Changes in the Degree of Contamination of Organic Horizons of Al–Fe-Humus Podzols upon a Decrease in Aerotechnogenic Loads, the Kola Peninsula

    Contamination levels of the organic horizon of Al–Fe-humus podzols (Albic Rustic Podzols) in the zone affected by atmospheric emissions of the Severonikel smelter (Murmansk oblast) within a 20-yearlong period are compared. The spatiotemporal changes in the total content of heavy metals in the soils in response to a decrease in aerotechnogenic loads have a complicated pattern. As the content of heavy metals in the soils varies widely, the correlation between their amount in the organic soil horizon and the distance from the contamination source is absent. In response to the ninefold decrease in the amount of atmospheric emission of Ni compounds, the bulk content of Ni in the organic horizons of podzols reliably decreased by 2.5 times. The threefold decrease in the emission of Cu compounds proved to be insufficient for a significant decrease in the Cu content in the soils. In 2016, the content of heavy metals in some sampling points even increased in comparison with the earlier periods. The Ni-to-Cu ratio in the soil samples changed significantly. In 1989–1994, bulk forms of heavy metals in the soil samples formed the sequence Ni > Cu > Co; in 2016, it changed to Cu > Ni > Co, which corresponds to the proportions of these metals in the aerial emissions. Under conditions of the continuous input of heavy metals from the atmosphere, the contamination of the organic horizons of podzols with heavy metals remains at the high or very high levels. Keywords: heavy metals, environmental pollution, Al–Fe-humus podzol, Albic Rustic Podzol, the Kola Peninsula, decreasing aerotechnogenic loads
DOI: 10.1134/S106422931803002X

    The negative impact of atmospheric contamination on natural ecosystems near nonferrous plants is well studied [50]. In the impact zone, terrestrial ecosystems are disturbed up to the state of the complete degradation with the formation of technogenic barrens [3, 4, 6, 8–10, 15, 27–30, 40, 46, 49, 50]. Eroded soils of such barrens are often acid, poor in nutrients, and contaminated by various heavy metals (HMs). In contrast to the soil pollution with oil products, when organic compounds undergo microbiological or chemical degradation and may be subsequently consumed by plants, micromycetes, and soil fauna, HMs at high concentrations are xenobiotics that may accumulate in soils for a long time. It has been shown that the period of half removal of HMs from the surface soil layer under the impact of leaching, uptake by plants, erosion, and deflation is from 13 to 110 years for Cd, from 70 to 510 years for Zn, from 310 to 1500 years for Cu, and from 740 to 5900 years for Pb [14].
    In Russia, the major sources of the environmental pollution with HMs are enterprises of the Norilsk Nickel mining and smelting metallurgical company (Severonikel, Pechenganikel, and Norilsk Nickel plants), the Sredneural’skii copper smelter (SUCS), and the Karabash copper smelter (KCS), The emission from these enterprises into the atmosphere was the greatest (140000–360000 t/yr) in the period from 1970 to the late 1980s. By the beginning of the 2000s, the emissions were reduced by three–five times; the reduction of emissions continued in the 2000s. At present, the do not exceed 5000–35000 t/yr [7, 25, 50]. The reduction of emissions has led to smaller aerotechnogenic loads on the ecosystems. In this context, it is interesting to study dynamic trends of changes in the particular ecosystem components and in the entire ecosystems in response to the decreasing loads. The first results of such studies have already been published [7, 8, 11, 13, 17, 18, 35, 41–45]. However, they are not numerous, and many aspects of this problem have yet to be examined. The aim of this work was to evaluate the response of organic horizon of Al–Fe-humus podzols (Albic Rustic Podzols) to a decrease in the atmospheric emissions from the Severonikel smelter in Murmansk oblast.

