ASSOCIATION OF CHERNOBYL-DERIVED PU-239-241, SR-90 AND CS-137 WITH ORGANIC-MATTER IN THE SOIL SOLUTION(240, AM)

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 for est soil within the 10km zone of the nuclear reactor af Chernobyl/Ukra ine. In the five fractions isolated (fraction 1: nominal molecular wei ght Mw less than or equal to 2000, fraction 2: Mw = 1300-1000, fractio n 3: Mw = 800, fraction 4: Mw = 400 daltons, fraction 5: inorganic com pounds), Pu-239+240, Pu-238, Am-241, Sr-90 and Cs-137 were determined. For that purpose, an efficient method for the simultaneous determinat ion of the actinides and Sr-90 was developed. The data show that pluto nium and americium are associated mainly with the high molecular fract ion 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 A Of horizon is associated almost exclusively with fraction 4. In the ot her two soil layers, however, this radionuclide is present essentially only infraction 5 (inorganic compounds). Caesium-137 from the soil so lution of the AOf horizon is associated essentially only with the frac tion 3, but in the deeper layers progressively also with all other fra ctions. Thus, in the third layer, Cs-137 is distributed almost uniform ly between all five fractions. Because the mobility and biological ava ilability 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 radio-nuclide in the soil solutio n might not be sufficient to interpret or predict adequately the fate of radionuclides in the soil.