An Assessment of the Impact of Gypsum Deposit Development on Changes in the Radiation Environment

Abstract

The aim of the conducted research was to assess the impact of gypsum deposit development on changes in the radiation levels of the abiotic components of the environment. For this purpose, a study of the radioactivity of water, bottom sediment, soil, gypsum and loam samples was performed. Ground-based studies of the distribution of the values of the ambient dose equivalent rate of gamma radiation and radon flux density were also carried out. It was shown that due to the high solubility of gypsum, the degree of karstification of the territory increases under the influence of meteoric waters, and as a result of the intensification of anthropogenic impact, the degree of chemical weathering of rocks increases. This leads to a coordinated change in not only the chemical but also the radiation conditions. In particular, radioactive contamination of quarry waters and areas of increased radon flux density in soil air were established. In bottom sediments, the significant correlations of ¹³⁷Cs, ²³⁸U and ²³⁴U activity concentrations with carbonates, organic matter and soluble salts contents, as well as Fe, Zn, Cu, Cr, Pb, Ni, Mo, Cd, Co, Ti and V, indicate a significant role of the anthropogenic factor in the accumulation in bottom sediments. This factor is associated with both regional atmospheric transport (¹³⁷Cs) and the activity of the mining enterprise in the study area (²³⁸U and ²³⁴U).

1. Introduction

Mining operations involve the extraction of huge masses of rock and soil to the surface. In addition to significant changes in the morphology of the territory, this also leads to changes in the distribution of chemical elements and radionuclides in the environment. First of all, the quality of water and bottom sediments in nearby watercourses and reservoirs suffers due to the discharge of quarry waters, as well as the soil cover due to aero-technogenic pollution [1,2]. The Glubokoye gypsum deposit is located in the northern taiga zone in northwestern Russia. It has been developed since 2008. The annual extraction of gypsum rock averages 600 thousand tons. Gypsum rock is mined by using the quarry method with blasting operations. It is generally recognized that the open-pit development of bedded deposits has the most significant impact on the environment. The extraction of gypsum is accompanied by a number of successive operations, including deforestation on part of the license area, the excavation of the layer of quaternary deposits lying above the gypsum rocks, blasting operations, the movement of gypsum using excavators and loaders, the crushing of gypsum using a crushing and sorting plant, the transportation of gypsum rock and overburden rocks and the storage of finished products. A complicating factor is the location of the quarry, two hundred meters from the State Natural Landscape Reserve of Regional Significance “Chugsky”. It was created to preserve the unique Chug relief, which is characterized by a large area of exposure of the gypsum basement under the influence of glacial exaration, the presence of large formations of dissected preglacial relief, small thickness of the Quaternary sediment cover and a hydrographic network with canyon- or trough-shaped valleys [3,4]. As a result, unique forms such as shelopnyakovye fields, collapsed cirques and ravines, disappearing lakes, towers and caves are widely represented on the territory of the Chugsky Reserve. The specificity of the lithogenic basis of ecosystems at dense gypsum outcrops, which determines the structure and geochemistry of the soil cover, vegetation and natural waters, made it possible to identify a new sulfate-calcium geochemical class in the family of northern taiga landscapes [5]. The four largest caves of the Chugsky Natural Reserve are included in the Cadastre of Large Gypsum Caves of the World and are worthy of being declared natural monuments [6]. The territory of the reserve contains habitats of rare and protected epiphytic lichens and coprophilous mosses, as well as protected animal species such as the osprey (Pandion haliaetus), the peregrine falcon (Falco peregrinus), the eagle owl (Bubo bubo) and the great grey shrike (Lanius excubitor) [7].

Previously, we studied the features of chemical pollution of the environment during the development of gypsum deposits with an assessment of pollution sources and transfer rates [8]. The aim of this study was to assess the impact of gypsum deposit development on changes in the radiation environment. The relevance of such a study is associated, in particular, with the high radon hazard of cave complexes widely visited by tourists [9,10,11]. Radon and its short-lived decay products pose a health hazard due to their carcinogenic effects. The radiation safety standards of the Russian Federation establish limits for the radon flux density (RFD) on land plots before construction—80 and 250 mBq (m²s)⁻¹ for residential/public buildings and industrial buildings, respectively. With such RFD values, the values of the equivalent equilibrium radon concentration (EERC), which is used instead of the specific radon activity (SAR) in Russia, are 100 and 300 Bq m⁻³, respectively [12]. In some cases, caves have been found to exceed the permissible levels for the population and workers by 100 to 500 times [13,14,15,16]. Of equal importance is the study of the distribution of ²³⁸U, as the progenitor of the decay chain involving ²²⁶Ra and ²²²Rn. Radionuclides such as ²³²Th and ⁴⁰K can also have negative effects when used in building materials [17]. ¹³⁷Cs is of interest as a modern human-made radioactive pollutant released into the environment as a result of global nuclear weapon testing and nuclear accidents such as Chernobyl and Fukushima, making it an environmental concern [18,19,20,21,22].

The scientific novelty of our research is related to the fact that currently, the world literature mainly evaluates the impact of gypsum deposit development on the chemical characteristics of environmental components [23]. This paper shows that due to the high solubility of gypsum, under the influence of meteoric waters that are not in equilibrium with it, the degree of karstification of the territory increases, and as a result of the intensification of anthropogenic impact, the degree of chemical weathering of rocks increases. This leads to a coordinated change in not only the chemical but also the radiation conditions in the area of gypsum deposit development.

2. Materials and Methods

2.1. Study Area

The study area is located in the northern taiga zone on a rolling plain formed over several ice ages. At the end of the last glaciation, huge volumes of meltwater seeped through soluble rocks and formed a karst relief with a large number of sinkholes, streams, lakes, caves and other karst formations. The soil cover depends on the composition of the underlying rocks and the degree of their drainage. Illuvial–ferruginous and illuvial–humus–ferruginous podzols are confined to moraine sandy–clayey deposits with a thickness of about 2–10 m and develop in well-drained areas of the relief—the tops of moraine hills and moderately steep slopes under bilberry–green moss spruce forests. In meso-depressions, where the thickness of the Quaternary deposits can reach 30 m, peaty–podzolic–gley soils are widespread. Unique soils with coarse, weakly and moderately decomposed litter and purely gypsum mineral horizons are developed at the gypsum outcrops on the surface [24].

Sedimentary deposits of the Sakmarian stage of the Lower Permian (P1s) lie under the cover of the Quaternary deposits. The upper part of the sedimentary layer, 80 m thick, is composed of gypsum and anhydrite containing lenses and interlayers of dolomites and sandstones (Figure 1). Admixture of terrigenous material is noted everywhere, both in the groundmass and along cracks. Deeper lies a 140 m layer of limestones and dolomites of the Sakmarian and Asselian (P1a) stages of the Lower Permian [8].

Groundwater is confined to the fractured and karstified zone in the upper part of the gypsum strata. It has a Ca-SO₄ composition and total dissolved solids (TDS) of 2.3 g L⁻¹, corresponding to full saturation with respect to gypsum. Two rivers, the Pozera and the Chuga, 24 and 34 km long, respectively, flow in the immediate vicinity of the mining allotment of the deposit. The Pozera River, 24 km long, originates from a swamp massif and has low total mineralization values in the source area, which is explained by its predominant snow and rain feeding. Six kilometers downstream, TDS increases from 0.5 to 1.8 g L⁻¹ due to a consistent increase in underground recharge. In the longer Chuga River, TDS at the quarry latitude reaches 2.3 g L⁻¹; i.e., it is completely formed due to the dissolution of gypsum.

