The Tisza River originates in the Transcarpathian region of Ukraine and flows downstream along the Ukrainian-Hungarian border, tracing the boundary with Romania. The Tisza River Basin, the largest sub-basin within the Danube River Basin, spans an area of 157,186 km² and extends across Ukraine, Romania, Hungary, Slovakia, and Serbia. The territory of the Upper Tisza River Basin consists mainly of nature reserves (Figure 1). The studied area lacks industrial facilities, and tourist and recreational activities take place there.

National Nature Park “Uzhansky” (49.005790, 22.676833), the Ukrainian part of the Biosphere Reserve “Eastern Carpathians”, mainly foothill landscapes, elevation 850–230 m above sea level, high forest cover of the territory with beech primeval forests.

National Nature Park “Synevyr” (48.544595, 23.597198) is characterized by predominantly mountainous landscapes, with elevations ranging from 440 to 1,720 m above sea level. The area is heavily forested, featuring extensive spruce-beech primeval forests (Ustymenko et al., 2022).

National Nature Park “Zacharovanyi Krai” (48.418366, 23.123090) is mainly characterized by lowland landscapes, with elevation differences ranging from 130 to 630 m above sea level. The area has extensive forest cover, primarily consisting of primeval beech forests (Shyshkanynets et al., 2024).

Investigated soil profiles: humus horizon −0 ÷ 20 cm, eluvial horizon −20 ÷ 50 cm, carbonate geochemical barrier >50 cm. Soil samples were collected using cores, following the procedure outlined by Tan (2005). Soil samples were cleaned of organic residues and stones, averaged, dried at room temperature to “air-dry” and sieved through a sieve (hole diameter 1.0 mm). Humidity was controlled gravimetrically.

In each National Nature Park, ten soil sampling sites were selected considering the landscape features and geological characteristics of the territories. The sampling strategy assumed that all soil types of the territory (taking into account the area ratio), all landscape features (mountain peaks, slopes, foothills, etc.), and forest cover with a characteristic plant type, as well as geological anomalies, if they occur, were covered.

Radiological studies of soil state were carried out directly without additional sample preparation. Determination of HMs content included: 1. Determination of total content: samples were digested using the aqua regia method according to the procedures (Hseu et al., 2002; Soodan et al., 2014), resulting in an acid mineralizate solution; 2. Determination of acid-soluble forms: an acid extract was obtained from the soil using nitric acid (HNO₃, 1 mol⋅L⁻¹). All reagents utilized in the study were of analytical-grade purity and obtained from Sigma-Aldrich (St. Louis, MO, United States) or Merck (Darmstadt, Germany). Double-distilled water (DDW) was utilized in all experiments.

An atomic absorption spectrometer, Shimadzu AA-7000 (Shimadzu, Japan), equipped with a graphite tube atomizer, was used for the determination of the HMs content in soil samples. The soil mineral solution and the acid extract were investigated, respectively. Palladium nitrate (Merck, CAS No.: 10102-05-3, Palladium Matrix Modifier for graphite furnace-AAS) served as a chemical modifier. The measurement parameters were as follows: sample volume was 20 μL; wavelengths for Cd were 228.8 nm (slit 0.7 nm), Pb were 283.3 nm (slit 0.7 nm), Zn were 213.9 nm (slit 1.0 nm), and Cu were 324.8 nm (slit 0.5 nm). The Zeeman effect for the background correction was applied. Pure argon (99.99%, flow is 100 mL⋅min⁻¹) was used as a shielding gas.

The furnace program for atomic-absorption measurements consists of several stages, each with specific temperature settings, heating speeds, durations, and argon flow conditions. We performed the first drying stage at 80°C with a high heating speed for 20 s, including argon flow. Then, we carried out the second drying phase at 160°C with a slow heating speed, also for 20 s, while maintaining argon flow. Next, pyrolysis is conducted at 950°C with a high heating speed for 10 s, again with argon flow included. Atomization is performed at a variable temperature depending on the element being analyzed. It occurs at a high heating speed over 5 s, but with argon flow excluded during this process. Optimal atomization temperatures are set at 2,300°C for cadmium (Cd), lead (Pb), and zinc (Zn), while copper (Cu) requires a slightly higher temperature of 2,400°C. Finally, the cleaning stage takes place at 2,800°C with a high heating speed, lasting for 3 s, with argon flow included to ensure the removal of residual materials. This structured sequence ensures precise atomic-absorption measurements while maintaining control over sample preparation and analysis conditions. Each soil sample was analyzed six times in parallel, and the results were statistically processed.

A multi-element standard solution (Merck, Multielement Standard Solution 6 for ICP, TraceCERT^®, UNSPSC Code: 41116107) was used to prepare the standard calibration solutions.

