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

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 85Sr and 137Cs 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 — TFm (dimensionless) and TFa (m2 kg− 1-dry) for the pre- and post-transplanting depositions, respectively. For brown rice, the TFm values of 85Sr 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 TFa values were 2 (2.5 × 10− 4 and 4.4 × 10− 4) for 85Sr and 3 (1.1 × 10− 3 and 2.9 × 10− 3) for 137Cs. Straws had several times higher TFm and TFa values of 85Sr than of 137Cs. The surface-water concentrations were substantially higher for the TFa than for the TFm, indicating the possibility of a much higher plant-base uptake for the TFa. In the TFa soils, 137Cs and, to a lesser degree, 85Sr 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 Kd values of 85Sr measured in a parallel experiment ranged from 20 to 170, whereas 137Cs had much higher Kd values. The use of TFa values instead of TFm values turned out to be a reasonable approach to the evaluation of a vegetation-period deposition.