Factors Affecting Long-Term Decline of 137 Cs and 90 Sr Concentrations in Plowed Layer of Paddy and Upland Soils of Japan. Noriko Yamaguchi 1)* , Katsutoshi Seki 2) , Misako Komamura 1) , Hideshi Fujiwara 1) , 1) National Institute for Agro-environmental Sciences, Tsukuba, Japan. 2) Univ. of Tokyo, Tokyo, Japan *E-mail: [email protected]
1．．．Introduction Atmospheric Nuclear test Atmospheric nuclear tests from 1945 to 1980 distributed radioactive fallout all over the world. 137 90 Since 1959, we have monitored the concentrations of Cs (T 1/2 = 30.2 y) and Sr (T 1/2 = 28.8 60 137 year) in the plowed layer of paddy and upland soils collected annually from agricultural fields in T- Cs, paddy soil T- 137 Cs upland soil 137 90 Japan. In the plowed layer of agricultural soils, the concentrations of Cs and Sr tended to ) Ex- 90 Sr upland soil -1 decrease after 1964 (Fig.1). The decreased concentrations of 137 Cs and 90 Sr are attributed to Ex- 90 Sr paddy soil 40 90 radioactive decay and downward migration. The physico-chemical properties of soil appear to T- Sr paddy soil Ex- 137 Cs paddy soil 137 90
affect the decline trend of Cs and Sr from plowed layer of the soil. (Bq kg Sr 90 20
Objective Cs and 137 137 90 To identify the factors affecting the long-term decline of Cs and Sr 0 in agricultural soils 1960 1970 1980 1990 2000 Year 2. Materials and Methods Fig. 1. 137 Cs and 90 Sr concentrations in soils Sampling site Soil properties
Paddy Soils Phosphate pH Particle size distribution wt% Bulk absorption Exchangeable cations Annual CEC -1 precipi- Soil samples were taken from a plowed Sampling Site density (H 2O) Org-C coefficient cmol kg PC1 c tation ●● Sapporo P 2O5 2.0 ～～～ 0.2 ～～～ 0.02 ～～～ <2 µm layer of paddy and upland fields of -1 -1 -1 -1 (mm) kg L g kg g kg cmol c kg CaO MgO K2O Na 2O 0.2 mm 0.02 mm 0.002 mm national or prefectural agricultural Sapporo Melanaquands 0.75 6.2 34 13.8 28.9 9.4 1.7 1.3 0.5 19 22 32 27 0.8608 1111 Akita Aquepts 0.81 5.0 22 8.9 25.7 7.2 1.7 0.5 0.5 6.0 25 39 31 0.6240 1750 experimental stations every year from Joetsu Aquepts 0.88 5.3 32 9.8 30.8 15 3.7 0.6 0.4 3.0 23 35 40 2.024 2756 1959 to 2003 after harvesting rice or Nonoichi Aquepts 1.11 5.8 27 4.9 16.4 6.7 0.5 0.7 0.3 5.9 34 37 23 0.0735 2482 ● Tottori Aquepts 1.09 5.4 23 6.7 19.2 8.0 0.8 0.8 0.2 4.7 31 36 29 -1.120 1953 Akita ● wheat. ● Morioka Melanaquands 0.66 5.9 90 18.6 55.8 27 3.7 0.5 0.7 15 26 28 32 5.276 1242 Morioka Mito Melanaquands 0.84 5.8 56 16.6 36.4 13 1.4 0.7 0.3 26 23 24 28 1.255 1347 Futaba Aquepts 1.05 5.3 12 4.1 10.3 4.2 0.1 0.9 0.2 36 35 13 15 -4.115 1115 ● Iwanuma Habikino Epiaquepts 1.13 6.2 24 5.4 17.9 13 1.3 0.7 0.3 15 40 24 22 -1.814 1205 Japan Sea Sanyo-cho Aquepts 1.11 5.6 17 4.9 13.8 7.4 0.2 0.7 0.2 29 24 24 23 -2.634 1198 Joetsu ● Nagaoka Nonoichi ● Tsukushino Aquepts 1.07 5.7 19 4.9 16.4 9.9 0.5 1.3 0.2 35 27 21 17 -2.863 1782 ●● Upland soils Kumagaya ●●Mito Sapporo Melanaquands 0.70 5.4 76 13.3 48.0 16 0.9 1.7 0.3 18 18 31 33 1.461 1111 ● Tottori ● ●● Nagaoka Aquepts 0.81 5.9 52 13.5 40.0 25 0.6 3.0 0.3 3.7 6.8 38 52 3.387 2304 ●● Tsukuba Futaba ● Morioka Melanaquands 0.63 5.9 96 19.2 45.0 18 0.6 1.8 0.2 19 28 33 20 1.064 1242 ● Iwanuma Aquents 1.51 6.1 14 10.9 23.8 14 4.0 1.1 0.1 16 17 29 39 -1.527 1275 ●● Pacific