Measurements of the absolute activity of RNs were carried out under low-background conditions (combined protection) on a gamma-spectroscopy complex “SBS-40” with a cooled 100 cm³ and 175 cm³ HPGe coaxial semiconductor Ge(Li)-detector. The measurement was carried out according to the procedure outlined by Symkanich et al. (2023) with a measurement time of 5,000 s. As a reference source for calibration (energy and efficiency calibration), KOUHC (¹⁵²Eu and combined samples ⁴⁰K–¹³⁷Сs) was used (bulk source of cuvette size). Each soil sample underwent six repeated analyses, followed by statistical processing of the results.

Soil acidity was determined by the procedure (ISO, 2005). The actual acidity of the soils determined potentiometrically by measuring the pH of an aqueous soil extract (soil: water ratio = 1:5). Measurements were performed using an HI-2211 pH/ORP meter (HANNA Instruments, USA). Determination of organic carbon in soils was carried out according to international standard (ISO, 1998) and the determination of the potential cation exchange capacity of soils was carried out according to standard (ISO, 2023). The organic carbon content was determined by a spectrometric method by oxidizing an accurate amount of soil in a sulfochromic medium and measuring the absorbance of the solution after the oxidation procedure (at a wavelength of 585 nm, spectrophotometer Shimadzu UV-2600, Shimadzu Co., Japan). The potential cation exchange capacity of soils was determined chemically using ammonium acetate solution at pH 7.

Mapping of territories was carried out using the “ArcGIS 10.2.1” program (Esri, 2019).

Mapping was carried out using the Kriging method (Kruglov model) as a prediction based on the coordinates of the soil sampling sites. The selection of adjacent sites was standard.

Cluster analysis was carried out applying Statistica 12.0 software using the hierarchical clustering method. The variables were standardized before clustering.

The properties of the soil of these territories play an important role. The soils of the National Nature Park “Synevyr” (mountain landscapes) are quite acidic (pH water extraction 4.0 ± 0.2), have a low organic carbon content (0.73% ± 0.11%), and have a low cation exchange capacity (29.9 ± 3.1 meq⋅100 g⁻¹). The soils of the National Nature Park “Uzhansky” (foothill landscapes) are less acidic (pH water extraction 4.6 ± 0.5), have a higher organic carbon content (1.41% ± 0.49%), and have a higher cation exchange capacity (34.7 ± 3.2 meq⋅100 g⁻¹). The soils of the National Nature Park “Zacharovanyi Krai” (lowland landscapes) are more acidic (pH water extraction 4.3 ± 0.4) than those of the National Nature Park “Uzhansky”, have a lower organic carbon content (0.89% ± 0.31%), but a higher cation exchange capacity (37.4 ± 3.9 meq⋅100 g⁻¹).

Table 1 presents a summary of the radiological studies conducted on the soils of National Nature Parks in the Tisa River Basin. The data in Table 1 show a significant difference in radioactivity of RN both for soils of different landscape zones and across soil profiles. A comparative analysis of the data in Table 1 shows a number of patterns. Firstly, the radioactivity of natural ⁴⁰K varies significantly within the boundaries of each national natural park, which is obviously associated with different types of soils within the territory. Therefore, data on the radioactivity of natural ⁴⁰K are not particularly informative. In general, the radioactivity of ⁴⁰K in soils is highest for foothill landscape zones. The radioactivity of ⁴⁰K varies slightly across soil profiles. Secondly, the radioactivity of technogenic ¹³⁷Cs is the highest for the humus soil profile (0–20 cm), especially for lowland landscape zones. Meanwhile, the distribution of technogenic ¹³⁷Cs within the humus soil profile varies considerably across the national nature parks, largely influenced by the geological and morphological characteristics of the respective territories. As an example, Figure 2 shows the distribution of ¹³⁷Cs in humus horizons within the National Nature Park “Uzhansky” (foothill landscapes). The data in Figure 2 show that technogenic ¹³⁷Cs is concentrated in the soils of the lowland part of the National Natural Park “Uzhansky”. Third, the total radioactivity of natural radionuclides (²³⁸U and ²³²Th series), excluding ⁴⁰K, differs significantly for soils of different landscape zones: mountainous (254 ± 27 Bq⋅kg⁻¹), foothill (239 ± 52 Bq⋅kg⁻¹), and lowland (106 ± 38 Bq⋅kg⁻¹). This is obviously related to the tectonic activity of the young Carpathian Mountains. The distribution of natural radionuclides (total radioactivity) in the humus soil horizon within individual National Natural Parks depends on the geology and tectonics of their territories. For example, Figure 3 shows the distribution of the total radioactivity of natural RNs within the National Natural Park “Synevyr”. The maximum radioactivity of RNs is concentrated within the mountain ranges (gray mountain-meadow soils). These patterns in the distribution of natural gamma-active RNs seem to be associated with the tectonic and geological structures of the studied territories (Sukharev et al., 2020). This geological structure causes the release of ²²²Rn (²³⁸U series) from mountain faults, which ensures the dominance of ²³⁸Uranium series RNs in mountain landscapes. There is no significant difference in the distribution of natural RNs in soil profiles, as can be seen from the data in Figure 4, which shows such a distribution for soil profiles of the National Nature Park “Zacharovanyi Krai”. The nature of the localization and vertical distribution of natural RNs in soil profiles is associated with lithological variations and soil type. As can be seen from Figure 4, the localization of natural radionuclides is observed in the western part of the National Nature Park “Zacharovanyi Krai”, where foothill landscapes and brown mountain-forest medium-deep podzolized heavy loam soils prevail. The localization of natural RNs within the territory is distinct, since the difference in RNs radioactivity for different areas differs by 2–3 times. With the vertical transition from the carbonate geochemical barrier (>50 cm) to the eluvial horizon (20–50 cm) and the humus horizon (0–20 cm), the area of localization of natural RNs increases somewhat. This natural process is associated with different properties of soil horizons. At the same time, the averaged data on RNs’ radioactivity (Table 1) do not show this. Fourth, in the soils of mountain zones, particularly within the humus soil profile, natural RNs from the ²³⁸U series (Σ ²³²Th/Σ ²³⁸U = 0.87 ± 0.19) are predominant. In contrast, RNs from the ²³²Th series prevail in the foothills (Σ ²³²Th/Σ ²³⁸U = 2.26 ± 0.41) and lowland areas (Σ ²³²Th/Σ ²³⁸U = 2.32 ± 1.47).

