Radioactivity in Soils (Cesium-134 and Cesium-137) 1991
A basis for work was provided by a body of data available from the Radiation Measuring Office of the Berlin Department of Urban Development and Environmental Protection. Measurements were made at 218 measuring points in the city and its near surroundings from 1989 to 1991. Samples were taken by soil bore cylinders from soils as undisturbed as possible, from the surface to a depth of 12 cm. Two horizons were differentiated; 0-6 cm and 6-12 cm. The selection of these horizons was based on the knowledge that most cesium-137 nuclides, representative of the nuclear tests of the 60’s, were not yet stored in deeper soil layers. Measurements documented that cesium-137 originating from nuclear testing reached only a penetration depth of 1.7 to 4.7 cm, even in sandy soils, with a transport rate of 0.1 to 0.3 cm/yr. (Gans/Arndt 1987). Soils repeatedly processed and thus possessing a relatively constant storage depth, such as agricultural fields, were only of secondary significance in Berlin urban areas (cf. Fig. 5). A decomposition of isotopes by soil organisms can also be ruled out.
The analysis of the nuclides could be concentrated on a few isotopes for judging long-term soil contamination. The activity and thus the health-endangering effects of iodine-131, initially considered the most important isotope, is limited to a half-life of 8 days (cf. Tab. 1). It quickly became insignificant. Strontium-90, important for its great radiotoxicity, and plutonium isotopes were found only in very low traces in the atmosphere. They could be disregarded in relation to soil contamination. Clearly recognizable in this investigation of radionuclide composition conducted directly after the reactor incident, however, was that the two isotopes cesium-134 and particularly cesium-137 would contribute to radiation exposition well into the future. They were thus assigned an indicator function for the evaluation of contamination diffusion.
The samples were analyzed for specific activity, measured in Bq/kg, and converted into area concentrations, expressed in Bq/m2.
Questions of relevant entry mechanisms of radioactive isotopes into soils are of further interest. Basic soil science data were determined for pH, soil type, humus, etc. Meteorological data were also included in the investigations, including precipitation amounts and distributions from a total of 77 measuring stations. This was related to the main precipitation events following the Chernobyl incident (7-8 May 1986).