Journal of Nuclear and Radiochemical Sciences, Vol. 3, No. 2, pp. 1-5, 2002

ArticlesGlobal Fallout Technetium-99 Distribution and Behavior in Japanese Soils
Keiko Tagami* and Shigeo Uchida
Environmental and Toxicological Sciences Research Group, National Institute of Radiological Sciences,
Anagawa 4-9-1, Inage-ku, Chiba-shi, Chiba, 263-8555, Japan
Received: April 11, 2002; In Final Form: August 7, 2002
In this study, concentrations of global fallout 99Tc in twenty-one surface soil samples collected in Japan were
determined by inductively coupled plasma mass spectrometry. The range of 99Tc concentrations in rice paddy
field, upland field and other soils were determined as 6 – 88, 4 – 8, and 7 – 29 mBq/kg-dry, respectively. The
highest average activity ratios of 99Tc /137Cs were found in paddy field soils followed by upland field and othersoils. The activity ratios in paddy field samples were one order of magnitude higher than the theoretical one estimated from nuclear fission yield, which was presently calculated as 3.3 × 10−4. Not only the activity ratio in deposition samples, but also mechanisms of 99Tc accumulation in paddy field soils from irrigation of surface water, canexplain the high 99Tc/137Cs activity ratios in soils. The results of vertical distributions of 99Tc and 137Cs in undisturbed soils showed that the migration rate of 99Tc would be slightly faster than that of 137Cs, but most of 99Tc
would still be retained in the 0 – 20 cm layer from the surface.

INTRODUCTION
Environmental amounts of 99 Tc have been accumulating
because of its long half-life of 2.1 × 105 y. This pure β-emitter(Emax= 0.29 MeV) is produced in the fission of 235U or 239Pu
with a relatively high fission yield of ca. 6%, and it has beenreleased to the environment through nuclear weapons tests andnuclear industries. The quantity of 99Tc produced by atmospheric nuclear weapon explosions was estimated to be 140
TBq using an equal fission yield to that of 137Cs and actual137
Cs deposition data.1 The release of 99Tc by nuclear industries through 1986, as estimated by Luykx,2 was of the order of1000 TBq. This was mainly from nuclear fuel reprocessing
and most of the 99Tc was discharged into the sea. The mostsignificant discharge of 99Tc nowadays is from Sellafield to the
Irish Sea; the reported total amount of 99Tc was ca. 550 TBq in
1995-1999. 3 Compared to this, the amount introduced via
nuclear medical use of 99mTc (half-life: 6.01 h) is negligible.
For example, the amounts from 99 Mo/ 99m Tc generators and99m
Tc used during 2000 in Japan were 180 TBq and 285 TBq,
respectively,4 which corresponds to ca. 7 MBq of 99Tc generation in total.
Technetium is known to exist in all valence states from +7
to –1 and the dominant species in natural aqueous solution in
equilibrium with atmosphere is the highly soluble chemical
form pertechnetate, TcO4−.5 Thus, 99Tc reaching the sea would
remain in seawater for a long time. However, in the terrestrial
environment, TcO 4 − has a high geochemical mobility 6 andavailability for plants.7−8 For this reason, it is thought to be
one of the most important radionuclides for long-term dose
assessment. From a radioecological viewpoint, analysis dataon global fallout 99Tc in environmental samples should give
useful information for predicting the nuclide behavior. At
present, however, due to very low concentration and analyticaldifficulties for determination of the nuclide in environmental
samples, there is a general lack of data on 99Tc levels in theliterature. Therefore, the behavior of the nuclide in the terrestrial environment is not well understood.
In this study, we determined concentrations of 99 Tc in*Corresponding author. E-mail:k_tagami@nirs.go.jp. Fax: +8143-206-3287.

surface soil samples. Also 137Cs was used as a comparative
indicator for discussion of the sources of 99Tc, because thefission yields from 235U and 239Pu are about the same (ca. 6%)
for the two isotopes, and the behavior and distribution of 137Cs
in the environment is reasonably well understood. The activity
ratio of 99Tc/137Cs was calculated to understand Tc mobility inthe soil environment.
EXPERIMENTAL
Sample Pretreatment. Twenty-one soil samples were collected from the surface layer (< 20 cm) of paddy fields, uplandfields and other areas (i.e., forests and orchards) in Japan(Figure 1). Table 1 lists the sampling sites and soil types. To

Paddy Field
Upland Field
Other
Other (soil core)

Figure 1. Soil collection sites in Japan.