The most complex geological conditions among the studied nature reserves are characteristic of the National Nature Park “Uzhansky” (Sukharev et al., 2020). Therefore, for this territory, we carried out a cluster analysis of the distribution of the total of RNs by the location of sampling points (Figure 5a) and individual RNs among themselves (Figure 5b). The results of cluster analysis show a pronounced correlation of the distribution of radionuclides within the territory of the National Nature Park “Uzhansky” and among themselves.

The summarized data of determination of HMs in soils of National Nature Parks (Tisa River Basin) are presented in Table 2. The overall distribution patterns of HM content in soils differ fundamentally from those of the radioactivity of RNs. The general trends of HM content in soils of protected areas (for different landscape zones) have been established. Firstly, the content of HMs in soils increases when moving from mountainous to foothill and lowland areas. To illustrate this pattern, Figure 6 displays the HM content in the humus soil horizon (0–20 cm) for all examined National Nature Parks. Secondly, the value of the lead (81%) migration coefficient (the ratio of the content of acid-soluble forms to the total content) for the humus soil profile of the National Nature Park “Zacharovanyi Krai” indicates that lead is obviously of anthropogenic origin. This is obviously due to atmospheric deposition. The areas surrounding the National Nature Park “Zacharovanyi Krai” consist of agricultural land with a well-developed transport infrastructure. Leaded gasoline was used in Ukraine for a long time, which is the reason for the increased lead content in the soil. There are no other anthropogenic sources of pollution. Third, the distribution of the total HMs content in the soils of individual National Nature Parks contrasts with the distribution of the total radioactivity of RNs. Figure 7 shows the distribution of the sum of HMs in the humus soil horizon of the National Nature Park “Synevyr” (mountain-forest light brown loam soils). Comparison of the data in Figures 3, 7 shows the opposite relationship for the same area. HMs accumulate primarily on the slopes and foothills of the mountains. Soil profiles show similar patterns. For example, Figure 8 shows the distribution of the total content of heavy metals in soil profiles of the National Nature Park “Zacharovanyi Krai”. As can be seen from Figure 8, HMs are localized mainly in the eastern part of the National Nature Park. The distribution is significantly influenced by lead content of anthropogenic origin, as the eastern part of the territory is much closer to agricultural areas with a well-developed transport system. With a vertical transition, the area of localization of HMs practically does not change. The pattern of distribution of HMs within the territory of the National Nature Park “Zacharovanyi Krai” is not sufficiently pronounced, unlike the RNs, since the total amount of HMs for different areas differs by less than 10%. Despite this, comparison of the data in Figures 4, 8 clearly highlights the inverse relationship between the distribution of HMs and RNs in soils.

The cluster analysis results indicate a strong correlation in the distribution of HMs within the National Natural Park “Uzhansky” (Figure 9a) and among themselves in the same territory (Figure 9b).