Dynamics of radioactive cesium behavior in agro-environment

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National Agriculture and Food Research Organization

Dynamics of radioactive cesium behavior in agro-environment

Noriko Yamaguchi, Kazunori Kohyama, Yusuke Takata, Sadao Eguchi

National Institute for Agro-environmental Sciences, NARO

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RCs concentration in farmland soils

Plant uptake

Criterion to decide whether or not decontamination is necessary

Soil to water distribution

What causes changes in RCs concentration in farmland soils?

3. Radiocesium Interception

Potential (RIP) of farmland soils

1. Changes in RCs

concentration of farmland soils for 5 years

2. Loss of RCs

from paddy fields Radioactive cesium (RCs):

¹³⁴

Cs+

¹³⁷

Cs

Three topics in this presentation

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2. Loss of radiocesium from paddy fields

3. Radiocesium Interception Potential of farmland soils

1. Changes in radiocesium concentration of farmland soils over 5 years

RCs deposition

Plough

15 cm

composite sample from 5 plots in each field

0-15cm

Ge detector

134Cs and ¹³⁷Cs activity

Yamaguchi et al., J. Environ. Radioactiv. (2012) 2/15

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RCs level in soil

(Bq / kg) Technology

～5,000

Inverting plow, reduction of RCs uptake by crop, topsoil removal

(uncultivated field)

5,000 ～10,000

Topsoil removal,

inverting plow, removal of soil after paddling with water

10,000 ～25,000 Topsoil removal

25,000 ～ Topsoil removal after solidification

RCs distribution map of farmland soils in Fukushima

8900 ha (2011)

2015

2200 ha (2015)

Kohyama, Takata, unpublished

0-500 500-1000 1000-5000 5000-10000 10000-25000 25000-50000 50000<

(Bq kg^-1)

Farmlands over 5000 Bq/kg

Technologies sorted by RCs levels in soil 2011

Decontamination Radioactive decay Erosion

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Rates of RCs decrease were typically faster than radioactive decay

RCs decrease over 5 years without decontamination

Paddy fields (average of 16 fields) Upland fields (average of 14 fields)

Relative RCs activity

year RCs concentration (Bq/kg)

RCs concentration in 2012

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 Wind erosion

 Surface run-off

 Downward migration

Accelerated by soil perturbation due to agricultural practices

Loss of RCs-bearing soil particles

17 y (9-24 y) 17 y (8-26 y) Paddy field Upland field

(wheat)

Effective half life of ¹³⁷Cs in ploughed layer Monitoring of ¹³⁷Cs

concentration in farmland soils for 25 years

(Komamura et al., 1999, 2004)

Global fall-out by atmospheric nuclear weapons test

137 Cs (Bq/kg DW)

Rates of decrease were faster than radioactive decay (T_1/2=30.1yr)

Radioactive decay

Average of 15 paddy fields

1960 1970 1980 1990 2000 2010

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1. Changes in radiocesium concentration of farmland soils for 5 years 2.

3. Radiocesium Interception Potential of farmland soils

2. Loss of radiocesium from paddy fields

Soil perturbation event by soil puddling with

irrigation water before transplanting rice seedlings Irrigation water

Atmospheric deposition

Surface drainage Tile drainage

Input

Output

Surface drainage during puddling

Surface drainage before transplanting

(5 days after puddling) SS: 65000 mg/L

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0.001 0.01 0.1 1 10 100 1000

0.1 1 10 100 1000 10000 100000

日和田降下物日和田灌漑水

日和田表面排水慣行日和田表面排水節水日和田暗渠排水小浜降下物小浜灌漑水小浜表面排水小浜湧水大波降下物大波灌漑水大波表面排水大波暗渠排水月舘降下物月舘灌漑水月舘表面排水月舘暗渠排水日和田・畑表面排水猪苗代・草地降下物猪苗代・草地表面排水比曽降下物

比曽表面排水比曽河川水樋渡降下物樋渡河川水南相馬灌漑水

SS (>0.025 μm) concentration (mg/L)

Total RCs vs suspended solids (SS)

Total RCs（Bq/L）

-3.0 -2.0 -1.0 0.0 1.0 2.0 3.0

RCs balance/inventory (%)

RCs Inventory (Bq/km²)

500 10³ to 10⁴

 Input and output of RCs was associated with those of suspended solids.