© 2002 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 09/06/2002

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TagamiJ. Nucl. Radiochem. Sci., Vol. 3, No. 2, 2002

TABLE 1: Sampling Sites, Types, and Some Characteristics of the Soil Samples
SampleSampling site
Soil typepH
code Prefecture/City or Town (FAO/UNESCO)
P-15
P-16
P-18
P-22
P-28
P-30
P-32
P-36
P-38
F-21
F-23
F-31
F-39
F-43
F-47
O-10
O-14
O-24
O-34
O-a
O-b

Ibaraki / Mito
Hiroshima / Fukuyama
Saga / Imari
Saga / Kawazoe
Akita / Omagari
Aomori / Misawa
Iwate / Morioka
Fukushima / Koriyama
Fukushima / Koriyama
Hiroshima / Fukuyama
Saga / Chinzei
Ibaraki / Asahi
Aomori / Kamikita
Shimane / Yokota
Okinawa / Ishigaki
Saga / Chinzei
Ibaraki / Tsukuba
Okinawa / Ginoza-son
Shizuoka / Gotenba
Fukui / Katsuyama
Akita / Akita

Fluvisols
Fluvisols
Cambisols
Fluvisols
Fluvisols
Andosol
AndosolCambisols
Fluvisols
Cambisols
Cambisols
Andosols
Andosols
Cambisols
Acrisols
Acrisols
Andosols
Acrisols
Andosols
–
–

5.47
5.95
6.06
6.42
5.92
6.70
6.28
6.31
6.36
5.36
5.98
6.49
5.73
6.01
8.29
4.72
5.14
8.28
6.16
–
–

CEC

AEC

Ex.-K2O Ex.-CaO Act.-Al Act.-Fe Total-C(meq/100g) (meq/100g)

9.0
11.0
16.0
23.5
18.4
36.5
27.4
9.6
15.4
6.0
14.4
13.2
28.2
6.2
15.6
18.1
26.2
15.0
24.8
–
–

<0.05
<0.05
<0.05
<0.05
<0.05
0.10
0.17
0.82
0.10
0.09
0.36
0.14
0.1
0.10
0.06
0.17
0.43
0.31
0.10
–
–

measure the depth profile of 99Tc in undisturbed soil, two coresamples were collected in Tsukuba (O-14) and Akita (O-b)
cities. The samples were air-dried and passed through a 2 mmmesh sieve. Some amounts of the air-dried soil was used forthe determination of soil characteristics, such as, pH, cationexchange capacity (CEC), anion exchange capacity (AEC),
exchangeable-K2O, exchangeable-CaO, active-Al, active-Fe,
total C, organic-C, and total-N. The results are listed in Table
1 as well.
A 100 mL (85 – 140 g) portion of each sample was used for137
Cs determination with a Ge detector (Seiko EG&G Ortec)
coupled to a multi-channel analyzer (Seiko EG&G, Model
7800). Decay was corrected to January 1, 2001. For the 99Tc
determination, air-dried samples were incinerated for 8 h at
450˚C to decompose organic matter before chemical separation.
Reagents. Nitric acid used was ultrapure-analytical grade(Tama Chemicals, AA-100). Deionized water (>17 MΩ) was
obtained from a Milli-Q water system (Millipore Co.). Prepacked columns of Tc-selective chromatographic resin, TEVA
(Eichrom Industries, Inc.) were used for separation of Tc. The
column was pretreated with 5 mL of 8 M HNO3 followed by
10 mL of 0.1 M HNO3.
A standard 9 9 Tc solution available from Amersham(TCZ.44) was used for calibrating ICP-MS. A 95mTc solution(half-life: 61 d) was used as a yield monitor. It was preparedby irradiation of Nb in a cyclotron to avoid any 99Tc contamination.9
Chemical Separation. The separation procedure used for99
Tc determination in soil samples was reported previously,10
hence, only a brief description of the experimental procedure isgiven here. Fifty to one hundred g of incinerated soil were
thoroughly mixed with a known amount of 95mTc. Then, Tc
was volatilized in a combustion apparatus for 3 h at 1000˚C
and Tc was collected in two vertical traps, each containing 150
mL of deionized water. After heating was completed, thissample was replaced with another 50 – 100 g batch of the same
incinerated soil sample and the combustion procedure wasrepeated until the accumulated weight of the treated sample
was 200 to 300 g. The volume of trap solution was reduced toapproximately 200 mL by evaporation on a hot plate at 100˚C.
Each solution obtained by the above mentioned extraction