Acid-forming sulfur compounds and HMs entering the atmosphere with the emissions of the largest in Europe Severonikel copper–nickel smelter are the main contaminants of natural ecosystems in the Kola Peninsula. The operation of the plant started in 1939, and the local low-sulfur ore was used [38]. Late in the 1960s, when the ore reserves were exhausted, highsulfur ore (up to 30%) of the Norilsk deposit was applied, which resulted in a sharp increase in the emission of sulfur dioxide and associated solid substances into the atmosphere. The fine-dispersed polymetallic dust is predominated by sulfides and oxides of metals, as well as by Ni and Cu [2, 47]. In addition to HM compounds, iron oxides and calcium, magnesium, and aluminum silicates are present in the metallurgical dust. Owing to reconstruction of the plant, the emission of contaminants into the atmosphere began to decrease since 1990. According to the latest data, the emission of sulfur dioxide and solid particles from the Severonikel smelter reaches 37300 and 2700 t/yr, respectively, which is about six times smaller in comparison with maximum emissions in the 1970s–1990s. The Kola Peninsula is located at the northern margin of forest biogeocenoses at the boundary between northern taiga and forest-tundra zones. The plains are mainly occupied by low-productive lichen and dwarfshrub–true-moss spruce and pine forests. According to the modern classification of 2004 (cited from [37]), the soils of the investigated region belong to the order Al–Fe-humus soils and the type of podzols; they are mainly represented by the subtypes of iron-illuvial and/or humus-illuvial podzols [37]. Their profile is characterized by a simple morphology: a raw-humus forest litter (the O horizon), a well-pronounced but thin podzolic (E) horizon, an illuvial (BH or BHF) horizon with sufficiently high content of illuviated humus, and parent material (the C horizon) [3]. The upper part of the soil profile (within the organic horizon) is specified by strongly acid reaction, high total (hydrolytic) acidity, moderate contents of potassium and phosphorus, and low nitrogen content. The organic horizon of soils in the northern taiga biogeocenoses is the main source of the mineral nutrition of plants; most of roots of forest-forming species and plants of the grass–dwarf-shrub layer are located in the organic horizon. This horizon is often considered a biogeochemical barrier for HMs entering the soil from the polluted atmosphere [8, 20, 26–29, 31, 32]. The organic horizon of podzols was sampled in the area under the aerotechnogenic plume of the Severonikel smelter or near it. This area is characterized by the strongest contamination by metal-containing emissions and includes the shores of Lake Moncheozero to the north of the smelter (19.4 km), the city of Monchegorsk, and the areas to the south and southwest of the smelter (the maximum distance from the contamination source is less than 5 km). The schematic map of soil sampling is presented in Fig. 1, and the geographical coordinates of sampling sites and their distances and directions from the smelter are given in Table 1. Samples of the organic horizon were taken at sites no closer than 300 m from local roads. At each site, the samples were taken at three points disposed in the corners of a triangle and spaced apart at 50–100 m from one another. Individual samples were then combined into the average mixed sample, and all foreign inclusions were removed from it. The mixed samples were dried at 40°Ñ; the mass of dried sample was no less than 50 g.

The total contents of Ni, Cu, and Co in the organic soil horizons were determined in air-dry weighed soil samples dissolve in in aqua regia (with heating). Cool solutions were filtered into polyethylene containers. The content of metals in solution was determined by the atom-absorption method on an ÀÀS-36 spectrophotometer. The relative error of determination was less than 10–15% and corresponded to the normal error of determination of the chemical composition of mineral raw materials according to the third accuracy category (OST 41-08-212-04). The correctness and accuracy of analytical data were controlled according to OST 41-08-214-04 and OST 41-08-265-04. The statistical treatment was performed with the use of Statistica 10.0 software. The significance of differences was evaluated by the Kruskal–Wallis and Wilcoxon tests.

The results of our study attest to complicated spatial and temporal patterns of changes in the degree of contamination of the organic horizons of Al–Fehumus podzol with HMs in response to the decrease in toxic loads on the ecosystems. It should be noted that the bulk contents of HMs widely varied both in 2016 and in the earlier years (Table 2).

