On the lognormality of radionuclide deposition.

The influence of the variation of soil density and the uncertainty of activity measurements on the statistical distribution of radionuclide concentrations on a site is considered. It is demonstrated that the influence of these factors adequately explains the observed deviation of radionuclide empirical probability distribution functions (empirical PDFs) from lognormal. In all probability lognormality of activity density distributions is the consequence of the atmospheric fallout process, as observed for deposition from Chernobyl and Fukushima.

Description of spatial patterns of radionuclide deposition by lognormal distribution and hot spots.

Spatial distributions of activity density (kBq/m(2)) and activity concentration (Bq/kg) are studied on sites with non-cultivated soils. Fitting datasets with lognormal, Weibull and normal distributions with sampling size n ≥ 60 showed that radionuclide deposition ((90)Sr, (137)Cs, (238)Pu, (239+240)Pu, (241)Am) due to Chernobyl fallout no more than in 10% of cases are described by Weibull distribution, and in the rest of the cases--by lognormal distribution. However asymptotics of "righthand tail" of empirical (sample) distribution quite often differs from the right-hand tail asymptotics of lognormal distribution.