Spatial Variability of the Distribution Coefficients of ¹³⁷Cs, ⁶⁵Zn, ⁸⁵Sr, ⁵⁷Co, ¹⁰⁹Cd, ¹⁴¹Ce, ¹⁰³Ru, ^95mTc, and ¹³¹l in a Cultivated Soil

To obtain information on the spatial variability of the sorption properties of a cultivated soil, the distribution coefficients K_d of the radionuclides ¹³⁷Cs, ⁶⁵Zn, ⁸⁵Sr, ⁵⁷Co, ¹⁰⁹Cd, ¹⁴¹Ce, ¹⁰³Ru, ^95mTc, and ¹³¹I were determined in batch experiments. Fifty soil samples were taken along each diagonal from a cultivated field (150 × 100 m) of Parabrown earth soil (Alfisol), and four replicate K_d measurements were performed for each soil sample in order to separate the spatial variability of the K_d values from the experimental error. The results show that the K_d values of the above radionuclides (with the exception of ⁵⁷Co) are not distributed randomly along each diagonal, but exhibit statistically significant trends or maxima and minima. The distribution coefficients increase on average in the sequence Tc < I < Sr < Ru < Cd ≃ Zn < Co < Ce < Cs. The spatial variability of the K_d values increases in the sequence Sr < Cs < Cd < I < Co ≃ Zn ≃ Tc < Ru < Ce by about one order of magnitude. For the soil investigated, if an error in the mean K_d value of 20 % is tolerated, at the 95 % confidence level, the minimum number of soil samples to be taken can be estimated for the above radionuclides as: strontium, 2; cesium, 4; cadmium, 8; iodine, 13; cobalt, 15; zinc, 15; technetium, 15; ruthenium, 23; and cerium, 140. Correlation analysis revealed that in many cases the K_d values of different radionuclides are closely correlated, i.e., that at locations, where one radionuclide exhibited relatively high K_d values, another showed either correspondingly high values (positive correlation) or low values (negative correlation). It is shown that in many cases these correlations are the result of the significant (positive or negative) correlation of the K_d value with the pH of the soil solution.