Spatial Patterns of Mercury and Geochemical Baseline Values in Arctic Soils

Abstract

The issue of formulating scientifically sound standards for mercury (Hg) content in Arctic soils is becoming increasingly pertinent in view of the rising human impact and climate change, which serve to augment the mobility of Hg compounds and their involvement in biogeochemical processes. In the absence of uniform criteria for regulating Hg concentrations, it is particularly important to determine its geochemical baseline values and the factors that determine the spatial and vertical distribution of the element in the soil profile. The study conducted a comprehensive investigation of Hg content and patterns of its distribution in various types of tundra soils in the European North-East of Russia. The mass fraction of total Hg was determined by atomic absorption spectrometry, and the spatial features of accumulation were analysed using geoinformation technologies. The distribution of Hg in the soils of the tundra zone was found to be distinctly mosaic in nature, determined by the combined influence of organic matter, granulometric composition, and hydrothermal conditions. It has been established that the complex influence of the physicochemical properties of soils determines the spatial heterogeneity of Hg distribution in the soils of the tundra zone. The most effective Hg accumulators are peat and gley horizons enriched with organic matter and physical clay fraction, while in Podzols, vertical migration of Hg is observed in the presence of a leaching water regime. In order to standardise geochemical baseline Hg values, a 95% upper confidence limit (UCL_95%) is proposed. This approach enables the consideration of natural background fluctuations and the exclusion of extreme values. The results obtained provide a scientific basis for the establishment of standards for Hg content in background soils of the Arctic.

1. Introduction

In today’s world of rapid industrial development and intensive natural resource use, human activity is leading to significant environmental pollution [1,2]. One of the most alarming groups of pollutants is heavy metals, which can build up in soil, water systems and living organisms through processes of bioconcentration [3,4]. Mercury (Hg) occupies a special place among them, as its high toxicity, volatility and tendency to bioaccumulate pose a threat to ecosystems at all levels, including humans [5,6].

Anthropogenic sources of Hg pollution cover a wide range of industrial and agricultural processes. Industrial enterprises in sectors such as metallurgy, chemistry and energy contribute to the release of Hg-containing compounds into the atmosphere, which subsequently leads to their deposition in soils and bodies of water [7]. Using mercury-containing materials in medicine, household products and agriculture further increases the ways in which this dangerous element enters the biosphere. Modern research shows that the global Hg cycle is characterised by the complex transfer of the element between the atmosphere, hydrosphere and lithosphere [8,9].

Soil plays a key role in Hg cycle, acting as a natural buffer that accumulates, transforms and redistributes this element [10,11]. Depending on the soil type and how it formed, the background Hg concentration can vary significantly. For instance, research indicates that Retisols naturally contain 200 ± 40 μg∙kg⁻¹ of Hg [12], whereas the median Hg value in the topsoil of European Union countries is estimated at 38.3 μg∙kg⁻¹, with 10% exceeding 84.7 μg∙kg⁻¹ [13]. These data enable us to assess not only the current state of pollution, but also to predict Hg accumulation dynamics under the influence of both natural and anthropogenic factors.

Over the past few decades, Hg levels have steadily increased due to the expansion of industrial activities, resulting in a greater toxic impact on ecosystems [14]. The accumulation of Hg in soils has complex effects ranging from disruption to plant growth processes to destabilisation of food chains, ultimately affecting human health [15]. Literature reviews emphasise that Hg compounds, particularly phenyl and alkyl derivatives, are highly toxic. This makes monitoring Hg levels a priority for environmental research [16,17].

Systematic monitoring of background Hg content in soils is crucial for the timely detection of geochemical anomalies and assessment of environmental risk. Regional studies enable us to identify areas with elevated Hg concentrations and develop comprehensive measures to optimise environmental protection technologies in industrial and agricultural sectors. This approach is particularly relevant in areas with diverse landscapes, where the geochemical characteristics of soils influence Hg accumulation and migration processes. For instance, studies conducted in the taiga zone of the Komi Republic emphasise the importance of considering landscape and geochemical characteristics when determining the vertical Hg distribution in soil profiles [12].

The importance of studying Hg pollution is determined by environmental and socio-economic factors. The accumulation of highly toxic Hg compounds in soil and their subsequent migration into plants and animals used for food can pose a serious threat to public health, as well as having a negative impact on biodiversity and the sustainability of ecosystems. In the context of global challenges related to sustainable development, continuous monitoring of soil conditions and assessment of Hg contamination levels allow timely environmental protection measures to be taken. The international community is developing uniform standards and recommendations for monitoring and managing Hg-related risks, as confirmed by the results of recent studies [18].

To adequately assess the environmental risks associated with heavy metals in soils, it is essential to distinguish correctly between geochemical background and baseline values. Geochemical background values reflect the elemental content of soils formed exclusively through natural geological and soil-forming processes, unaffected by anthropogenic activity [19]. Geochemical baseline values, in contrast, characterise the initial elemental composition of soils by taking existing anthropogenic impacts into account, reflecting the actual spatial and temporal variability of element concentrations in surface environments [20,21].

