Influence of the soil bioavailability of radionuclides on the transfer of uranium and thorium to mushrooms

The soil–mushroom transfer of thorium and uranium was analyzed in two ecologically similar but geographically separated Spanish ecosystems by means of the transfer factor, TF. Uranium TF values were in the range 0.043–0.49, and thorium TF values in the range 0.030–0.62. These values were similar to those of 90Sr, 239+240Pu, and 241Am found previously in the same ecosystems. Given the low availability of uranium and thorium, the available transfer factors, ATF, were also determined. These were higher than the TF values by one order of magnitude for 234, 238U, and by 2–3 orders of magnitude for 228, 230, 232Th. The ATF value of thorium was similar to that of 137Cs, and that of uranium similar to that of 40K. Hebeloma cylindrosporum presented the highest uranium and thorium transfer factors, confirming this species as a good bioindicator of a soil's radioactive content.