*Article* **Geochemical Indication of Functional Zones at the Archaeological Sites of Eastern Europe**

**Marianna Kulkova**

Department of Geology and Geoecology, Herzen State Pedagogical University, 191186 St. Petersburg, Russia; kulkova@mail.ru

**Abstract:** The article considers a new approach for determining the functional zones of the prehistoric archaeological sites in Eastern Europe by the method of geochemical indication: the use of mathematical statistics for processing the geochemical data of cultural deposits at archaeological sites, and the identification of groups of interrelated chemical elements and compounds that reflects the processes of natural sedimentation and anthropogenic activity. It makes it possible to separate the lithological and anthropogenic components. This approach is important for the identification of geochemical element groups associated with different functional zones. The reconstructions were conducted at the Neolithic, Early Metal Age, and the Bronze-Early Iron Age sites in Eastern Europe. Abnormal concentrations of the association (P2O5antr, CaOantr and Srantr) in sediments are attributed to zones of accumulation of bone remains. Anomalous concentrations of a group of elements (K2Oantr, Rbantr) in deposits are associated with wood ash and fireplaces, ash residues from ritual activities, and fires. The group of elements (Ba, MnO, Corg) reflects the accumulation of humus and organic remains, and can characterize areas with food residues, skins, and rotten wood. With the help of the distribution of the main lithological elements (SiO<sup>2</sup> , Al2O<sup>3</sup> ) in sediments, it is possible to reconstruct the paleorelief at the sites.

**Keywords:** geochemical indication; functional zones; archaeological sites; Eastern Europe; mathematical statistic; XRF analysis; cultural layers

## **1. Introduction**

The problem of the functional zone determinations on archaeological sites is very important for an understanding of the spatial distribution of different structures connected with ancient human activity. The living and household buildings had not been preserved on most sites of the Stone, Bronze, and Iron Ages and their space reconstructions are impossible through the archaeological methods only. The activities of prehistoric people influenced the variations in the chemical compositions of the soil by enriching it with or depleting it of certain chemical elements that form archaeological soils and cultural layers [1]. The geochemical markers of anthropogenic activity are conserved in the deposits for many years. In this connection, the application of the geochemical methods at the archaeological sites is a useful instrument for the analysis of cultural layers and determination of the ancient anthropogenic activity at the sites and the reconstruction of functional zones. The last time, the method of geochemical indication for the functional zone is actively being developed in different archaeological sites in Eastern Europe.

The spatial distribution of anomalous concentrations of some chemical elements in the places of ancient settlements makes it possible to establish the boundaries of archaeological sites and their locations; obtain information about the landscape features; establish functional zones and features of various structures within the settlements [2,3]. As a rule, elements for determining anthropogenic activity on ancient sites are P, Ca, K, Na, and Mg, as are the trace elements Cd, Cr, Cu, Pb, and Zn [4–10]. Aston et al. [4] identified the main functions of human activity in which the accumulation of anthropogenic chemical

**Citation:** Kulkova, M. Geochemical Indication of Functional Zones at the Archaeological Sites of Eastern Europe. *Minerals* **2022**, *12*, 1075. https://doi.org/10.3390/ min12091075

Academic Editors: Domenico Miriello and Donatella Barca

Received: 1 July 2022 Accepted: 23 August 2022 Published: 25 August 2022

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**Copyright:** © 2022 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