Ocean Sanyo-cho ●● Mito Melanaquands 0.65 6.1 49 14.7 39.4 18 3.2 2.1 0.2 25 22 28 26 -0.008 1347 Tsukusino ● Habikino Tsukuba Melanaquands 0.86 6.0 49 19.3 41.1 22 4.8 2.3 0.2 8.8 22 36 33 1.811 1159 ● Paddy field Kumagaya Udepts 0.83 5.0 11 4.10 16.3 7.0 0.6 1.4 0.2 12 37 31 21 -2.189 1284 Futaba Aquents 0.65 6.2 14 4.60 14.6 11 1.3 1.7 0.1 30 34 18 19 -3.883 1115 ● Upland field Sanyo-cho Aquepts 1.13 6.3 19 4.90 13.2 8.3 0.2 0.7 0.2 30 23 24 23 -3.590 1198 Japan Sea side Pacific Ocean side Soil is flooded during rice cultivation
137 Cs and 90 Sr analyses Principal Component Analysis 137 Cs → γ-ray spectrometry (Camberra GR 1820) Four factors were extracted by principal component analysis. 90 Sr → β-ray counting by 4 π or 2 π gas flow low-background counter % of the Cumulative % SPSS 12.0J Value Eigenvalues total of the total (Aloka LBC-480, SC-511) variance variance Exchangeable 90 Sr (Ex-90 Sr) 1 5.918 45.5 45.5 2 2.181 16.8 62.3 Paddy field → Extracted by 1M CH 3COONH 4 (Soil:Solution = 1:1) 3 1.553 11.9 74.3 4 0.957 7.4 81.6 1 5.645 43.4 43.4 2 2.635 20.3 63.7 Calculation of decrease constant in the plowed layer Upland field 3 1.819 14.0 77.7 Base year 4 1.146 8.8 86.5 -λλλt Factor loadings of PC1 70 Y=Ae Paddy field Upland field 137 -1 60 Y: Cs (Bq kg ) Bulk density -0.666 -0.290 pH (H 2O) 0.0190 -0.368 ) 50 t: Year
-1 Total carbon 0.719 0.798 40 P abs. coeff. 0.872 0.754 CEC 0.956 0.923 The extracted first 30 λ: Apparent decrease constant Ex-Ca 0.851 0.604 principal component Ex-Mg 0.791 0.051 Cs kg (Bq 20 Ex-K -0.173 0.600 was related to the 137 fast component Ex-Na 0.693 0.593 sorption characteristics 10 Coarse sand -0.561 -0.748 of soils Akita, Paddy soil Fine sand -0.474 -0.643 0 Silt 0.437 0.876 1960 1970 1980 1990 2000 Clay 0.832 0.741 Year Causes for the loss of 137 Cs and 90 Sr from the plowed layer of soils a.Radioactive decay (Decay constant: λ ) λ = λλλR + λ L R b.Leaching (Decrease constant by leaching: λL) Are there any c.Uptake by plant etc.. negligible relationships between λλλL 90 137 The decrease constant of Sr and Cs by leaching ( λL) was calculated by subtracting the and soil properties? contribution of radioactive decay from the apparent decrease constant in the plowed layer. The decrease constants between 1970-1980 were also calculated for Ex-90 Sr and defined as fast components of λ. 3．．．Results and Discussion T-90 Sr Ex-90 Sr Ex-90 Sr T-137 Cs Residence half time of 90 Sr and 137 Cs Fast + Slow components Fast component in the plowed layer of soils. (1970-2004) (1970-1980) Sapporo Akita Ave. Joetsu 47.5 y Nonoichi Paddy Tottori Ave. Ave. Ave. field Morioka 20.1 y 10.8 y 7.8 y Mito Futaba Habikino San-yo-cho ln2 Apparent residence Tsukushino half time λ 0 10 20 60 0 10 20 30 0 10 20 30 0 10203040 200 Sapporo Nagaoka Morioka Upland Iwanuma ln2 Residence half time field Mito Ave. λL by leaching Tsukuba 43.3 y Kumagaya Futaba Ave. Ave. San-yo-cho 15.5 y 13.3 y
0 10 20 30 0 10 2040 0 20 40 100 Residence half time (Year)
In the paddy fields, 90 Sr was preferably leached from the plowed layer compared to 137 Cs. Exchangeable 90 Sr was leached out from the plowed layer of paddy soils faster than from upland soils. Decrease constant for exchangeable 90 Sr could include two (fast and slow) components.