In 2016, variation ranges were 185– 1970mg/kg for Ni, 280–3910 mg/kg for Cu, and 25– 125 mg/kg for Co. In 1989–1994, the ranges were 370–5180 for Ni, 150–4290 for Cu, and 48–103 mg/kg for Co. Thus, maximum concentrations exceeded minimum concentrations by 11–14 times for Ni, 14– 29 times for Cu, and 2–5 times for Co.
    The samples from site no. 14 located at the greatest distance (about 23 km) to the northeast of the smelter t were characterized by the lowest total content of all the studied HMs in both observation periods. The highest total contents of Ni (in 2016), Cu (in 1994 and 2016), and Co (in 2016) in the organic horizons of podzols were at site no. 1 at a distance of 6.2 km to the northwest of the plant. In 1993–1994, the highest contents of Ni and Co were determined at sites no. 11 (3.1 km to the northwest of the smelter) and no. 4 (15.3 km to the north-northwest of the smelter, respectively. A high variability in the bulk content of Ni content in the organic horizons of podzols within 20 km from the Severonikel smelter (from 1160 to 9980 mg/kg) was also found in the study by Kashulina with coauthors [19].
    The correlation analysis has shown the absence of the dependence of the content of all the studied HMs on the distance from the smelter (r = –0.06–0.08, p > 0.05) both in 1989–1994 and in 2016. We tried to group initial data in various ways: compass direction, frequency of wind of particular direction, classification position of plant communities, and moistening rate of habitats. However, no reliable differences in the contents of the studied HMs in the soil samples regardless of these groupings were found for both according to the Fisher criterion, or the Kruskal–Wallis nonparametric criterion. This is explained by the great variability of the analyzed parameters, which may be illustrated by the example of HM contents in samples taken in the immediate vicinity of the smelter (sampling sites no. 2–4) (Fig. 1, Tables 1, 2). In the first sampling period, variations in the total contents of Ni, Cu, and Co in the organic horizon of podzols were insignificant (1.1–1.5 times); in 2016, they were 4.4 times for Ni and 4.8 times for Cu, but remained insignificant for Co.
    The absence of correlation between the contents of HMs in the organic horizon of podzols and the distance of sampling from the contamination source can be explained by the methodological specificity of our study. Soil samples were taken within a relatively small (up to 20–25 km) distance from the smelter. At such distances, variation in the contents of metals is not very large (about an order of magnitude), whereas local spatial variability typical of the aerotechnogenic contamination is considerable. If we study the samples arranged along the gradient of pollution with due account for the local conditions (i.e., geomorphic position, slope aspect, type of soils and parent materials, vegetation, etc.), certain correlation between the contents of HMs in the soils and the distance from the source of contamination can always be seen [2, 3, 8– 12, 19, 20, 27, 28, 31, 32, 34, 46, 49, 50].
    The Ni-to-Cu ratio in the soil samples was significantly different in the two observation periods (Table 2). In 1989–1994, the total Ni content in the samples was considerably higher than the total Cu content according to the Wilcoxon criterion (z = 3.62, p = 0.0003), which was probably related to the higher rates of emission of Ni compounds into the atmosphere (z = 3.06, p = 0.002) in 1990–2001 in comparison with Cu emissions (Fig. 2).

In 2002–2016, the opposite ratio between the volumes of emission of these two elements was observed: the annual Cu emission was considerably higher than the annual Ni emission (z = 2.27, p = 0.023); this led to a higher total Cu content in the studied samples in 2016 (z = 3.89, p = 0.0001) in comparison with the Ni content (Table 2). The total Co content in the organic horizon was lower in comparison with Ni and Cu; the difference between the two observation periods was insignificant (z = 0.71, p = 0.48). With respect to their concentrations in the organic horizons of podzols, the studied HMs can be arranged into the following sequences Ni > Cu > Co in 1989– 1994 and Cu > Ni > Co in 2016. A comparative analysis of data on the contents of HMs in the organic horizon of podzols for the two observations periods shows an ambiguous response of this horizon to the decrease in aerotechnogenic loads.
    The emission of Ni and Cu into the atmosphere in 1989–1994 averaged 2210 and 1400 t/yr; in 2016, it decreased to 245 and 462 t/yr, respectively. Thus, the emission of Ni decreased by nine times, whereas the emission of Cu decreased by three times. The nonparametric Wilcoxon criterion shows that Ni concentrations in the organic horizon significantly dropped in 2016 (z = 3.86, p= 0.0001; by 2.5 times on the average) in comparison with those in 1989–1994. For Cu and Co, the differences were not reliable. In addition, different sampling sites were characterized by different trends in the contents of HMs during the studied period. For example, the content of HMs decreased 5–11 times at some sampling points in 2016 (point no. 2) and increased 2–11 times at other sampling points (point no. 6). The ratio between Ni contents in the samples of 1989–1994 and the samples of 2016 varied from 0.4 to 10.9; for Cu, it varied from 0.1 to 5.0 (Fig. 3).

