In the early months after the accident, the levels radioactivity of agricultural plants and plant-consuming animals was dominated by surface deposits of radionuclides. The deposition of radio iodine caused the most immediate concern, but the problem was confined to the first two months after the accident because of fast decay of the most important isotope, 131I.
The radioiodine was rapidly absorbed into milk at a high rate leading to significant thyroid doses to people consuming milk, especially children in Belarus, Russia and Ukraine. In the rest of Europe increased levels of radioiodine
After the early phase of direct deposit, uptake of radionuclides through plant roots from soil became increasingly important. Radioisotopes of caesium (137Cs and 134Cs) were the nuclides which led to the largest problems, and even after decay of 134Cs (half-life of 2.1 years) by the mid-1990s the levels of longer lived 137Cs in agricultural products from highly affected areas still may require environmental remediation. In addition, 90Sr could cause problems in areas close to the reactor, but at greater distances its deposition levels were low. Other radionuclides such as plutonium isotopes and 241Am did not cause real problems in agriculture, either because they were present at low deposition levels, or were poorly available for root uptake from soil.
In general, there was a substantial reduction in the transfer of radionuclides to vegetation and animals in intensive agricultural systems in the first few years after deposition, as would be expected due to weathering, physical decay, migration of radio nuclides down the soil and reductions in bioavailability in soil, see Fig. 5. However, in the last decade there has been little further obvious decline, by 3-7 percent per year.
FIG. 5. Reduction with time of 137Cs activity concentration in milk produced in private and collective farms of the Rovno region of Ukraine with a comparison to the temporary permissible level (TPL).
The radiocaesium content in foodstuffs was influenced not only by deposition levels but also by types of ecosystem and soil as well as by management practices. The remaining persistent problems in the affected areas occur in extensive agricultural systems with soils with a high organic content and animals grazing in unimproved pastures that are not ploughed or fertilized. This particularly affects rural residents in the former Soviet Union who are commonly subsistence farmers with privately owned dairy cows.
In the long term 137Cs in milk and meat and, to a lesser extent, 137Cs in plant foods and crops remain the most important contributors to human internal dose. As 137Cs activity concentration in both vegetable and animal foods has been decreasing very slowly during the last decade, the contribution of 137Cs to internal dose will continue to dominate for decades to come. The importance of other long lived radionuclides, 90Sr, plutonium isotopes and 241Am, in terms of the human dose will remain insignificant.
Currently, 137Cs activity concentrations in agricultural food products produced in areas affected by the Chernobyl fallout are generally below national and international action levels. However, in some limited areas with high radionuclide contamination (parts of the Gomel and Mogilev regions in Belarus and the Bryansk region in Russia) or poor organic soils (the Zhytomir and Rovno regions in the Ukraine) milk may still be produced with 137Cs activity concentrations that exceed national action levels of 100 Bq per kilogram. In these areas environmental remediation may still be warranted.