Transport behavior and rice uptake of radiostrontium and radiocesium in flooded paddy soils contaminated in two contrasting ways

摘要

In order to investigate the transport behavior and rice uptake of radiostrontium and radiocesium in flooded rice fields, lysimeter experiments with two paddy soils were performed in a greenhouse. A solution containing ~(85)Sr and ~(137)Cs was applied in two different ways - being mixed with the top soil 27 d before transplanting or being dropped to the surface water 1 d after transplanting. Rice uptake was quantified with two kinds of transfer factor - TF_m (dimensionless) and TF_a (m~2 kg~(-1)-dry) for the pre- and post-transplanting depositions, respectively. For brown rice, the TFm values of S5Sr and 137Cs differed between the soils by factors of 2 (1.6 × 10~(-2) and 2.5 × 10~(-2)) and 7 (2.2 × 10~(-2) and 1.5 × 10~(-1)), respectively. Corresponding factors by the TF_a values were 2 (2.5 × 10~(-4) and 4.4 × 10~(-4)) for ~(85)Sr and 3 (1.1 × 10~(-3) and 2.9 × 10~(-3)) for ~(137)Cs. Straws had several times higher TF_m and TF_a values of ~(85)Sr than of ~(137)Cs. The surface-water concentrations were substantially higher for the TF_a than for the TF_m, indicating the possibility of a much higher plant-base uptake for the TF_a. In the TF_a soils, ~(137)Cs and, to a lesser degree, ~(85)Sr were severely localized towards the soil surface, probably leading to an increased root uptake. The activity loss due to plant uptake and water percolation was generally inconsiderable. Time-dependent K_d values of ~(85)Sr measured in a parallel experiment ranged from 20 to 170, whereas ~(137)Cs had much higher K_d values. The use of TF_a values instead of TF_m values turned out to be a reasonable approach to the evaluation of a vegetation-period deposition.