Table 1. Correlation coefficient between the decrease constants by leaching ( λλλL) and soil properties
bulk base coarse fine annual pH total C P-abs CEC Ex-Ca Ex-Mg Ex-K Ex-Na silt clay PC1 density saturation sand sand precip.
Ex- 90 Sr (n=15) 0.084 0.372 0.283 0.167 0.181 0.286 0.017 0.347 -0.012 0.116 0.121 0.212 -0.036 -0.392 0.110 -0.015 paddy ('70- T- 90 Sr (n=10) -0.488 0.134 0.434 0.485 0.588 0.372 0.570 -0.293 0.601 -0.532 -0.311 -0.484 0.321 0.658* 0.668* 0.353 '04) T- 137 Cs (n=13) -0.104 0.211 -0.115 0.069 0.124 0.128 0.658* 0.570* 0.646* 0.458 -0.063 0.382 -0.363 0.112 0.104 0.254
90 λλλL paddy Ex- Sr (n=14) 0.393 0.184 -0.194 -0.374 -0.429 -0.270 -0.578* 0.426 -0.641* 0.282 0.392 0.498 -0.350 -0.584* -0.540 0.218 (fast Ex- 90 Sr (n=6) comp.) 0.741 0.420 -0.446 -0.579 -0.759 -0.153 -0.673 0.639 -0.882* 0.590 0.630 0.486 -0.726 -0.730 -0.876* -0.102 Japan Sea Side
upland Ex- 90 Sr (n=8) 0.306 0.066 -0.802** -0.901** -0.962** -0.857** -0.194 -0.624 -0.626 0.602 0.405 0.630 -0.660 -0.500 -0.899** -0.377 ('70- '04) T- 137 Cs (n=9) 0.632 0.406 -0.336 0.135 0.000 0.272 0.711* 0.053 -0.360 0.638 -0.145 -0.493 -0.020 0.582 0.057 0.139 ** p < 0.01 , *p < 0.05
The leaching of 90 Sr from the plowed layer of the upland field was Fast component Fast +slow component controlled by the sorption capacity of soils. 0.20 (’70-’80) 0.20 (’70-’04) Japan Sea side 0.15 0.15 Pacific Ocean side 90 In the paddy field, λλλL for Sr calculated from the data of 1970-2004 (fast and slow λ components) was not related to the soil sorption characteristics. On the other hand, L 0.10 0.10 90 fast component of λλλL for Ex- Sr was related to PC1 in the paddy field of Japan Sea 0.05 0.05 side. It would be possible that most part of easily exchangeable 90 Sr had been lost from plowed layer before 1980 in the paddy field of Japan Sea side. A metrological 0 0 factor other than precipitation amounts should be taken into account. -4 -2 0 2 4 6 -4 -2 0 2 4 6 PC1 4. Conclusion
90 Sr was leached from the plowed layer of agricultural soils faster than 137 Cs. In the upland field, the soil sorption characteristics controlled the leaching rate of 90 Sr from the plowed layer. However, in the paddy field where soil is flooded during rice cultivation, the soil sorption characteristics alone could not explain the leaching rate of 90 Sr. The relationship between less-mobile 137 Cs leaching and soil properties was not obvious. 5. References • Komamura et al. (2005) Monitoring 90 Sr and 137 Cs in rice, wheat, and soil in Japan from 1959 to 2000. Miscellaneous Publication of National Institute for Agro-Environmental Sciences, No. 28 in press . • Komamura et al. Residence half-time of 137 Cs in Japanese paddy top soils. Radioisotopes vol. 48 p. 635-644. (in Japanese with English abstract) 6. Acknowledgement This work was supported by a fund for a radioactivity survey in Japan from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.