Environmental impacts of the reactor disaster of Chernobyl

In Germany, systematic measurements, particularly of radioactive caesium and strontium in various environmental media were started in the late 50s of the twentieth century. Until 1964 the Federal Agency for Food (now Max Rubner Institute, Federal Research Institute of Nutrition and Food) observed in all animal and plant foods a sharp increase in the activity of the radionuclides measured which was a result of the fallout due to atmospheric nuclear weapons tests. The relatively rapid decrease until 1970 can be explained by the decline in direct deposition on plants owing to the test ban for atmospheric nuclear weapons tests.

After that the activity levels in food diminished continually until 1986 when the Chernobyl fallout significantly increased the contamination again. Radioactive caesium (caesium-137 and caesium-134) and iodine (iodine-131) were particularly significant for the radiation exposure of the population as a consequence of the reactor disaster of Chernobyl. Today virtually only the long-lived caesium-137 is of importance for Central Europe. Due to its half-life of about 30 years, this radionuclide has only decayed by about half from 1986 until now.

Radioactive air masses over Germany

As a result of the Chernobyl accident large amounts of radioactive substances were released into the atmosphere. They were dispersed into heights of more than one kilometre due to thermal lift caused by the graphite fire. The initially prevailing airstream transported the radioactive substances over Poland to Scandinavia. A second cloud moved over Slovakia, the Czech Republic and Austria to Germany. The third cloud eventually reached Romania, Bulgaria, Greece and Turkey.

Already shortly after the radioactive air masses had reached Germany in late April/early May 1986, the direct deposition of radioactive substances on pastures and on a few crops ready for harvesting led to high levels of iodine-131 in cow's milk and leafy vegetables such as spinach in southern Germany. In early May 1986, the German Commission on Radiological Protection recommended for a short period to only authorise fresh milk containing less than 500 becquerels of iodine-131 per litre for human consumption. Several federal states (Bundesländer) applied much more stringent standards, for example by recommending not to consume fresh milk with concentration levels of iodine-131 exceeding 20 becquerels per litre.

On account of its short half-life of about 8 days, most of the iodine-131 had already decayed after a few weeks. The total exposure due to radioactive iodine stemmed from an amount of less than 1 gram deposited over the then Federal Republic of Germany. According to the information provided by the Gesellschaft für Strahlen- und Umweltforschung (Company for Radiation and Environmental Research, GSF; now HMGU, Helmholtz Zentrum München – Deutsches Forschungszentrum für Gesundheit und Umwelt (German Research Centre for Environmental Health)) the amount of caesium-137 deposited on the same area was about 230 grams.

Consequences for food and feed

Also plant foods and animal feed not yet ready for harvesting were affected by the direct deposition of radionuclides from the atmosphere onto the above-ground plant parts. A major part of the deposited radiocaesium (caesium-137 and caesium-134) was absorbed by the plant's leaves in this case and was then distributed within the plant. Over the long term, however, radiocaesium was mostly taken up from the soil by the roots.

Radiocaesium may be strongly bound to certain clay minerals in the mineral soils of many areas of arable land. For this reason it is only available for uptake by the roots to a very limited extent. Already in the summer of 1986, agricultural crops which had only been sown or planted after the Chernobyl reactor accident were contaminated with not more than just a few becquerels of radiocaesium per kilogram. Also today, the caesium-137 contents in agricultural products from domestic production are at this level or below.

Daily intake of caesium-137, caesium-134 and strontium-90 with the total food in becquerel per person and day

In Germany, the mean intake of caesium-137 through foods from agricultural production is about 80 becquerels per person and year. The figure on the left provides an overview of the daily intake of caesium-137, caesium-134 and strontium¬90 in the total diet.

Forest foods

The situation is quite different in the case of forest foods. Also 30 years after the Chernobyl reactor accident, significantly enhanced activity concentrations of caesium-137 can be measured especially in mushrooms and game. The reason for this is the different forest soil structure compared to that of arable land. Forests are characterised by so-called organic surface layers on top of the mineral soils. In these layers which are formed from decomposing litter and are rich in soil organisms, radiocaesium is easily available and is taken up quickly by soil organisms, mushrooms and plants.

Radiocaesium remains within the highly effective nutrient cycles which are characteristic for nutrient-poor ecosystems. Therefore, it migrates only very slowly down into the mineral soil layers where it may be fixed by certain clay minerals similarly as in agricultural soils. For this reason, the radiocaesium levels of forest products generally decline only slowly. Forest foods with higher contamination levels are expected in those parts of Germany which were particularly affected by Chernobyl fallout. This applies especially to the Bavarian Forest and the areas south of the Danube. In other regions such as northern Germany the activity levels are correspondingly lower because of the lower deposition of radiocaesium. Caesium¬137 levels of wild mushrooms and game can vary strongly also within small areas.