Association of Chernobyl-derived 239 + 240Pu, 241Am, 90Sr and 137Cs with organic matter in the soil solution

To investigate the extent of association of fallout radionuclides with soil organic matter, gel filtration was applied to the soil solution obtained from the three top horizons AOf, AOh and AOh + A1/A2 of a forest soil within the 10 km zone of the nuclear reactor at Chernobyl/Ukraine. In the five fractions isolated (fraction 1: nominal molecular weight Mw 2000, fraction 2: Mw = 1300-1000, fraction 3: Mw = 800, fraction 4: Mw = 400 daltons, fraction 5: inorganic compounds), 239 + 240Pu, 238Pu, 241Am, 90Sr and 137Cs were determined. For that purpose, an efficient method for the simultaneous determination of the actinides and 90Sr was developed. The data show that plutonium and americium are associated mainly with the high molecular fraction 1 and to a much smaller percentage also with the fraction 2. While the differences between plutonium and americium were rather small in the top two horizons, americium in the third soil layer is present to some extent also in the fractions 3, 4, and 5. Strontium-90 from the AOf horizon is associated almost exclusively with fraction 4. In the other two soil layers, however, this radionuclide is present essentially only in fraction 5 (inorganic compounds). Caesium-137 from the soil solution of the AOf horizon is associated essentially only with the fraction 3, but in the deeper layers progressively also with all other fractions. Thus, in the third layer, 137Cs is distributed almost uniformly between all five fractions. Because the mobility and biological availability of these radionuclides will depend on their association with soil organic matter, the present data suggest that the determination of only the total concentration of a radionuclide in the soil solution might not be sufficient to interpret or predict adequately the fate of radionuclides in the soil.