisture extraction
Site
Iboishi Yama
Watershed
Grassland
Kotaishi Watershed
Young Conifer
Forest
Cedar Trees
Matured Conifer
Forest
Cedar Trees
Land
Grassland
domination
water cycle
Recharge area
Discharge area
Recharge area
Recharge area
condition
Depth
10cm 30cm 50cm 10cm 20cm 30cm 10cm 30cm 50cm 10cm 30cm 50cm
2011 8/29
800 1000
200
300
200
150
100
150
50
50
200
450
9/7
1100
800 1000
300
300
450
100
200
50
100
230
450
9/23 1100
800 1000
500 1000
500
200
450
80
200
350
500
10/8
500 1100 1200 1400
900
500
300
400
150
180
300
550
10/22
600 1100 1100
300
50
80
200
450
80
200
200
500
11/5
250 1000
700
280
250
210
50
200
0
200
210
500
11/19
600 1000
500
50
100
150
100
150
50
150
150
460
12/5 1000
550
500
530 1010
300
130
130
50
150
110
700
2012 1/12
C
C
C
C
C
C
C
C
C
C
C
C
2/17
C
C
C
C
C
C
C
C
C
C
C
C
3/9
C
C
C
C
C
C
C
C
C
C
C
C
4/14
C
C
C
C
C
C
C
C
C
C
C
C
C: Soil core sample
13
Figure 8 Soil Core Sampling in winter
Figure 9 Soil Core Sampling in Winter
Figure 10 Soil Water Extraction by Centrifugal Process
14
Figure 11 Soil Core Sample Shipped and Sieving
Figure 12 Particle Size Separation
15
Figure 13 Soil Moisture Monitoring Sensors installed at Multiple Depth (Left) and Data
Logging System (Right)
16
Chapter 3 Results
3-1 Temporal and Spatial Variation of Cesium Concentration of Soil Water
Iboishi Yama Watershed (grassland):
The vertical profiles of soil water with concentration of radiocesium are plotted against the
depth from the soil surface as shown in figure 13 and 14. The temporal variation of Cesium
was found in the surface soil water at 10 cm depth and then exponentially decreasing with
depth. Both the Cs-137 and Cs-134 had found higher concentration in the beginning of July
2011. Because of its decaying in the nature, the concentration of Cs-134 is found to be lower
in the beginning of September 2011.
The vertical profile of Cesium in soil water shows that Cesium has reached in all the depth of
10 cm, 30 cm and 50 cm depths with the rainfall input. In the Iboishi Yama Watershed,
Cesium concentration varies from 0.009 Bq/Kg~2.5 Bq/Kg.
Kotaishi Watershed (Farmland):
The Cesium concentration (Cs 134 and Cs 137) varies in Kotaishi Watershed is 0.026 Bq/Kg~
0.111 Bq/Kg respectively. From the result shown in Figure 16, cesium does not move until the
30 cm depth. The soil texture result in figure 21 shows that the in every depth of 10cm, 20 cm
and 30 cm, high percentage of clay content and trap the Cesium.
Young Conifer Forest:
In the case of Young Forest, higher value of cesium concentration shows in 10 cm depth. The
cesium value ranging from 0.141 Bq/Kg~0.466 Bq/kg. The figure 17 shows the temporal
variation of Cesium in Young Conifer Forest. The vertical profile of Cesium in Young
Conifer forest shows that in every depth, cesium has been reached and in 50 cm high
concentration of Cs-137 has been detected. The forest sites contents high volume of litters and
roots.
Matured Conifer Forest:
In the case of Matured Forest, the cesium concentration in 10 cm is higher than 30cm and 50
cm depths. The cesium value is ranging from 0.036Bq/Kg~0.682 Bq/kg. The profile of
vertical distribution and rainfall input is shown in the figure 18.