This distinction is particularly relevant for Hg, which is highly mobile and capable of long-range atmospheric transport. Even in areas far from local sources of pollution, the influx of mercury compounds from the atmosphere can result in elevated concentrations in the upper soil horizons. This makes the use of a ‘clean’ geochemical background methodologically incorrect. Under these conditions, geochemical baseline values more accurately reflect the combined influence of lithogenic factors, soil formation processes, climatic conditions and diffuse anthropogenic contributions associated with global and regional Hg transport [22,23].

Many authors therefore consider geochemical baseline values to be a more informative and practical parameter than geochemical background when assessing areas already affected by industrial, mining or agricultural activities [24,25].

This study is based on the working hypothesis that the capacity and economy of soils determine their baseline Hg values, taking into account their physicochemical properties, geochemical landscape conditions, and distance from sources of Hg emissions. Taking these factors into account allowed us to identify patterns in the spatial and vertical distribution of Hg. In this regard, the aim of the study was to assess geochemical baseline Hg values in the profiles of various types of tundra soils in the European North-East of Russia, taking into account the natural conditions of the region.

2. Materials and Methods

2.1. Study Area

The research was carried out on background soils located within the European Northeast of Russia, specifically in the Vorkuta District of the Komi Republic (Figure 1). The identification and naming of soil types were performed in accordance with the diagnostic horizons, properties and classification criteria defined by the World Reference Base for Soil Resources [26], ensuring the compatibility of the obtained data with international soil classification standards. The studied sites are primarily situated in the territory of the Bolshezemelskaya Tundra, which forms one of the key landscape regions of the Arctic zone. This tundra area is conventionally divided into northern and southern subzones and occupies approximately 10% of the total territory of the Komi Republic [27].

The region is characterised by the widespread presence of permafrost. In the northern subzone, permafrost occurs as a continuous layer, while in the southern subzone it has a discontinuous or island-like pattern. The depth to the permafrost table ranges between 0.3 and 2 m, and the total thickness of the frozen layer varies from 40 to 100 m. The temperature of the permafrost reaches −2 to −5 °C within the zone of continuous occurrence, and approximately −2 to 0 °C in areas where it is discontinuous or insular in nature. Climatically, the southern tundra subzone is part of the Atlantic–Arctic region, characterised by pronounced seasonality of air mass circulation: during winter, warm southern winds prevail, while in summer, the area is influenced by cold northerly flows. Overall, the local climate exhibits a distinctly severe and continental character, typical of high-latitude permafrost-affected environments [28].

The climate of the tundra zone is distinctly harsh and cold, with an average annual air temperature of approximately –6 °C at the latitude of Vorkuta. Snow cover persists for the majority of the year, typically lasting between seven and eight months. The depth of the snowpack varies considerably across the landscape—from as little as 0.1 m in wind-exposed areas, to over 1.5 m in depressions or locations protected by dense vegetation. Such spatial irregularity is largely controlled by strong and frequent winds, which can reach speeds of up to 30 m∙s⁻¹, redistributing snow and shaping the winter surface pattern.

The period with sub-zero air temperatures extends from October through May. January represents the coldest month of the year, when the mean monthly temperature averages around −20 °C, whereas July is the warmest, with mean temperatures near +12 °C. The duration of the transition period, when temperatures fluctuate around 0 °C, is about 125 days, while days with air temperatures exceeding +5 °C occur for roughly 90 days annually. The frost-free season is extremely short, lasting only about 52 days, and biologically active temperatures above +10 °C are recorded for approximately 43 days.

The annual average precipitation is 550 mm, with most of the rainfall occurring between June and September. During the short warm season, atmospheric humidity remains high while evaporation rates are relatively low. These conditions contribute to the waterlogged soil typical of tundra landscapes [29].

The prevailing climatic conditions of the Vorkuta District exert a decisive influence on the processes of soil formation and transformation. The onset of soil freezing typically occurs towards the end of September, coinciding with the establishment of stable sub-zero air temperatures. A continuous snow cover usually forms in October, further contributing to the insulation of the frozen ground. As winter progresses, soil freezing intensifies, and, in permafrost-affected landscapes, the seasonally frozen layer merges seamlessly with the underlying permafrost horizon.

The thawing of soils begins only after the complete melting of the snowpack, usually in early June. However, due to low summer temperatures and limited heat input, the seasonal thawing depth remains relatively shallow. In certain years, particularly during cooler summers, the active layer may fail to thaw completely, resulting in the persistence of perennially frozen sections near the surface throughout the warm period [30].

A detailed account of the soils and vegetation in the Vorkuta District was provided in earlier studies [31].

2.2. Sampling Sites

To obtain a statistically reliable and representative collection of soil materials from the tundra zone of the Komi Republic, a route-based sampling strategy was employed. The selection of sampling sites considered the morphological heterogeneity of the soil cover, variations in vegetation structure, and features of the local microrelief. Such an approach ensured that the diversity of soil-forming conditions within the studied territory was adequately represented in the dataset.

Samples from organic horizons were collected as composite specimens, each comprising five individual subsamples taken by the envelope method within an area of approximately 100 m². This technique effectively minimised micro-scale spatial variability inherent to organic horizons, resulting in a more homogeneous representation of their properties.

For the mineral part of the soil profiles, sampling was performed at designated key sites, where full soil pits were excavated. Given the relatively stable structure and lower heterogeneity of mineral horizons, samples were obtained by combining equal portions of material collected from three vertical walls of each profile. This procedure ensured the representativeness of the collected material across the entire horizon, and enhanced the comparability of analytical results among sampling locations.

