Peculiarities of ²²²Radon and ²³⁸Uranium Behavior in Mineral Waters of Highland Terrains

Abstract

Mineral waters from two tectonically active mountain systems within the Alpine-Himalayan orogenic belt, the Pamir and the Greater Caucasus (Elbrus region), were analyzed for ²²²Rn activity and ²³⁸U concentrations to establish correlations with geological conditions, physicochemical characteristics of water, and to assess the potential health risk associated with ²³⁸U and ²²²Rn. It was found that in mineral waters of the Pamir, the concentrations of ²³⁸U (0.004–13.3 µg/L) and activity of ²²²Rn (8–130 Bq/L) are higher than in the Elbrus area: 0.04–3.74 µg/L and 6–33 Bq/L, respectively. Results indicate that uranium mobility in water is strongly influenced by T, pH, and Eh, but is less affected by the age of host rocks or springs′ elevation, whereas radon activity in waters depends on the age of rocks, spring elevation, ²³⁸U content, and values of δ¹⁸O and δ²H in water. This study reveals fundamental geological distinctions governing uranium and radon sources in the mineral waters of these regions. Isotopic evidence (²²²Rn and ³He/⁴He) demonstrates crustal radon sources prevail in Pamir, whereas the Elbrus system suggests mantle-derived components. The U concentrations do not exceed 30 µg/L, and most water samples (94%) showed ²²²Rn activities below 100 Bq/L, complying with the drinking water exposure limits recommended by the World Health Organization and European Union Directive. However, in intermountain depressions of the Pamirs, at low absolute elevations (~2300 m), radon concentrations in water can increase significantly, which requires special attention and study.

1. Introduction

Mineral waters originate from water–rock–gas interactions and serve as a valuable balneological resource, attracting significant research interest (e.g., [1,2,3,4]). During geological evolution, mineral waters may become enriched with various chemical elements and radionuclides, which can present health risks if consumed without regulation (e.g., [5,6,7]). The content of natural radionuclides in mineral waters can vary within several orders of magnitude and depends on the physical, chemical, and geological properties of the aquifer (e.g., [8,9,10]). Since the mid-1900s, numerous studies have explored the relationship between radionuclides in water and geological factors (e.g., [10,11,12,13,14,15]). As a result, radioactive isotopes are now widely applied in geology for tracking uranium deposits (using ⁴He, ²²²Rn), assessing degassing depth in hydrothermal systems (²²²Rn), predicting earthquakes and volcanic activity (²²²Rn, ²²⁶Ra, ⁴He, ⁴⁰Ar), studying groundwater dynamics in oil provinces (³H), monitoring soil gases induced by hydraulic fracturing (²²²Rn), and many other applications.

The radionuclides uranium-238 and radon-222 share the same geologic source because radon-222 is a daughter isotope in the uranium-238 decay series (e.g., [16]). In groundwater, radon activity typically correlates with the local abundance of uranium/radium in rocks and soil (e.g., [17,18]). Groundwater in granitic massifs exhibits elevated radon activity and is widely used in balneotherapy (e.g., in Georgia [19], Romania [20], Poland [21], Russia [22], Kyrgyzstan [23], Tajikistan [24], and others). Most such sites are located at the foothills of mountain systems, within an elevation range of 100–2000 m above sea level (asl). Two of the most prominent radon therapy destinations in the Tien Shan highlands are Dzhete-Oguz (Kyrgyzstan, 2000 m asl) [23] and Khodzha-Obi-Garm (Tajikistan, 1960 m asl) [24]. Data on radionuclide distribution in mineral water above 2000 m elevation remain scarce (e.g., [25]).

The population in the high-altitude regions of the Pamirs (2000–7500 m) and the Elbrus area (1200–5000 m) is relatively small; approximately 36,000 people reside in the Elbrus region and about 250,000 in the Pamirs. However, these regions hold significant tourism potential, partly due to the abundance of natural mineral springs [26].

Over the past decades, extensive isotopic and geochemical data on groundwater in the Elbrus region have been collected [7,27]. In contrast, research on the mineral waters of the Pamirs was most active in the mid-20th century [28], with limited modern data available (e.g., [26,29]). Radionuclide behavior in these groundwaters and related health risks have never been assessed.

For the first time, this study analyzes the distribution of radon and uranium in mineral springs across two mountain systems within the Alpine-Himalayan orogenic belt—the Pamirs and the Elbrus area (Greater Caucasus), which are situated at elevations ranging from 1300 to 4000 m.

The aim of this work is to assess the level of uranium concentrations and radon activity in mineral waters of the Pamirs and the Elbrus area, to study the relationship with geological and geographical conditions, chemical and isotopic composition of water, and associated gases, and to assess the health risk of drinking mineral waters. A comprehensive understanding of the behavior and concentration of radionuclides in groundwater may be important for the development of effective strategies for the management of mineral water resources.

2. Objects and Methods

2.1. Study Area

During 2024–2025, we investigated CO₂-rich and N₂-rich mineral water manifestations (springs and wells) in two key segments of the Alpine-Himalayan orogenic belt: the Pamir (Tajikistan) and the Greater Caucasus (Elbrus area, Russia) (Figure 1). The geological and tectonic frameworks governing mineral water formation in these regions have been detailed in recent studies [7,26,30]. The geological settings of the Caucasus and Pamir exhibit key differences in their orogenic formation (types of orogenesis). The Caucasus formed under conditions of continental collision, specifically, the collision of the Arabian and Eurasian plates. This process was characterized by horizontal compression, leading to the development of fold-and-thrust structures, as well as volcanism resulting from partial crustal melting in the collision zone. In contrast, the Pamirs’ orogenesis involved both accretion (the incorporation of microcontinents and island arcs) and subsequent collision with Eurasia. While the Northern Pamirs display a collision-type origin similar to the Caucasus, the Central and Southern Pamirs preserve remnants of oceanic crust from the Tethys paleo-ocean. Accretion led to a key difference between the geological structure of the Central and Southern Pamirs and that of the Caucasus: vertically oriented rocks of oceanic origin and deep vertical faults (e.g., [28]). These features create favorable conditions for fluid circulation within the Pamirs and promote the formation of thermal waters. In contrast, natural thermal water discharges are rare within the Caucasus’ mountain structures.

The geological base of both regions is anchored by Precambrian rocks (gneisses, marbles, and migmatites), constituting the most ancient formations and covering significant areal extents. Paleozoic granites, granodiorites, and volcanic rocks are widespread in the central Caucasus and northern Pamir. The Elbrus area of the Greater Caucasus lacks Mesozoic units in its central orogenic structure (Figure 1), with such rocks occurring only as marginal elements, whereas the Pamirs’ southern and central sectors contain voluminous Jurassic-Cretaceous granitoid complexes. While Tertiary eruptives mark the Pamirs’ youngest formations (e.g., [31]), the Greater Caucasus shows recent geological activity through both Quaternary granites and Holocene volcanism (last Elbrus eruption, 1.5–1.7 ka) (e.g., [32]). Mineral springs rarely reach the surface in the mountain regions under study; the majority of the waters were found in the middle of the 20th century as a result of drilling operations (e.g., [27,28]). The physico-chemical composition of waters in the study areas also exhibits some differences (e.g., [7,26]). First of all, is the water temperature (

Table 1. The isotope-geochemical characteristics of mineral waters and associated gases of the studied highland regions. (*-data from [19]; Still—no babble gas). The age abbreviations: KZ—Cenozoic Era, PZ—Paleozoic Era, MZ—Mesozoic Era, PR—Proterozoic Eon. The ‘Host rocks’ refers to permeable geological formations (aquifers) that store and transmit groundwater.