 RCs was lost from paddy fields at rates of less than a few % per year of the soil RCs inventory

RCs balance/Inventory

RCs inventory (Bq/m²)

RCs balance/inventory (%)

8

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1.E+2 1.E+3 1.E+4 1.E+5 1.E+6 1.E+7

0.001 0.01 0.1 1

日和田降下物日和田灌漑水日和田表面排水慣行日和田表面排水節水日和田暗渠排水小浜降下物小浜灌漑水小浜表面排水小浜湧水大波降下物大波灌漑水大波表面排水大波暗渠排水月舘降下物月舘灌漑水月舘表面排水月舘暗渠排水日和田・畑表面排水日和田・果樹表面排水日和田・果樹浸透水猪苗代・草地降下物猪苗代・草地表面排水比曽降下物

比曽表面排水比曽河川水樋渡降下物樋渡河川水南相馬灌漑水

Electrical conductivity (EC) (dS/m) K

d

(L/ kg )

K

_d

10

³

～ 10

⁷

L/kg

Decrease with increasing EC

   

 Bq/L 

filtrate in

conc Cs

Bq/kg SS

in conc L/kg Cs

K

₁₃₇

137

d



1 10 100 1000 10000

10³ 10⁴ 10⁵ 10⁶ 10⁷

K d

RIP (mmol kg^-1)

Distribution coefficient

Source of plant uptake

K

d

(L/ kg)

10³ 10⁴ 10⁵ 10⁶ 10⁷

1 0.1

10² 0.01 0.001 9

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Intrinsic parameter describing RCs mobility in soil independent of various environmental factors

1. Changes in radiocesium concentration of farmland soils for 5 years 2. Loss of radiocesium from paddy fields

RCs in plant RCs in soil Transfer factor (TF)

Simple and Important parameters to describe/predict RCs behavior

Water Soil

Plant

RIP

TF

K_d

= RCs in soil RCs in water Distribution coefficient

3. Radiocesium Interception Potential (RIP) of farmland soils

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Radiocesium interception potential (RIP)

Parameter to evaluate selectivity and quantity of highly selective sites for Cs sorption

 Cation exchange sites except FES masked by Ca²⁺

 FES  saturated by K⁺

Spiked ¹³⁷Cs is exchanged with K on FES

COO^-

O O COO

HO －－

－

－－

－

－－－

－

－－

－

－－

－

Experimental method

137Cs

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Geometric mean: 1280 ± 2.16 mmol/kg Minimum: 73 mmol/kg

Maximum: 12700 mmol/kg Median: 1300 mmol/kg

RIP distribution map

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<500 500 - 1000 1000 - 1500 1500 - 2000 2000 - 2500 2500 - 3000 3000 - 3500

>3500

RIP (mmol kg^-1)

Fluvisols Cambisols Andosols Soil Group

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Values within a column followed by different letters are significantly different according to Steel-Dwass test at p≦0.01 for soil group and p≦0.005 for clay mineral composition

Mica ×

× 〇

Sm ×

〇〇

Ave 625 1530 1560

N 37 28 487

a b b

Andosols Fluvisols Cambisols TC % >6% 3~6

％

<3% >3% <3%

Average 486 812 1360 1260 1870 2410

N 52 69 96 79 486 143

a b c c d e

Mica: vermiculite, illite Sm: smectite

Soil Group Clay mineral composition

Low RIP Andosols, High TC, w/o crystalline clay minerals

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RCs distribution map RIP map

Prediction of RCs uptake by crop +

RCs concentration of farmland soil changes over time Loss of Cs-bearing soil particles by erosion

Summary

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Acknowledgement

This work was performed under a contract with the Agriculture, Forestry and Fisheries Research Council, MAFF, Japan.

We wish to thank the staff at prefectural institutes and farmers in Fukushima, Ibaraki, Tochigi, Miyagi for their cooperation with

sample collections.

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Dynamics of radioactive cesium behavior in agro-environment