All soil sampling locations were deliberately positioned at considerable distances from any potential sources of anthropogenic influence (Figure 1). Specifically, the selected sites were situated: more than 20 km away from the city of Vorkuta, active coal-mining areas, and thermal power plants; at least 10 km from smaller settlements; and over 5 km from major transportation corridors, including railways and motor roads. Such spatial isolation of the sampling points ensured that the collected materials reflected baseline geochemical conditions with minimal human-induced alteration.

In total, 280 composite (mixed) soil samples were collected and analysed during the course of the study. This dataset encompasses a broad spectrum of natural environments and soil types characteristic of the tundra landscapes of the Komi Republic, thereby providing a statistically robust basis for assessing the regional variability and baseline Hg values and other physicochemical soil parameters.

2.3. Methods

The chemical analysis of the soil samples was carried out in the eco-analytical laboratory of the Institute of Biology (Syktyvkar, Russia), This laboratory is accredited by the Rosstandart analytical laboratory accreditation system. All soil samples were oven-dried at 30 °C to minimise the release of gaseous mercury, and then sieved using a 2 mm mesh sieve.

Soil organic carbon (SOC) content was analysed using the Walkley–Black method with colourimetric termination on a Unico 2100 spectrophotometer (United Products and Instruments, Inc., Dayton, NJ, USA), measuring absorption at 600 nm [32]. Horizons containing ≥20% SOC were categorised as organic, while those with less were categorised as mineral [26].

The soil particle size distribution was determined according to the Kachinskii’s method [33], implemented in two successive stages. First, the soil samples were dispersed to break up the aggregates: this was achieved by treating them with an HCl solution, followed by boiling them in an alkaline medium (NaOH). The particles were then separated using electrostatic sieves with defined pore sizes. In the second stage, six granulometric fractions were isolated. The fine fractions were recovered from the suspension and oven-dried at 105 ± 5 °C. Their relative content was then calculated with respect to the oven-dry soil mass.

The pH values of the soil solution were measured in accordance with MI № 88-17641-004-2018 [34] using a HANNA HI 8519 N pH meter fitted with a HI 1332 combination electrode (Hanna Instruments, Póvoa de Varzim, Portugal). The pH values were measured in an aqueous suspension; the ratio of soil mass to the volume of distilled water was 1:5 for the mineral horizons and 1:10 for the organic ones.

The total Hg content (ω) was determined by atomic absorption spectrometry, using pyrothermal decomposition of the sample on an RA-915+ mercury analyser with a PYRO-915+ pyrolytic attachment (LUMEX, Saint Petersburg, Russia), in accordance with PND F 16.1:2.23-2000 [35]. This method is characterised by its high sensitivity (with a detection limit of 5 μg∙kg⁻¹) and acceptable accuracy. Quality control of the analytical procedures was performed at every stage using certified reference materials (CRMs, Halifax, NS, Canada), with at least 10% of the samples analysed being duplicates. This approach ensured the accuracy and reproducibility of the results obtained were validated.

In accordance with the recommendations of the US Environmental Protection Agency (EPA) [36], the 95% upper confidence limits (UCL_95%) of the average Hg concentration in the upper soil horizons were calculated for each soil type. The upper horizons were chosen because they are all organic (with the exception of horizon A0 of Umbric Fluvisols and O of Histic Cryosols) and have a high capacity for accumulation. They also serve as an important indicator of the aerotechnogenic load on soils.

A preliminary exploratory analysis was conducted to identify outliers or a substantial number of non-detect values, as these could have an adverse effect on the outcome of statistical analyses.

The Shapiro–Wilk test was used to assess whether the distribution of Hg concentration values in the upper (organic) horizons was normal or log-normal. First, the null hypothesis that the initial (non-normalised) data conformed to a normal distribution was tested. In all cases, the obtained p-values were less than 0.05, indicating that the Hg concentration distributions deviated from normal distribution. At the second stage, a similar analysis was performed on the logarithmically transformed Hg concentration values. The p-values obtained were all greater than 0.05, indicating that there were no statistically significant deviations from a lognormal distribution. The nature of the data distribution was taken into account when processing the results statistically, selecting the UCL_95% calculation method and interpreting the obtained data.

The UCL_95% was calculated based on the parameters of the normal distribution of logarithmic values (ln(ω)) using equation [36]:

$${\mathrm{U}\mathrm{C}\mathrm{L}}_{95\%}=\mathrm{e}\mathrm{x}\mathrm{p}(\overline{\ln \left(\omega \right)}+t_{0.95;n-1}·S_{\ln \left(\omega \right)})$$

To visualise the spatial patterns of the UCL_95%, a soil map of the study area (scale 1:1,000,000) was digitised alongside existing contours of different soil types (subtypes) [37]. Using a GIS programme (ArcView GIS 3.2a), each soil contour was assigned a colour depending on the UCL_95%.

A one-way ANOVA test was performed using Statistica 10.0 (p < 0.05) to determine differences in Hg content in the upper (organic) soil horizons. Two-dimensional correlation analysis was performed using Pearson’s correlation coefficient (r), and the statistical significance of this was assessed using the Neyman–Pearson approach. The observed value of the coefficient was then compared with the critical value for a two-way test and a significance level of 0.01.

