Bio-availability and transfer of natural radionuclides in a mediterranean ecosystem

We have studied the soil to plant transfer coefficients for the natural radionuclides 226Ra, 228Ra, and 228Th in the species Cistus ladanifer in a Mediterranean type ecosystem. The secular equilibrium that exists between 228Ra and 228Th in the total fraction of the soil is broken in the available fraction and, consequently, in the different vegetation fractions considered. The bio-availability of 226Ra is similar to that of 228Ra. There exists, nevertheless, a positive discrimination by the plant in favour of the former. The transfer from the available fraction of the soil to the different fractions in the plant present a characteristic time-depedence for each vegetation fraction.