Concentrations of Natural Radionuclides (40K, 226Ra, 232Th) at the Potash Salts Deposit

Specific activity of natural radionuclides 226Ra, 232Th and 40K was measured by gamma spectrometry in samples of soil and bottom sediments of rivers on the territory of potassium salt deposit development in order to collect information on radionuclide distribution. According to the results of examinations, the range of 226Ra, 232Th and 40K activity values in soils and bottom sediments of the territory of influence was <2.17–19.7 Bq/kg, <2.5–20.5 Bq/kg, 123–500 Bq/kg. Migration of the studied radionuclides is limited to 2 km zone. The results obtained provide useful information on the levels of naturally occurring radionuclides in the zone of influence of mining objects and are of interest for a comparative evaluation of the impact of salt mining on the radiation environment of the territory.


INTRODUCTION
The natural radiation background of the territory is conditioned by cosmic radiation as well as by the presence of radioactive elements in the Earth's crust, water bodies and atmosphere. Among the radioactive isotopes of natural origin the most widespread and important from the ecological point of view are 238U, 232Th, 226Ra and 40K [Paschoa et al., 2010]. The last decades of civilization development have led to radical changes in ecological conditions, in particular, to intensive radioactive contamination of the main components of the natural environment (soils, natural waters, rocks, bottom sediments) [Mirzoyeva et al., 2015]. The radiation background of any territory consists of a natural background and radioactive contamination caused by anthropogenic impact. Often high levels of radiation background are associated with natural radioactivity of rocks, as well as with geochemical features of behavior of radionuclides themselves. Therefore, when characterizing the radioecological state of the territory it is necessary to assess not only anthropogenic impact, but also the natural component and compare the data obtained with the regional background [Markelov, 2008].
Gamma spectrometric measurements of the activity of natural radionuclides in various objects of the environment are now actively used to assess the ecological state of territories. Such studies are actively conducted for soils, bottom sediments and beach sands [Abusini et al., 2008; Guerrero et al., 2020]. The attention of researchers to the radiation situation in salt deposits is associated with the promising development of speleotherapy [Calin et al., 2012]. In particular, the use of materials based on natural potassium salt for interior decoration is a known tool to create a therapeutic effect due to air ionization [Chernyi, 2019].
The study area is one of the most developed areas of the Verkhnekamskoye potash salt deposit (Ural region, Russia). Underground mining of sylvinite (raw material for the production of potash fertilizers), carnallite (production of artificial carnallite for the magnesium industry) and brines (raw material for the production of soda) is conducted here. The geological reserves of the deposit are huge and are estimated at 96.4 billion tons for carnallite rock, 113.2 billion tons for sylvinite and 4.650 billion tons for rock salt.
The purpose of the research is to estimate the activity of natural radionuclides 226Ra, 232Th and 40K in soils and bottom sediments within the territory of potassium salt deposit development, their spatial distribution and possible anthropogenic sources. In the investigated group of natural radionuclides, 40K is of the greatest interest for the territory under consideration. Natural potassium is a mixture of three isotopes 39K, 40K, 41K, while only 40K is radioactive. It is a natural radionuclide of non-serial origin that is present everywhere on the Earth. The results presented in the paper provide useful information on the content of natural radionuclides in the influence zone of mining objects and are of interest for comparative evaluation of the impact of salt mining on the formation of radiation situation of territories.

METHODS
Soil, bottom sediments and core samples were taken from an exploration well within the study area of the potassium deposit. Soil sampling points were relatively evenly dispersed throughout the territory adjacent to the mining operation. A total of 44 soil samples were taken by the dug-in method from the top soil layer (0-0.4 m interval). Bottom sediments (6 samples of fine-grained fraction) were taken from watercourses within the territory adjacent to the enterprise.
Radiation examination of the core was carried out on the exploration well in the depth interval of 3.5-349.0 m. Samples were taken from the minable seam and characteristic lithological types of rocks of the overlying strata. A total of 15 core samples were examined. To compare the results obtained from the core samples, the crushed rock in the underground space of the mine was examined. A total of 4 such samples were examined.
Specific activity of natural radionuclides (40K, 226Ra, 232Th) was studied for all samples under laboratory conditions. Gamma-spectrometric measurements of samples were done on scintillation gamma-spectrometer MKS-01A "Multirad" (Russia). Gamma-spectra were processed using the programs included in the Progress package. Errors of specific activity measurements were ±(20-30)% of the measured value.
For the energy calibration of the spectrometer, a reference source based on 137Cs and 40K was used. The source is a 1-liter Marinelli vessel filled with KCl, closed with a lid, in the base of which there is a cylindrical recess, where the 137Cs source is fixed (a disc 29 mm in diameter, in the center of which there is an active spot, sealed with two polyamide films 50 µm thick each). The source activity is 1500 Bq, and the error (at 0.95 confidence probability) is 20%. The spectrum from the gamma spectrometer's calibration source shows maximums of 137Cs (energy 662 keV) and 40K (energy 1461 keV) total absorption peaks, count rate in a certain energy range (usually 620-700 keV). In order to control own noises of gamma-spectrometric path and take into account the impact of external gamma radiation on the hardware radiation spectrum of the investigated sample, after calibration the background measurement was made (during 1800 sec). In the investigated samples of soil and bottom sediments their organic part was removed by calcination before measurement, the samples were mixed and placed in a Marinelli vessel up to the mark of 1 L.
Rationing of exposure to natural radionuclides in accordance with the requirements of radiation safety standards (Radiation Safety Standards, Russia) was carried out according to the calculated value of specific effective activity A eff , which characterizes the total specific activity of natural radionuclides in the studied material, determined taking into account their biological impact on the human body (Bq/kg). The formula for its calculation is as follows: where: A Ra and A Th are the specific activities of 226Ra and 232Th (Bq/kg), which are in equilibrium with the remaining members of the uranium and thorium series, A K is the specific activity of 40K (Bq/kg).