3. Results

3.1. The Physical and Chemical Characteristics of Soils

The physicochemical parameters of the investigated soils exhibit pronounced heterogeneity (

Table 1. Soil characteristics (Vorkuta District, Komi Republic, Russia).

Soils	Horizons	Depth, cm	pH (H2O)	SOC, %	Clay (<0.01 mm), %
Stagnic Cambisols	O	0–7	4.8	40.9	– *
	G	20–50	5.3	1.4	37
	Bg	50–90	5.4	0.86	40
Histic Gleysols	O	0–21	5.8	32.6	–
	G	21–40	6.2	1.4	33.8
	Bg	60–95	6.1	0.55	28.4
Histic Cryosols	O	0–41	5.7	18.7	–
	G	41–55	6.1	0.89	31.6
	Gf	55–70	5.3	1.14	28.3
Entic Podzols	A0	0–4	4.2	8.3	–
	Eg	4–18	4.6	0.21	1.6
	Bhf	18–65	5.0	0.21	3.6
Folic Stagnic Retisols	O	0–12	4.8	39.4	–
	Eg	15–40	5.1	0.85	13.7
	Bg	50–140	5.6	0.17	26.2
Histic Stagnic Retisols	O	0–30	4.5	35.6	–
	Ehg	30–40	4.8	10.6	12.1
	Bg	55–90	6.0	0.36	23
Albic Podzols	A0	0–4	4.3	25.4	–
	Eg	4–10	3.9	0.86	2.5
	Bf	10–50	4.8	0.61	6.2
Stagnic Podzols	O	0–10	4.9	22.2	–
	Ehg	10–25	5.0	1.11	3.6
	Bg	25–60	5.0	0.44	2.6
Fibric Histosols	O	0–20	4.4	55.7	–
	H	20–50	4.3	60.7	–
	Chg	95–110	4.4	4.2	33.6
Umbric Fluvisols	A0	0–5	5.2	1.21	12.4
	A1	5–20	5.4	1.18	9.5

* Not analysed.

), which is expected to exert a substantial influence on the geochemical pathways and intensity of Hg compound migration and accumulation. Soil acidity displays a wide range of variation, ranging from strongly acidic conditions (pH 3.9 in the Eg horizon of Albic Podzols) to weakly acidic environments (pH 6.2 in the G horizon of Histic Gleysols). Numerous studies have demonstrated that under acidic conditions, particularly within the pH interval of 3–5, the solubility of inorganic Hg forms increases significantly, thereby facilitating their downward translocation through the soil profile [38]. Furthermore, at pH values below 5, the reduction in the sorptive capacity of clay minerals results in enhanced mobility and redistribution potential of Hg species [39]. Consequently, soil acidity can be regarded as one of the primary controlling factors determining the speciation, transformation, and migration dynamics of mercury in natural terrestrial systems.

The SOC content demonstrates marked variability, ranging from minimal concentrations in mineral horizons (from 0.17 to 10.6%) to remarkably elevated levels within the peat layers of Fibric Histosols, reaching up to 60.7% (Table 1).

The granulometric composition of the examined soils displays substantial heterogeneity. The highest proportions of clay are characteristic of the illuvial horizons of Stagnic Cambisols, whereas the lowest values are recorded in the podzolic horizons developed on sandy fluvioglacial deposits (Eg horizons of Podzols). It has been demonstrated by a considerable number of studies that fine-grained clay particles, particularly those associated with iron and aluminium oxides, exhibit a pronounced sorption affinity for heavy metal cations, including Hg [40]. Consequently, soils with a high clay content often act as effective geochemical barriers, immobilising Hg and thereby restricting its downward migration through the soil profile.

3.2. The Total Hg Content

A series of studies were conducted with the objective of ascertaining the total Hg content in various types of tundra soils in the Vorkuta District. The findings of these studies revealed significant variability, both between soil types and between their genetic horizons. The Hg content exhibited significant variation, ranging from trace amounts (i.e., less than 5 μg∙kg⁻¹) to a maximum of 210 μg∙kg⁻¹ (

Table 2. The total Hg content present within the tundra soils (Vorkuta District, Komi Republic, Russia), μg∙kg⁻¹.

