ASSOCIATION OF CHERNOBYL-DERIVED PU239-241, SR-90 AND CS-137 WITH DIFFERENT MOLECULAR-SIZE FRACTIONS OF ORGANIC-MATTER IN THE SOIL SOLUTIONOF 2 GRASSLAND SOILS(240, AM)

Radiocesium is normally bound only rather weakly and unspecifically by humic substances, in contrast to the actinides Pu and Am. Recently, h owever, it was observed that fallout Cs-137 in the soil solution from an Of-horizon of a podzol forest soil (slightly decomposed plant mater ial) was associated essentially only with one single size fraction of the humic substances. In deeper soil layers with well humified materia l (AOh-horizon), radiocesium was associated with all size fractions of the dissolved organic matter (DOM). To examine whether this unexpecte d behaviour is also observable for DOM isolated from other soils, we d etermined the association of fallout Cs-137, Sr-90, Pu-238, Pu239+240 and Am-241 with various size fractions of DOM from in situ soil soluti ons isolated from two layers (0-2 cm and 2-5 cm) of two grassland soil s (a soddy podzolic soil and a peat soil) within the 10 km zone of the nuclear reactor at Chernobyl (Ukraine). The four size fractions of DO M as obtained by gel filtration of the soil solution were (mean nomina l molecular weight in daltons): fraction I: greater than or equal to 2 000, fraction II: 1300; fraction III: 560, fraction IV: inorganic comp ounds. The results for the well humified DOM (humus accumulation horiz on of podzol, deeper layer of peat soil) showed that Pu and Am are ess entially associated with the high molecular weight fractions, while Sr is present only in the 'inorganic' fraction. Radiocesium is found in all the size fractions separated. A quite similar pattern was also fou nd for Pu, Am, and Sr in the soil solution from only slightly decompos ed plant material(0-2 cm of peat soil), but not for radiocesium. This radionuclide was again essentially only observable in one single low m olecular weight fraction of DOM. The above results thus support our re cent observations in the different horizons of a forest podzol mention ed above, even though no reason for the different binding of radiocesi um by well humified soil organic matter and by only slightly decompose d plant material can be given at present. The data demonstrate, howeve r, that information on only the total amount of a radionuclide in the soil solution will not be sufficient to interpret or predict its fate adequately in the soil.