№	Name	Age	Host Rocks Types	Elevation m. a.s.l	Type	TDS, g/L	T, °C	pH	Eh, mV	238U	Th	δ18O	δ2H	222Rn	N2	CH4	CO2	He	δ13C(CO2)	δ13C(CH4)
										µg/L		VSMOW, ‰		Bq/L	%				‰, VPDB	‰, VPDB
		Rock			Water										Associated Gases
Mineral Waters of the Elbrus (№ 1-15) and Pamir (№ 16-34) Areas
1	Tyrnyauz park (well)	KZ	Granites (Eldzurty)	1296	Na-HCO3-Cl	5.4	17	6.7	−71	0.07	0.02	−10.6	−72.1	33	21.4	1.22	76.7	0.032	−11	−61.1
2	Parametric (well)			1320	Na-HCO3	5.7	17	6.4	−50	0.25	0.002	−9.6	−70.5	10	8.6	0.45	90.4	0.001	−7.3	−22.3
3	Shaushiib (spring)		Granodiorite, andesite, dacite	1820	Na-HCO3-Cl	10.6	10	6.4	43	0.46	0.007	−9.6	−54.9	9	0.95	0.011	98.8	0.008	−5.7
4	Toxana (low spring)			2163	Na-HCO3	3.0	5.4	6.2	−48	0.04	0.03	−10.1	−61.7	7	0.45	0.012	98.9	0.001	−5.8	−63.1
5	Ingushli (upper spring)	PZ	Sedimentary and volcanic rocks	2209	Na-HCO3-Cl	1.9	6.4	5.9	54	0.22	0.009	−9.2	−56.7	10	0.7	0.008	98.7	0.001	-
6	Toxana (upper spring)			2677	Ca-HCO3-SO4	1.9	7.2	6.3	107	0.55	0.002	−9.8	−62.7	7	58.3	0.13	40.6	0.048	−9.5	−48.3
7	Djilusu (well)			2362	Na-Ca-HCO3	3.1	22	6.4	−14	0.41	0.007	−12.0	−84.2	9	0.6	0.005	98.9	0.001	−5.8	−18.3
8	Neutrino tunnel (lake)	PR	Granites, gneisses, crystalline schists	2060	Na-HCO3-Cl	8.5	34	6.8	92	0.05	0.02	−8.4	−83.9	7	1.8	0.39	97.1	0.001	−7.3	−26
9	Neutrino tunnel (spring)			2060	Na-Cl-HCO3	7.3	41	6.7	19	0.05	0.02	−10.5	−66.8	6	1.6	0.39	97.1	0.001	−8
10	Terskol (spring)			2121	Ca-HCO3-Cl	0.3	9.6	5.8	61	1.37	0.34	−13.3	−94.5	10	0.9	0.38	97.3	0.001	−7.2 *
11	Poliana Narzanov (well)			1954	Ca-Na-HCO3	2.3	12	6.1	10	1.51	0.007	−13.9	−98.3	7	0.9	0.001	97.8	0.003	−8.3
12	Badaevka (well)			1942	Ca-HCO3	1.3	17	6.7	39	0.63	0.002	−14.2	−97.5	6	59	0.15	37.7	0.009	−9.2
13	Ingushli (low spring)			1940	Na-HCO3-Cl	1.5	16	6.5	32	0.35	0.005	−7.9	−50.2	12	1	0.01	98.8	0.0004	−4.8
14	Serebryni Klych (well)			2050	Ca-HCO3	0.6	15	6.4	0	3.74	0.001	−13.3	−90.5	10	62.8 *	0.0002	30.1 *	0.014	−8.0 *
15	Baksan (upper spring)			1557	Ca-HCO3-SO4	2.4	14	6.1	35	0.13	0.0001	−10.6	−73.8	6	still	still	still	still
16	Madjura (spring)	MZ	Granitoids	3737	Na-HCO3	1.5	51	6.6	−140	0.17	0.006	−15.3	−123	80	40.9	3.55	53.8	0.48	−8.8	−27.9
17	Kizilrabat (spring)			3867	Ca-HCO3	1.3	39	6.3	0	3.6	0.006	−13.8	−98	27	19.6	0.16	79.7	0.021	−4.4
18	Yashikul (geyser)			3788	Na-HCO3	4.0	41	7.3	−15	0.05	0.014	−13.6	−117	10	2	0.47	97.2	0.11	−5.7	−29.2
19	Sasik-Bulak (well)			3792	Na-HCO3	0.4	32	8.7	27	1.7	0.007	−18.8	−140	24	90.6	0.001	8.1	0.36	−16.9
20	Djelandy (well)			3568	Na-SO4	0.4	79	9.4	−224	0.004	0.006	−17.7	−129	44	97	0.72	0.12	0.07	−20.9	−18.9
21	Zagitor (spring)	PZ	Igneous and metamorphic rocks	2349	Na-Ca-SO4-Cl	1.8	21	7.1	35	2.6	0.014	−16.5	−117	130	94.1	0.0002	3.2	0.3	−12.6
22	Modyan (spring)			3733	Na-HCO3	1.1	63	7.5	18	0.02	0.01	−15.8	−115	8	88.3	0.74	8.1	0.12	−13.9	−17.6
23	Garm-Chashma	PR	Gneisses, marbles, migmatites, evaporites	2566	Na-HCO3	3.3	60	6.7	−281	0.08	0.006	−14.7	−109	16	1.7	0.009	97.8	0.001	−4.5
24	Sist (spring)			2300	Ca-HCO3-SO4	0.9	11	5.9	98	0.5	0.014	−14.1	−102	34	still	still	still	still	s
25	Barshor (spring)			2390	Ca-Na-HCO3-Cl	2.9	15	6.1	95	13.3	0.006	−14.3	−104	47	2	0.001	97.2	0.003	−5.3
26	AVJD (low spring)			2431	Ca-Na-HCO3	1.8	31	6.4	135	3.9	0.014	−14.5	−106	99	still	still	still	still	still
27	AVJD (well)			2436	Ca-HCO3	1.8	35	6.2	69	4.6	0.015	−14.1	−103	45	still	still	still	still	still
28	Sherigin (spring)			2783	Ca-Na-HCO3-Cl	2.7	34	6.3	0	9.4	0.010	−15.8	−118	105	0.3	0.001	99.5	0.0001	−5.2
29	Vranch (spring)			2776	Ca-HCO3	1.9	19	6.1	26	4.7	0.005	−15.5	−113	92	6.7	0.013	92.9	0.053	−5.8
30	Sovetbond (spring)			3096	Na-HCO3-Cl	6.1	23	6.4	−6	0.4	0.010	−13.1	−114	12	0.28	0.0002	99.6	0.0009	−7.9
31	Sharalai (spring)			3010	Na-HCO3	2.7	44	7.1	71	0.6	0.011	−13.9	−110	34	2.2	0.09	97	0.01	−5.7
32	Kauk (spring)			3199	Ca-HCO3	0.5	34	7.3	76	1.7	0.003	−16.3	−120	21	still	still	still	still	still
33	Bibi-Fatima (spring)			3208	Ca-SO4-HCO3	0.7	45	7.6	83	0.75	0.005	−16.2	−118	37	still	still	still	still	still
34	Naspar (spring)			3614	Na-SO4-HCO3	0.6	22	9.4	−101	0.01	0.009	−15.8	−117	11	98	0.08	0.11	0.3	−10.9