It may be concluded that the ninefold decrease in the emission of Ni compounds into the atmosphere resulted in a 2.5-fold decrease in the bulk Ni content in the organic horizon; the threefold decrease in the emissions of Cu compounds did not cause a significant decrease in the Cu content in the soil samples. That is to say, a threefold fold decrease in the atmospheric emissions of the plant is insufficient for a significant drop in the degree of soil contamination with HMs. In the case of the ninefold reduction of atmospheric emissions, the decrease in the content of HMs in the organic horizons of contaminated podzols becomes more pronounced. However, the total content of HMs in the organic horizon of podzols in 2016 increased by 2–11 times at some sites in comparison with the first observation period.
    There are evidences [23] that only the content of mobile Ni compounds decreased by 1.3–1.9 times over the period of 2002–2011, while the Cu content remained unchanged. Our data indicate that contents of acid-extractable forms of HMs in 2016 in the soils of the buffer and impact zones of the Severonikel smelter was 2–4 times higher than that in 1981–1999 [32, 34]. Numerous investigations prove that the contents of Ni, Cu, and Co in the upper organic soil horizon within the 15- to 20-km-wide impact zone of the Severonikel smelter increases by two–three orders of magnitude [3, 10, 11, 17, 19, 20, 31, 32, 34, 48, 51]. Acomparison of our data with maximum permissible concentrations (MPCs) established for the mineral soil horizons shows that, in 2016, they were exceeded for the studied metals at all the sampling sites [39]: 5– 50 times for Ni and 8–120 times for Cu. MPCs were exceeded even at the most distant sampling site no. 14 (5.0–8.5 times), and the greatest excess of the MPC was found at site no. 1 6.2 km from the smelter (50times for Ni and 120 times for Cu).
    According to the scale of ecological norming [36], a very high contamination level was seen in 50% (for Ni) and in 92% (for Cu) of samples taken in 2016; a high contamination level was determined in 37% and 8% of the soil samples, respectively; and only 13% of samples were assigned to moderately contaminated soils with respect to the Ni content. Hence, the contamination of the organic horizon of podzols by HMs remains high or very high despite the 3–9-fold decrease in atmospheric emissions and 2.5-fold drop in the Ni content of the soil. The similar conclusion was made by all investigators of soil contamination near the enterprises of nonferrous metallurgy over a long period, when the regimes of atmospheric emissions of plants differed [7, 10, 16, 17, 42–45]. Though there are some evidences of a 1.4–5.7-fold decrease in the total HM content in soils early in the 21th century in comparison with 1980–1990, soil phytotoxicity changed slightly, and soil contamination by HM remains very high [10, 17, 45]. It is known that organic soil horizon is an important biogeochemical barrier, where pollutants are fixed and are prevented from penetration deep into the soil and into adjacent media [5, 8, 20, 26–29, 31, 32]. Continuous input of polymetallic dust from the air results in constantly high and very high contamination of podzols in the studied area.
    The metallurgical dust—the component of atmospheric emissions of the smelter—is a finely dispersed mixture of sulfides and oxides of metals: chalcocite (Cu2S), chalcopyrite (CuFeS2), pyrrhotine (Fe7S8(Nix)), pentlandite ((Ni, Fe)9S8), covellite (CuS), cuprite (Cu2O), melaconite (CuO), and metallic Ni and Cu [2, 46, 47]. As shown earlier [33, 34], the particles of ashed samples of organic horizon of podzols are mainly represented by various primary minerals and iron oxides, and the soil material in the impact zone contains a large amount of spherical particles enriched with HMs. The study of the internal structure of the spherical particles shows that their basis is mainly formed by magnetite with admixtures of Ni and Cu.
    The amount of melt silicate composed of olivine is smaller: it fills the space between dendrites. Sulfides of Ni (heazlewoodite) and Cu (chalcocite) occur as small phases inside the most spherical particles. This composition and structure of phases is typical for converter slag formed at smelting of nonferrous metals [1]: the spherical particles are the drops of slag carried into the atmosphere with final gases. A field experiment has shown that 14 years after the application of polymetallic dust, some spherical particles are still preserved unchanged in the organic soil horizon [33], i.e., their transformation is strongly retarded.
    The compounds of HMs in spherules are difficultly soluble in water, but may participate in complex-forming reactions with soil organic acids (humic and fulvic acids) and undergo transformation from difficultly soluble sulfides into water-soluble sulfates. With time, the portion part of easily soluble complexes and salts derived from difficultly soluble compounds becomes greater. Nevertheless, this transformation may take a lot of time, which is proved by experimental data on transformation of particles of polymetallic dust [33]. In addition, Ni and Cu differ in fixation rate of their organomineral compounds in soil. It is known that Cu is a strong complex-forming element, whose complexes with organic ligands are stable, while the complexing capacity of Ni is significantly lower. At the same time, the leaching rates of these elements from contaminated soils are different. According field experiments [8, 31, 48], after the simultaneous Ni and Cu application, the Ni content in the soil decreases more actively within a shorter time in comparison with the separate application of these elements. The period of soil purification to the initial status was determined as 21–190 years for Ni and 42–700 years for Cu.
    Therefore, the continuous additional input of HMs from the atmosphere in the industrial regions prevails over their geochemical leaching and dispersion. Hence, it is necessary to elaborate a system of remediation measures for contaminated soils. In recent years, numerous remediation technologies have been elaborated [21, 22]. Soil remediation via application of a fertile layer composed of organic and mineral components is one of the approaches tested in the Kola Peninsula. Its application resulted in a decrease in the soil acidity and enrichment of the soil with carbon and nutrients [23, 24]. The remediation efficiency depends on the composition and thickness of the newly applied layer and accompanying measures, including repeated lime application.