Two lakes, Sennoe and Karasevoye, are moraine lakes with virtually no underground recharge. Their water has a Ca-HCO₃-SO₄ composition and minimal TDS values (0.1 g L⁻¹). Three small unnamed lakes with difficult underground recharge, due to partial colmatation of the bottom, are classified as karst and have intermediate TDS values (0.8–1.8 g L⁻¹). Quarry waters and waters from the quarry water settling tank have a Ca-SO₄ composition. The TDS value in quarry waters was increased compared with groundwater to 2.6 g L⁻¹ due to additional enrichment with calcium sulfates and sodium chlorides. In the quarry water settling tank, the TDS value corresponded to groundwater (2.3 g L⁻¹).

2.2. Sample Description and Preparation

In August 2023, 14 water samples were collected to determine the activity of the uranium isotopes ²³⁴U, ²³⁵U and ²³⁸U. Within a 3 km radius of the quarry (N 64°05′04″–64°08′10″, E 42°33′21″–42°41′40″), samples were collected from two rivers, five lakes and one groundwater source. Samples of quarry water and water from a settling pond were also collected. Following the detection of high uranium concentrations in the quarry water, additional samples were collected from it in January and February 2025 (W-11-1 and W-11-2, respectively, in

Table 1. Activity concentrations of uranium isotopes and uranium concentration in water samples.

ID	Water Point	N	E	238U (mBq L−1)	234U (mBq L−1)	235U (mBq L−1)	234U/238U (Bq Bq−1)	U (µg L−1)
W-1	Pozera River 1 a	64.089	42.635	3.04 ± 0.46	3.14 ± 0.47	<0.38	1.03 ± 0.15	0.25 ± 0.04
W-2	Pozera River 2	64.106	42.626	10.5 ± 1.6	17.2 ± 2.6	1.14 ± 0.56	1.63 ± 0.24	0.85 ± 0.13
W-3	Lake Sennoe	64.110	42.591	0.96 ± 0.14	1.37 ± 0.20	<0.27	1.43 ± 0.21	0.08 ± 0.01
W-4	Chuga River 2	64.125	42.688	22.1 ± 3.3	33.0 ± 5.0	0.81 ± 0.55	1.50 ± 0.22	1.78 ± 0.27
W-5	Lake 5	64.126	42.673	0.18 ± 0.02	0.24 ± 0.03	<0.01	1.33 ± 0.19	0.02 ± 0.00
W-6	Lake 6	64.130	42.674	9.38 ± 1.40	10.0 ± 1.5	0.4 ± 0.21	1.07 ± 0.16	0.76 ± 0.11
W-7	Lake 7	64.135	42.659	5.11 ± 0.76	5.78 ± 0.86	<0.12	1.13 ± 0.16	0.41 ± 0.06
W-8	Chuga River 1	64.112	42.689	8.51 ± 1.27	11.8 ± 1.8	0.78 ± 0.42	1.38 ± 0.20	0.69 ± 0.10
W-9	Spring	64.112	42.689	16.7 ± 2.51	18.6 ± 2.8	1.34 ± 0.49	1.12 ± 0.16	1.35 ± 0.20
W-10	Karasevoe Lake	64.108	42.661	9.83 ± 1.47	13.0 ± 1.9	1.34 ± 0.91	1.32 ± 0.19	0.79 ± 0.12
W-11	Quarry	64.112	42.654	91 ± 14	111 ± 17	4.53 ± 1.35	1.22 ± 0.18	7.34 ± 1.10
W-11-1	Quarry	64.112	42.654	68 ± 10	103 ± 13	1.92 ± 0.89	1.51 ± 0.22	5.52 ± 0.82
W-11-2	Quarry	64.112	42.654	65.7 ± 9.9	88 ± 12	1.29 ± 0.69	1.34 ± 0.20	5.30 ± 0.80
W-12	Settling tank	64.108	42.645	24.3 ± 3.6	29.4 ± 4.4	0.52 ± 0.22	1.21 ± 0.18	1.96 ± 0.29
W-13	Pozera River 4	64.129	42.628	18.0 ± 2.7	21.3 ± 3.2	<0.38	1.18 ± 0.17	1.45 ± 0.22
W-14	Pozera River 3	64.116	42.632	5.77 ± 0.86	10.1 ± 1.5	1.14 ± 0.56	1.75 ± 0.26	0.47 ± 0.07
Mean b				16.1 ± 2.4	20.4 ± 3.1	1.01 ± 0.45	1.31 ± 0.19	1.30 ± 0.19

^a The numbering of river water samples is carried out in the direction of the flow; ^b the average values were determined without taking into account samples W-11-1 and W-11-2.

). Samples of 20 L were acidified with hydrochloric acid to pH 1–2, and the required amount of the isotopic tracer ²³²U was added so that its activity was comparable with the activity of the isolated natural uranium isotopes. After 4 h, the pH was brought to 4.5–5.5 by using a 20% solution of hexamethylenetetramine (urotropine), verifying it with universal indicator paper. After about 1–2 h, 15 g of sorbent in the form of specially prepared activated carbon of the BAU-A brand was added to the sample under study and thoroughly mixed with a stirrer for 10 min, after which it was allowed to settle for 24 h. The water was drained with a hose, and the carbon with the remaining water was poured into 1.5 L plastic containers and transported to the laboratory.

In August 2023, ten bottom sediment samples, fourteen soil samples, three gypsum samples and three samples of overburden moraine deposits were also collected to determine the activity of the uranium isotopes ²³⁴U and ²³⁸U, as well as ¹³⁷Cs, ²²⁶Ra, ²³²Th and ⁴⁰K. The sample mass was about 1 kg. Bottom sediment samples were collected by using a Peterson hand sampler at a distance of 0.5 m from the shore from silt deposits of the same two rivers and five lakes where water sampling was carried out. Soil samples were collected from the litter–peat soil horizon “O” with a thickness of 2–7 cm. This is a surface horizon consisting of organic material of varying degrees of decomposition (no more than 50%) and various botanical compositions. The organic matter content is >35% of the horizon mass. It is underlain by a clay-illuvial soil horizon “B1” with a thickness of 5–15 cm [25]. Samples of gypsum and overburden moraine loams were collected directly on the territory of the quarry.

Figure 2a shows the study area and sampling locations.

2.3. Analytical Procedures

The desorption of uranium from BAU-A-grade coal in water samples was carried out under laboratory conditions in the laboratory of environmental radiology of the N. Laverov Federal Center for Integrated Arctic Research of the Ural Branch of the Russian Academy of Sciences (Arkhangelsk, Russia), which complies with the accreditation criteria for testing laboratories established in ISO/IEC 17025 [26]. The laboratory has a wide range of reference radionuclide sources for equipment calibration and quality control procedures for measurements.

Activated carbon with the remaining water was filtered with a Buchner funnel with a diameter of 90 mm and a “white ribbon” filter. The sorbent was dried in a drying cabinet at a temperature of 105 °C. The dried carbon with precipitated isotopes was placed in a 250 mL beaker, filled with hot 5% soda solution and thoroughly mixed with a stirrer for 3 h. After that, the resulting mixture was filtered with a vacuum filtration funnel through a “white ribbon” filter with a diameter of 90 mm, and the carbon was additionally washed several times with distilled water. Then, the carbon was discarded, and the filtrate was brought to pH 1–2 with hydrochloric acid and boiled for 30 min in a beaker under a watch glass.