). If the average water temperature for Pamir waters is 36.7 °C, then the waters of the Elbrus region are more than twice as cold—16.4 °C. Waters of the regions also differ in the values of mineralization (TDS). The TDS of Pamir waters ranges from 0.4 to 6.1 g/L (average: 1.9 g/L), whereas in the Elbrus area, mineralization reaches up to 10.6 g/L (average: 4.8 g/L). In the majority of the studied mineral waters from the Pamir and Caucasus regions, the dominant anion is HCO₃, and HCO₃-Cl-type waters are frequently observed (Figure 2). This is particularly characteristic of the Caucasus. HCO₃-SO₄, SO₄-Cl, and SO₄-type waters are also found in the Pamirs but are not typical of the Elbrus area.

2.2. Methods

A total of 34 mineral water discharges from the Elbrus area (15 samples) and the Pamir (19 samples) were collected for hydrochemical, isotopic, and gas composition analyses. Surface water samples were also collected, including five river water samples and two lake water samples. Several rock samples were also collected for chemical analysis and microscopic examination. The latitude and longitude of the sampling sites were recorded by a handheld GPS (eTrex 20, GARMIN, Olathe, KS, USA). Altitude above sea level, air temperature, and pressure data were collected on-site by a Multifunctional Environmental Meter (Kestrel 2500 NV, Boothwyn, PE, USA).

Measurements of electrical conductivity (EC), pH, Eh, and water temperature were performed in situ using hand-held meters Mettler Tolledo S2 Seven2Go (Zürich, Switzerland) calibrated before sampling.

For chemical and isotopic analyses, water samples were filtered through 0.45 µm mixed cellulose ester filters (Advantec, Tokyo, Japan) and stored in acid-washed high-density polyethylene bottles. Samples intended for cation analysis were acidified to pH < 2 using ultrapure HNO₃. Dissolved carbonate species were determined by titration (e.g., [33]) with reagent-grade HCl (0.1 mol) and NaOH (0.02 mol) with phenolphthalein and methyl orange immediately after sampling. The concentrations of ²³⁸U and ²³²Th in waters were measured by inductively coupled plasma mass spectrometry (ICP-MS, X7 Quadrupole, Thermo Scientific, Waltham, MA, USA) in the Analytical Center of the Institute of Microelectronics Technology and High-Purity Materials, Russian Academy of Sciences, Chernogolovka, Moscow [34]. A Picarro 2170i laser isotope analyzer (Santa Clara, CA, USA) was used at the Collective Use Center of the Geological Institute of the Russian Academy of Sciences (Moscow, Russia) to determine the values of δ²H and δ¹⁸O.

Bubbling gas samples were collected from the natural springs and boreholes using standard techniques [35]. The composition of associated gases was determined in the Heat and Mass Transfer Laboratory (Geological Institute of the Russian Academy of Sciences, Moscow, Russia) using a gas chromatograph “Kristall 5000” (JSC SDB Chromatek, Yoshkar-Ola, Russia) with an inaccuracy no greater than 2–3 vol.%. The values of δ¹³C(CO₂) and δ¹³C(CH₄) were determined using the Thermo Electron system consisting of the Delta V Advantage mass spectrometer and the Trace GC Ultra gas chromatograph in the Laboratory of Isotope Geochemistry and Geochronology, Geological Institute, Russian Academy of Sciences, with an error of ±0.2%. All δ¹³C values are given in per mille (‰) relative to the VPDB standard.

Radon activity in water was measured in situ using a portable Alfarad+ radon monitor (NTM Zashita Co., Moscow, Russia). The method relies on the circulation and transfer of radon from the water sample into the working chamber of the radon measurement block via bubbling. A 50 mL water sample was collected in a capsule and then placed in the bubbler system, where air was passed through the water to facilitate the rapid release of ²²²Rn from the liquid into the gas phase. The released radon was transported through connecting tubing to the radiometer’s chamber, with complete transfer achieved in five minutes as per methodological specifications [36]. The instrument is capable of quantifying radon activity in water samples at concentrations as high as 800 Bq/L, with an associated uncertainty of ±30%, which is suitable for preliminary screening/field measurements [37]. To minimize statistical counting errors, extended measurement times were employed, water from each spring/well was analyzed five times, and the average value was calculated. Thus, measurement error did not exceed 5% (e.g., [38]). In this study, radon was measured as activities and uranium was measured as a concentration, which is consistent with common practice and with the MCLs or other human-health benchmarks available for these constituents.

The analyses of rocks were performed in the Geological Institute RAS, Moscow, Russia. Microelements Th and U were analyzed using the ICP MS technique with an Agilent 7500c spectrometer (Agilent Technologies Inc., Santa Clara, CA, USA). Analytical errors are 1–10%, depending on the concentrations. Uranium minerals were studied using Tescan Lyra 3 XMH (Tescan, Brno, Czech Republic) dual beam scanning electron microscope equipped with EDS Aztec X-Max 80 Standart (Oxford Instruments, Abingdon, UK).

Thermodynamic calculations were performed using the Aquachem 5.11 software with PhreeqC code [39,40]. The degree of statistical dependency of the system’s physicochemical properties and different constituents was determined using a nonparametric measure of Spearman’s rank correlation, which falls between r = +/−1. High correlations are indicated by absolute values of 0.8 < r ≤ 1, intermediate and high correlations are characterized by values of 0.5 < r ≤ 0.8, while independence across variables is shown by r = 0.

3. Results and Discussion

3.1. Geology, Orogeny, and Radionuclides Behavior

Rocks serve as the main source of radionuclides, and underground fluids (water and gases) contribute to their transport to the surface (e.g., [41]). To elucidate the origin of radionuclides in mineral waters, the relationship between the concentration (activity) of ²²²Rn and ²³⁸U in these waters, types of host rocks, and their geologic age was studied. According to the geological structure of the Pamirs, the collected mineral waters are associated with three main types of rocks: (1) Mesozoic granitoids (5 samples), (2) Paleozoic igneous and metamorphic rocks (2 samples), and (3) Precambrian metamorphic strata including gneisses, marbles, and migmatites (12 samples). Mineral springs of the Elbrus area also occur in three main rock associations: Cenozoic (Quaternary) granites/granodiorites/sedimentary rocks (4 samples), Paleozoic sedimentary/volcanic rocks (3 samples), and Precambrian gneisses/schists/migmatites (7 samples). This hierarchy demonstrates increasing mineral water affinity with older, more metamorphosed lithologies (Table 1). The analysis of geological conditions of Pamir and the Elbrus region revealed the main similarities and differences in the distribution of mineral waters in the studied territories, as well as variations in the levels of radionuclide content (Figure 3).

