Uncertainty analysis of the external gamma-dose rate due to the variability of the vertical distribution of 137Cs in the soil

The external gamma-dose rate at 1m height above a flat area due to the presence of fallout radiocesium in the soil is frequently calculated from the observed depth profile of the Cs-137 activity as well as the soil mass per unit area. At a given site, these depth profiles may. however, vary considerably, thus introducing an uncertainty to the external gamma-dose calculated in this way. To assess this source of uncertainty for a typical grassland site, the activity of Chernobyl-derived Cs-137 and the wet bulk density in the three upper soil layers at 100 plots in a 100m x 100m pasture were determined. Analysis of these data shows that the frequency distribution of the dose rates calculated from the corresponding depth profiles of all plots is similar to a log-normal distribution (mean 25 nGy h (-1), median 22 nGy h (-1), standard deviation 11 nGyh(-1); range 1.6 56 nGy h(-1)). The various sources which contribute to the uncertainty of the dose rate are quantified. The semi-variogram indicates that any spatial dependence of the dose rates occurs on this pasture only over distances that are smaller than the shortest sampling interval (here about 10m). It is estimated which errors have to be expected for the median dose rate when the depth profiles of Cs-137 and of the wet bulk density are determined only for a small number of plots. It is preferable to calculate the mean dose rate as a mean from the n individual dose rates rather than from an averaged Cs-137 depth profile of the n plots. (C) 2001 Elsevier Science Ltd. All rights reserved.