Radioactivity in Soils (Cesium-134 and Cesium-137) 1991
The appended maps describe the contamination of the radioactive isotopes cesium-134 and cesium-137 in topsoils in the depth of 0-12 cm. Different reference times are depicted.
Figure 9 gives an analysis of pre-contamination levels, input amounts, and drops of activity in soils due to radioactive decay. The spatial distribution is clear in individual maps. Long-term inputs refer only to cesium-137, caused by the global fallout from atmospheric nuclear tests in the 50’s and 60’s. Cesium-134 emitted at that time has no relevance today because of its short half-live of 2.1 years. This “pre-Chernobyl contamination” of soils in Berlin reaches an average activity, in relation to area, of about 1,600 Bq/m2, as depicted in Fig 9. Cesium-137 activity was tripled by the Chernobyl reactor incident, while new inputs of cesium-134 correspond to about 1,600 Bq/m2. The average total cesium activity directly after the reactor accident is thus about 6,100 Bqm2. The drop in soil contamination is more determined by the decay of cesium-134 than it is by any transferring process.
Map 01.09.1 Cesium-137 before the Chernobyl Reactor Accident
The calculated spatial distribution of cesium-137 in topsoil before the reactor incident at Chernobyl is given in Map 01.09.1. The average value of all calculated single values is about 1,300 Bq/m2. The distribution of values and amounts can be gathered from Figure 10. A comparison with the situation in Bavaria shows that the location of the area to be tested is also decisive, even in the case of fallout caused by atmospheric nuclear tests.
Previous contamination in the greater Berlin area was mainly in the range of 300 to 2,500 Bq/m2. Contamination in Bavarian soils was higher. Values in Bavaria were concentrated in the range of 1,200 to 5,000 Bq/m2; individual values were sometimes 10 times higher. Even in consideration of the sample depth of up to 30 cm, which collected activity not only in disturbed soils (tilled fields) but also in greater depth, the causes of disparities in pollution distribution and amounts are due primarily to factors effective in small areas, as well as meteorological conditions such as long-term precipitation frequency, amounts, and wind direction dispersal.
An interpretation of the spatial distribution of pollution in Berlin shows concentration peaks in West Berlin due to long-term average prevailing west and southwest winds combined with dry and wet deposition. Forest locations are to be given particular attention for the highest concentrations of activity appear here, in the humus layer, which is only a few centimeters thick. A particularly strong binding of the isotope cesium-137 with soil components takes place in this humus layer. In urban areas, in contrast, differentiating influence factors for small areas must often be expected. They are related to the external influence path (emissions) and to the soil substrates as well. The small-area variance of radionuclides can be further clarified only by very complicated investigations.
Map 01.09.2 Depositions of Cesium-134 and Cesium-137 Resulting from the Chernobyl Reactor Accident
The input situation of emitted material during the reactor accident was basically determined by meteorological conditions existing from 26 April to 13 May 1986. The rain-free, high-pressure weather conditions which prevailed during the first 10-12 days led to deposition distribution primarily dependent on the filtering capacities of topsoil structures, as well as the moisture content of the atmosphere. Besides forest locations, areas particularly affected were those where high humidity prevailed all day, or where atmospheric water vapor condensed with cooling in the night. The formation of dew leads to increased soil inputs, similar to wet depositions.
The first precipitation took place from 7 to 8 May. There were very different deposition rates, paralleling precipitation amounts. Additional depositions of the more relevant cesium-137 at single measuring points reached 860 to about 5,600 Bq/m2; new inputs of cesium-134 swung between 430 and 2,600 Bq/m2.
A comparison with the situation in Bavaria, Figure 10, shows how decisive the location of the test area is for new inputs too (cf. Fig. 11).
Map 01.09.3 Cesium-134 and Cesium-137 on 1 May 1987
Total contamination loads (old and new inputs), calculated for 1 May 1987, show cesium-134 to have an activity of only 75% of its original load levels, due to its rate of decay. Contaminated areas were defined by new inputs from the Chernobyl incident, and old deposits.
Map 01.09.4 Cesium-134 and Cesium-137 on 1 May 1991
The progression of radioactive decay of tested isotopes produced the results of 1 May 1991. Surface activity of cesium-134 is only 20% of original load levels, while cesium-137 concentrations have declined only about 7.5% in comparison to new inputs.
Evaluation of Potential Dangers of Soil Loads
Radioactive substances deposited in soils can reach humans through:
- soil-plant transfer
- washout into ground water
- disturbed soils
- direct radiation.
It is difficult to make a direct evaluation for the dangers posed to humans by soil loads, because the transfer of pollutants such as cesium-137 from topsoil into cultivated plants is determined by many factors; particularly the type of soil, the type and age of plants, the concentration of pollutants in the soil and ground water, and other parameters. Transfer factors are disputed (Litz/Tietz 1987). These factors are used for the estimation of threats arising from soil contamination loads and as measure for the soil-plant transfer. A transfer factor is defined as the concentration of pollutants in the plant, divided by the concentration of pollutants in the soil. The transposition of individual transfer factors, always determined by selected conditions for the respective individual situation, is linked to the observance of these boundary conditions. New investigations show that the concentration of a pollutant in a plant is directly related to the concentration of the pollutant in the waters of the soil, but not to the concentrations in the soil itself (Schüttelkopf/Schmidt 1990).
Despite these limiting statements, some statements can be made about possible health threats posed by existing soil contamination loads in Berlin.
Dosage through direct soil radiation – based on cesium-137 – is negligible. Measuring techniques cannot document it and it disappears in the background radiation.
The inhalation of radioactive cesium through dust can be evaluated by measurements of activity concentrations in the air. It amounts on the average to less than 0.00001 Bq/m2 and is negligible, even in comparison to the mean natural levels (such as radon-226 and its resulting products).
Foodstuffs are therefore the only significant factor. They are investigated at both the production and the trade level. The Radiation Measurement Office of the Berlin Department of Urban Development and Environmental Protection publishes measuring results every week in “Weekly Reports” and distributes them to various information media, consumer associations, etc. Certain products continue to be listed among the more highly contaminated foodstuffs. Those are mainly forest fruits, mushrooms and animals that live on undisturbed, humus-rich forest soils.