After the solution cooled, 1 mL of ferric chloride solution was added to it and mixed, and iron hydroxides were precipitated with a 30% aqueous ammonia solution. The hydroxides thus obtained were filtered with a “blue ribbon” filter and dissolved in 70 mL of a 7 M HNO₃ solution. At this stage, there occurred separation from the interfering isotopes ²¹⁰Po (5.305 MeV) and ²²⁶Ra (4.777 MeV), as well as from iron due to the very low coefficient of their transfer from the HNO₃ solution to tributyl phosphate (TBP). Next, the uranium isotopes were extracted with 15 mL of a 30% tributyl phosphate solution in toluene in a separatory funnel using a stirrer for 5 min. The lower part of the solution was discarded, and the upper extract was washed twice for 1 min with 15 mL of 7 M HNO₃. At this stage, separation from ²³⁰Th (4.685 MeV) took place, since in a 30% TBP solution, uranium extraction occurs more than 10 times more efficiently than thorium. Next, to prepare for the re-extraction of uranium isotopes from tributyl phosphate, the extract was washed with a solution of a complexing agent (0.04 M HF in 0.25 M HNO₃) for 1 min. The re-extraction of uranium isotopes from TBP was performed by stirring it 3 times in portions of 15 mL with distilled water for 1 min each. The combined aqueous re-extract (45 mL) was evaporated to dryness in a quartz crucible, the dry salts were treated with 5 mL of concentrated HNO₃ and evaporated again, and the dry residue was dissolved in 20 mL of 2% Na₂CO₃ with heating. The resulting solution was poured into a Teflon cell for electrolysis. Deposition was carried out on a stainless steel disk with a diameter of 34 mm for 30 min at a current of 2 A. A platinum electrode was used as an anode. The disk was washed with distilled water and dried, and activity was measured with a Multirad-AS alpha spectrometer (NPP DOZA, Moscow, Russia).

Bottom sediment samples were dried in a BINDER E28 drying oven at 105 °C. A sample used as a tracer solution with known activity of the internal standard ²³²U was also added to a 10 g sample of each bottom sediment sample. The method for extracting uranium isotopes after complete decomposition was described in a previous study [27]. A 10 g sample was placed in a porcelain crucible and fired at 500 °C until the organic matter was completely burned out. The fired sample was transferred to a Teflon cup and moistened with distilled water, and 1 cm³ of the ²³²U isotope indicator was added (the accuracy of introducing the indicator into the sample should not be worse than 3%). Then, 40 cm³ of HF and 10 cm³ of HClO₄ were added, covered with a Teflon lid, and heated until HClO₄ vapors appeared. The HF treatment was repeated 2 more times, each time cooling the cup before adding the acid and evaporating until HClO₄ began to smoke.

After decomposition was complete and the solution cooled, the edges of the cup and the lid were washed with water and evaporated again until thick white HClO₄ vapors appeared with the cup open. This operation was repeated twice more. The solution was finally evaporated to wet salts. The salts were dissolved by boiling in 50 cm³ of 7 M HNO₃. If there was an insoluble residue, it was filtered through a 9 cm diameter “blue ribbon” filter and washed 3 times with hot 7 M HNO₃ in portions of 5–10 cm³. The solution containing uranium isotopes was transferred to a separatory funnel, a 30% solution of freshly purified TBP in toluene was added (aqueous and organic phase ratio of 4:1), and the radionuclides were extracted for 5 min. After the separation of the phases, the mother liquor (lower layer) was poured back into the beaker, and the organic extract was washed for 1 min, first 2 times with an equal volume of 7 M HNO₃ and then 1 time with an equal volume of a solution of 0.25 M HNO₃ in 0.04 M HF. The mother liquor and washings were discarded. Then, the re-extraction of uranium isotopes was carried out by washing the organic phase for 1 min 3 times with an equal volume of distilled water. The combined aqueous re-extract was evaporated to dryness, treated with 5 cm³ of concentrated HNO₃ to remove traces of organic substances and evaporated to dryness again. The dry residue containing uranium isotopes was dissolved in 10 cm³ of a 2% soda solution with heating, filtered through a “blue ribbon” filter and transferred to an electrolytic cell. The filter and beaker were washed with 5 cm³ of electrolyte and added to the main solution. The electrodeposition of uranium isotopes was carried out on a stainless steel substrate for 30 min at a constant current of 2 A. The substrate (disk) was cleaned with fine sandpaper and wiped with acetone immediately before use. After electrolysis, the disk was washed with distilled water and dried in air. The resulting preparation (counting sample) was measured on an alpha spectrometer with an uncertainty of 10–15%. The total uncertainty of the analysis was defined as st + sys (statistical + systematic), the errors of U measurement are given individually, and the extraction deficit of ²³²U was 40–50%. The conversion of ²³⁸U activity concentrations into U concentrations was carried out by using a calculation method.

Gamma spectrometric measurements were carried out as described previously [28] (see Supplementary Materials, Text S1). The radionuclides ¹³⁷Cs, ²²⁶Ra, ²³²Th and ⁴⁰K were determined on a low-background semiconductor gamma-spectrometer (ORTEC, Perkin Elmer, Inc., Shelton, CT, USA), based on a coaxial germanium detector of high purity (HPGe; GEM10P4-70) with a pulse signal processor (SBS-75) and Gamma-pro software (Baltic Scientific Instruments, Riga, Latvia. https://timet.by/wp-content/uploads/2023/08/analytical-software-gammapro.pdf (accessed on 9 June 2025). The resolution of the gamma-spectrometer along the line of 1.33 MeV (⁶⁰Co) was 1.75 keV, and the relative efficiency was 15%. The calibration and quality control of gamma-spectrometric measurements were carried out by using measurements of volumetric activity—Marinelli vessels of different densities (1 L) (RITVERTS, Russia-Germany). Each reference source of radionuclides had a calibration certificate. The sample acquisition time was from 7000 to 10,000 s.

In addition to determining the activity concentrations of radionuclides, pH and the content of carbonates, organic matter and water-soluble salts were also determined in the bottom sediments, according to the method described in [29] (see Supplementary Materials, Texts S2 and S3). The granulometric composition of the bottom sediments was determined by using an AS 200 Control vibrating sieve machine (Retsch Gmbh Germany, Genprice, San Jose, CA, USA) with a set of sieves from 45 μm to 2 mm.

2.4. Area Measurements of Ambient Dose Equivalent Rate (ADER) of Gamma Radiation and Measurements of Radon Flux Density (RFD) in Soil Air

In August 2024, an expedition was conducted to comprehensively study the radiation background of the area in the immediate vicinity of the gypsum deposit (Figure 2b). The study included the following experiments.

We conducted a gamma survey of the mining allotment area, as well as adjacent areas at a distance of up to 300 m. To determine the ADER [30], a dosimeter–radiometer (MKS-AT1117M) with a BOI information processing unit and a BDKG-11 detection unit (NPUP “ATOMTEKH”, Minsk, Republic of Belarus); ambient dose equivalent rate measurement ranges from 0.01 to 100 μSv h⁻¹; limits of permissible basic relative error ±20%) was used. The gamma dose rate measurements in air were performed at a height of 0.1 m above the ground [31,32]. A lightweight tripod made of aluminum was used to support the dosimeter during the measurements. Due to the low weight of such tripods, their effect on gamma radiation is considered to be insignificant, and the measurement results can be directly used to calculate the effective dose for mining workers [33,34]. A total of 229 ADER measurements were performed (Table S5).