Org.-C

Total-N(g/kg)(g/kg)(g/kg)(g/kg)

(%)

(%)

(%)

0.14
0.24
0.14
0.22
0.30
0.19
0.16
0.08
0.19
0.19
0.83
0.57
1.32
0.23
0.27
0.42
0.28
0.09
0.31
–
–

1.18
1.76
2.59
4.56
3.60
2.89
2.88
1.92
2.42
0.79
1.74
2.42
3.46
1.30
6.19
0.97
1.33
3.38
4.06
–
–

4.5
0.9
2.4
1.6
2.2
39.8
65.1
3.6
2.8
0.9
4.0
43.2
30.2
0.8
5.4
4.4
36.9
5.9
37.2
–
–

9.5
4.2
7.4
6.0
4.2
22.0
36.0
9.8
12.8
1.5
5.7
28.3
18.0
4.6
11.7
4.5
20.5
8.7
42.0
–
–

2.40
1.46
2.24
2.42
1.87
9.48
6.66
0.32
1.54
0.90
1.26
2.81
4.35
1.60
1.11
1.69
11.15
0.86
5.04
–
–

2.16
1.37
2.06
2.00
1.84
8.92
6.64
0.32
1.42
0.82
1.15
2.74
4.17
1.53
1.00
1.38
10.14
0.79
5.00
–
–

0.26
0.15
0.23
0.19
0.16
0.59
0.56
0.05
0.15
0.10
0.12
0.25
0.4
0.13
0.13
0.17
0.26
0.14
0.46
–
–

procedure was adjusted to 0.1 M HNO3 and passed through a
TEVA resin column to separate and concentrate Tc isotopes.11
Because ICP-MS cannot differentiate between 99Ru and 99Tc, itis necessary to remove all Ru from the final sample solution
prior to ICP-MS measurement. Ruthenium present in thesample solution is not effectively retained on the TEVA resin.
Then, the column was washed with 2 M HNO3 to remove any
remaining traces of Ru. Technetium isotopes were eluted with
5 mL of 8 M HNO3. The separation steps completely removed
Ru; only < 0.1% was found in the 8 M HNO3 eluate. The
eluate containing Tc was evaporated to dryness at lower than70˚C, and the residue was dissolved in 5 mL of 2% HNO3 for
ICP-MS.
Measurement. The radiochemical recovery of Tc wasdetermined by comparing the count of 95mTc in the sample withthat in the standard solution using a NaI (Tl) scintillation
counter (Aloka, ARC-380). The 99Tc content of the sample
solution was then determined by ICP-MS (Yokogawa, PMS2000) with 180 s counting time at mass 99. Total measurement time was approximately 10 minutes per solution. A 99Tc
standard solution (Japan Isotope Association, NH4 TcO 4 in
0.1% ammonia solution) was used to determine detection efficiency. To check levels of potential interference elements(e.g., Ru, Mo), m/z = 98, 101, and 102 were also scanned at
the same time.
The average radiochemical recovery of 95mTc from the soil
samples was 63 ± 6% by the present method. The instrumental
detection limit for 99Tc by ICP-MS was 0.03 mBq/mL of thesample solution which corresponds to 0.014 mBq/g-dry of soil
sample under the operational conditions.
RESULTS AND DISCUSSION
Paddy field soils. The data for concentrations of 99Tc and
Cs in paddy field soils are listed in Table 2. The ranges of99
Tc and 137Cs concentrations are 6 – 88 mBq/kg-dry (average:
29 mBq/kg-dry) and 1.4 – 14 Bq/kg-dry (average: 6.3 Bq/kgdry), respectively. The concentrations of 99Tc in paddy fieldsoils are the same level as those we found previously.12 Then,the concentration of 99Tc was compared on the basis of several
characteristics of the soils. The results are not listed here, butthey showed that there is no correlation between 99Tc concentrations and soil characteristics for paddy field soils. This
137