17
Figure 14 Temporal Variation of Cesium Concentration in Iboishi Yama Watershed
(Grassland)
18
Rainfall
7/1
7/8
7/15
7/22
7/29
8/5
8/12
8/19
8/26
9/2
9/9
Day
mm/hr
0
10
Rainfall
20
30
Figure 15 Vertical Profile of Cesium Concentration in Soil Water
19
Rainfall
Day
7/1 7/31 8/30 9/2910/2911/2812/28
mm
0
10
Rainfall
20
30
Figure 16 Vertical Profile of Cesium Concentration in Kotaishi Watershed (Farmland)
20
Bq/Kg
Figure 17 Temporal Variation of Cesium Concentration in Young Conifer Forest
21
Bq/Kg
Figure 18 Temporal Variation of Cesium Concentration in Matured Conifer Forest
Day
Rainfall
7/1 7/31 8/30 9/29 10/2911/2812/28
mm
0
10
Rainfall
20
30
Figure 19 Vertical Profile of Cesium Concentration in Matured Forest
22
3-2 Soil Texture
Iboishi Yama Watershed (Grassland)
In the Iboishi Yama Watershed, the presence of root is dominated in the 10 cm depth to 30 cm
depth. In the case of 30 cm depth, the presence of sand and silt is higher than clay. In 50 cm
depth, the presence of gravel and clay is higher as shown in Figure 20.
Kotaishi Watershed (Farmland)
Soil core sample was analyzed particle distribution by sieving method as shown in figure 12.
Each component of soil texture root, gravel, sand, silt and clay was separated. In every depth
of 10cm, 30 cm and 50 cm were separately analyzed. As shown in figure 21, in 10 cm depth,
soil is dominated by gravel and silt. Similarly in 30 cm and 50 cm depth, soil are dominated
by clay, silt and sand respectively.
Young Conifer Forest (Kawamata Riko)
In the Young Conifer Forest, root was present in all the depth of 10 cm, 30 cm and 50 cm
depth. It means presence of root channel upto 50 cm depths. In the 50 cm depth, soil is
dominated by high clay but 30 cm depth, very few percentage of clay as shown in figure 22.
Matured Conifer Forest (Kawamata Riko)
In case of Matured Conifer forest, equal proportion of root is present in all depths. Except in
10 cm depth, high clay content in the 30 and 50 cm depths of matured conifer forest as shown
in figure 23.
23
Figure 20 Iboishi Yama Watershed (Soil Texture)
Figure 21 Kotaishi Watershed (Soil Texture)
24
Figure 22 Young Conifer Forest (Soil Texture)
Figure 23 Matured Conifer Forest (Soil Texture)
25
3-3 Soil Moisture Change
Iboishi Yama Watershed (Grassland)
The temporal variation of soil water movement in grassland shows the direct effect of rainfall
input until 50 cm depth. The higher the rainfall input, higher the movement of water in soil
surface. From starting of August 5th to end of October 2011, soil water movement is highly
affected by rainfall input as shown in figure 24. In the winter, the change of soil water
movement is not effected as there is no rainfall event.
Kotaishi Watershed (Farmland)
The farmland is highly dominated by gravel. It became difficult to installed suction lysimeter
until 50 cm depth. So, for the soil water extraction, only the 10cm, 20 cm and 30cm depth
were chosen. The soil moisture logger and sensor were not installed because of difficulties in
digging the hole.
Young Matured Forest
The temporal variation of soil water movement in the Young forest with the rainfall input, is
highly affected in 10cm and 30 cm depth whereas in the 50 cm depth, the soil water
movement is comparatively lower. From July 1st to end of Sept 2011, the soil water
movement is affective in all three depths. In the winter season, there is only small movement.
But in 10 cm depth, soil water movement is changing than 30 cm and 50 cm depths as shown
in figure 25.
Matured Conifer Forest
The temporal variation of soil moisture in Matured forest shows that only the little amount of
rainfall input reached to the 50 cm depth. In the case of 10 cm and 30 cm depth, the
movement of soil water is similar. Even in winter season when there is no rainfall input, the
soil water movement is differently in 50 cm depth as shown in figure 26.
