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 (90)Sr, (239+240)Pu, and (241)Am 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, 238)U, and by 2-3 orders of magnitude for (228, 230, 232)Th. The ATF value of thorium was similar to that of (137)Cs, and that of uranium similar to that of (40)K. Hebeloma cylindrosporum presented the highest uranium and thorium transfer factors, confirming this species as a good bioindicator of a soil's radioactive content.