Determination of RFD on the North–South profile. The ADER often correlates with another exposure source, radon. ADER data were used to predict the radon flux from soil [32,35,36,37]. For field measurements of RFD, we used the Measuring Complex for Monitoring Radon, Thoron and Their Daughter Products “Alfarad Plus”—AR (manufacturer: Manufacturing Company “NTM-Zashita”, Russia) with an autonomous air blower, filter-driers and a collection chamber. The measurement range of radon flux density from the soil surface is from 20 to 1000 mBq (m²s)⁻¹. The limit of permissible relative error in measuring the ²²²Rn flux density from the soil surface is no more than ±30%. A total of 14 radon flux density measurements in soil air were performed.

3. Results and Discussions

3.1. Water

The obtained results of determining the activities of uranium isotopes and uranium concentration in water are presented in Table 1. The results of the study of the chemical composition of water according to the data [8] are shown in Table S2.

The maximum total alpha activity of uranium isotopes, 0.202 ± 0.030 Bq L⁻¹, was recorded in quarry waters (sample W-11) (Table 1). It is higher than the maximum permissible concentration (MAC) for drinking water [38]: 0.2 Bq L⁻¹. The alpha activity of water in the quarry water settling pond (Table 1) and in the lower reaches of the Chuga River (Figure 3a) (samples W-12 and W-4) was at the level of 0.05–0.06 Bq L⁻¹. The activity of 0.04 Bq L⁻¹ was established in the groundwater of the spring (Table 1) (sample W-9) and in the lower reaches of the Pozera River (Figure 3a) (sample W-13).

The minimum total alpha activity of uranium isotopes of 0.4–11 mBq L⁻¹ was recorded in three lakes and in the upper reaches of the Pozera River (samples W-5, W-3, W-1 and W-7). In the middle reaches of the Pozera River, the upper reaches of the Chuga River and two lakes, the activity concentration of uranium isotopes was 16–28 mBq L⁻¹ (samples W-14, W-6, W-8, W-10 and W-2).

Elevated ²³⁴U/²³⁸U activity ratios of 1.3–1.8 were prevalent in the river (samples W-14, W-2, W-4 and W-8) (Figure 3b) and lake (samples W-3, W-5 and W-10) waters (Table 1). Decreased uranium isotope ratios of 1.0–1.2 were characteristic of spring groundwater and quarry waters (samples W-9, W-11 and W-12). They were also found in two river (Figure 3b) (W-1 and W-13) and two lake (Table 1) (W-6 and W-7) water samples.

The range of uranium precipitation from infiltration waters can be constructed by using the equations Eh_max = 0.40–0.06 pH and Eh_min = 0.40–0.08 pH, obtained by comparing the data of detailed hydrogeochemical sampling and analysis with the theoretically expected equilibria. The intervals of the onset of uranium precipitation are expected at the following extreme pH values: at pH 6.5 Eh from +20 to −150 mV and at pH 8.5 Eh from −90 to −200 mV [39]. According to Table S2 and Figure 4a, almost all analyzed samples were above these ranges. The only exceptions were samples W-1 and W-5 with minimum Eh values of −40 and −52 mV, pH values of 6.59 and 7.01 and, accordingly, minimum uranium concentrations of 0.02 and 0.25 μg L⁻¹. There is no clear correlation between the uranium concentration and Eh; however, if we exclude the samples least affected by hydrochemical processes with Ca-HCO₃-SO₄ composition with minimum TDS values at the level of 0.1 g L⁻¹ and leave only samples with Ca-SO₄ composition with TDS values of 0.5–2.3 g L⁻¹, then there is a linear correlation with the determination coefficient R² = 0.57 (Figure S1).

The isotopic composition of uranium in water reflects the balance between (i) the α-recoil effect with the transition of nonequilibrium ²³⁴U into water from rocks [40,41,42,43,44] and (ii) the rate of rock dissolution with the transition of both isotopes into water in an equilibrium ratio. The lower the rate of rock dissolution, the less equilibrium uranium will pass into water. This was well demonstrated by Andrews et al. (1982) [45] by using a granite massif as an example. The intense fracturing of insoluble rocks contributed to the creation of very high ²³⁴U/²³⁸U ratios in water due to the alpha-recoil effect. Therefore, the lower the ²³⁴U/²³⁸U ratio in water, the higher the rate of rock dissolution. This pattern can explain the lower values of the uranium isotope activity ratio in quarry waters, since the crushing of rocks during mineral extraction occurs most intensively and contributes to their increased dissolution. The uranium isotope ratio is virtually independent of Eh in the oxidizing environment of the aquifer [46,47] (Figure 4b).

The graphs in Figure 4c–f indicate a correlation of uranium concentrations with TDS, calcium, sulfate ion and strontium (R² = 0.50, 0.48, 0.46 and 0.36, respectively). The correlation of calcium and sulfate ion with TDS is in turn explained by their content in the chemical composition of water at the levels of 90 and 80 mg-eq %, respectively. Strontium is approximately uniformly distributed in gypsum and passes into water synchronously with calcium and sulfate ion.

The maximum uranium concentration (7.34 ± 1.10 μg L⁻¹) was found in the quarry waters sampled in August 2024. These waters were also characterized by the maximum TDS value of 2.6 g L⁻¹ (see Table S2). In the spring groundwater, in the quarry water settling basin and in the Chuga River (samples W-9, W-12 and W-4), the TDS values were 0.3 g L⁻¹ lower. These waters also had the highest uranium concentrations (1.35–1.96 μg L⁻¹). All these waters had a calcium sulfate composition and were formed by dissolving gypsum to complete saturation [48]. A distinctive feature of the quarry waters is their enrichment with 0.3 g L⁻¹ in Na₂SO₄ and NaCl salts, apparently of anthropogenic origin (see Table S2).

In fresh groundwater circulating in sulfate and carbonate rocks of the North Dvina artesian basin (Mezen syneclise), uranium concentrations above 2 μg L⁻¹ have not been detected [49]. Average concentrations of 6.6 μg L⁻¹ with maximum concentrations of up to 18.5 μg L⁻¹ have been established in the aquifer complex of the Padunskaya suite of the Vendian, composed of red-colored ferruginous siltstones and sandstones [50]. Therefore, the most likely source of high uranium contents in quarry waters may be overburden rocks, represented by reddish-brown moraine loams with gravel and pebbles of igneous rocks carried out during the last glacial period from the Baltic (Fennoscandian) shield. They are stored in dumps (see Figure 1) and are subject to weathering with the removal of soluble compounds, including uranium sorbed by iron hydroxides and organic matter (see Section 3.3.1). Peatlands, widespread in the study area, can also be enriched in uranium. Even the so-called bog uranium ores are known and are found in areas with a humid climate due to the transfer of uranium by soil and groundwater and sedimentation on natural organogenic reducing barriers [51,52].

In January and February 2025, the uranium concentrations in quarry waters consistently decreased to 5.52 ± 0.82 and 5.30 ± 0.80 μg L⁻¹, while the ratio of uranium isotope activities increased slightly (samples W-11-1 and W-11-2 in Table 1). This indicates a decrease in the intensity of overburden dissolution processes in winter.

Minimum uranium concentrations (0.02–0.25 μg L⁻¹) were observed in two lakes and in the upper reaches of the Pozera River (samples W-3, W-5 and W-1). In these reservoirs, groundwater recharge was minimal or absent altogether (total mineralization of 0.1–0.8 g L⁻¹). In the remaining water samples, a variable ratio of groundwater and atmospheric components was observed, and accordingly, uranium concentrations had intermediate values.

3.2. Bottom Sediments

3.2.1. Activity Concentrations of Radionuclides in River Sediments

Table 2. Specific activity of radionuclides in bottom sediments (BS).

