Role of soil organic matter in the mobility of radiocesium in agricultural soils common in Japan

Abstract Cesium-137 is an important radionuclide because of its long radiological half-life (30 years) and high fission yield from 235 U and 239 Pu. In order to evaluate the radiation dose from 137 Cs, it is important to know its mobility in different soils. Soil organic matter (SOM) might affect the 137 Cs mobility because SOM is a reactive colloid. Therefore, tracer studies were carried out to clarify the effect of SOM on Cs mobility in different soils. Soil–soil solution distribution coefficients ( K d s) of 137 Cs were measured for 87 Japanese agricultural soil samples to evaluate the adsorbability of 137 Cs. The soil samples were selected from three common and one miscellaneous soil groups in Japan. The percentage of SOM-bound 137 Cs and the exchangeability of SOM-bound Cs were evaluated by selective extraction. To evaluate the effect of dissolved organic matter (DOM), which is a portion of the SOM fraction, on Cs mobility, the amount of 137 Cs bound to DOM was also determined. The observed K d values ranged from 215 to 43 400 L/kg with a geometric mean of 2210 L/kg. Among the four soil groups, relatively low K d values were observed for Andosol samples. Their high SOM content would possibly affect 137 Cs mobility. Then the effect of SOM was verified for the four soil groups. The selective extraction showed that 3–17% of soil-adsorbed 137 Cs was in an SOM-bound form. Moreover, it was observed that the 27–100% (average: 63%) of 137 Cs adsorbed by the soil humic acid (HA) fraction was exchangeable. The effect of DOM in soil solution was also verified. About 5–82% (average: 45%) of 137 Cs in soil solution was in HA-bound form. These results suggested that SOM could increase the mobility of 137 Cs in soils by binding it in exchangeable and DOM-bound forms.