Natural radionuclides in soil and evaluation of their exposure in specific areas on the territory of Lithuania

Abstract

The following three unique areas in Lithuania have been chosen for the research: Vilnius, Juodkrantė and Visaginas. Vilnius is the city with the largest population in the country which is dominated by an anthropogenic environment. Juodkrantė is characterized by a very clean environment; it is a recreation area located near the Baltic Sea and Curonian Spit which is included on the UNESCO World Heritage List. Juodkrantė has a natural environment and minimal urbanization. Visaginas is a special for the nuclear power plant (which is currently being decommissioned). It is the only nuclear power plant in the Baltic States built close to the border with Belarus on the shore of Drūkšiai Lake. The specific activities of naturally occurring radionuclides 232Th, 226Ra and 40K were determined experimentally in the soil samples taken from Vilnius and Juodkrantė districts and in the technogenic soil samples from the territory of Visaginas Nuclear Power Plant. Among all identified natural radionuclides, the highest level was that of 40K, i.e. the mean value was 425 ± 71 Bq/kg, while the minimum level was that of 232Th, i.e. the mean value was 3 ± 1 Bq/kg. The mean value of 226Ra specific activity was 15 ± 2 Bq/kg. The contribution of the gamma radiation of each 226Ra, 232Th and 40K (separately) on the external equivalent dose rate (EDR) was calculated. The EDR level in Vilnius city varied from 43 to 98 nSv/h. The highest EDR levels caused by gamma radiation were measured above asphalt surfaces, concrete surfaces and structures. The lower EDR levels were found in parks, above natural cellular surfaces. The EDR levels measured on the Curonian Spit varied from 22 nSv/h to 60 nSv/h. The lowest EDR levels were found on the beaches by the sea, in the sand dunes and above the forest cover. The higher levels have been measured in Juodkrantė, on the coast of the Curonian Lagoon since a major part of the coast of the Curonian Lagoon is covered by stone and special concrete embankments. The EDR levels on the territory of Visaginas varied from 30 to 130 nSv/h. The wider distribution of the EDR values was potentially caused by the distribution of bulk soil and building materials within the research area. Having carried out the comparison with the EDR pollution on the other territories of Lithuania, it was found that the EDR level measured in Visaginas had varied within the limits of the environmental background level of Lithuania, and the measured specific activities of radionuclides are close to the measured specific activities of building materials found in other cities.