It can be seen that radon activity and uranium concentration are noticeably higher in Pamir waters than in Elbrus waters. The concentration of 232-thorium is very low in all studied waters (Table 1). These findings demonstrate that Precambrian basement rocks constitute a fundamental control on mineral water genesis in both studied regions. Simultaneously, Cenozoic rocks in the Caucasus and Mesozoic rocks in the Pamirs represent significant sources of mineral waters. Regarding rock age, an important clarification is needed. When discussing the influence of water-bearing rock age, we posit that older rocks are generally more susceptible to weathering, destruction, and typically exhibit higher fracturing density. This enhanced fracturing increases rock permeability to water and gases while expanding the reactive surface area with radioactive minerals. These effects are particularly pronounced in rocks involved in orogenic processes and tectonic activity, which explains the observed correlations. As direct quantification of rock fracturing in the study areas was unfeasible at this research stage, we used geological age as a proxy (assuming fracturing increases with age), through which certain dependencies were identified.

The distribution of 222-radon and 238-uranium in the mineral waters of the studied territories, based on the data from Table 1, is shown in Figure 4. The lack of elevation data in such representations severely undermines the reliability of conclusions, as critical altitude-related factors—climatic, orographic, geological, and hydrological—are disregarded. Consequently, this may result in oversimplifications or incorrect assumptions regarding radionuclide distribution and dynamics in mountainous regions. Due to the significant difference in the elevation of these mountain ranges—reaching up to 7649 m in the Pamirs and 5642 m at Mount Elbrus—the altitudes of the springs also vary considerably (Table 1). The mineral springs of the Pamirs are located between 2349 and 3867 m asl, while those in the Elbrus region occur at elevations of 1296 to 2667 m asl. Figure 5A shows the correlation between radon activity in the studied waters and spring elevation.

Radon levels show an inverse relationship with decreasing altitude for both the Pa-mir (r = −0.53) and the Elbrus area (r = −0.54) (Figure 5A). In Pamir waters, this dependence is most pronounced in Precambrian rocks (r = −0.5), and in Elbrus waters it is associated with Paleozoic and Cenozoic rocks (r = −0.8–−0.9). Based on the fact that this dependence is observed for both mountain structures, its essence is related to the physical properties of radon (high density of 9.73 kg/m³, about 7.6 times higher than the density of air, and short half-life—3.8 days). Geological conditions (fracturing, water availability, fluid flows, etc.) can also limit the access of radon to the upper parts of mountain structures. ²²²Rn is formed in the upper part of the Earth’s crust by the natural radioactive decay of uranium, when ²²⁶Rn ions formed during decay of ²³⁸U diffuse from the rock matrix to fracture surfaces. Separation of uranium and radium is often observed in the hypergenesis zone. Radium is deposited separately from uranium on the walls of fractures in rocks, tuffs, clays, etc. Emanating reservoirs are formed in this way [42]. Chemical peculiarities of uranium behavior in water determine the ways of its migration in the environment, while the main factor allowing radon to be fixed in mineral waters at significant altitudes is the proximity of the source of radon generation in rocks.

Uranium concentrations decrease with altitude only in the Pamir region (r = −0.54) (Figure 5B). However, in the highest Pamir mineral springs (above 3500 m asl)—those associated with Mesozoic rocks—a strong positive correlation is observed (r = 0.9, Figure 5B). Also, the highest uranium concentrations (18.6 µg/L) were determined in Sasykul Lake water, at an altitude of 3830 m asl. Samples of surface water (streams) located at altitudes above 3500 m also show uranium concentrations higher than in underground sources (9.0–11.6 µg/L). Thus, at altitudes above 3500 m, elevated uranium content in groundwater may be an indicator of its surface supply. The lowest position in the Pamir section is occupied by springs in Precambrian rocks (from 2400 to 3200 m above sea level). The largest number of natural mineral springs is located here. This indicates both the high cracking of ancient Precambrian rocks, which creates the best conditions for water circulation, and the influence of regional groundwater flow, which contributes to water unloading in the lowest structures of the Pamirs.

Uranium concentrations do not show significant correlations with elevation in waters from the Elbrus area (r = 0.3; Figure 5B). However, intermediate correlations are observed for waters circulating through Paleozoic and Cenozoic rocks (r = 0.7), likely due to a shift from reducing (Eh = −225 mV) to oxidizing conditions (Eh = +55 mV). The redox potential (Eh) shows a moderate positive correlation with uranium concentrations (r = 0.55) in these waters. This provides conditions for uranium oxidation to the hexavalent form, which is very soluble in water. At the same time, correlations of uranium with altitude are not observed for waters circulating in Proterozoic rocks (r = 0.2). Uranium concentrations in these mineral springs exhibit substantial variability (0.05–3.74 μg/L), likely reflecting differences in local uranium source availability. Some recent evidence suggests that the pronounced variability in dissolved uranium levels might reflect spatial variations in uranium enrichment within the granite body (e.g., [10,43]). Nevertheless, a moderate but significant positive correlation exists between redox potential (Eh) and uranium concentrations (r = 0.51), suggesting redox conditions influence uranium mobility.

3.2. Geochemical Aspects of ²³⁸U and ²²²Rn Migration

Radon, as an inert gas, does not react chemically with other elements, unlike its progenitor uranium. Consequently, the concentration of uranium in water can be used only indirectly to judge the pathways of radon input. Under favorable physicochemical conditions, uranium in water can be transported by groundwater to considerable distances and have large concentrations, but there will be no radon in the water. In turn, radon emanations in water can be caused by secondary mineral deposits (containing U and ²²⁶Ra) on rock fracture walls (e.g., [14,15,42]).

In order to establish relations between radon and uranium, as well as their sources in mineral waters of high-mountainous areas, it was necessary to consider geochemical patterns of migration of these elements.

3.2.1. Uranium

Uranium migration in the studied waters depends on water temperature, pH, and Eh (Figure 6A–C) and is independent of salinity (TDS). The correlation coefficients of uranium with water temperature for Pamir and Elbrus are −0.7 and −0.5, respectively (Figure 6A). The dependence of uranium concentrations on pH (Figure 6B) is more pronounced for Pamir mineral waters (r = −0.56) than in Elbrus waters (r = −0.32). A strong correlation (r = 0.76) of Eh with ²³⁸U was observed for all Pamir mineral waters (Figure 6C). For the mineral springs of Elbrus, the correlation is weaker, almost two times (r = 0.41). These correlations reflect the peculiarities of uranium migration in different chemical environments and, to a certain extent, characterize the hydrogeological conditions of the studied regions.