Soils	Horizon	n	ωmin–ωmax	$\overline{\mathit{\omega }}$	±S	V, %	$\overline{\mathbf{ln}\left(\mathit{\omega }\right)}$	$\pm {\mathit{S}}_{\mathbf{ln}\left(\mathit{\omega }\right)}$
Stagnic Cambisols	O	20	28–210	100 cd	70	70.5	4.36	0.69
	G	12	14–20	17	5	17.6	–	–
	Bg	12	18–23	21	5	10.1	–	–
Histic Gleysols	O	20	104–200	150 e	40	20.3	5.01	0.21
	G	12	14–22	19	6	23.9	–	–
	Bg	12	18–23	25	8	22.3	–	–
Histic Cryosols	O	15	46–150	110 d	40	30.2	4.65	0.37
	G	9	11–18	14	5	24.5	–	–
	Gf	9	17–27	20	6	20.6	–	–
Entic Podzols	A0	10	20–80	60 bc	30	46.5	3.91	0.61
	Eg	6	5–11	7	4	24.5	–	–
	Bhf	6	7–15	12	4	27.7	–	–
Folic Stagnic Retisols	O	10	70–210	160 e	50	29.9	5.04	0.40
	Eg	6	16–28	22	7	21.4	–	–
	Bg	6	30–48	37	11	19.7	–	–
Histic Stagnic Retisols	O	10	80–200	130 de	50	34.4	4.83	0.34
	Ehg	6	18–36	27	10	27.4	–	–
	Bg	6	22–43	33	11	23.0	–	–
Albic Podzols	A0	10	18–46	32 b	11	35.8	3.40	0.39
	Eg	6	5–9	5.8	2.1	27.5	–	–
	Bf	6	5–12	9	3	27.1	–	–
Stagnic Podzols	O	10	28–70	43 b	23	47.8	3.69	0.44
	Ehg	6	6–13	10	3	25.1	–	–
	Bg	6	8–20	15	6	31.0	–	–
Fibric Histosols	O	15	34–117	80 c	30	33.2	4.41	0.26
	H	9	39–120	80 c	30	37.2	–	–
	Chg	9	17–33	21	8	29.3	–	–
Umbric Fluvisols	A0	10	7–26	18 a	9	42.6	2.88	0.58
	A1	6	5–20	14	7	41.6	–	–

n—number of mixed soil samples; ω_min–ω_max—range of values; $\overline{\omega }$—arithmetic mean; S—standard deviation; V—variation coefficient. Different lowercase letters (i.e., a, b, c, d, and e) show significant differences among total Hg contents in the upper (organic) horizons (p < 0.05).

). The lowest values were recorded in mineral horizons of soils developed on sandy soil-forming rocks (Podzols). The maximum values were recorded in the upper (organic) horizons of Cambisols, Gleysols and Retisols.

Elevated levels of total Hg are consistently detected in the upper (organic) layers of tundra soil. According to most published research, the uppermost soil layers of tundra landscapes are active zones of Hg accumulation. This phenomenon is primarily governed by several interrelated environmental and biogeochemical factors, including the substantial presence of organic substances, persistently low temperatures, limited decomposition and mobility of organic matter, and overall suppression of microbial activity. These features are characteristic of tundra ecosystems, where peat horizons form and thicken at an extremely slow rate. Consequently, Hg becomes tightly bound to stable organic components, which leads to its long-term preservation and gradual accumulation in the soil profile.

In the upper (organic) horizons of tundra soils developed on loamy substrates, the concentration of total Hg varies within a wide range (100 to 150 μg∙kg⁻¹), accompanied by high coefficients of variation (up to 70.5%). This variability reflects pronounced spatial heterogeneity in Hg distribution, which is most likely controlled by microtopographic differences, variations in the composition and quality of soil organic matter, and fluctuations in soil moisture content. The enrichment of mineral horizons with Hg (up to 48 μg∙kg⁻¹) may be attributed to cryoturbation processes, which mechanically mix and displace surface material downward into deeper soil layers [1,41]. Additionally, enhanced vertical migration of Hg can occur during seasonal thawing and snowmelt when the mobilisation of water-soluble organic compounds facilitates its downward transport, a phenomenon previously observed during the warm summer period [42]. Recent research conducted in Alaska has also documented notable Hg concentrations in the mineral fractions of tundra soils [43]. Furthermore, where mineral horizons remain water-saturated for part of the year, the vertical translocation of Hg-bearing compounds is likely to be intensified, contributing to the redistribution of this element within Arctic soil profiles.

Podzolization processes are particularly well expressed in soils developed on sandy parent materials, such as Albic and Stagnic Podzols. The concentrations of total Hg in these soils are generally lower, particularly in the mineral horizons (Eg and Bf), where the Hg content does not exceed 15 ± 6 μg∙kg⁻¹. This pattern likely results from a combination of factors. Firstly, the dominance of eluviation processes promotes the leaching of Hg together with water-soluble organic and inorganic complexes. Conversely, the inherently low SOC content in these sandy substrates limits the sorptive capacity of the solid phase, thereby reducing the soil’s ability to retain Hg.

The greatest accumulation of Hg was recorded in Retisols and Histic Gleysols, particularly in the peat horizons of these soils, where concentrations reached 160 ± 50 μg∙kg⁻¹. The observed differences in Hg content among these soil types were statistically insignificant, indicating similar sorption behaviour and suggesting that comparable geochemical mechanisms govern Hg retention. These findings emphasise the critical role of the peat layer as an effective sink for Hg, capable of binding this element predominantly through interactions with organic matter. This accumulation is further enhanced under acidic conditions and periodic water saturation, as these favour the stability of organo-mercury complexes and limit the mobility of the element within the soil profile.

Of particular interest are the peat soils of high moor bogs (Fibric Histosols), where the mean Hg concentration is approximately 80 ± 30 μg∙kg⁻¹ in both the O and H horizons. Despite their high moisture content and abundance of organic material, the Hg levels in these soils are statistically lower than in other peatland types. This may be due to the specific characteristics of the hydromorphic transformation of organic matter. In anaerobic conditions, the oxidation of plant residues and the formation of oxygen-containing functional groups (carbonyl, carboxyl and phenolic), which are responsible for binding Hg ions and affecting their mobility and retention in the peat profile, is not promoted [44,45].