Global Fallout Technetium-99

J. Nucl. Radiochem. Sci., Vol. 3, No. 2, 2002
137

99

TABLE 2: Concentrations of Cs and Tc in Paddy Field
Soils in Japan on a Dry Weight Basis and Activity Ratio of99
Tc to 137Cs
137

Code
P-15
P-16
P-18
P-22
P-28
P-30
P-32
P-36
P-38

Cs (Bq/kg)

Decay corrected to 2001.1.1

2.7 ± 0.3
1.4 ± 0.7
14.0 ± 0.6
4.1 ± 0.4
14.0 ± 0.5
6.3 ± 0.5
8.4 ± 0.5
1.7 ± 0.2
4.2 ± 0.4

99

Tc (mBq/kg)

99

Activity ratio
Tc/137Cs (×10−3)

8.4 ± 1.3
6.1 ± 0.5
88 ± 1522 ± 334 ± 56.8 ± 0.752 ± 1012 ± 1.729 ± 43.2 ± 0.6
4.4 ± 2.3
6.3 ± 1.1
5.4 ± 0.9
2.4 ± 0.4
1.1 ± 0.1
6.2 ± 1.3
7.0 ± 1.4
7.0 ± 1.1

Note) Error shows statistical errors in calculation (1σ).

agrees with our previous results obtained by tracer
experiment.13 We observed that Tc sorption ratios onto soil
surface showed no correlation with any soil characteristics
after a 3-day contact with the soils. Presumably, immobilization of Tc in soil was mainly controlled by redox conditions
and was not affected by soil types.
The activity ratios of 99 Tc to 137 Cs are given in the lastcolumn of Table 2, and they range from 1.1 × 10−3 to 7.0 × 10−3
with an average of (4.8 ± 2.1) × 10 −3 . Theoretically, theactivity ratio from nuclear fission yield is presently calculated
as 3.3 × 10−4 with correction for radioactive decay, because it
is assumed that the major source of 99Tc in Japan to now arises
from fallout. Compared to the theoretical ratio, the activity
ratios in the paddy field soils are one order of magnitude
higher. However, the activity ratios in paddy field soils of thisstudy are the same order of magnitude or one order of magnitude less than those in undisturbed soils in Japan reported
previously as (1.2 – 39) × 10−3.14−16 The 99Tc/137Cs activity
ratio in soil is influenced by several factors: one of them isdepositions containing rain and dry fallout. Ehrhardt andAttrep 17 reported that the range of the activity ratio in rainsamples which were collected in the U.S.A. was (0.11 – 2.5) ×
10−2 during 1961 – 1974, those were already higher than thetheoretical ratio. In Spain, García-León et al.18 measured
similar or higher values of (0.3 – 12.3) × 10−2 in rain samples
collected during 1984 – 1987. These ratios in the deposition
samples are higher than those in soils obtained in this study.
The activity ratio in atmospheric samples, except samples
affected by the Chernobyl accident, seem to increase withtime. However, it should be noted that the activity ratios inrecent precipitations would not affect those in soil samples,
because 137Cs deposited on Japanese soil was derived fromfallout of atmospheric nuclear weapons testing and the deposition level which peaked in 1963 had decreased to insignificant
levels by the 1990’s. The amounts of 99Tc and 137Cs deposited
onto Japanese soils for the last two decades are negligible
compared to total amounts already present in natural soils.
One thing which is hard to explain is where the excess amountof 99Tc comes from. Further studies are necessary to understand the 99Tc behavior in the atmosphere.
The other Tc accumulation mechanisms in paddy fields arealso explained by the change of Tc chemical form in soil undera waterlogged condition. The nuclide is expected to be in asoluble form as pertechnetate under an aerobic condition, suchas in surface soils and water. However, paddy fields are generally waterlogged during the planting period and subsequently,
the redox potentials of Eh decrease because microorganisms
use up the oxygen present in the water or trapped in the soil.19
The relatively low redox condition is common in paddy fields
during the planting period. Indeed, when we previously did atest using three soils to measure Eh and pH changes with time
under the waterlogged condition, the Eh values were 0.36 –3