In natural conditions, uranium can be in tetravalent and hexavalent forms. In the active water exchange zones, where groundwater contains free oxygen, uranium undergoes oxidation to the hexavalent state (U⁺⁶), forming compounds that are well soluble in water. In groundwater, uranium is mainly present in its most mobile oxidized form of uranyl +6 as UO₂²⁺. In deep horizons under reducing conditions (in the zone of difficult water exchange), U in rocks is in the tetravalent form, forming very poorly soluble compounds. The redox potential (Eh) is a characteristic that affects uranium behavior in water. In accordance with the above, the maximum U concentrations in the studied waters are observed at positive values of Eh (0–+100 mV), and the minimum ones at negative values of Eh (−300–0 mV).

Redox conditions of waters characterize the depth of water circulation, which is confirmed by the increase in temperature of waters with negative Eh. The correlation coefficient of Eh with temperature is −0.6. Waters with high temperature generally contain less uranium (Figure 6A) and are discharged at higher absolute elevations (r = 0.56) (waters in Mesozoic granites, Figure 5B). As the altitude decreases, the temperature of the waters decreases (r = 0.57), and the uranium content increases (Figure 6A). Thus, the content of uranium is relatively higher in the waters of the zone of active water exchange compared to the zone of impaired water exchange.

Uranium concentrations in Pamir waters are also controlled by calcium and magnesium concentrations (r = 0.74). This indicates that, under certain conditions, uranium can move into the water by dissolution of carbonates and is also removed from the water together with secondary carbonates. For mineral waters of Elbrus, no such dependences were revealed (r = 0.1). The results align well with the geological framework of the studied regions. Water-rock interactions with carbonate formations are particularly evident in Pamir groundwaters, reflecting the region’s abundance of marine-origin rocks. In contrast, the Elbrus area shows limited carbonate distribution in its upper stratigraphic sections. The mineral springs north of Elbrus (#3–7, 13; Figure 1), where groundwater possibly interacts with Cretaceous marine sediments of the cover sequence, exhibit a strong positive correlation between calcium and uranium (r = 0.8). In other words, calcium, magnesium, and uranium share common sources in Pamir waters but have distinct sources in the Elbrus region.

Associated gases also affect the processes of interaction of water with rock (e.g., [44]). The calculated correlation coefficients between ²³⁸U and CH₄ for the waters of Pamir and Elbrus are very close and are −0.66 and −0.61, respectively, which is in good agreement with the fact that the presence of CH₄ and other reducing agents reduces the solubility of uranium [45]. The presence of CO₂ in water, on the contrary, promotes the solubility of uranium [28]. CO₂ acidifies the solution (reduces pH), enhancing dissolution of uranium-bearing minerals. In slightly acidic to neutral waters (pH 5–7), uranium (U⁶⁺) forms stable complexes with carbonate ions (CO₃²⁻): UO₂²⁺ + 3CO₃²⁻ ↔ UO₂(CO₃)₃⁴⁻. These complexes enhance uranium solubility, preventing its precipitation. A positive correlation between uranium and CO₂ is characteristic of the Pamir waters (r = 0.69), while in the Elbrus area, only waters with CO₂ content > 90 vol.% show a positive correlation (r =0.69). A group of mineral waters (#1, 2, 6, 12, and 14) with concentrations < 90 vol.% show a strong negative correlation (r = −0.8). However, with the observed decrease in CO₂ in these waters, pH and Eh conditions remain favorable for uranium accumulation (e.g., [46]). This could result from oxidizing conditions (e.g., dissolved O₂), competition for sorption sites (e.g., [10]), or other factors. Further research is required to resolve these complex interactions. The δ¹³C determinations of methane showed its thermogenic genesis for the Pamirs (from −17.1 to −29.2‰), while sources with microbial methane are also characteristic of the Elbrus region (Table 1). Studying the genesis of methane in the Elbrus region, Lavrushin [27] concluded that methane is of crustal origin, and the heterogeneity of values is associated with the influence of magmatic activity, which determines the generation temperatures and isotopic characteristics of CH₄. The formation of isotopically heavy methane in Pamir gases is associated with crustal melting and isotope exchange processes in the CO₂-CH₄ system.

3.2.2. Radon

As mentioned above, the studied mineral waters of Pamir and the Elbrus region significantly differ from each other in temperature and TDS (Table 1). At the same time, general dependences of radon concentration on temperature or TDS for both Pamir and Elbrus waters were not found (r = 0.2). For mineral waters of the Pamir, statistically significant patterns are not revealed, but the highest correlation coefficient (r = 0.4) is observed in CO₂-rich waters confined to Proterozoic rocks. Correlations of radon with pH and Eh for waters of both regions are below the significance level, and the correlation coefficient is from −0.3 to 0.1.

Significant correlations ²²²Rn are established only for waters distributed in Proterozoic rocks of Elbrus: r_temp = −0.72, r_TDS = −0.81. This occurs because the solubility of radon decreases with increasing temperature and mineralization [47]. A strong positive correlation (r = 0.93) exists between TDS and temperature in these waters, suggesting that elevated temperatures enhance rock leaching processes.

As shown in Figure 7A, ²²²Rn concentrations in waters from the Pamir moderate correlate with uranium concentrations (r = 0.6). The strongest correlation (r = 0.8) is observed in Proterozoic rocks, while waters circulating through Mesozoic formations show a weaker relationship (r = 0.4). This pattern aligns with the diffusion model of radon origin (e.g., [15]).

Figure 7B demonstrates a moderate but significant positive correlation (r = 0.5) between ²²²Rn and uranium concentrations in Elbrus waters. Only one well within Elbrus shows elevated radon concentrations (#1, Table 1), approximately three times higher than other mineral waters. The well is about 500 m deep, and the uranium-to-radon ratios indicate that these mineral waters have an additional source of radon, which is not directly related to uranium entering the water. Potential additional radon sources include: (i) secondary mineral phases precipitated on fracture surfaces containing trapped ²²⁶Ra, and (ii) associated carrier gases that mobilize and transport radon to the surface (e.g., [41]). These processes increase radon concentrations near the surface, and do not affect U concentrations in the water (e.g., [15]).

Correlations between the daily activity of radon and dissolved CO₂ in fresh groundwater were found in the territory of the Western Caucasus [48]. Statistical calculations on CO₂-rich waters (CO₂ > 90%, Table 1) show that the correlation coefficients of ²²²Rn with the Σ U + Th for Pamir reach 0.89, and for Elbrus 0.56. This again confirms that CO₂ has a great influence on the radionuclide geochemistry of the studied areas.

3.3. Probable Sources of Radionuclides: Insight from Mineral Equilibrium Calculations

The results of chemical analysis of rocks show that the highest concentrations of U are observed in Mesozoic granites of Pamir (4–12 ppm), whereas quaternary granites from the Elbrus area contain from 4 to 5 ppm of U. Microprobe studies have shown that the main U and Th mineral concentrator is silicates ((Th,U)SiO₄, uranothorite) and uranyl and thorium oxides (thorianite and uraninite, Figure 8A,B). The low concentrations of uranium in the rock, along with the rare and microscopic grain sizes of uranium minerals, generally correspond to the overall low level of groundwater radioactivity compared to the waters in the Tien Shan granites [49]. The uranium concentrations in gneisses and carbonates are lower than in granites: 1–2 ppm and <0.25 ppm, respectively.

