Uptake of technetium-99 by food crops under field conditions

Long-term field experiments have been carried out in the Chornobyl Exclusion zone to determine parameters describing technetium (⁹⁹Tc) transfer into five food plants (Lettuce, Radish, Wheat, Bean, and Potato) from four types of soil, namely Podzoluvisol, Greyzem, Phaeozem, and Chernozem. Technetium was added to the soils under field conditions in a pertechnetate form. In the first two years, soil type had little effect on Tc uptake by plants. In the first and second years after contamination, the concentration ratios (CR), defined as ⁹⁹Tc activity concentration in the crop (dry weight) divided by that in the soil (dry weight), for radish roots and lettuce leaves ranged from 60 to 210. For potato tubers, the CR was d 0.4-2.3, i.e., two orders of magnitude lower than for radish and lettuce, and for summer wheat grain it was lower at 0.6 ± 0.1. After 8-9 years, root uptake of ⁹⁹Tc by wheat decreased by 3-7 fold (CR from 0.016 ± 0.005 to 0.12 ± 0.034) and only 13-22 % of the total ⁹⁹Tc added remained in the upper 20 cm soil layers. The time taken for half of the added ⁹⁹Tc to be removed from the 20-cm arable soil layer due to vertical migration and transfer to plants was short at c. 2-3 years.