Assessment of reference levels of radionuclides in soil in the areas of the Kaluga oblast affected by the Chernobyl accident

The paper presents values of reference levels of 137Cs, 90Sr and 239Pu in soil, adjusted to different land-use scenarios and specific characteristics of the Kaluga oblast areas, contaminated with radionuclides following the Chernobyl accident. The levels were assessed on the basis of radiation-hygienic and ecological criteria. According to radio-hygienic criterion the annual effective radiation dose from the radionuclides in soil should not exceed 1 mSv/year. The reference levels meeting the criterion were calculated for the following land-use scenarios: 1 - permanent stay in the territory contaminated with radionuclides and unlimited agricultural exploitation; 2 - limited agricultural exploitation of the territory (homesteading); 3 - permanent stay in urban area; 4 - temporal stay of people in contaminated area (industrial usage); 5 - recreational land use (tourism). Values of transfer factors of the radionuclides from soil to food products were calculated with the use of data of ecological and radiation-hygienic monitoring in the settlements of the Kaluga oblast and other settlements contaminated with radionuclides as a result of the accidents at the Chernobyl NPP and Mayak Production Association. For calculating reference levels on the basis of ecological criteria the following reference species were used: the Scots pine ( Pinus sylvestris ), red clover ( Trifolium pratense ), the earthworm ( Lumbricus terrestris ), the honey bee ( Аpis melliféra ), the grass-snake ( Natrix natrix ), the mollard duck ( Anas platyrhynchos ), the elk( Alces alces ). In areas of unlimited agricultural exploitation internal radiation dose from 137Cs is formed mainly due to intake of mashrooms and berries (79%), potato, milk and meat (19%), the dose from 90Sr - due to intake of milk (56%), vegetables (36%); from 239Pu - due to intake of fish, meat and milk (98%). The lowest reference levels of the radionuclides in soil should be in agricultural lands (scenario 1), the highest - in lands used for industrial purposes (scenario 4). Reference levels of soil contamination with 137Cs in urban territories (scenario 3) was 1.9 times higher than in lands for agricultural exploitation, with 90Sr - 105 times and for 239Pu - 156 times. Internal radiation dose within the town is lower than in rural territories, however, the contribution of gamma-radiation from 137Cs to the external dose remains the same. If the land is used in accordance with the scenario 5, reference levels values depend on natural food chains. In total, assessed reference levels of the radionuclides in lands for unlimited agricultural exploitation meet the criteria used in territories affected by radioactive contamination as a result of the Kyshtym disaster and the Chernobyl accident. The use of regional specific characteristics of radionuclides accumulation in farm products and natural foods for the assessment allows reducing conservatism in assessment of reference levels of radionuclides in soil, as compared to the assessment with the use of summarized reference values of the parameters. The radiation-hygienic criterion, the basis for estimating reference levels of 137Cs, 90Sr and 239Pu in the scenario 1 is more rigid than the ecological criterion, whereas, in contrast, ecological criterion used for estimating reference levels for 90Sr and 239Pu in scenarios 3 and 4 is more rigid. It is recommended to use the most conservative value of the reference level that meets both radiation-hygienic and environmental requirements as a single indicator of the permissible content of radionuclides in soil, that ensuring the radiation safety of the population and the preservation of a favorable environment.