Patterns of Spatial Distribution of Radium-226 Specific Activity in Soils and Radon Exhalation Rate from the Soil Surface in the Territory of Pyatigorsk

Abstract

Purpose: To identify spatial distribution patterns of ²²⁶Ra specific activity in soils and radon flux density (RFD) from the soil surface within the territory of Pyatigorsk, and to determine the correlation between these parameters and indoor radon concentrations (IRC) for the purpose of developing medium-and large-scale mapping of areas by potential radon hazard levels. Material and methods: RFD measurements were primarily conducted during the dry season using the open-chamber method with activated charcoal, employing equipment from the “Kamera-01” measuring complex. Measurements of dose rate were performed using DRG-01T and DKg-07D “Drozd” gamma dosimeters at a height of 1 m above the ground surface. Measurements of specific activity of natural radionuclides in soil samples were carried out using a gamma spectrometric system equipped with a NaI(Tl) 63×63 mm scintillation detector and “Progress-2000” software in a 1-liter Marinelli beaker geometry. The samples were preliminarily stored for two weeks in hermetically sealed Marinelli containers to achieve radioactive equilibrium between radium and radon. In total, the study included 224 measurements of RFD from the ground surface, 47 determinations of specific activity of natural radionuclides in samples of bedrock and unconsolidated deposits, and approximately 200 measurements of dose rate. Results: Radium specific activity values within the city ranged from 15–195 Bq/kg, and the RFD ranged from 10–567 MBq/m2∙s. The lowest values of radiation parameters are typical for the floodplain and the I-II upper floodplain terraces of the Podkumok River, the highest values are for territories composed of Maikop clays. In most of the city’s territory, except for the floodplain of terraces I–II, the average RFD values exceed the level of 80 mBq/m2∙s, and the proportion of private values exceeding this level is more than 50%, which makes it possible to classify this territory as a potentially radon-hazardous category. Extremely high226Ra concentrations (477–868 Bq/kg) and RFD (679–2760 mBq/m2∙s) were recorded in the distribution sites of fresh travertines precipitating from hot hydrogen sulfide-carbon dioxide waters in the Goryachaya Mountain area. Several local radon anomalies have also been detected in the Pyatigorsk (RFD values reach 5260 mBq/m2∙s), which appear at separate points and do not repeat over time, which are apparently associated with local convective radon transport in the soil. The paper presents the results of radon monitoring at anomalous points in different seasons of the year.