Spatial distribution of 137Cs, 40K, 226Ra, 232Th in soils of Olenegorsk city (Murmansk region)

Abstract

The goal of the study is obtain original data and identify the main patterns of spatial distribution of radionuclides in the soils of Olenegorsk, depending on zoning and urban development, focusing one specific activity of 137Cs and natural radionuclides, namely 232Th, 226Ra, 40K, in soils. Such studies of urban soils in Olenegorsk have not been carried out earlier. Location and time of the study. Soil samples in the city of Olenegorsk, Murmansk region, were collected in the summer of 2018. Methods. Soil samples were collected at 24 study sites on the territory of the Olenegorsk city from the upper 0-5 cm layer. Under laboratory conditions, the samples were dried at 105° C to constant mass. Then soil aliquots were placed in a Marinelli vessel to measure isotope specific activity using gamma spectrometry. Registration of gamma radiation from a countable soil sample, as well as processing of spectra, was carried out using the Progress-gamma software and hardware complex FVKM.412131.002-03. Calibration of the gamma spectrometer by energy to monitor the safety of the installation parameters was carried out after each measurement using a combined control source OISN-137-1 in a 1-liter Marinelli vessel. The minimum exposure time for the counting sample was 3600 seconds. However, depending on the activity of a sample, the exposure time in individual samples with weak activity was increased. Main results: The maximal specific activity of 137Cs in urban soils of Olenegorsk was 47.5 Bq/kg in the northwestern part of the city in the undisturbed soils of the natural landscape. Maximal values of 40K specific activity reached 293.8 Bq/kg in the central and south-eastern parts of the city within the boundaries of multi-storey residential district. The maximal specific activity of 226Ra in soils reached 14.6, and that of 232Th reached 12.2 Bq/kg, being confined to soils in the area of high-rise and mid-rise buildings, most likely, due to the differential vegetation density since the latter easily involves 40K into the biotic turnover. Maximal values of soil 226Ra and 232Th specific activity (14.6 and 12.2, respectively) were determined in areas with multi-storey and mid-rise buildings. The correlation analysis between the radionuclides and soil skeleton particles (≥2 mm) and finer (≤ 1 mm) fractions showed rather close association of radionuclides with soil skeleton and <45 µm particles, as well as a moderately strong correlation with the 45 µm fraction. Moderately strong correlation was found between 232Th and 226Ra, and between 40K and soil skeleton particles. Conclusion. The obtained data about the accumulation of 137Cs and natural radionuclides (232Th, 226Ra, 40K) in the soils of Olenegorsk do not exceed the maximum permissible concentrations, characteristic of the urbanized areas of the Murmansk region.