Availability of 99Tc to rye grass grown on agricultural soils

This study was conducted to describe the time change in the uptake rates of 99Tc by rye grass (Lolium perenne L. cv. Callan) on contaminated soils, to calculate the transfer factors from soil to plants, and to evaluate the means of predicting the accumulation of Tc in plants. Rye grass was grown for 22 weeks on two agricultural soils contaminated with increasing concentrations of 99Tc (2.5, 25 and 250 kBq/kg). The aerial parts were harvested every 4 weeks, and the dry matter yield and uptake of 99Tc by the shoots were determined. The effect of phosphate fertilization on the availability of 99Tc to rye grass was also tested. Chemical extractions (i.e. water, CaCl2 (0.01M) and Na2CO3 (0.5M)) were performed on freshly contaminated and unplanted soils after the 22 week growth period. The results showed a stimulation in growth at low 99Tc concentrations (2.5 and 25 kBq/kg) at the first cutting. Application of 250 kBq/kg, however, had no significant effect on plant growth. Accumulation of 99Tc in shoots reached 62-78% of the initial 99Tc after 22 weeks, and only 2% was found in the roots. The uptake rates decreased sharply after the first cutting and followed an exponential model with time. A decrease in uptake and accumulation of 99Tc was observed with increasing phosphate fertilization. The time change in the accumulation of 99Tc by rye grass followed a y = a (1 - e-ct) model, where the a value is the maximum accumulation of 99Tc. The a value, which was reached at the end of the 22 week growth period, could be predicted before culture from Na2CO3 extractions of 99Tc from freshly contaminated soil. (author). 16 refs, 3 figs, 2 tabs