Can Pu239+240 replace Cs-137 as an erosion tracer in agricultural landscapes contaminated with Chernobyl fallout?

Erosion studies often use Cs-137 from the global fallout (main period: 1953 -1964) as a tracer in the soil. In many European countries, where Cs-137 wa s deposited in considerable amounts also by the Chernobyl fallout in 1986, the global fallout fraction (GF-Cs) has to be separated from the Chernobyl fraction by means of the isotope Cs-134. In a few years, this will no longe r be possible due to the short half-life of Cs-134 (2 yr). Because GF-Cs in the soil can then no longer be determined, the potential of using Pu239+24 0 as a tracer is evaluated. This radionuclide originates in most European c ountries essentially only from the global fallout. The activities and spati al distributions of Pu and GF-Cs were compared in the soil of a steep field (inclination about 20%, area ca. 3 ha, main soil type Dystric Eutrochrept) , sampled at 48 nodes of a 25x25 m(2) grid. The reference values were deter mined at 12 points adjacent to the field. Their validity was assured by an inventory study of radiocaesium in a 70 ha area surrounding the field sampl ing 275 nodes of a 50x50 m(2) grid. In the field studied, the activity conc entrations of GF-Cs and Pu in the Ap horizon were not correlated (Spearman correlation coefficient R = 0.20, p > 0.05), and the activity balance of Pu differed from that of GF-Cs. Whereas no net loss of GF-Cs from the field w as observed as compared to the reference site, Pu was more mobile with an a verage loss of ca. 11% per unit area. In addition, the spatial pattern of G F-Cs and Pu in the field differed significantly. The reason may be that due to their different associations with soil constituents, Pu and Cs represen t different fractions of the soil, exhibiting different properties with res pect to erosion/deposition processes. This indicates that both radionuclide s or one of them may not be appropriate to quantify past erosion. When trac er losses are used to calibrate or verify erosion prediction models, system atic deviations may not only stem from model shortcomings but also from tra cer technique. (C) 2000 Elsevier Science Ltd. All rights reserved.