Technogenic radionuclides in undisturbed Bulgarian soils

•137Cs, 90Sr and 239+240 Pu concentration of soils in Bulgaria is presented.•The radio-ecological status of Bulgarian soils for period 1985÷2010 is discussed.•137Cs and 90Sr migration processes in soil are considered.•The established radioactivity is а result of global fallout and Chernobyl accident.

This work presents data for the activity concentration of artificial radionuclides in soils from different regions in Bulgaria for the period 1985–2011. The subjects of the study were undisturbed soils from mountainous, hilly, and plain areas. Special attention has been paid to the contamination with the long-lived technogenic radionuclides cesium-137, strontium-90, plutonium-238, and plutonium-239 + 240. 137Cs was measured by low level gamma spectrometry and 90Sr by radiochemical separation and low level beta counting. The isotopes of plutonium were measured by alpha-spectrometry after a radiochemical procedure of purification and concentration.Cesium-137 and strontium-90 were the main technogenic radionuclides detected in the examined Bulgarian soils few years after the Chernobyl NPP accident. Their content in the soils from high mountain areas is several times higher than that in the soils from the plane areas in the northern part of the country. High heterogeneity in the pollution has been observed. Mean soil activity concentration levels of up to 250 Bq kg− 1 for 137Cs and 14 Bq kg− 1 for 90Sr were found in different years. The maximal values registered in separate soil samples were about 1300 Bq kg− 1 (137Cs) and 24 Bq kg− 1 (90Sr). The activity concentration levels for 239 + 240Pu varied between 0.1 and 3.7 Bq kg− 1. The results indicate that the radioactive pollution of the Bulgarian soils with artificial radionuclides is a result of global fallout and Chernobyl nuclear power plant accident.