Calculations of mineral saturation indices (SI) can help clarify the nature of processes controlling the concentration of elements in groundwater. Using the PhreeqC code [40], saturation indices were calculated for typical minerals present in the rocks under study: aluminosilicates, carbonates, halides, and sulfates. The obtained values were compared with uranium and radon concentrations. The dynamics of changes in saturation indices, i.e., saturation of water in relation to the minerals of the rocks, represented in Figure 9, approximate the evolution of the mass transfer process (e.g., [50]).

Statistical analysis revealed that against the general background (gray dots), a distinct group of springs stands out (#10, 14, 15, 19, 21, 25, 26, 27, 28, 29, 32, in Table 1), showing uranium enrichment as waters reach saturation with respect to aluminosilicates (SI albite, r = 0.78, Figure 9A). An intermediate correlation was observed between uranium and halides (SI halite, r = 0.68, Figure 9B), and a weak correlation for sulfates (SI gypsum, r = 0.48) and carbonates (SI dolomite, r = 0.41, Figure 9C). This group primarily includes springs from the Pamirs (8 springs) and the Elbrus region (3 springs). Most of the springs are related to the Proterozoic rocks, and only three springs on the Pamir are related to the Mesozoic and Paleozoic rocks (#17, 18, 20). Based on these findings, the most probable source of uranium in these waters is aluminosilicates and evaporites.

In the remaining waters (gray dots in Figure 9, characterized by low uranium concentrations), either the necessary conditions for uranium accumulation are absent, or the waters circulate in uranium-depleted rocks. For example, the mineral waters of the Elbrus region, which have the highest U contents (1.5–3.7 µg/L), are undersaturated with respect to albite (SI = −1.1), dolomite (SI = −2.4), and gypsum (SI = −1.9). This suggests that uranium could be released through the dissolution of these minerals. These waters exhibit low radon activity (7–10 Bq/L). Conversely, the waters in the Elbrus region with the highest radon concentrations contain very low U concentrations. These waters are supersaturated with respect to albite (SI = 1.5), potassium feldspar (SI = 1.5), and dolomite (SI = 0.5), indicating that dissolution of these minerals is unlikely. Moreover, uranium (or radium) may be scavenged during carbonate precipitation [51]. The low saturation indices for gypsum (SI = −3.7) suggest its possible dissolution and subsequent release of uranium into the water.

The observed correlation between ²³⁸U and ²²²Rn with the saturation indices of evaporites (halides and sulfates) suggests favorable conditions for the release of radionuclides into water during the dissolution of these mineral phases. Elevated radioactivity in anhydrite-bearing strata and carbonate reservoirs has been documented in sedimentary oil and gas basins (e.g., [52,53]). The radioactivity of evaporites is attributed to the fact that seawater evaporation leads to the enrichment of dissolved elements, including uranium and thorium. Based on the data in Figure 9B,E, water-rock interaction with ancient evaporites appears to be a common process, to varying degrees, in most of the studied waters from both regions. However, these processes seem to play a more significant role in the mineral waters of the Pamirs compared to the Elbrus region. Only three springs in the Elbrus area exhibit a clear relationship between uranium and mineral saturation indices (Figure 9A–C), whereas such associations are more pronounced in Pamir waters.

The processes of pre-concentration and evaporite formation appear to be currently active in the Pamirs. A notable example is the Sasik-kul Salt Lake, which has a salinity of about 100 g/L and a uranium concentration exceeding 18 mg/L. Studying radon distribution in this lake is an important objective for upcoming research.

Interestingly, the designated “uranium-rich group” (Figure 9) also shows a correlation with radon. The highest correlation coefficients are observed between: radon and sulfates (r = 0.71); radon and carbonates (r = 0.57); and radon and aluminosilicates (r = 0.37) (Figure 9D–F). This suggests that sulfates and carbonates are the primary minerals controlling radon activity in these waters. This process may occur through the release or uptake of ²²⁶Ra by secondary carbonates or sulfates (e.g., [10]).

According to a comparative investigation, these processes, which are so typical of the mineral waters of Pamir, very slightly reflect in the hydrochemical environment of Elbrus. This highlights the different mechanisms controlling radon influx in mineral waters of these regions. The interaction of water with carbonates and sulfates in the Pamirs is facilitated by the abundant marine-derived rocks in the region. The dissolution of evaporites has been identified as one of the dominant processes controlling the chemical composition of mineral waters in the Pamir region [26]. In contrast, in the studied area of the Elbrus region, this phenomenon is observed only in the eastern part, where a well with high radon concentrations is located. The primary process controlling the chemical composition of the mineral water of the Elbrus region is interaction with aluminosilicates [7]. The limited distribution of evaporites in the upper geological section of the Elbrus region indicates a fundamentally different geological control of radon behavior compared to the Pamirs.

3.4. Radionuclides, Stable Isotopes, and Associated Gases

Figure 10A shows δ¹⁸O and δ²H isotope ratios in mineral waters of the Pamir and the Elbrus Mountains. For the mineral waters of Elbrus, δ¹⁸O values range from −14.2 to −7.9‰ (SMOW), while δ²H values vary from −98.3 to −50.2‰ (SMOW). For Pamir waters, δ¹⁸O values range from −18.8 to −13.1‰ (SMOW), and δ²H values range from −140.0 to −98.0‰ (VSMOW). These data indicate that meteoric precipitation is the typical source of all the types of studied mineral waters (Figure 10A).

The altitude effect (δ¹⁸O vs Elevation), which is well manifested in surface waters of regions [54,55], is not found in underground mineral waters of single mountain structures of Pamir (r = −0.3) or Elbrus (r = −0.1). For groundwater in these regions, this is attributed to changes in the isotopic composition of water during its evolution within rock formations: the mixing of atmospheric precipitation with older waters, varying circulation times for different sources, and other isotopic effects (temperature-related, gas-related, etc.). Nevertheless, the altitudinal dependence is clearly visible when examining the isotopic values of mineral groundwater from the Pamir and Elbrus regions together (r = −0.66, Figure 10B). The correlation coefficient between elevation and δ²H is more strong: r = −0.72. The δ¹⁸O values in water also correlate with groundwater temperature (r = −0.56). Thus, the regional isotopic characteristics of these mountain systems are reflected in groundwater and follow altitude effects (Figure 10B).

A notable distinction between the mineral waters of the studied regions lies in the composition of associated gases. The Elbrus area exclusively hosts CO₂-dominated cold mineral waters, reflecting mantle-derived fluid inputs [27]. The concentration of CO₂ in this area ranges from 30.1 to 98.9 vol.% (Table 1), whereas N₂ contents exhibit significant variation, from 0.7 to 62.8 vol.%. Only four samples (#1, 6, 12, 14) contained significant nitrogen, with concentrations ranging from 21.4 to 62.8 vol.%. The Pamirs, in contrast, exhibit both CO₂-rich waters (cold and thermal) and a significant group of thermal N₂-rich mineral waters, suggesting varied fluid origins and pathways [26].CO₂ and N₂ show significant variability, with concentrations ranging from 0.11 to 99.6 vol.% and 0.28 to 98.0 vol.%, respectively (Table 1).