elements occurs: the development of ancient settlements, animal breeding in close quarters, the use of fire (fireplaces, slash-and-burn cultivation), ancient metallurgy, or subsistence activities (production of leather, processing of crops). One of the important anthropogenic indicators is phosphorus. Phosphorus is the main component of human and animal bones, a component of living tissues (in the form of nucleic acids, phospholipids, nucleotides, and so on), and a component of everyday products (such as wood, plant, or meat products) [11]. Phosphorus can accumulate in the soils of ancient settlements as a result of food preparation and the utilization of waste products [8]. The concentration of organic materials containing phosphorus, accumulated in the process of human activity, is proportional to the time of human occupation and the growth of the population [12]. Therefore, the phosphorus content in cultural layers is an indicator of the intensity of human occupation in the area. The phosphorus concentration in the soil increases depending on the supply of different organic materials, such as plants and animals, that are used by people. The decomposition of animal and plant organisms in settlement areas, the use of animal dung as fertilizer, and the physiological activity of humans and animals in their habitation areas increase phosphorus concentration in soils [7]. The different phosphorus compounds in soils are stable to oxidation, reduction, leaching, and dissolution [13,14] and newly mineralized inorganic phosphorus, while generally retained in the soil, is subject to some vertical translocation in the soil due to factors directly affecting adsorption, such as pH, cation exchange capacity, and cation availability [15,16]. The distribution of inorganic phosphate is therefore determined by the chemistry and adsorption kinetics of the soil throughout decomposition and precipitation [7].

At archaeological sites, cultural deposits usually consist of household waste, bones, metal slag, ashes, dung, and the remains of burials and cremations, etc. Application of other geochemical elements and indicators derives additional information about site occupation. Such elements as potassium (K) and sodium (Na) can be connected with the presence of fire ash in the areas of fireplaces [17]. However, it needs to be noted that these elements are the main components of feldspar and plagioclase minerals from lithological sediments. Rubidium (Rb) could be taken up by plants as a substitute for potassium (K), which it chemically resembles. These elements can be considered as anthropogenic components at the archaeological site just in comparison with the background or a correlation between high concentrations and any evidence of charcoal or fire ashes at the ancient places.

According to Wells et al. [10], the complex of elements (Fe, Mn, Zn, and Cu) can indicate areas of waste disposal, burials, cesspools, or rubbish after feasts. Increased concentrations of iron (Fe) and mercury (Hg) at settlements can be explained by the use of different natural pigments in rituals. It should also be noted that the accumulation of geochemical components depends also on the different natural factors of sedimentation such as geomorphology, diagenetic transformations of sediments, climatic conditions, and others. For example, if the archaeological sites are located in areas with complex relief, the primary distribution of chemical elements may have changed as a result of erosion after the elements were originally deposited in the soil. The destroyed material accumulates in depressions and at the feet of slopes as colluvium [1]. These cases require the development of a uniform methodological approach, the determination of a precise geochemical background, and the understanding of geochemical processes at the site. Application of indicator ratios of chemical elements reflects the degree of enrichment of anthropogenic elements in comparison with their background concentrations in the soils of the settlement and outside of it [1,18]. The method of "multi-element" analysis [3,8,13], which has been applied relatively recently, allows us to establish a complex of several chemical components and to consider their connection with different functional zones. Scholars established some groups of the chemical elements characterizing certain functional zones connected with anthropogenic activity [1]: abnormal concentrations of P (phosphorous), Cu (copper), Mn (manganese), Ca (calcium) connect with burials/cemeteries [19–21]; zones of fireplaces–P, K, Mg (magnesium), Zn (zinc), Rb [22,23]; waste piles-P, K [10,24,25]; farm areas, inner space of living buildings-P (phosphorus), Ca, Mg, Fe, K, Th (thorium), Rb, Cs (cesium), Pb

(lead), Zn (zinc), Sr (strontium), Ba (barium) [3,10,24–29]; paints–the heavy metals [10]; ore mines, metallurgy-Cu (copper), Pb (lead), Mn (manganese) [30–34]; places of archaeological sites-B (boron), Cu (copper), Mg, Mn, Ni (nickel), P, Se (selenium), Zn (zinc), K (potassium), Ba (barium), Ca (calcium), and Na (sodium) [13,19,21,34]. metals [10]; ore mines, metallurgy-Cu (copper), Pb (lead), Mn (manganese) [30–34]; places of archaeological sites-B (boron), Cu (copper), Mg, Mn, Ni (nickel), P, Se (selenium), Zn (zinc), K (potassium), Ba (barium), Ca (calcium), and Na (sodium) [13,19,21,34]. At the archaeological sites of Eastern Europe, the geochemical approach using multi-

of fireplaces–P, K, Mg (magnesium), Zn (zinc), Rb [22,23]; waste piles-P, K [24,10,25]; farm areas, inner space of living buildings-P (phosphorus), Ca, Mg, Fe, K, Th (thorium), Rb, Cs (cesium), Pb (lead), Zn (zinc), Sr (strontium), Ba (barium) [3,10,24–29]; paints–the heavy

*Minerals* **2022**, *12*, x FOR PEER REVIEW 3 of 23

At the archaeological sites of Eastern Europe, the geochemical approach using multicomponent analysis based on the mathematical statistics was applied [18,35–38]. component analysis based on the mathematical statistics was applied [18,35–38].