ID	137Cs	226Ra	232Th	40K	238U	234U	234U/238U
	(Bq kg−1)						(Bq Bq−1)
BS-1	4.50 ± 0.88	20.9 ± 2.7	13.2 ± 2.3	373 ± 43	9.93 ± 1.49	11.1 ± 1.6	1.11 ± 0.17
BS-2	3.52 ± 0.62	16.6 ± 2.2	11.3 ± 1.9	350 ± 38	7.52 ± 1.13	6.51 ± 0.97	0.87 ± 0.13
BS-3	6.24 ± 1.25	11.2 ± 2.1	3.33 ± 0.51	338 ± 39	3.13 ± 0.47	2.97 ± 0.44	0.95 ± 0.14
BS-4	1.61 ± 0.57	10.8 ± 1.4	4.67 ± 1.15	251 ± 26	3.87 ± 0.58	4.28 ± 0.64	1.11 ± 0.17
BS-5	6.12 ± 1.12	17.5 ± 2.2	19.9 ± 2.3	407 ± 42	17.6 ± 2.64	17.5 ± 2.6	0.99 ± 0.15
BS-6	20.8 ± 3.0	23.5 ± 3.6	11.9 ± 2.0	302 ± 41	73 ± 11	89.5 ± 13.4	1.22 ± 0.18
BS-7	10.2 ± 2.1	13.3 ± 2.5	8.67 ± 1.71	208 ± 31	62.6 ± 9.4	54.8 ± 8.2	0.87 ± 0.13
BS-8	1.33 ± 0.30	20.6 ± 2.8	12.1 ± 2.2	380 ± 38	4.38 ± 0.66	3.63 ± 0.54	0.83 ± 0.12
BS-10	10.3 ± 2.5	27.0 ± 4.1	5.12 ± 0.92	199 ± 28	12.5 ± 1.9	9.24 ± 1.38	0.74 ± 0.11
BS-13	1.52 ± 0.51	18.1 ± 2.5	5.93 ± 1.74	396 ± 46	5.68 ± 0.85	5.07 ± 0.76	0.89 ± 0.13
Mean	6.61 ± 1.29	18.0 ± 2.6	9.61 ± 1.68	320 ± 37	20.0 ± 3.0	20.5 ± 3.1	0.96 ± 0.14

shows the results of determining the activities of radionuclides in bottom sediments. The results of the study of the physical and chemical parameters of the bottom sediments are given in

Table 3. Physical and chemical parameters of bottom sediments.

ID	Particle size distribution, %					LOI (%)	CO32− (%)	OM (%)	pH	Eh, (mv)	SS (mg g−1)
	>500 µ	500 µ	250 µ	100 µ	<45 µ
BS-1	4.70	16.3	46.5	21.4	11.1	0.46	0.63	6.47	5.90	118	0.86
BS-2	6.48	16.5	57.9	13.6	5.52	0.39	0.53	5.37	6.63	83	0.65
BS-3	28.2	44.6	22.6	4.04	0.56	0.10	0.14	0.39	5.92	127	0.16
BS-4	42.5	46.5	7.80	2.29	0.91	0.83	1.13	0.74	6.58	96	0.39
BS-5	14.4	21.3	37.7	15.6	11.0	0.96	1.31	6.05	6.19	85	1.14
BS-6	64.9	15.8	10.9	4.90	3.50	1.77	2.41	45.6	6.65	67	3.30
BS-7	11.5	24.1	30.8	17.8	15.8	1.18	1.61	44.5	5.95	116	3.52
BS-8	10.3	36.5	46.6	5.00	1.60	0.97	1.32	3.72	6.97	73	0.92
BS-10	20.2	14.5	33.9	24.2	7.20	0.33	0.45	38.2	5.54	132	0.63
BS-13	7.00	34.0	43.6	11.0	4.40	0.44	0.60	5.83	6.50	63	0.79
Mean	21.0	27.0	33.8	12.0	6.20	0.70	1.00	15.7	6.30	96	1.20

. Table S1 contains descriptive statistics of the considered parameters of bottom sediments.

The maximum ¹³⁷Cs activity concentration, from 10.2 ± 2.1 to 20.8 ± 3.0 Bq kg⁻¹, was found in three lakes (BS-6, BS-7 and BS-10) (Figure 5c). These lakes also had the maximum organic matter content, from 38.25 to 45.61% (Figure 5a). Two of these lakes (BS-6 and BS-7) also had the maximum CO₃²⁻ (1.61 and 2.41%) and soluble salts (3.30 and 3.52 mg g⁻¹) contents (Figure 5b). In addition, the maximum contents of dusty and fine-grained particles (15.8 and 17.8%, respectively) were recorded in the lake from which the BS-7 sample was taken (Table 3).

The maximum activity concentrations of ²³⁸U (62.6 ± 9.4 and 73 ± 11 Bq kg⁻¹), which correspond to uranium contents of 4.3–6.8 mg kg⁻¹, were found in two of these lakes (BS-6 and BS-7) (Figure 5e). Also, in two lakes (BS-6 and BS-10), the maximum activities of ²²⁶Ra (23.5 ± 3.6 and 27.0 ± 4.1 Bq kg⁻¹) were recorded. In the BS-6 sample, the maximum uranium isotope ratio of 1.22 ± 0.18 was established (Figure 5f). The maximum activities of ²³²Th (19.9 ± 2.3 Bq kg⁻¹) and ⁴⁰K (407 ± 42 Bq kg⁻¹) were determined in the BS-5 sample (Figure 5c,d), for the composition of which no features were found in the distribution of physical and chemical parameters (Table 3).

Overall, the average values of ²²⁶Ra, ²³²Th and ⁴⁰K activities were 18.0, 9.6 and 320 Bq kg⁻¹, respectively, which are lower than the world average values (33.0, 45.0 and 420 Bq kg⁻¹, respectively) [53,54,55].

3.2.2. The Ratio of Radionuclide Activity, the Concentrations of Chemical Elements and the Main Parameters Characterizing Bottom Sediments

Table 4. Correlation matrix of radionuclide activities, concentrations of chemical elements and main parameters characterizing bottom sediments.

	226Ra	232Th	40K	238U	234U	100 e	45 e	<45 e	CO32−	OM c	SS d	Na	K		Ca
137Cs	0.42	0.10	−0.42	0.86 a	0.89 a	−0.48	−0.18	0.19	0.61	0.85a	0.73 b	0.56	0.62		0.77 a
226Ra		0.27	−0.02	0.17	0.22	0.24	0.47	0.11	0.16	0.45	0.14	0.21	0.06		0.32
232Th			0.52	0.20	0.22	0.37	0.23	0.44	0.43	−0.04	0.26	0.34	0.48		0.18
40K				−0.40	−0.32	0.50	−0.21	−0.18	−0.13	−0.68 b	−0.34	−0.16	−0.19		−0.30
238U					0.98 a	−0.41	−0.04	0.41	0.81 a	0.85 a	0.98 a	0.82 a	0.91 a		0.88 a
234U						−0.45	−0.06	0.28	0.84 a	0.80 a	0.93 a	0.81 a	0.87 a		0.91 a
	Mg	Sr	Fe	Mn	Zn	Cu	Cr	Pb	Ni	Mo	Cd	Co		Ti	V
137Cs	0.34	−0.01	0.33	−0.43	0.55	0.92 a	0.74 b	0.95 a	0.72 b	0.49	0.61	0.09		0.94 a	0.80 a
226Ra	0.24	0.19	0.05	0.24	0.17	0.36	0.19	0.54	0.21	−0.30	0.22	0.05		0.27	0.29
232Th	0.42	0.21	0.5	0.24	0.38	0.18	0.37	0.15	0.43	0.08	0.43	0.58		0.19	0.42
40K	−0.05	0.10	−0.06	0.51	−0.28	−0.30	−0.32	−0.48	−0.31	−0.54	−0.24	0.27		−0.39	−0.27
238U	0.54	0.18	0.67 b	−0.21	0.82 a	0.91 a	0.96 a	0.90 a	0.94 a	0.71 b	0.88 a	0.32		0.96 a	0.95 a
234U	0.58	0.20	0.60	−0.17	0.75 b	0.97 a	0.91 a	0.90 a	0.89 a	0.65 b	0.84 a	0.28		0.95 a	0.94 a