A correlation was found between the isotopic composition of water and the content of associated gases: CO₂ (r¹⁸_O = 0.6; r²_H = 0.5) and N₂ (r¹⁸_O = −0.6 and r²_H = −0.5). This is because the isotopically light waters of the high mountains contain N₂ and little CO₂. The proportion of CO₂ increases significantly with decreasing altitude (r = −0.58). In very rare cases, the groundwater CO₂ flux reaches high altitudes (3788 m asl, #18); mostly, CO₂ dominates at altitudes below 3000 m asl (Figure 10B).

Figure 10A shows that nitrogen-rich waters (#19, 20, 21, 22, and 34, Table 1) exhibit the lightest isotopic signatures (δ¹⁸O from −15.8 to −18.8‰, δ²H from −140 to −115‰). Notably, these are thermal waters with temperatures ranging from 20 to 80 °C. Such a depleted isotopic composition in thermal waters is evidently linked to their formation at high elevations under cold paleoclimatic conditions (e.g., [56]). The δ¹⁸O and δ²H values likely reflect the influence of Pamir glacial meltwater, which serves as the source for modern nitrogen-rich thermal springs. As seen in Figure 10B, these nitrogen-rich thermal waters are predominantly found above the lower glacial boundary (3500–4000 m asl).

The majority of nitrogen-rich mineral waters in the Pamirs exhibit low radon levels (up to 44 Bq/L, Figure 10C) and show no correlation between radon and δ¹⁸O (r = −0.30). Only one spring—Zagitor (#21), the sole natural N₂-rich manifestation at a lower elevation (2350 m asl)—displays high ²²²Rn activity in water (130 Bq/L). Figure 10B,C demonstrate that this spring’s isotopic characteristics align with trends and fields typical of CO₂-rich waters. Field surveys further revealed that the spring is located atop a large travertine deposit and has very low water and gas discharge rates. It is highly probable that the CO₂ source of this once evidently powerful CO₂-rich spring has been depleted, possibly due to tectonic shifts, resulting in nitrogen becoming the dominant gas. The elevated radon concentrations further support this hypothesis.

Analysis of data excluding the Zagitor source revealed that in nitrogen-rich waters, radon content correlates with δ¹⁸O values (r = −0.52, Figure 9C) and with water temperature (r = 0.56). The inverse correlation between radon and δ¹⁸O suggests dilution of groundwater by atmospheric precipitation, which leads to decreased radon concentrations and heavier water isotopic composition.

The positive correlation with spring temperatures (22–79 °C) may indicate active water-rock interaction processes occurring at elevated temperatures in deep aquifers. This leads to the release of radon from mineral formations. The inverse correlation between ²³⁸U and temperature (r = −0.55) points to reducing conditions characteristic of deep aquifer systems. Also, some radon escapes from the water due to a decrease in solubility at higher temperatures. Consequently, the free gases of nitrogen-rich thermal waters should exhibit elevated radon concentrations.

CO₂-rich waters from Pamir and Elbrus exhibit a moderate δ¹⁸O-elevation correlation (r = −0.66; Figure 10B). Interestingly, the correlation between elevation and radon activity in water shows exactly the same coefficient (r = −0.67, Figure 10C). The strongest correlation between radon and δ¹⁸O is observed in Pamir’s CO₂-rich waters (r = −0.86). This indicates predominant radon transport with isotopically light (glacial) waters. As the water’s isotopic composition becomes heavier (due to dilution by atmospheric precipitation), the waters become depleted in radon.

Despite the relatively high temperatures of Pamir’s CO₂-rich mineral waters (15–60 °C), radon-temperature correlations are very weak (r = −0.43). We suggest that active mixing of water by CO₂ gas bubbles disrupts radon-water equilibrium effects, which cancel out temperature dependencies while not affecting water isotopes.

For Elbrus CO₂-rich waters, the correlation between radon and δ¹⁸O is insignificant (r = 0.23). This is likely due to the region’s hydrogeological conditions [26]: most waters are shallow and influenced by atmospheric precipitation, while the sample also includes deep wells with low discharge rates (>radon half-life period) that exhibit active degassing.

Thus, it was established that radon activity in underground mineral waters within the altitudes from 2300 to 3792 m asl is related to isotopic characteristics of water, temperature, and gas factor. Such effects have not been previously observed in mineral waters of the highlands. The identified correlations suggest complex physical processes differentially control radon distribution in these waters. Probably, the established dependences can be considered as indicator characteristics. However, further research is required to fully characterize these mechanisms.

It should be noted that correlations between uranium and water isotopes in mineral waters are either absent or statistically insignificant (r = −0.36 to 0.0). This is primarily due to the distinct chemical behavior of uranium under different geochemical conditions. In contrast, radon exhibits more conservative behavior.

Furthermore, radon appears unrelated to the CO₂ source, as no significant correlations with δ¹³C(CO₂) were identified. Based on available geophysical and geochemical evidence, the most probable CO₂ origin for the Pamir region is intracrustal melting [26,57]. For the Elbrus area, however, the CO₂ shows a clear mantle-derived signature (−4 to −8‰ δ¹³C), consistent with deep-seated mantle sources (e.g., [58]).

3.5. ³He/⁴He Ratio and Radon Sources

During the radioactive decay of the U series, a large amount of ⁴He is formed, which changes the ³He/⁴He ratio and can also be used to determine the origin of radon. For mineral springs in the Pamir and Elbrus regions, limited determinations of ³He/⁴He ratios and ²²²Rn concentrations are available (e.g., [58,59,60]). However, even the available data clearly demonstrate the difference between the sources of helium and radon in the studied regions. Figure 11 demonstrates distinct differences in ³He/⁴He ratios and ²²²Rn concentrations between Pamir and Elbrus mineral waters. According to helium systematics, mantle-derived gases ascending through deep faults exhibit significantly higher ³He/⁴He ratios, typically several times greater than atmospheric values (³He/⁴He)_air = 1.39 × 10⁻⁶). Elevated ³He/⁴He ratios are also characteristic of regions with high heat flow values [59]. Recent estimates indicate that the mantle helium fraction in associated gases of the Elbrus region may exceed 30% [58].

In contrast to the Elbrus region, Pamir springs exhibit low ³He/⁴He ratios and elevated ²²²Rn concentrations, consistent with crustal radiogenic contributions. The correlation coefficient (r = −0.73) indicates an inverse relationship between mantle-derived gas inputs and radon concentrations in water. Specifically, increased mantle degassing corresponds to decreased radon levels, while reduced deep degassing leads to higher radon activity.

3.6. Health Risk Assessment from Mineral Water Consumption

This section’s goal is to evaluate the potential risk that human health poses from mineral waters containing the studied radionuclides.