26
Figure 24 Temporal Variation of Soil Water Movement in Iboishi Yama Watershed
27
Figure 25 Temporal Variation of Soil Water Movement in Young Conifer Forest
28
Figure 26 Temporal Variation of Soil Water Movement in Matured Conifer Forest
29
Chapter 4 General Discussion
The concentration of radiocesium in the surface soil is not uniform. It is well known that
Cesium-137 is supplied through deposition like rain or dry fallout and absorbed into the
surface soil (McCallan et al., 1980). The radiocesium behavior is highly attached with soil
particle and 80% of radiocesium attached in top soil (Tamura, 1964, Kato et al., 2012). Only
the small portion of radiocesium will undergo into the soil surface with the rainfall input in a
preferential path i.e root channel and worms hole and then by the process of infiltration and
percolation. It is also proved that the depth distribution of radiocesium in soil also very clear
with decreasing order of concentration with depth.
4-1. Iboishi Yama Watershed
The depth distribution of radiocesium in soil water shows that the contamination of soil water
in 10 cm depth is higher than the 30 cm and 50 cm depth. When comparing with soil depth
distribution of radiocesium in soil and water, very few amount of radiocesium will move
through the preferential path like root channel and worm holes proceeding the infiltration and
percolation. If it is compared with the soil moisture data, the movement of water is very
effective during rainfall input. In grassland, the water is reached until 50 cm very clearly but
the cesium concentration is very low in 50 cm depths. It proves that the cesium is strongly
attached in soil particle rather than water drops.
4-2. Kotaishi Watershed (Farmland)
In Kotaishi Watershed, very low level of cesium is reached upto 30 cm depth. The soil is
largely dominated by clay silt sand. The vertical profile of Cesium (figure 16) shows that Cs137 is higher concentration in 20 cm depth. The equal proportion of soil particle is present as
shown in figure 21 i.e. sand, silt and clay. The presence of sand, silt and clay dominated sites;
soil water does not go clearly in deeper depth.
4-3. Conifer Forest (Young and Matured)
The soil water movement in Matured forest and Young forest are similar. With the rainfall
input, it is clear that soil water reached until 50 cm depth. It means there is a chance to reach
the radiocesium in 50 cm depth. In case of forest areas, the highest concentration of Cs 137
0.466 Bq/ Kg at 50 cm depth at young conifer forest. It is observed that near the trunk of the
tree, soil water was able to collect plenty than other places. So, it is easy to move the soil
water near the rhizoids and tree roots zones. It means the rainfall input through stem flow and
through fall (which is highly contaminated by cesium, Kawamori 2012, Figure 28) to the
subsurface water is easy to undergo. The highly contaminated water is moved to soil water of
every depth in Forest sites. Which made the high cesium concentration is soil water in Conifer
forest areas.
30
In the matured conifer forest, every 10 cm depth, high concentration of Cesium have been
detected. The cesium value of 10 cm depth is Matured conifer forest is about 0.682 Bq/Kg.
When the rainfall event occurs, the water moves in the ground surface through stem flow,
through fall and interception, the thick litter of Forest (shown in figure 7) is soaked with
contaminated water in upper surface level of 10 cm depth. So, high concentrations have been
found in 10 cm depth in Matured Conifer forest.
Hypothetically, the cesium should move in subsurface by rainfall input, process of infiltration
and percolation. Rainfall event in mountain area like Yamakiya is uneven. The rain fall data
from one place to another place can be differ (P.C. et al., 2012). Also during the Field study
sampling, it was clearly observed that precipitation pattern was differed from every 1000
meters. It could be clear either the movement of Cesium in Soil water is in vertical profile
condition or not. If the rain gauge were installed in the study sites, it will make the clear idea
of soil water movement with cesium.
