Long-term availability of Tc deposited on soil after accidental releases

Considerable uncertainties are associated with the transfer of Tc from soil to plant. Mathematical models built to simulate that transfer generally use a single value derived from short-term experiments. Recent observations have, however, given a clear indication that the soil-to-plant transfer varies with time, depending on the biogeochemical cycling of this element. The long-term behavior of Tc in soils accidentally contaminated has been studied for 4 y in lysimeters exposed to natural climatic conditions. An important fraction of the deposit (70%) is rapidly removed from the 20-cm plow layer with a half-time of 54 +/- 2 d; the remaining fraction tends to persist in the soil for long periods of time (half-time = 30 +/- 16 y). Immediately after the deposit, the plant-to-soil concentration ratios are very high (about 400); they decrease to six 3 y after the contamination. The shape of the evolution of the contamination measured in grass after an accidental deposit is best-fitted to a sum of two exponential functions; the environmental half-times estimated for the two components are about 30 d and 2 to 3 y, respectively.