Temporal changes of the radiocesium activity concentration in river bottom sediment and suspended sediment in Eastern Japan

Yuichi Onda1, Chen Tang1, Xiang Gao1, Yukio Takeuchi1,2, Keisuke Taniguchi2, Momo Kurihara1,3, and Katsumi Hirose11University of Tsukuba, Center for Research in Isotopes and Environmental Dynamics, Tsukuba, Japan2Fukushima Prefectural Centre for Environmental Creation3National Institutes for Quantum and Radiological Science and Technology

Introduction

1.Purpose of this research

Temporal trends in Cs-137 concentrations and suspended solids in river sediments from September 2011 to January 2017 in eastern Japan, based on Ministry of the Environment data from 716 monitoring 461 sites, with particle size corrections applied.

Also. In some locations, the active concentration of suspended solids was compared with dissolved Cs-137.

Methods

2、Samplings and dataDatabase of this research is based on the Report on Radioactive Substance in Public Water Environment Monitoring Surveys(2011-2018). Data size :

Cs-137 concentration in bottom sediment and dissolved:

in Hamadori, Nakadori from 2011 to 2018; other areas in eastern Japan from 2013-2018;

Cs-137 concentration in suspend sediment: in Hamadori, Nakadori from 2011 to 2017.

Sampling took by every month or season.

Number of samplings:Bottom sediment: 3006 in east JapanSS: Combined with bottom sediment 30 groups in

Fukushima.

Method

2、Correction and calculation

The bottom sediment standard particle size as 1 m2/g(89.61 cm2/g in Hamadori and Nakadori), the suspend sediment is 0.202 m2/g. The correction index P can be described by(v=0.65):

And the calculation of Kdac(apparent Kd in fresh water) can be described by:

Cs+

He and Walling(1996) Journal of Environmental Radioactivity,Vol. 30, pp. 117-137

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E+07

0 1 2 3 4 5 6 7

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

0 1 2 3 4 5 6 7 8 9

Ota (SS) -Otagawa River (bottom sediment)

SS, dissolved bottom sediment Years from the accident

Suspended solid Bottom sediment

Suspended solid Bottom sediment

Nor

mal

ized

Cs-

137

conc

entra

tion

[m2 /k

g]K

d(ac

) [L/

kg]

3、Correlation of Cs-137 concentration in bottom sediment and SSThe change of SS and bottom sediment coincided

(under equilibrium condition) —— 15 sites

Results

3、Correlation of Cs-137 concentration in bottom sediment and SSSS and bottom sediment changes do not match

(disequilibrium) —— 4 sites

Nihonmatsu (SS)-Takadabashi Bridge (bottom sediment)

SS, dissolved bottom sediment

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E+07

0 1 2 3 4 5 6 7Years after the accident

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

0 1 2 3 4 5 6 7

Nor

mal

ized

Cs-

137

conc

entra

tion

[m2 /k

g]K

d(ac

) [L/

kg]

Suspended solid Bottom sediment

Suspended solid Bottom sediment

Results

5、Results of correction in bottom sediment(in other sites)

Whole situation(2013-2018):

Without correction(213 declining, 23 increasing);

With correction(212 declining, 24 increasing);

0 221

169

42

20

20

40

60

80

100

120

140

160

180

(-∞,-0.5] (-0.5,-0.25] (-0.25,0] (0,0.25] (0.25,0.5] (0.5,+∞)

λbefore

0 123

167

41

40

20

40

60

80

100

120

140

160

180

(-∞,-0.5] (-0.5,-0.25] (-0.25,0] (0,0.25] (0.25,0.5] (0.5,+∞)

λafter

Discussion

5、Relationship of entrainment coefficient A0 and declining rate -λ

The relationship between declining rate -λ and the initial fractions of the entrainment coefficient

A0 2011-2012

y = -0.477ln(x) - 3.4481R² = 0.6355

-4

-3

-2

-1

0

1

2

3

4

1.00E-04 1.00E-03 1.00E-02 1.00E-01 1.00E+00

The

decl

inin

g ra

te -λ

(yr-1

)

The initial fractions of the entrainment coefficient A0

(m2 kg-1)

y = -0.088ln(x) - 0.8658R² = 0.4788

-4

-3

-2

-1

0

1

2

3

4

1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02 1.00E-01 1.00E+00

The

decl

inin

g ra

te -λ

(yr -

1)

The initial fractions of the entrainment coefficient A0(m2 kg-1)

The relationship between declining rate -λ and the initial fractions of the entrainment

coefficient A0 2013-2018

y = -0.029ln(x) - 0.2399R² = 0.117

-4

-3

-2

-1

0

1

2

3

4

1.00E-06 1.00E-04 1.00E-02 1.00E+00 1.00E+02

The

decl

inin

g ra

te -λ

(yr -

1)

The initial fractions of the entrainment coefficient A0 (m2 kg-1)

The relationship between declining rate -λ and the initial fractions of the entrainment

coefficient A0 2013-2018(except Hamadori，Nakadori)

Negative correlation and range of –λ is big Negative correlation and range of –λ is getting smaller in last 5 years.

Discussion

5、Relationship between declining rate -λ and the initial inventory of the Cs137 C0

The relationship between decline rate λ and the initial inventory of the Cs-137 C0 2011-2012

-4

-3

-2

-1

0

1

2

3

0 500000 1000000 1500000 2000000 2500000 3000000

The

chan

ging

rate

-λ (y

r-1)

The initial inventory of the Cs-137 C0(Bq m-2)

-1

-0.5

0

0.5

1

0 500000 1000000 1500000 2000000 2500000 3000000

The relationship between decline rate λ and the initial inventory of the Cs-137 C0

2013-2018

-1

-0.5

0

0.5

0 50000 100000The initial inventory of the Cs-137 C0

(Bq m-2)

The relationship between decline rate λ and the initial inventory of the Cs-137 C0 2013-

2018 (others sites)

some point is over the x axis and near by the y axis. It is also same in other areas.

Discussion

Discussion

5、Rate of Kd/SS and Kd/sediment

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

Kd/

SS

Kd/sediment

2013Above 1:1 line(rate>5 times)Near 1:1 line

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

Kd/

SS

Kd/sediment

2014

5、Rate of Kd/SS and Kd/sedimentAbove 1:1 line(rate> 5 times)Near 1:1 line

Discussion

Conclusion

The logarithm of the initial inventory was found to be negatively correlated with the rate of decline of Cs-137 in the sediment. The sediment of Cs-137 and the Kdac of SS were found to be similar over time. These results suggest that geographic conditions and temporal factors also influence the decline in Cs-137 contamination levels in river systems.