#### **2. Materials and Methods 2. Materials and Methods**  The samples of cultural deposits were collected at sites of different ages (Figure 1).

The samples of cultural deposits were collected at sites of different ages (Figure 1). The main characteristics of sites are presented in Table 1. The sediment samples were taken at intervals of 0.6 m on the coordinate grid on the surface of cultural layer at each archaeological site under consideration. The coordinates of the primary point of sampling were registered by GPS. The main characteristics of sites are presented in Table 1. The sediment samples were taken at intervals of 0.6 m on the coordinate grid on the surface of cultural layer at each archaeological site under consideration. The coordinates of the primary point of sampling were registered by GPS.

**Figure 1. Figure 1.** Map of studied archaeological sites of Eastern Europe. Map of studied archaeological sites of Eastern Europe.


#### **Table 1.** The main characteristics of places of sampling.

The Serteya archaeological microregion (Figure 1) is located in the basin of the Dvina-Lovat' basin, in the southern part of the Pskov and northwestern parts of the Smolensk region. Ancient lakes in the area of the Serteya archaeological microregion were formed after the retreat of the glacier and were located in a chain, separated by narrow isthmuses, which were eroded during periods of rising water levels. Along their shores, as well as in the central parts of the lake basins, there are unique archaeological sites that were dated from the 8th millennium BC up to 10 c. AD. At present, the lake basins are swamped and inherited by the narrow channel of the Serteya River, which flows into the Western Dvina River. The archaeological excavations are conducted by the northwestern expedition of the State Hermitage Museum (St.Petersburg, the head of the expedition is A. Mazurkevich) [36,37]. Thirty-five samples from the 1st cultural layer, three control samples beyond the archaeological area and 28 samples from the 2nd cultural layer, and three control samples were taken for analysis at the Serteya 3-3 site.

Rocky remains "Bratja"—an object of the post-glacial geological history of Fennoscandia, located directly on the seashore, in the northwestern part of the Sredny Peninsula (Figures 1 and 2). Two picturesque stone pillars of a bizarre shape stand out in the surrounding landscape, which is similar to natural "statues" with a mythological meaning [39]. The pillars are located on a flat, first sea terrace-an ancient pebble beach, at an altitude of about 15 m above sea level. Sediments of medium grain size sand were sampled under the turf layer for determination of cultural layer features. Forty samples from the cultural layer at the Bratja object and eight control (background) samples were taken outside of the place. The investigations were provided together by the archaeologist M. Shakhnovich (National Museum Republic of Karelia, Petrozavodsk, Russia).

inherited by the narrow channel of the Serteya River, which flows into the Western Dvina River. The archaeological excavations are conducted by the northwestern expedition of the State Hermitage Museum (St.Petersburg, the head of the expedition is A. Mazurkevich) [36,37]. Thirty-five samples from the 1st cultural layer, three control samples beyond the archaeological area and 28 samples from the 2nd cultural layer, and three con-

Rocky remains "Bratja"—an object of the post-glacial geological history of Fennoscandia, located directly on the seashore, in the northwestern part of the Sredny Peninsula (Figures 1 and 2). Two picturesque stone pillars of a bizarre shape stand out in the surrounding landscape, which is similar to natural "statues" with a mythological meaning [39]. The pillars are located on a flat, first sea terrace-an ancient pebble beach, at an altitude of about 15 m above sea level. Sediments of medium grain size sand were sampled under the turf layer for determination of cultural layer features. Forty samples from the cultural layer at the Bratja object and eight control (background) samples were taken outside of the place. The investigations were provided together by the archaeologist M. Shakhnovich

trol samples were taken for analysis at the Serteya 3-3 site.