0.48 V and the pH values were 5.8 – 6.3 just after becomingwaterlogged, under which condition Tc should exist as TcO4−.
Then after a 7-day waterlogged period, although hardly any pH
values of the samples changed, the Ehs dropped to ca. 0.2V,
which is close to the boundary between water-soluble TcO 4−
and other lower soluble Tc species in the pH-Eh diagram given
by Brookins.5 For a longer waterlogged period, the Ehs would
drop near or lower than 0V due to microbial activity. 1 3
Probably, Tc is transformed from TcO4− to a lower oxidation
form such as TcO2, TcO(OH)2, or TcS2 under a relatively low
Eh condition.5, 20 Sulfides may be formed because H2S gas canbe produced in waterlogged soil when rice plants are growing.
These lower oxidation forms of Tc have lower solubility andcould possibly be sorbed onto soil. After harvesting rice frompaddy fields, the fields are kept dry for the next several months
until the next planting season to prevent plant diseases. In ourprevious radiotracer study, we found that most of the Tc
immobilized onto the soil surface was not in a readily soluble
form even under aerobic conditions.21 When Tc was transformed from TcO4− to other chemical forms that have a low
bioavailability, plants would not absorb Tc in the soil. Thus,
99
Tc would accumulate in the paddy field soils.
Although paddy field soils could immobilize global fallout
99
Tc that was deposited directly onto them, these relatively
high 99Tc concentrations in paddy fields suggest other 99Tc
sources. There is a possibility that the paddy field soils had
trapped 99Tc not only from deposition, but also from irrigationwater through the mechanism as mentioned above. According
to Watanabe,22 the average Japanese paddy field receives 1800mm of irrigation water during the irrigation season, which is
twice as much as the water supply from precipitation. The irrigation water would contain 99Tc leached from other soil sites,e.g., forest soils. Some part of the 99Tc in the water could betrapped onto the paddy field soil surface, though there are nodata on 99Tc in water for irrigation.
Upland Field Soils and Other Soils. The data for concentrations of 99Tc and 137Cs in upland field soils and other soils
are shown in Table 3. The averages of 99Tc and 137Cs concentrations in upland field soils are 5.9 mBq/kg-dry (range: 4.3 –7.7 mBq/kg-dry) and 6.3 Bq/kg-dry (range: <1.1 – 7.7 Bq/kgdry), respectively. Those in other soils are 16.4 mBq/kg-dry(range: 7 – 29 mBq/kg-dry) and 45 Bq/kg-dry (range: <1.1 –144 Bq/kg-dry), respectively. The concentrations of 99Tc in
upland field soils are about one order of magnitude lower thanthose in paddy field soils but other soils are the same order as
in paddy field soils. The activity ratios of 99Tc/137Cs in uplandfield soils and other soils are (0.7 – 3.8) × 10−3 and (0.2 – 1.1)
TABLE 3: Concentrations of 137Cs and 99Tc in Upland Field
and Other Soils in Japan on a Dry Weight Basis andActivity Ratio of 99Tc to 137Cs
137

Code

Cs (Bq/kg)

Decay corrected to 2001.1.1

Upland field
F-21
3.4 ± 0.5F-23
1.1 ± 0.4F-31
3.1 ± 0.4F-39
7.7 ± 0.6
F-43
<1.1
F-47
2.7 ± 0.5
OthersO-1010.1 ± 0.4
O-1456.1 ± 1.2
O-24
<1.1
O-34
19.9 ± 0.8
O-a143.8 ± 1.4
O-b37.6 ± 0.4

99

Tc (mBq/kg)

99

Activity ratio
Tc/137Cs (×10−3)

4.9 ± 0.5
4.3 ± 0.4
5.8 ± 0.8
5.6 ± 0.4
6.9 ± 0.4
7.7 ± 1.2

1.5 ± 0.3
3.8 ± 1.4
1.9 ± 0.3
0.74 ± 0.08–
2.9 ± 0.7

7.0 ± 0.3
15.4 ± 0.5
12 ± 1.5
20 ± 1.6
29.2 ± 3.2
13.9 ± 0.8

0.69 ± 0.04
0.27 ± 0.01–
1.01 ± 0.09
0.20 ± 0.02
0.37 ± 0.02

Note) Error shows statistical errors in calculation (1σ).

