Distribution Coefficients and Accumulation Rates of Ra-226 and Po-210 in Some Agricultural Soils

Solid-liquid partition coefficients ( K d ) of Ra-226 and Po-210 were measured for five agricultural soils in five fields in the south of Vietnam. Rice, maize, water spinach and tomato were grown on Fluvisols, while sweet potato – on Arenosols. The obtained values were very different between radionuclides, cultivation types, and agricultural practices. For Ra-226, they were 2783, 1189, 907, 431, and 1117 L kg –1 in soils under rice, maize, water spinach, sweet potato, and tomato, respectively. The values of 826, 332, 409, 217, and 451 L kg –1 were estimated for K d in soils under rice, maize, water spinach, sweet potato, and tomato. For a particular soil, the K d of Ra-226 was higher than that of Po-210. It proves that in the solid phase of the soil, Po-210 was absorbed more strongly than Ra-226. In this study, a new method for determining the leaching rates of Ra-226 and Po-210 from the topsoil to the deeper layer of soil was introduced. It was based on the K d value and the leaching rate of water for a specific soil type. The leaching rates of 7.4, 37, 63, 36.6, and 61 were estimated for Ra-226 in rice, maize, water spinach, sweet potato, and tomato soils respectively. Similarly, the leaching rates of 22.1, 111, 111, 69.1, and 133 were estimated for Po-210 in rice, maize, water spinach, sweet potato, and tomato soils respectively. The long-term assessment of Ra-210 and Ra-226 in the selected soil was based on the mass balance between the daily input and output of radionuclides in topsoil. It was found that activity concentrations of Ra-226 and Po-210 increase significantly in some soil soils for example under sweet potato and tomato soil.