Migration of 137Cs, 90Sr, and 239+240Pu in Mediterranean forests: influence of bioavailability and association with organic acids in soil

• Downward migration velocities of 137Cs, 90Sr and 239+240Pu in two Mediterranean forests were determined.
• Downward migration velocities: 90Sr ≈ 239+240Pu > 137Cs.
• Bioavailability by itself (90Sr > 239+240Pu > 137Cs) does not explain differences in migration velocities.
• Humic + Fulvic acids: 239+240Pu > 90Sr > 137Cs.
• Humic + Fulvic acids and bioavailable fraction can explain the downward migration velocities.

The understanding of downward migration of anthropogenic radionuclides in soil is a key factor in the assessment of their environmental behavior. There are several factors that can affect this process, such as the radionuclide source, their chemical form, soil and environmental characteristics, etc. Two Mediterranean pinewood ecosystems in Spain, which were affected mainly by global fallout, were selected to assess the migration of 137Cs, 90Sr, and 239+240Pu. Using auxiliary modeling (diffusion–convection equation and compartmental model), it followed from field observations that the migration velocities of 90Sr and 239+240Pu were similar and higher than that of 137Cs. The downward migration of radionuclides can be considered a consequence of their association with soil particles. A sequential speciation procedure also confirmed that 90Sr was the most bioavailable radionuclide followed by 239+240Pu and 137Cs. Although this can explain the different velocity of 90Sr and 137Cs, bioavailability could not explain by itself the similar velocities of 239+240Pu and 90Sr. The presence of organic acids in the soil can also influence the migration of radionuclides attached to them, which decreased in the order: 239+240Pu > 90Sr > 137Cs. Thus, the joint consideration of bioavailable and humic + fulvic acid fractions can explain the observed differences in the downward velocities.