(National Museum Republic of Karelia, Petrozavodsk, Russia).

**Figure 2.** Rocky remains "Bratja"—an object of the post-glacial geological history of Fennoscandia. The photo is by M. Shakhnovich [39]. **Figure 2.** Rocky remains "Bratja"—an object of the post-glacial geological history of Fennoscandia. The photo is by M. Shakhnovich [39].

Sites of the Neolithic-Early Metal Age (Okhta 1 and Podolye 1) are located in the Ladoga Lake basin (Figure 1). The excavations were conducted under the head of T. Gusentsova and P. Sorokin (Institute of the Cultural Heritage, St. Petersburg, Russia). The Okhta 1 site is located on the border of the Russian Platform and the Baltic-Scandinavian crystalline shield, on a sandy cape formed after the Littorina transgression [18,36,40,41]. On the Okhtinsky Cape, at the confluence of the Okhta and Neva rivers, a unique multilayered monument was discovered containing cultural layers of the Neolithic and Early Metal Ages, the Early Iron Age, and Roman times. The site Podolye 1 is located on the southern coast of Lake Ladoga, about 4 km from the coast [41,42]. At the Podolye 1 site, as well as at Okhta 1, wooden fishing structures of different ages were found. The lower cultural layer is light yellow silt with thin interlayers of peat. The upper cultural horizon consists of sandy-silty sediments with thin black organic layers. Dense grey loam overlaps these layers. Twenty-five samples from the cultural layer at the Okhta 1 site, three control Sites of the Neolithic-Early Metal Age (Okhta 1 and Podolye 1) are located in the Ladoga Lake basin (Figure 1). The excavations were conducted under the head of T. Gusentsova and P. Sorokin (Institute of the Cultural Heritage, St. Petersburg, Russia). The Okhta 1 site is located on the border of the Russian Platform and the Baltic-Scandinavian crystalline shield, on a sandy cape formed after the Littorina transgression [18,36,40,41]. On the Okhtinsky Cape, at the confluence of the Okhta and Neva rivers, a unique multi-layered monument was discovered containing cultural layers of the Neolithic and Early Metal Ages, the Early Iron Age, and Roman times. The site Podolye 1 is located on the southern coast of Lake Ladoga, about 4 km from the coast [41,42]. At the Podolye 1 site, as well as at Okhta 1, wooden fishing structures of different ages were found. The lower cultural layer is light yellow silt with thin interlayers of peat. The upper cultural horizon consists of sandy-silty sediments with thin black organic layers. Dense grey loam overlaps these layers. Twenty-five samples from the cultural layer at the Okhta 1 site, three control samples and 32 samples from the upper cultural layer at the Podolye 1 site and five control samples were sampled.

At the Tarkhankut-18 site (Figure 1), located on the Tarkhankut Peninsula in Crimea, there are the Late Bronze-Early Iron Age complexes. The excavations were provided by M.T. Kashuba (IIMC RSA, St. Petersburg, Russia) and T.N. Smekalova (Crimea Federal University, Simferopol, Autonomous Republic of Crimea) [43]. The cultural layer has been formed on the surface of the aeolian deposits. The samples have been taken from the surface of the cultural layer and from the aeolian deposits (background pit) to establish the background contents of geochemical components. The background pit was laid outside the archaeological site for determination of the composition of geologically similar deposits that are not subject to anthropogenic influence. Forty-seven samples at the Tarkhankhut\_18 excavation were taken, and 16 probes were sampled from the background pit.