The comparison of contamination of the organic horizon in Al–Fe-humus podzols in the area affected by atmospheric emissions of the Severonikel smelter determined in 1989–1994 and 2016 has shown that the spatiotemporal changes in the contents of HMs in these soils in soils in response to a decrease in the aerotechnogenic loads are different. As a result of the great range of HM concentrations in the organic horizon of podzols within the 20-kmwide impact zone of the smelter both in 1989–1994 and in 2016, no definite correlation between the total content of the main contaminants (Ni, Cu, and Co) in the soils and the distance from the contamination source was found.
    The Ni-to-Cu ratio in the soil samples significantly differed in the two observation periods: with respect to the total content, HMs may be arranged in the sequence Ni > Cu > Co in 1989–1994 and Cu > Ni > Co in 2016. These sequences are probably related to the ratios between these elements in the emissions. The ninefold decrease in the Ni emission into the atmosphere resulted in a significant (2.5 times on the average) drop in its total content in the organic horizon of podzols. A threefold decrease in the Cu emission was insufficient for decontamination of the organic horizon of podzols: bulk contents of Cu (and Co) in the soil samples taken in 1989–1994 and 2016 did not significantly differ.
    The total content of HMs even increased by 2–11 times at some sampling sites in 2016 in comparison with the previous period. Despite the fact that annual Ni emissions into the atmosphere have lowered by 3–9 times, and Ni concentrations in the soil samples have decreased by 2.5times, the contamination of the organic horizon of podzols by HMs remains high or very high according to the scale of ecological norming. Continuous input of HMs from the air to the soil surface in the impact zone of the working plant retards self-purification of the organic horizon of podzols. Therefore, a system of efficient measures for remediation of contaminated soils should elaborated.

The authors are grateful to T.V. Mushnikova, the chief of the Laboratory of the Kola Geological Information and Laboratory Center for chemical analyses of the samples.

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            Translated by I. Bel’chenko

EURASIAN SOIL SCIENCE Vol. 51 No. 3 2018