^a Significance value p ≤ 0.01; ^b significance value p ≤ 0.05; ^c organic matter; ^d soluble salts; ^e particle size distribution: <45, 45–100 and 100–250 μ.

shows the correlation matrix of radionuclide activities, the concentrations of chemical elements and the main parameters characterizing the bottom sediments. A more complete correlation matrix is provided in the Supplementary Materials (Table S4). The distribution of chemical elements in the bottom sediments of rivers and lakes in the area of the Glubokoye gypsum deposit is presented in Table S3.

A significant correlation is observed between the activity concentrations of cesium and uranium isotopes and the contents of organic matter, soluble salts and carbonates in the bottom sediments. ¹³⁷Cs, ²³⁸U, ²³⁴U and OM correlate with each other with p = 0.001–0.002; for ¹³⁷Cs and SS, p = 0.016 was obtained and for ¹³⁷Cs and CO₃²⁻ p = 0.06. As shown in Section 3.1, oxidation–reduction and acid–base conditions do not prevent uranium from being found in the surface waters of the region mainly as part of uranyl–carbonate complexes. UO₂(CO₃)₂²⁻ accounts for about 55–57% and UO₂(CO₃)₃⁴⁻ for 12–27%. Uranium precipitates only in the reducing environment of groundwater circulating relatively deep below the earth’s surface [56]. Therefore, the accumulation of isotopes in the bottom sediments apparently occurred through sorption (see Figure 5a,b,e).

A significant correlation was also established among ¹³⁷Cs, ²³⁸U, ²³⁴U, Ca, Cu, Cr, Pb, Ni, Ti and V. The significance value for isotopes and these chemical elements was p ≤ 0.01 (see Table 4). For uranium isotopes, there was also a significant correlation with Na, K, Fe, Zn, Mo and Cd. For ¹³⁷Cs, it is weaker (on average, p = 0.15, r = 53). A specific set of elements and the correlation with the contents of organic matter, soluble salts and carbonates confirm the conclusion about the significant role of the anthropogenic factor in the accumulation of cesium and uranium isotopes in the bottom sediments [8].

The contamination of bottom sediments with ¹³⁷Cs is a result of the Chernobyl accident in 1986, when not only soils and bottom sediments but also vegetation were contaminated over large areas of Europe [20,57,58,59,60]. Uranium isotopes are mainly of natural origin, as they are ubiquitous in near-surface sediments, soils, surface and groundwater (see Section 3.1 and Section 3.3). The anthropogenic contribution is mainly associated with the disintegration of rocks and soil cover during mining operations (see Figure 1) and the subsequent transfer of dissolved uranium to bottom sediments.

The isotopes ²³²Th and ⁴⁰K weakly correlate with each other (r = 0.52, p = 0.16). The correlation of ²³²Th and potassium is approximately of the same order (r = 0.48, p = 0.12). In addition, a connection is observed between ⁴⁰K and the fraction of the grain size distribution 45–100 µ (r = 0.50, p = 0.14). This may reflect the lithogenic nature of these isotopes. Potassium is one of the main rock-forming elements, second in average (clarke) content in clayey deposits (2.7%) only to silicon, aluminum and iron [61]. In carbonate minerals, its content is ten times lower. However, the isotopic content of ⁴⁰K is very low and amounts to 0.0117% [61,62]. That is, the content of ⁴⁰K in clays and carbonates is about 3.16 and 0.316 mg kg⁻¹, respectively. The average concentrations of thorium in clays and carbonates are 12 and 1.7 mg kg⁻¹, respectively [61], with an isotopic content of ²³²Th of 99.98% [62,63]. In dissolved form, the ²³²Th concentrations in surface waters in the study area did not exceed 0.02 μg L⁻¹. The average concentration of potassium was 1.2 mg L⁻¹ (Table S1); that is, the concentrations of ⁴⁰K, taking into account the isotopic content of 0.0117%, were about 0.14 μg L⁻¹. This may indicate the transport and precipitation of these isotopes mainly as part of mechanical suspensions.

For radium-226, only a weak correlation can be noted with Pb (r = 0.54, p = 0.11), the ≤45 μ fraction (r = 0.47, p = 0.17), OM (r = 0.45, p = 0.19) and ¹³⁷Cs (r = 0.42, p = 0.23). This indicates the isolated nature of ²²⁶Ra. It is a consequence of the radioactive decay of uranium isotopes, but its relationship with uranium isotopes and chemical elements correlating with uranium is weak. This is explained by the low migration capacity of radium compared with uranium. Being an analogue of alkaline earth elements, especially calcium and barium [48], it actively precipitates as RaSO₄ in the sulfate-rich environment of gypsum deposits. This is also indicated by the weak correlation with Pb, fraction ≤ 45 μ, OM and ¹³⁷Cs.

3.3. Soils, Quaternary Deposits and Gypsum

3.3.1. Activity Concentrations of Radionuclides in Soils, Overburden Deposits and Gypsum

Table 5. Specific activity of radionuclides in soils (S), sandy–clayey deposits (SC) and gypsum (G).