The mean Hg concentration in Umbric Fluvisols is relatively low, reaching up to 18 ± 9 μg∙kg⁻¹, while the coefficient of variation exceeds 40%. These patterns reflect the influence of alluvial deposition processes and the limited thickness of the humus horizon, both of which constrain these soils’ capacity to accumulate Hg. The comparatively small variations in Hg content are therefore largely controlled by the heterogeneity of the alluvial material and the weaker development of organic-rich layers, which typically serve as major sorption sites for Hg in other tundra soil types.

4. Discussion

Analysing the physicochemical properties of the studied soils enables us to distinguish the characteristic features of each soil group and evaluate their influence on the behaviour of Hg within the soil environment. The obtained data suggest that peat soils provide the most favourable conditions for the accumulation of Hg compounds owing to their high SOC content and persistent water saturation. Under these conditions, a reducing redox potential promotes the retention of Hg and the formation of methylmercury species [46,47]. Stagnic Cambisols, characterised by moderately acidic pH values and a high clay and SOM content in the upper soil layers, demonstrate a significant capacity for Hg immobilisation and accumulation.

Conversely, Entic, Albic and Stagnic Podzols have a low sorption capacity. The combination of low SOC content, a small proportion of fine particles and highly acidic conditions increases the mobility of Hg compounds within these soil profiles. Consequently, such soils pose a potential risk with regard to the downward translocation of Hg into deeper soil layers and, ultimately, into groundwater systems [38,48].

Comparisons with similar studies conducted in other northern regions [43,49,50] suggest that Hg concentrations in the upper (organic) horizons of tundra soils in Alaska, Canada and Scandinavia usually fall within the range of 100 to 160 μg∙kg⁻¹, which is consistent with the results obtained in the present study. These similarities suggest that the mechanisms governing Hg accumulation in tundra soils are largely universal, reflecting global patterns of Hg transformation, fixation, and retention under cold-climate conditions.

The upper (organic) soil horizons act as key geochemical barriers within the soil profile where elemental fractionation occurs according to physicochemical characteristics and mobility. Elements such as Hg concentrate in these horizons due to their uptake by vegetation and subsequent incorporation into decomposing organic residues.

It is acknowledged that organic matter exerts a complex and frequently dual influence on the behaviour of Hg in soils. On the one hand, humic substances—particularly humic acids—have been shown to form strong complexes with Hg ions. This substantially reduces their solubility and mobility within the soil profile [10,51,52]. Conversely, organic compounds have been observed to engage in mercury methylation processes, particularly under conditions of reducing or anaerobic activity. This process results in the generation of methylmercury, a highly mobile and toxic species [39,53]. Furthermore, the presence of fulvic acids and low-molecular-weight organic acids has been documented to enhance the lability and transport potential of Hg ions through the formation of soluble complexes [54]. It is therefore evident that the type and SOC composition play a decisive role in regulating both the immobilisation and transformation of Hg in terrestrial environments.

The extent of this enrichment depends on a variety of interconnected factors, such as atmospheric deposition (both natural and human-induced), cryogenic and hydromorphic processes, the binding capacity of organic compounds, the kinetics of complex formation, the degree of organic matter decomposition and the ecosystem’s overall biological productivity [55]. However, this natural enrichment process is often overlooked in contemporary environmental assessments, where elevated concentrations of Hg and other trace elements in surface horizons are frequently attributed solely to anthropogenic contamination.

In recent decades, the issue of large-scale human impact on the natural environment has attracted growing scientific interest, particularly with regard to the transboundary movement of pollutants. Hg is of particular concern among these contaminants due to its high mobility and persistence in the atmosphere. Numerous studies based on diverse environmental archives, such as polar ice and snow cores [56,57], as well as terrestrial mosses used as bioindicators [58], have convincingly demonstrated that Hg can be transported over vast distances from its emission sources. However, establishing the relative contribution of anthropogenic and natural sources to the Hg pool in soils distant from industrial centres remains a complex and unresolved issue. Current analytical data suggest that Hg concentrations often differ significantly between organic and mineral soil horizons. These discrepancies are generally interpreted as reflecting the combined influence of multiple factors, including the mineralogical and chemical composition of the parent materials, the accumulation and transformation of organic matter, and the specific features of regional biogeochemical cycling. However, it is unclear to what extent these variations are controlled by geogenic and biogenic processes, or whether they are the result of long-term atmospheric deposition of anthropogenic mercury. This ambiguity highlights the need for further comprehensive studies to distinguish natural background levels from the effects of global, human-induced Hg contamination.

According to environmental regulations in Russia [59] and the UK [60], soil contamination levels should be assessed based on regional background concentrations, or natural reference levels, for each element. Alternatively, they can be assessed based on the 95th percentile of mean element concentrations (UCL_95%) [36]. This study clearly demonstrates that total Hg content exhibits considerable spatial variability, even within a single genetic soil type. Consequently, from an environmental monitoring perspective, it is more reasonable to rely on the UCL_95% of Hg concentration characteristic of a particular soil type than on measures of central tendency such as the mean or median. Concentrations of Hg that do not exceed this threshold can be considered the geochemical baseline level specific to the corresponding soil group [36]. Although using the 95th percentile as a boundary criterion is somewhat conventional, this method effectively captures the predominant range of natural variability while excluding anomalously high, non-representative values. Therefore, this statistical framework provides a more objective basis for defining regional baseline Hg values.