4TagamiJ. Nucl. Radiochem. Sci., Vol. 3, No. 2, 2002

× 10 , respectively, with the averages of (2.1±1.2) × 10 and(5.1±3.4) × 10 −4 , respectively. Slightly high correlations(r>0.5) between 99Tc concentrations and some soil properties,
such as CEC, act-Fe, total-C, org-C, and total-N, for uplandfield soils and other soils are found. No correlation appears forthe paddy field soils. The relation has not been explained yet,though there was a possible influence from organic matter andmicroorganisms in the soils.
The average activity ratios found in this study for uplandfield soils are seven times higher than the theoretical ratio but
those for other soils are close to the theoretical one. Amongthe soils used in this study, the average activity ratio for paddy
field soils is the highest. The ratios in paddy field soils aretwice as much as those in upland field soils and one order ofmagnitude higher than those in other soils maybe because ofdifferent degrees of the soil reduction.13 The higher or similar
activity ratios found in this study as compared to the theoretical one suggest 99Tc accumulation in the soils, even underaerobic conditions. Generally, the most preferable chemical
form of Tc is TcO 4 − which shows a small possibility forchanging to a lower oxidation state or less soluble form underaerobic conditions. Also, the reported distribution coefficient(Kd) of 99Tc in Japanese soil samples generally showed low
values,23, 24 that is, no significant Tc fixation to surface soils is
expected when Tc is added as TcO4−. Subsequently, TcO4− isthought to migrate much faster than cations, e.g., 137 Cs + ,because of the highly soluble chemical form. However, thestudied soils seemed to retain 99Tc. It is necessary to identify
chemical forms of Tc in natural soil conditions.
Vertical distributions in undisturbed soils. To know thedetails of 99Tc and 137Cs behaviors in other soils, 2 soil coresamples were used for the measurement of vertical distributions of these radionuclides. The results are shown in Figure 2.
Within 20 cm, the average activity ratios in both cores seem tobe the same as the theoretical ratio. However, as it is clear
from Figure 2 (a), the concentration of 137Cs is highest in thefirst 5 cm while the highest 99Tc concentration is found in the
10 – 15 cm portion. The same result is found in the Figure 2
−3

−3

99

(b), and, moreover, Tc is determined at a soil depth of 40 –55cm. 99Tc migrates faster than 137Cs, but the difference issmaller than expected because most of the 99Tc was retained in
the 0 – 20 cm portion.
Although local reduction conditions can be seen even in the
aerobic surface soil, the mechanisms of Tc sorption onto thesoil should not be explained by only redox factor. Some
reports suggested that the soil organic fraction would play arole in immobilization of Tc, possibly through sorption byexchange mechanisms, reduction to less soluble forms andcomplexation.7, 25 Moreover, Morita et al.16 reported that there
was a relatively good relationship between 99Tc concentrationand soil organic matter contents. Also, we found previouslythat there were high correlations between carbon contents andTc adsorption rate at an early stage of the contact. 1 3Technetium might combine with solid organic matter. As wefound in this study, some soil properties would support TcO4−
transformation into immobilized forms. However, the mechanism of how Tc is accumulated in the surface soil underaerobic conditions is not clear. Our results, at least, lead to atentative conclusion that more Tc might have been fixed on
soil as expected before. Further studies are needed to clarify
the phenomenon of 99Tc accumulation in surface soil layer,
particularly, in an upland field environment.
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Activity ratio of 99Tc / 137Cs
10-5

10-4

Activity ratio of 99Tc / 137Cs
10-4

10-3

10-3

10-2

10-1

0

5

0

5

Cs-137
Tc-99
Activity ratio

10
Depth / cm

Depth / cm

L+H

Cs-137
Tc-99
Activity ratio

15

10

20
(a) Akita

20
1

10

100

30

Concentrations of 137Cs (Bq/kg) and 99Tc (mBq/kg)
40(b) Tsukuba

55
0.1

1

10

100

Concentrations of 137Cs (Bq/kg) and 99Tc (mBq/kg)

Figure 2. Vertical distributions of 99Tc, 137Cs and the activity ratio of 99Tc/137Cs in undisturbed soil core samples collected in Akita (a) and Tsukuba(b) cities, Japan. Error bar shows statistical error in calculation (1σ).

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