Radioactivity in Soils (Cesium-134 and Cesium-137) 1991
The methodological starting-point was the knowledge that the isotopes cesium-134 and cesium-137, to be viewed as the key isotopes, were emitted from the reactor core in a precise ratio of 1:2. It was thus possible to separate the “Chernobyl-related” cesium-137 from the global fallout of the early nuclear tests. The cesium-134 from this time was no longer measurable in 1986. An expert opinion (Kannenberg 1991) spatially interpolated the activity concentrations of cesium-134 and cesium-137 at each measuring point, and related them to the following time periods:
- cesium-137 before the reactor accident in Chernobyl (Map 01.09.1)
- deposition of cesium-134 and cesium-137 resulting from the reactor accident in Chernobyl (Map 01.09.2)
- cesium-134 and cesium-137 as of 1 May 1987 (Map 01.09.3)
- cesium-134 and cesium-137 as of 1 May 1991 (Map 01.09.4).
Interpolation of Measuring Values
The methodic instruments for spatial interpolation of single values were the semi-variogram analysis, and the Kriging process, both used in the Berlin Environmental Information System. The variogram describes the variance of measure values for all pairs of measuring points in relation to their distance from each other. It gives a basis of comparison for the similarity of measurement results between point pairs of equal distance (cf. Fig. 6). Corresponding variograms were determined for different directions in those cases where variance depended not only on the distance, but also on how point pairs were situated (anisotropy). In contrast to linear interpolation processes, for example, the Kriging interpolation takes the determined spatial variability into consideration and thus gives assessment values with minimal incorrect ratings. A further advantage of this process is that the minimization of incorrect ratings gives a measure for the certainty of the determined assessed values (cf. Fig. 7).
Consideration of Precipitation Quantities
Radioactivity in the soil was compared with the diffusion of precipitation quantities on 7 and 8 May (Schlaak 1989) in order to clarify the interactions which influenced the structure of the radionuclide contamination. Dry high-pressure weather conditions prevailed in the first 10 days after the reactor incident. Thunderstorm precipitation began in the night of 7 May. Rain levels in some localities reached more than 40 mm of precipitation (the long-term average for May at the Dahlem measuring point is 49 mm). This rain enabled a washout of activity from the atmosphere and entries into soils and surface waters.
A comparison of the isohyete (precipitation contours) map of amounts of rain from 7 and 8 May 1986, and the map of radioactivity shows that they coincide with each other very much, even when a portion of inputs previously entered soils by dry deposition. There are however, no measuring values available for the Berlin area in order to separate dry nuclide depositions as aerosol (fallout) from wet depositions with precipitation (washout). Figure 8 explains the influence of the precipitation previously mentioned.