Calculating the UCL_95% of Hg concentrations in the soil samples (Figure 2) showed that Retisols, Stagnic Cambisols and Histic Gleysols have the highest permissible background values. The latter two soil groups are notably the most widespread in the Vorkuta District of the Komi Republic, collectively accounting for nearly half of the region’s total land area. These distribution patterns reflect the dominance of hydromorphic conditions and the accumulation of organic matter, which strongly influence Hg retention in the soil profile.

Hg analysis with UCL_95% is a common approach for determining and distinguishing background and baseline Hg levels in soils from those in areas with anthropogenic influence. For instance, studies in northern and temperate regions have produced the following benchmarks. A study of Hg concentrations in the surface (organic) horizons of Arctic soils in the Svalbard archipelago revealed Hg content ranging from 41 to 254 μg∙kg⁻¹, with median values in these natural Arctic soils comparable to background levels in the Arctic [61]. While no specific UCL_95% values were calculated in this study, the concentration range indicates the upper limit of natural Hg content in Arctic soils as being close to 250 μg∙kg⁻¹.

In the context of temperate northern soils in Europe and North America, background values for Hg have been proposed based on statistics from a large number of observations. For instance, an analysis of the regional distribution of Hg in surface soil layers in the EU indicates that 84% of Hg values lie below ~85 μg·kg⁻¹, while the top 1% exceed 422 μg·kg⁻¹ [13,15], reflecting the influence of both natural factors and anthropogenic sources. Although the UCL_95% has not been calculated directly for this dataset, the percentile distribution can be used to understand Hg levels in different soils across Europe.

Studies of mercury (Hg) distribution in the upper soil horizons of temperate latitudes in the United States indicate wide variability. Statistical models have shown that concentrations of Hg exceeding 200–300 μg·kg⁻¹ are extremely rare and are usually the result of human activity or local sources [62]. These data enable high percentile values (e.g., 95% or above) to be used as thresholds for evaluating background conditions in comparable geochemical landscapes in North America.

The Canadian Soil Quality Guidelines for the Protection of Environmental and Human Health for inorganic Hg concentrations emphasise that natural Hg levels in surface soils vary by region and organic matter content, with the UCL_95% serving as a benchmark for identifying outliers [63]. These measurements consistently exceed background levels in many temperate forest soils in Europe, highlighting regional differences driven by climate, biomass and historical sedimentation.

A comparison of Hg distribution data in soils from various northern regions highlights the need for regional interpretation of UCL_95% values. In Arctic soils, an increase in the natural distribution of Hg in the upper (usually organic) horizon is primarily associated with high SOC content and stable hydrological conditions. In contrast, in temperate forest and peat landscapes, threshold values shift towards lower levels. The UCL_95% values established in this study for each soil group are consistent with the statistical characteristics of Hg content in northern soil ecosystems. However, local calibration is required, taking into account the composition of organic matter, pH and soil texture.

It is also important to note that inter-study comparisons demonstrate the extent to which UCL_95% depend on local geochemical conditions. This emphasises that there is no single ‘universal’ boundary for baseline Hg content in all northern regions, and that regional UCL_95% values provide the most reliable basis for environmental soil assessment.

A statistically significant, positive correlation was identified between Hg concentrations and the SOC across all horizons studied (r = 0.687). This clearly indicates that Hg accumulation in soils is largely governed by the quantity and composition of organic matter. Humic and fulvic substances have a strong binding capacity, primarily due to the presence of reactive functional groups, such as carboxyl, phenolic and thiol moieties, which readily form stable organometallic complexes with mercury ions. This markedly decreases their solubility and mobility in the soil environment [10]. Similar patterns have been observed in several independent studies, in which the concentration of organic carbon accounted for 60–80% of the total variability in Hg content within surface horizons. Taken together, these findings confirm the central role of soil organic matter as a major geochemical barrier controlling mercury retention and limiting its redistribution within terrestrial ecosystems [13,64,65].

Furthermore, elevated levels of organic matter in soil can have a dual effect on Hg dynamics. On the one hand, organic compounds enhance Hg retention through complexation and sorption processes, thereby contributing to its accumulation within the solid phase. Conversely, under conditions of excessive moisture, periodic waterlogging and active lateral or vertical runoff, these same organic ligands can facilitate the remobilisation of Hg by forming soluble organomercury complexes. This secondary mobilization mechanism leads to the partial transfer of Hg from stable soil fractions into more mobile, dissolved forms. Numerous hydrological observations provide evidence for this dual behaviour, demonstrating that increases in dissolved organic carbon concentration are frequently accompanied by corresponding rises in dissolved Hg content. These correlations highlight the complex and sometimes contradictory role of soil organic matter as both a geochemical sink and a potential source of Hg under varying hydrological regimes [66].

A statistically significant positive association was also found between total Hg concentrations and the proportion of clay particles (<0.01 mm) in the soils (r = 0.497). This finding lends support to the idea that the fine-grained fraction acts as an additional geochemical sink for Hg due to its high specific surface area and the abundance of reactive mineral surfaces, including those of clay minerals, as well as iron, manganese and aluminium oxides. The enhanced sorption capacity of these components provides multiple binding sites for Hg ions, thereby stabilising the element within the soil matrix. Similar dependencies have been observed in various environmental settings. For example, studies of Hg distribution in polluted agricultural soils in north-eastern Thailand [67] and sandy substrates in southern Sweden [68] have shown that Hg concentrations increase proportionally with the amount of fine particles present. These findings emphasise the vital role of mineral surface sorption in regulating Hg retention and restricting its downward or lateral migration.