ID	137Cs	226Ra	232Th	40K	238U	234U	234U/238U
	(Bq kg−1)						(Bq Bq−1)
S-1	39.4 ± 5.8	26.1 ± 3.8	14.4 ± 2.4	216 ± 33	2.49 ± 0.36	1.74 ± 0.27	0.70 ± 0.11
S-2	45.3 ± 6.8	39.9 ± 5.8	22.3 ± 3.4	256 ± 39	3.58 ± 0.57	3.79 ± 0.56	1.06 ± 0.16
S-3	9.14 ± 2.07	12.5 ± 1.9	12.6 ± 2.0	118 ± 18	6.29 ± 0.91	6.71 ± 1.02	1.07 ± 0.15
S-4	30.9 ± 4.5	39.5 ± 5.8	29.0 ± 4.4	196 ± 30	4.33 ± 0.62	4.41 ± 0.64	1.02 ± 0.15
S-5	48.2 ± 7.3	12.1 ± 2.0	11.4 ± 1.8	187 ± 28	1.47 ± 0.28	1.32 ± 0.20	0.90 ± 0.14
S-6	24.5 ± 3.9	29.2 ± 4.3	27.1 ± 4.2	111 ± 17	9.39 ± 1.43	9.98 ± 1.51	1.06 ± 0.15
S-7	23.3 ± 3.6	37.9 ± 5.7	23.3 ± 3.5	251 ± 38	1.22 ± 0.19	1.19 ± 0.18	0.98 ± 0.15
S-8	31.3 ± 4.8	18.7 ± 2.8	22.1 ± 3.3	262 ± 40	10.2 ± 1.5	9.49 ± 1.41	0.93 ± 0.15
S-10	28.8 ± 4.8	12.5 ± 2.0	8.16 ± 2.5	127 ± 20	2.42 ± 0.38	2.30 ± 0.31	0.95 ± 0.15
S-11	33.1 ± 4.8	26.2 ± 4.0	21.4 ± 3.3	496 ± 73	10.9 ± 1.5	10.1 ± 1.5	0.93 ± 0.15
S-12	32.7 ± 5.0	33.5 ± 5.1	31.2 ± 4.7	352 ± 51	10.5 ± 1.5	10.4 ± 1.5	1.00 ± 0.15
S-13	34.4 ± 5.4	37.3 ± 5.7	23.1 ± 3.5	166 ± 25	6.63 ± 0.97	5.86 ± 0.86	0.88 ± 0.13
S-14	36.8 ± 5.6	18.1 ± 3.0	26.2 ± 3.9	223 ± 34	6.89 ± 0.96	8.47 ± 1.24	1.23 ± 0.18
S-15	35.1 ± 5.3	31.0 ± 4.6	21.5 ± 3.2	346 ± 52	8.02 ± 1.23	5.56 ± 0.83	0.69 ± 0.10
Mean	32.4 ± 5.0	26.8 ± 4.0	21.0 ± 3.3	236 ± 36	6.02 ± 0.88	5.81 ± 0.85	0.96 ± 0.14
SC-1	nd 1	24.6 ± 3.8	13.6 ± 3.0	318 ± 62	10.9 ± 1.6	8.37 ± 1.26	0.77 ± 0.12
SC-2	nd	18.8 ± 2.6	20.5 ± 3.8	379 ± 72	12.2 ± 1.8	8.81 ± 1.32	0.72 ± 0.11
SC-3	nd	18.8 ± 2.6	16.1 ± 3.4	367 ± 70	12.3 ± 1.8	8.79 ± 1.32	0.72 ± 0.11
Mean	nd	20.7 ± 3.0	16.7 ± 3.4	355 ± 68	11.8 ± 1.8	8.66 ± 1.30	0.74 ± 0.11
G-1	nd	4.02 ± 1.12	nd	nd	2.78 ± 0.42	1.98 ± 0.30	0.71 ± 0.11
G-2	nd	2.81 ± 0.94	nd	nd	0.99 ± 0.15	1.14 ± 0.17	1.15 ± 0.17
G-3	nd	4.24 ± 1.18	nd	nd	2.11 ± 0.32	1.44 ± 0.22	0.94 ± 0.14
Mean	nd	3.69 ± 1.08	nd	nd	1.96 ± 0.29	1.52 ± 0.23	0.93 ± 0.14

¹ nd—not determined (concentration below detection limit).

shows the results of determining the activities of radionuclides in soils, overburden deposits and gypsum.

The ¹³⁷Cs activity in the litter–peat soil horizon “O” averaged 32.4 ± 5.0 Bq kg⁻¹. This is five times higher than in the bottom sediments. However, it should be noted that in the three lakes with the maximum organic matter content, the ¹³⁷Cs activity ranged from 10.2 ± 2.1 to 20.8 ± 3.0 Bq kg⁻¹ (see Section 3.2.1). Earlier, in the eutrophic peat horizon “TE” 150 km northwest of the study area, average ¹³⁷Cs activity values of 50.9 Bq kg⁻¹ were established [20]. The background activity of ¹³⁷Cs at the level of 50 Bq kg⁻¹ was determined for peat soils in the northwest of the Kola Peninsula [64]. The higher activity concentration of ¹³⁷Cs is likely due to the presence of clay, as ¹³⁷Cs has a strong affinity for clay and organic matter. In moraine loams and gypsum, the activity of ¹³⁷Cs was below the detection limit.

The average activities of the uranium isotopes ²³⁸U (6.02 ± 0.88 Bq kg⁻¹) and ²³⁴U (5.81 ± 0.85 Bq kg⁻¹) in soil were three times lower than in bottom sediments. They corresponded to a uranium concentration of 0.48 ± 0.07 mg kg⁻¹. At the same time, the average values of the ²³⁴U/²³⁸U activity ratio in soil and bottom sediments were the same and amounted to 0.96 ± 0.14. This corresponds to the conditions of secular equilibrium associated with the formation of the mineral part of soils and bottom sediments due to rock dissolution and the absence of geologically long-term contacts with moving water [65]. In moraine loams, the uranium content was twice as high as in soils (0.95 ± 0.14 mg kg⁻¹), while in gypsum, it was minimal, amounting to 0.16 ± 0.02 mg kg⁻¹.

The activity of ²³²Th in moraine loams was 1.3 times lower than in soils (16.7 ± 3.4 Bq kg⁻¹) and 1.7 times higher than in bottom sediments. The activity of ⁴⁰K in moraine loams was almost the same as in bottom sediments (355 ± 68 Bq kg⁻¹), while in soils, it was 1.5 times lower. In gypsum, the activities of these isotopes were below the detection limit. The highest activities of the ²²⁶Ra isotope were in soils, from 12.1 ± 2.0 to 39.9 ± 5.8 Bq kg⁻¹, with an average of 26.8 ± 4.0 Bq kg⁻¹. In moraine loams, they were comparable to the activities in bottom sediments (20.7 ± 3.0 Bq kg⁻¹), and in gypsum, they were minimal, amounting to 3.69 ± 1.08 Bq kg⁻¹.

3.3.2. Ambient Dose Equivalent Rate (ADER) of Gamma Radiation

The range of variation in the ambient dose equivalent rate (ADER) of gamma radiation in the quarry and adjacent areas was from 11 to 69 nSv h⁻¹, with an average value of 38 nSv h⁻¹. Based on the obtained data (Table S5), the areal distribution of the ADER was constructed, as shown in Figure 6a.

The results of the study showed that the lowest ADER values are observed in the areas of direct exposure of the gypsum formation surface, as well as in the areas of the sorting and storage of mined gypsum rock (see the white color in Figure 2b). This is due to the low radioactivity of gypsum [65,66] (Table 5). Higher values are confined to the locations of overburden dumps, which are represented by a mixture of clayey sediments with gypsum fragments (see Figure 1). Sandy–clayey sediments contain significant amounts of radionuclides [66,67] (Table 5); therefore, the ADER intensity is directly related to the relative content of sand and clay in this mixture. That is, the redistribution and artificial concentration of natural radionuclides in overburden dumps is a direct consequence of mining operations.

In accordance with [68], the effective dose of radiation from natural radiation sources for all workers, including personnel, should not exceed 5 mSv yr⁻¹ under production conditions (any professions and industries). The maximum ADER values obtained in this study are significantly lower than the established level of effective dose for quarry workers. When converted to an 8 h working day with a total number of working days per year of approximately 250, they amount to 0.14 mSv yr⁻¹. At the same time, it should be noted that the Japanese government established a maximum permissible level (MPL) for the public of 1 mSv yr⁻¹ as an acceptable level of airborne exposure, which is calculated as 0.23 μSv h⁻¹ (0.19 μSv h⁻¹ from the Fukushima Dai-ichi Nuclear Power Plant accident and an average of 0.04 μSv h⁻¹ background radiation in Japan) [69,70].

In general, it is worth noting that the obtained ADER values do not exceed the background values typical for the region. They also comply with the requirements of SP 2.6.1.2612-10 [71], according to which when selecting areas for the construction of residential and public buildings, areas with an equivalent dose rate of gamma radiation of less than 0.3 μSv h⁻¹ should be selected.

3.3.3. Radon Flux Density (RFD) from Soil Air

The studies showed that the RFD varied within the range of 6 ± 1 to 181 ± 54 mBq (m²s)⁻¹ (

Table 6. Results of radon flux density measurement, mBq (m²s)⁻¹.

