Foliar contamination of Phaseolus vulgaris with aerosols of 137Cs, 85Sr, 133Ba and 123mTe: influence of plant development stage upon contamination and rain

As part of a requirement to improve the assessment of the impact of radioactive fallout on consumed agricultural products, bean plants at four development stages (seedlings, preflowering, late flowering and mature plants) were contaminated by dry deposition of 137Cs, 85Sr, 133Ba and 123mTe aerosols. The influence of two rain scenarios and of the development stage upon contamination on interception, retention, and translocation to pods was studied. Interception of the four radionuclides was almost identical and varied from 30 to 60% with increasing development stage. The most important rain parameter was the time which elapsed between contamination and the first rain. Whatever the development stage, rain washed off more cesium from the leaves when it occurred 2 days after the deposit (37% at the seedling stage, for example) rather than later on (6 days, 27%), due to rapid migration of Cs in the plant. The first rain washed off nearly 40% of Ba whatever the scenario. For later stages, Sr and Ba were more washed off by heavy weekly rains than by weak twice-a-week rains, perhaps because of the Sr/Ba-contaminated material loss associated with wash off (desquamation of cuticles). Te showed little wash off (less than 5%). Wash off decreased with an older development stage for a weak rain intensity, due to the superimposition of leaves. Heavy rains removed this shelter effect. At harvest, rain effect was no longer detectable as foliar activity was similar for both rain scenarios. Translocation factors (TF) for strontium and barium increased from 6x10-3 to 1x10-1 with the plant development stage upon contamination, whereas those for cesium remained almost unchanged between 2x10-1 and 4 x 10-1. Flowering is the most critical stage towards residual contamination in pods at harvest, with the exception of direct deposit on pods at the mature stage (TF values are one order of magnitude higher). TF value for Te was 6.5x10-2 and was due to direct deposit. Modelling reflected the trends, through the differential values of the wash off and absorption coefficients, of what was reported for experimental results