Additionally, the influence of the clay fraction is closely linked to its interaction with organic matter. Within illuvial horizons (Bf), the formation of stable clay–humus complexes creates microenvironments favourable for the secondary accumulation of Hg. These organo-mineral associations act as composite sorbents, enhancing the immobilisation of Hg and contributing to the vertical redistribution of the element within the soil profile [69,70].

Soil acidity is widely recognised as one of the key environmental parameters that govern the mobility and speciation of Hg within terrestrial ecosystems. This is primarily due to its influence on the chemical form of Hg and its interaction with the organo-mineral matrix of soils [71,72]. Mercury is least soluble near pH 3, a range at which humic acids—major constituents of soil organic matter—tend to precipitate, thereby immobilising a considerable portion of mercury through complexation and co-precipitation processes. Under such strongly acidic conditions, sorption processes are further enhanced by the high affinity of Hg²⁺ ions for reactive mineral surfaces, particularly goethite and manganese hydroxide, which have maximum adsorption capacity at pH values below 4 [73,74].

As soil pH increases towards slightly acidic or near-neutral conditions (approximately pH 5–7), a notable shift in Hg behaviour is observed. The partial dissolution of humic substances that previously retained Hg in solid form leads to an increase in the concentration of dissolved Hg species, thus enhancing the element’s mobility within the soil solution [75,76]. This transition reflects the delicate balance between sorption and desorption processes, which are pH-regulated. The negative correlations observed between total Hg concentration and soil acidity (r = −0.373) and between pH and the SOC content (r = −0.533) are consistent with this interpretation. This indicates that, within the available pH range (3.9–6.2), soils with lower pH values tend to exhibit stronger Hg fixation and higher organic matter accumulation. Together, these findings highlight the critical role of soil acidity in regulating the partitioning of Hg between solid and dissolved phases, as well as its subsequent environmental mobility.

5. Conclusions

This study provides a comprehensive assessment of the spatial and vertical patterns of total Hg distribution in tundra soils in the European North-East of Russia. The results suggest that observed variability in Hg concentrations is primarily influenced by the interaction of landscape and geochemical factors, the qualitative composition of soil organic matter, particle size characteristics and acid–base conditions. The highest Hg accumulation was recorded in the upper (organic) horizons of Retisols and Histic Gleysols, with concentrations reaching up to 210 μg∙kg⁻¹. This enrichment is largely due to the high sorption capacity of humified organic material, the prevalence of reducing environmental conditions and the relatively slow rates of organic matter mineralisation typical of cold, waterlogged tundra ecosystems. Conversely, the lowest Hg concentrations were found in soils formed on sandy fluvioglacial deposits, where the scarcity of fine particles and organic carbon, coupled with pronounced soil acidity, increases the mobility and potential leaching of Hg compounds. These findings emphasise the vital role of soil-forming processes and substrate properties in determining the geochemical behaviour and environmental fate of Hg in northern landscapes.

The findings of this study demonstrate that geochemical baseline Hg values can vary considerably even within a single soil type. This significantly limits the reliability of using simple arithmetic averages as reference values. A more robust and scientifically sound approach is to establish the 95% upper confidence limits of the average Hg concentration. UCL_95% effectively accounts for geochemical variability while minimising the influence of outliers and anomalously high measurements. The highest UCL_95% of Hg values were determined for Retisols, Stagnic Cambisols and Histic Gleysols within the Vorkuta District (Komi Republic, Russia). These results emphasise the necessity of incorporating the specific properties of the upper (organic) soil layers into regional soil quality standards and environmental risk assessments. The study therefore provides a scientific basis for harmonising regional regulatory frameworks governing Hg concentrations in soils in the Vorkuta District, ensuring that heterogeneity and the unique features of the Arctic pedosphere are adequately reflected in environmental monitoring and risk evaluation practices.

Correlation analysis has demonstrated that the Hg accumulation in tundra soils is predominantly controlled by the SOC content, whereas the effects of grain-size composition and actual acidity are of secondary importance. The strong association between Hg and organic matter provides compelling evidence of the central role played by humic and fulvic complexes in Hg immobilisation. However, this relationship also implies potential environmental vulnerability, as changing hydrothermal conditions, increased soil moisture and temperature fluctuations may enhance the dissolution of organic compounds and promote Hg remobilisation and migration within the soil profile.

Further research into the Hg speciation in soils is promising, including that on the ratio of elemental, inorganic and organically bound Hg and the potential formation of methylmercury in organic horizons. Considering the different Hg forms will enable a more accurate evaluation of changes in its mobility and ecological hazard in the context of permafrost degradation and climate-induced alterations to the hydrothermal regime.

The outcomes of this research are therefore fundamental for refining regional criteria for geochemical baseline Hg values, improving environmental monitoring protocols and predicting possible shifts in Hg behaviour in the event of permafrost degradation and climate-induced changes to Arctic hydrothermal regimes.