ID	Location		Height m.a.s.l.	RFD	Lithological Composition
	N	E			Natural Conditions	Quarry Area
R-1	64.10660	42.64184	71.2	100 ± 30	Soil and loam
R-2	64.10746	42.64268	71.5	181 ± 54	Soil and loam
R-3	64.10852	42.64508	75.0	89 ± 26	Soil, loam and sand
R-4	64.11017	42.64747	75.0	20 ± 6		A mixture of sandy–clayey deposits of Quaternary age with fragments of gypsum rocks in waste rock dumps
R-5	64.11200	42.64834	69.2	25 ± 5
R-6	64.11373	42.64879	71.3	22 ± 6
R-7	64.11549	42.64790	59.8	20 ± 6
R-8	64.11712	42.64643	61.6	27 ± 8
R-9	64.11895	42.64646	53.7	61 ± 18
R-10	64.12066	42.64514	62.5	18 ± 5		Gypsum, sand and loam
R-11	64.12237	42.64329	54.3	6 ± 1		Gypsum and loam
R-12	64.12383	42.64226	63.3	10 ± 3		Gypsum and сlay
R-13	64.12585	42.64069	67.5	159 ± 47	Soil and clay
R-14	64.12752	42.64116	65.7	92 ± 28	Soil, loam and sand

). The maximum values, from 89 ± 26 to 181 ± 54 mBq (m²s)⁻¹, were observed outside the mining area and were confined to areas with natural occurrence of Quaternary moraine loams with undisturbed soil cover and forest vegetation. The minimum values, from 6 ± 1 to 18 ± 5 mBq (m²s)⁻¹, were established in the quarry area, in areas where gypsum layers were directly exposed on the surface. Intermediate values, from 20 ± 6 to 61 ± 18 mBq (m²s)⁻¹, were confined to overburden dumps, represented by clayey deposits with gypsum rock fragments (see Figure 1) (Figure 6b).

The results indicate that the gypsum layers in the quarry are a virtually impermeable screen preventing radon from exiting from the earth’s interior [11]. Outside the quarry, the Quaternary moraine deposits are permeable due to their porosity and humidity. In addition, in sedimentary basins, ²²⁶Ra concentrations in near-surface sediments and soils play a major role in radon production. This is a zone of a specific boundary soil layer with a thickness of 1.5–3.0 m, in which active gas exchange with the atmosphere occurs [72]. According to [31], the average RFD values in watersheds composed of clayey soils were higher than in river valleys dominated by sandy deposits. The average ²²⁶Ra activity concentration in loams was 15–25 Bq kg⁻¹ and in sands 10–15 Bq kg⁻¹. We obtained average ²²⁶Ra activity values of 20.7 ± 3.0 Bq kg⁻¹ in loams and 26.8 ± 4.0 Bq kg⁻¹ in soils. In gypsum, ²²⁶Ra activity is minimal, amounting to 3.69 ± 1.08 Bq kg⁻¹ (Table 5).

Thus, the RFD intensity within the quarry, like the ADER, is directly related to the relative contents of clay, loam, sand and gypsum in the overburden dumps and to the exposure of gypsum layers. The potential radon hazard of the territory is assessed in accordance with the Basic Sanitary Rules for Ensuring Radiation Safety [71], which establish the maximum permissible radon flux density (RFD) from the soil surface at construction sites, amounting to 80 mBq (m²s)⁻¹ for residential and public buildings. Table 6 shows that such areas may occur outside the quarry, and this should be taken into account during their development. In addition, caves are widespread in the study area, usually associated with zones of an increased fracturing of rocks, in which advective transfer is possible—a flow of radon in the volume of the moving gas phase [72]. Such transfer is significantly higher than the diffusion transfer of radon from soils in an undisturbed state and requires separate study.

4. Conclusions

The objective of the conducted research was to assess the impact of gypsum deposit development on changes in the radiation levels of the abiotic components of the environment. It was found that U concentrations in ground and surface waters did not exceed 2 μg L⁻¹. In quarry waters, its concentration reached 8.5 μg L⁻¹, and the total alpha activity exceeded the MAC for drinking water (0.2 Bq L⁻¹). The most probable source of high U content may be overburden rocks stored in dumps. During weathering, uranium sorbed by iron hydroxides and organic matter is removed from them. Relatively low ratios of uranium isotope activities in quarry waters are due to the crushing of rocks during mining operations, which increases the rate of their dissolution and the transition of both isotopes into water in an equilibrium ratio.

In bottom sediments, the maximum activity concentrations of ¹³⁷Cs, ²³⁸U, ²³⁴U and ²²⁶Ra were found in lakes with the maximum contents of CO₃²⁻, OM and SS, as well as dusty and fine-grained particles. Uranium concentrations reached 6.8 mg kg⁻¹. The maximum activities of ²³²Th were determined in samples for which no features in the distribution of physical and chemical parameters were revealed. In general, the average values of ²²⁶Ra, ²³²Th and ⁴⁰K were 18.0, 9.6 and 320 Bq kg⁻¹, respectively, which are below the world averages. Significant correlations of ¹³⁷Cs, ²³⁸U and ²³⁴U activities with CO₃²⁻, OM and SS contents, as well as Fe, Zn, Cu, Cr, Pb, Ni, Mo, Cd, Co, Ti and V, indicate a significant role of the anthropogenic factor in their accumulation in bottom sediments. This factor is associated with both regional atmospheric transport (¹³⁷Cs) and the activity of the mining enterprise in the study area (²³⁸U and ²³⁴U).

The isotopes ²³²Th and ⁴⁰K are weakly correlated with each other. The correlation of ²³²Th and potassium is approximately of the same order. In addition, a relationship is observed between ⁴⁰K and the content of dusty particles. This may reflect the lithogenic nature of these isotopes and indicate their transport and sedimentation mainly in mechanical suspensions. Radium-226 is a consequence of the radioactive decay of uranium isotopes, but its relationship with uranium isotopes and chemical elements that correlate with uranium is weak. This is explained by the low migration capacity of radium compared with uranium in the sulfate-rich environment of gypsum deposits.

The highest values of the ambient dose equivalent rate are associated with the locations of waste rock dumps, represented by a mixture of sandy–clayey deposits with gypsum fragments. Clayey deposits and soil contain a significant amount of radionuclides; therefore, the ADER intensity is directly related to the relative clay content in this mixture. However, these ADER values are significantly lower than the established effective dose level for quarry workers of 5 mSv yr⁻¹ and do not exceed 0.14 mSv yr⁻¹. They also comply with the requirements for selecting construction sites (≤0.3 μSv h⁻¹). The gypsum layers brought to the surface by mining operations in the quarry are a virtually impermeable screen that prevents radon from the earth’s interior. Outside the quarry, the radon flux density in some areas does not comply with the requirements for selecting construction sites, ≤80 mBq (m²s)⁻¹. It reaches a value of 181 ± 54 mBq (m²s)⁻¹ due to the fact that the soils have increased permeability and ²²⁶Ra activity at a level of 26.8 ± 4.04 Bq kg⁻¹. In the waste rock dumps in the quarry area, RFD, like the ADER, is directly related to the relative content of clay, loam, sand and gravel and has intermediate values. It is also necessary to pay special attention to the fact that caves are widespread in the study area. They are usually associated with zones of increased rock fracturing in which advective transfer is possible—a flow of radon in the volume of the moving gas phase. Such transfer is significantly higher than the diffusion transfer of radon from soils in an undisturbed state and requires separate study.