RESULTS AND DISCUSSION
Soil cover, being at the intersection of migration ways of chemical elements, including radioactive elements, is the main component of the natural environment, carrying the cumulative effect of long-term anthropogenic impact [Grebenshchikova et al., 2017]. In the upper layer of soils and grounds (up to 5 cm), 95-98% of anthropogenic activity is concentrated. The natural activity changes insignificantly with depth, which is why the radiation characteristics of this particular layer are the main in the analysis of soils and grounds.
According to the results of examinations of soils and bottom sediments, the maximum values of specific effective activity of the studied natural radionuclides (value range 62-88 Bq/kg) are observed in the soils of the sites in the immediate vicinity of the operating salt mine administration and to the north-east of it (Table 1, Table 2, Fig. 1). The obtained outlines of specific effective activity distribution of the investigated natural radionuclides correspond to the outlines of 40K activity distribution (Fig. 2). The formed zone of higher concentrations of natural radionuclides is conditioned by the wind regime of the territory of research, where a high frequency of southern direction winds is observed during a year. Taking into account the prevailing wind direction, there is a significant removal of substances with atmospheric flows in the northern and northeastern directions.
Developed potassium ores are the source of 40K entry into the environment. Radiation study of the exploration well core (Table 3) demonstrates high values of 40K activity in productive potassium formations which is caused by their mineral composition. For the considered deposit the highest values are typical for mottled sylvinites (up to 4967 Bq/kg), which is significantly higher than the 40K activity for generalized data on sedimentary rocks and continental crust. All examined samples of potassium salts do not exceed the permissible value of 740 Bq/kg by the rated specific effective activity (Radiation Safety Standards, Russia).
The mine site administration monitors atmospheric air at a number of locations on the border of the sanitary protection zone of industrial facilities on a monthly basis. Potassium chloride and sodium chloride are defined among other constituents. The control results show that the maximum permissible concentrations are not exceeded (0.3 mg/m 3 for KCl, 0.5 mg/m 3 for NaCl). Taking into account the volume of emissions into the atmospheric air, the main entry of potassium chloride and sodium chloride is connected with the emissions of the drying department of the fertilizer production plant. According to the results obtained, migration of the studied radionuclides is limited to the zone of 2 km. Only insignificant part of the investigated samples (8%) exceeds the worldwide average activity of 40K for soils. In bottom sediments the average activity of 40K does not exceed the values for other territories with anthropogenic impact. The situation is different for the content of 226Ra and 232Th. Even their maximum activities in the investigated soil samples are essentially lower than the worldwide average values.
The quantitative state of radioactive equilibrium in various objects of the environment is characterized by the coefficient of radioactive equilibrium, which is the ratio of the activity of a given radionuclide to the activity of the radionuclide that originated or preceded it in the radioactive series [Vertman et al., 2002]. If the radioactive equilibrium is not disturbed, the ratio will be 1. Thus, the ratio of activity values 232Th/226Ra<1 in soils can serve as an indicator of anthropogenic impact. The results of the conducted assessment for soils of the study area showed several zones with disturbed ratio of these natural radionuclides (Table 4, Fig. 3). The configuration of the isolated zones of imbalance indicates the absence of their connection with the activities of the mine of potassium salts.

CONCLUSIONS
Monitoring of changes in radiation background and radionuclide activity is currently an important task in order to assess the radiation situation in industrial and urbanized areas. In spite of the high activity of 40K in the developed potassium ores, the value of this index for the majority of investigated soil samples at the Verkhnekamskoye salt deposit does not exceed worldwide average values, for samples of bottom sediments -values of other territories with anthropogenic influence. Migration of the investigated radionuclides is limited to the zone of 2 km. Even the maximum activities of 226Ra and 232Th in the investigated soil samples are significantly lower than the worldwide average values. All this allows us to conclude that the underground development of the deposit has no effect on the radiation situation of the territory under study. The results of the studies can be used in solving the problems of radiation monitoring of mining enterprises and environmental assessment of territories, taking into account the radiation loads.