31
Figure 27 Vertical Soil Profile of Cesium Concentration in Kawamata Town
(Source: Kato el al., 2012)
32
Activity（Bq/Ｌ）
Through fall in Young Conifer Forest
1200
1000
800
600
400
200
0
Cs-134
Cs-137
Figure 28 Cesium Concentration in Through Fall Water in Kawamata Riko
(Source: Kawamori et al 2011)
33
Chapter 5 Conclusion
In this study, radionuclides (radiocesium) behavior of subsurface water in different vegetation
types were studied. The fallout of radiocesium in first time in surface depends upon the wind
pattern, velocity and rainfall events. Based on experienced of field visit to Fukushima, rainfall
event was also uneven in different location of sites. When we measured the radiation air dose,
Kotaishi Watershed (Farmland) was quite higher than other sites. It shows that trap of
radiocesium and radiation dose in the air depends on the topography, landscape, trees, and
small vegetation.
The previous study by Kato et al. 2012 in the same sites shows that 80% of radiocesium
strongly deposit in the top soil of upper 2 cm soil surface. That shows that upper surface level
of soil is highly contaminated with radiocesium. It means hypothetically, with the typhoon
and heavy rainfall pattern, infiltration and percolation, the radiocesium presence in the upper
surface level should be moved to the deep groundwater. However, in soil water, the cesium
concentration was found very low comparing to the surface soil.
From the initial result of radiocesium in soil water, the concentration value is ranging from
(Cs-134 and Cs-137) 0.009 Bq/Kg ~ 2.38 Bq/kg and 0.021 Bq/Kg~2.48Bq/kg respectively. It
means the concentration of cesium is very low in soil water. After the Fukushima Dai Ichi
Accident, in the initial monitoring of radionuclides both Cs 134 and Cs 137 have been found
higher. Only from the heavy rainfall input, radiocesium should be forced to move in the deep
surface but the presence of soil particle, the movement of Soil water is different from place to
place as shown in figures 24, 25, 26.
The soil water movements are different in different vegetation types. In Iboishi Yama
Watershed (grassland), movement of soil water is more effective in every depth because of
small types of vegetation shown in figure 3 and 17. Also, the vertical profile of cesium in soil
water in grassland shows the cesium concentration in every depth and decreasing the every
value with time period. From the study, it is also clear that an isotope of cesium decaying in
the environment because of its half-life. The cesium concentration in forest sites are
comparatively higher than other sites like grassland. This shows that when the fallout of
cesium in ground surface, the vegetations differs the cesium deposition. From the result, it is
known that small grass does not trap the cesium but large canopy of tree trap the cesium and
moves to the ground surface with forest hydrological cycle.
Similarly, in forest site, soil water movement is different from Iboishi Yama Watershed
(grassland). Because of presence of trees, inception occurs and presence of litter in ground
surface, the movement of soil water reach until the 50 cm depth even the rainfall input is
small. Because of thick layer of litter in forest areas, in every depth, soil water movement is
effective. From the soil water movement phenomena, chance of reaching of radiocesium upto
50 cm is highly possible. The result obtained in Young conifer forest of 50 cm depth is higher
value of Cesium proves that the possibility of Cesium movement by preferential path of root
34
channel and also it proves the presence of roots in every depth of Forest sites as shown in
figure 22 and 23.
35
Acknowledgements
This study was carried under financial assistance of FMWSE project funded by (MEXT
Special Coordination Funds for Promoting Science and Technology 2011), Japan.
I am extremely thankful to my supervisors Prof Maki Tsujimura, who provided me chance to
study hydrology and EDL program in the University of Tsukuba.
I am also thankful to Professor Yuichi Onda, Professor Keisuke Sueki, Department of Life
and Environmental Science, The University of Tsukuba, for their noble guidance, support
with full encouragement and enthusiasm.
I am grateful to Assistant Professor Atsushi Kawachi and Dr. Yutaka Abe for his valuable
suggestions, ever encouraging and motivating guidance. Very special thanks to team mate Ms.
Manami Hada, who was always there with me during sampling in the field as well as sorting
and identification of the samples in the lab.
I would also like to thank all the laboratory members of watershed and hydrology for their
kind assistance in helping and encouragement.
Last but not the least I would also like to thank all of my friends, EDL members, family
members for encouraging and supporting me whenever I needed them.
36
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