The Yudinovo site is one of the most studied sites of the Upper Paleolithic (15–12 thousand years ago), located in the Desna river basin, in the Pogarsky district of the Bryansk region of Russia (Figure 1). The excavations were carried out by G. Khlopachev (Kunstkamera Museum, St. Petersburg, Russia). The household objects of different types have been discovered on the site: the remains of five dwellings of the Anosovo-Mezin type, six large household pits, ten large "ash pans"—the accumulations of bone coal and

ash, two small hearths, two hearth pits, several "clad" pits, numerous production sites associated with the primary treatment of flint and tusk, and cutting carcasses of Arctic foxes [44]. The strategy of sample preparation is the following: the total weight of the sample was about 100 g, taking into account possible duplicates. The deposit samples were dried in an oven for 24 h at a temperature of 105 ◦C. For geochemical analysis, after each crushing cycle, control sieving was carried out, the residue is crushed to the desired grain size. Then, all crushed material is combined. Manual reduction is carried out by the method of multiple (no more than 3 with one crushing) quartering and combining the material of two opposite quadrants to continue processing according to the accepted scheme. Due to the heterogeneity of the distribution of components in large and small fractions, before reducing the samples, they are mixed and then quartered. The sample is grinded up in powder to a particle size of 200 mesh, 0.074 mm (to the state of powder). Next, the loss on ignition (LOI) at a temperature of about 550 ◦C for 40 min was determined for the powder samples. For X-Ray Spectral fluorescence analysis using the Spectroscan Max-GV spectrometer, Spectron, St.Petersburg (Lab of Geochemistry of the Environment of Herzen State University, St.Petersburg, Russia), 300 mg samples were pressed into a tablet using a boric acid substrate. The measuring system of the spectrometer is in the vacuum chamber, while the samples are at the ambient pressure, so no He is required for the sample chamber and all samples (including liquid and powder) may be studied with no special measures taken. XRF-WD with an energy resolution of 9 eV (Si Kα) and 60 eV (Fe Kα). Spectrum scanning and high resolution of crystal analyzers exclude close lines of different elements overlapping (therefore, there is no need in their mathematical differentiation) and provides correct background accounting. The Specroscan determines the elements from <sup>11</sup>Na to <sup>92</sup>U. A vacuum line along the optical way allows us to determine the light elements including (11 Na). All 15 chemical elements have been determined. The range of determined concentrations from 0.0001% to 100%. Limits of detection, L for light matrix (exposure-100 sec): Na–1 <sup>×</sup> <sup>10</sup>−1%, Mg–1 <sup>×</sup> <sup>10</sup>−2%, Al–1 <sup>×</sup> <sup>10</sup>−3%, Si–5 <sup>×</sup> <sup>10</sup>−4%, P–5 <sup>×</sup> <sup>10</sup>−4%, Cd, Pb–5 <sup>×</sup> <sup>10</sup>−4%; S, Ti, V, Cr–1 <sup>×</sup> <sup>10</sup>−4%, Co, Ni–5 <sup>×</sup> <sup>10</sup>−5%. Each sample was measured for 2 h once. The main elements are transformed into oxides by a spectrometer automatically. Several samples were measured twice for control of stability.

The Surfer Mapping Software, Golden Software, Colorado, USA (Versions 10.0, 13.0) was applied to map the distribution of geochemical indicators of anthropogenic activity in the studied area. According to geochemical indicators and groups of indicators, the functional zones were reconstructed.

After measuring, the results of geochemical elemental composition were processed by the method of multivariate statistical analysis (correlation analysis and factor analysis on the base of PCA) to determine the main factors influencing sedimentogenesis. Correlation analysis allows the grouping of the geochemical elements and their compounds depending on their genesis and certain conditions of sedimentogenesis basing the mineralogical and geochemical composition of sediments. A method of multicomponent analysis based on the identification of groups of geochemical components of anthropogenic genesis on the base of correlation and method PCA of factor analyses was developed by the author. This gives the possibility to characterize various functional zones at ancient settlements [18,35,36,38]. The data on the chemical composition of deposits from two sites (the Serteya and the Bratja) were separately calculated by means of the factor analysis based on the principle component method (PCA) for determination of environmental and ancient anthropogenic factors that influenced the sedimentation. The key concept of factor analysis is that multiple observed variables have similar patterns of responses because they are all associated with a latent variable. The number of principal components was determined according to how complex our model will be. As a rule, the factor corresponding to the largest eigenvalue (7.14, 7.8 for the Serteya 3-3 for the 1st and 2nd cultural layers separately and 7.1 for the Bratja site, respectively) accounts for approx. 27%, 30%, and 26% of the total variance. The second factor corresponding to the second eigenvalue (4.9, 5.2, and 4.7) accounts for approx. 19%, 21%, and 18% of the total variance, and so on. When analyzing correlation

matrixes, the sum of the eigenvalues is equal to the number of (active) variables from which the factors were extracted (computed). Four main factors were taken into account for determination of sedimentation characteristics for sites. For other sites, the geochemical modules were applied without PCA calculation because the results can be compared with archaeological information directly.