Uranium and radon have distinct toxicological profiles due to differences in their chemical properties, routes of exposure, and mechanisms of biological damage (e.g., [6,16,19,51]. Due to the low specific activity of ²³⁸U (radioactive concentration), the mass of uranium required to produce significant activity is quite large (1 mg of ²³⁸U has an activity of only 12.5 Bq) [6]. But, natural uranium induces nephrotoxicity, which has a lower threshold than radiotoxicity. Therefore, the guideline value for U based on chemical toxicity (µg/L), which poses significant health risks, particularly kidney damage, when ingested in high concentrations through drinking water.

The primary health effect of radon is lung cancer, resulting from inhalation in indoor air. Along with soil and atmospheric air, water, and food, trace amounts of radon enter living organisms, where they become sources of ionizing radiation. Elevated levels of radon cause radiation sickness, damage blood-forming organs, and lead to lung cancer.

The content of radon and uranium in water is regulated. The concentrations of ²³⁸U in mineral waters from the Pamir and Elbrus regions do not exceed the limits for potable water (15–30 µg/L) according to many international standards, including the US EPA [61], the World Health Organization [62], and Russian water quality standards [63]. The maximum permissible concentrations of radon in drinking water differ significantly across regions, from 11 Bq/L (United States) to 100 Bq/L (Europe, [37]). According to Russian Radiation Safety Standards [64], the radon content in drinking water should not exceed 60 Bq/L. Mineral waters with at least 185 Bq/L are classified as radon waters and can be used in balneotherapy [65].

The European Union (EU), the World Health Organization (WHO), and the U.S. Environmental Protection Agency (EPA) continually develop and refine regulations to limit public exposure to radionuclides in drinking water [66]. However, ²²²Rn (radon) and its decay products are excluded from the Total Indicative Dose (TID)—a parametric value of 0.1 mSv/year [66]. This 0.1 mSv/year threshold represents a very low risk level, equivalent to a cancer risk of 5.5 × 10⁻⁶. Such values are comparable to carcinogenic chemicals studied in the Elbrus area [7]. Notably, this dose is only achievable if exposure persists continuously over a full year. Calculations reveal that ²²²Rn contributes no more than 0.9 mSv/year (average: 0.3 mSv/year) to the total annual effective dose in Pamir’s mineral waters, and even less in Elbrus—0.24 mSv/year (average: 0.08 mSv/year) (Figure 12A,B). These low values suggest minimal activity from regulated radionuclides (²²⁴Ra, ²²⁶Ra, ²³⁴U, ²³⁸U) (e.g., [9,67]).

Figure 12 is based on the above results, indicating that groundwater at high altitudes (more than 3500 m asl) practically does not contain radon. It can be seen that only one sample of mineral water taken from a well (#1, Table 1) in the granite massif of the Elbrus region exceeds the maximum permissible concentration of ²²²Rn (Figure 12A).

In accordance with previous studies, this water has exceeded normative values for other elements as well [7]. The radon activity in subsurface mineral fluids is obviously greater east of Mount Elbrus, in the direction of young granite batholiths and Mesozoic sedimentary coverings. According to [68,69], these formations include substantial uranium deposits with strong radon emissions (Rn > 200 Bq/L). Mineral waters in this Caucasus region have served balneotherapeutic purposes for a century (e.g., [70]).

As can be seen, the ²²²Rn radiation hazard of mineral springs of Pamir is noticeably higher: out of 19 springs, 12 confined to granites and gneisses show values higher than recommended—0.1 mSv/year. Exceeding the TID of 0.1 mSv/y does not mean that the water is unsuitable for drinking. However, further investigation of radium content is required.

In the highlands, the distribution of radon in the groundwater is greatly influenced by the many mountain peaks. In the studied Pamir region alone, there are five mountain peaks exceeding 5000 m in elevation (Figure 12B). Under these conditions, radon activity—both in air and groundwater—remains very low, ensuring safe exposure levels for humans. However, in intermountain depressions at lower elevations (~2300 m), radon concentrations in water can rise substantially, warranting targeted investigation and monitoring.

4. Conclusions

This study presents the first comparative analysis of ²²²Rn and ²³⁸U distribution in mineral waters of two contrasting highland systems of the Alpine-Himalayan orogenic belt: the Pamir and the Elbrus area. The mineral springs occur at elevations ranging from 1296 to 3867 m asl, with temperatures reaching up to 79 °C. The gas composition is dominated by CO₂ and N₂, with CO₂ being particularly prevalent in most springs.

The following significant findings have been drawn after comprehensive investigations:

1.

It has been established that the concentrations of ²³⁸U and the activity of ²²²Rn in the mineral waters of the Pamirs are higher than in the waters of the Elbrus area. In both regions, radon activity shows a strong correlation with uranium concentrations in mineral waters associated with Proterozoic-age rocks, the most ancient geological formations in these areas.

2.

Altitude dependencies of uranium and radon distribution have been established. With increasing altitude, radon activity in groundwater decreases. This can be explained by both the physical properties of ²²²Rn (high density) and hydrogeological characteristics (low circulation rate exceeding the half-life of radon). On the contrary, the concentration of ²³⁸U tends to increase with increasing altitude of the mountain structure, which is associated with oxidizing conditions of the groundwater environment, favorable for its migration.

3.

Thermodynamic calculations revealed key processes influencing uranium and radon incorporation into groundwater. The results demonstrate that: Uranium mobilization is controlled by water-aluminosilicate interactions, which are enhanced in the presence of CO₂; Radon activity is governed by both aluminosilicate and evaporite interactions. These processes are particularly pronounced in Pamir waters due to abundant marine-origin rocks. In contrast, the Elbrus region shows generally low radon concentrations due to limited evaporite distribution in its upper geological sections.

4.

The obtained data align well with previous studies, confirming that waters in the Elbrus region exhibit mantle-derived signatures, whereas Pamir waters reflect intracrustal melting processes. Pamir springs show low ³He/⁴He ratios and elevated ²²²Rn levels, consistent with a dominant crustal radiogenic contribution. In contrast, Elbrus groundwaters display higher ³He/⁴He values, indicating mantle input—a feature typical of regions with high heat flow.

5.

It was found that radon activity in high-altitude mineral waters (2300–3792 m above sea level) has been linked to isotopic composition, temperature, and gas content. Such effects had not previously been reported in high-mountain mineral waters. The observed correlations suggest complex physical processes governing radon distribution, which may serve as potential indicator characteristics. However, further research is needed to fully elucidate these mechanisms.

The strongest radon–δ¹⁸O correlation occurs in CO₂-rich Pamir waters, implying that radon is primarily transported by isotopically light (glacial) waters. As the isotopic composition becomes heavier (due to mixing with atmospheric precipitation), radon concentrations decrease.

6.

Health risk assessments of the Pamir and Elbrus mineral waters have shown that neither uranium nor radon negatively impacts human health. Concentrations of ²³⁸U do not exceed drinking water limits (15–30 μg/L). Studies confirm that at altitudes above 3000 m, radon activity in groundwater is very low, ensuring safe human exposure levels. However, in intermountain basins at lower altitudes (~2300 m), radon concentrations in water can rise significantly, necessitating further study and monitoring.