Cs-137 mobility in soils and its long-term effect on the external radiation exposure

To predict the external gamma-dose rate of Chernobyl-derived Cs-137 for a period of about 100 years after its deposition, the vertical distribution of radiocesium in several meadow soils in the Chernobyl area and in Germany was determined, and the corresponding residence half-times of this radionuclide in the various soil layers were evaluated using a compartment model. The resulting residence half-times were subsequently used to calculate the vertical distribution of Cs-137 in the soil as a function of time and finally to predict the external gamma-dose rates in air for these sites at various times. A regression analysis of the data obtained showed that the time dependence of the relative gamma-dose rate in air D (t) at the Chernobyl sites can be described by an exponential equation D (t) = a + b.exp(-t/c), where t is the time after deposition. For the ten German sites the best fit was obtained using the two-exponential equation D (t) = a.exp(-t/b) + c.exp(-t/d). The gamma-dose rate of Cs-137 at the Chernobyl sites decreases significantly more slowly with time than ar the German sites. This means that after e.g. 30 years the mean relative gamma-dose rate at the German sites will have decreased from 100% (corresponding to an infinite plane source on a smooth surface) to 9% (95% confidence interval 8%-10%), while at the sites in the Chernobyl area it will have decreased only to 21% (20%-23%), This difference is the result of the longer residence half-times of Cs-137 in the soils at the Chernobyl sites. All results are compared with estimates from earlier studies.