The ratios of geochemical components and geochemical modules for individual elements and their oxides, for example, potassium, calcium, strontium, and phosphorus, which can form compounds associated both with the mineral composition of deposits and the activity of ancient people, were applied. This makes it possible to separate the components associated with the lithological component of the deposits from the anthropogenic ones [18,36,38]. Comparison with samples taken outside the anthropogenic activity from the same lithological layer. Other factors that affect the behavior of individual chemical elements are pH (alkaline acid), Eh (reductive-oxidative) potentials of the soil environment, the content of organic matter in soils, etc., which should also be taken into account when assessing the formation of a cultural layer at a settlement. To assess the overall anthropogenic impact on the settlements, the author proposed the following ratios of components for calculation: P2O5antr = P2O5/(P2O<sup>5</sup> + Na2O) (%), Caantr = CaOtot/(CaOtot + Na2O) (%), K2Oantr = K2O/K2O + Na2O) (%) and Rbantr = Rb/(Rb + Na2O) (%). Elevated values of indicators (P2O5antr, CaOantr) correlate well with the areas of sediments in which bone remains were found. In this case, such ratios make it possible to establish and separate anthropogenic calcium and phosphorus from lithogenic ones, which are associated with the lithology of the deposits. Indicators (K2Oantr, Rbantr) were used to identify focal clusters [18,35,38].

#### **3. Results**

#### *3.1. Serteya Archaeological Microregion*

The reconstruction of the functional zones was carried out at the multilayer Neolithic site Serteya 3-3 [37]. The remains of the material culture were found in sandy-silty deposits. Mineral composition of deposits: quartz, mica, clay minerals, feldspars, accessory minerals: zircon, garnet, titanomagnetite, hydrogoethite, and ilmenite. According to the correlation matrix basing on the results of the geochemical composition of deposits (*n* = 35), several geochemical groups of components can be distinguished. These groups will be associated with different minerals, organo-mineral complexes of the anthropogenic component of deposits at the site:


An analysis of the stratigraphy and paleorelief on the site allowed the conclusion that the place for the settlement was chosen taking into account the features of the ancient relief and composite from sandy dune deposits. According to the factor analysis, it is possible to identify the factors associated with anthropogenic transformations of sediments at the site. For the lower (1st) cultural horizon, the first factor (FI) characterizes the features of the relief and reflects the antagonism of the components of the sand (SiO2, Zr) and the components of the clay-mica minerals (Al2O3, TiO2, Fe2O3, MgO, Na2O). It is based on positive and negative correlation between components. Accumulation of the clay component enriched in iron oxides and hydroxides is typical for depressions and pits, including those left from plant roots. The second, third, and fourth factors can be interpreted as related to various anthropogenic activities at the site, which led to a change in the composition of sediments and the appearance of different types of components:

FII (CaO, Na2O, SiO2/K2O, Ba, TiO2);

FIII (K2O,Al2O3, Ba, CaO/Fe2O3, MnO, Ctot); FIV (MgO, MnO, Fe2O3,TiO2/P2O5, Ctot).

The second factor (FII) shows the antagonism of components associated with the fireplaces (K2O, Ba, TiO2) to lithogenic components (CaO, Na2O, SiO2). In places of increased concentrations of elements associated with wood ash, residues of charcoal and ash stains from hearths were found. The third factor (FIII) shows the antagonism between the complex of lithogenic components (K2O, Al2O3, Ba, CaO) and components (Fe2O3, MnO, Ctot), which are associated with the accumulation of humus and coincide with traces of habitation and internal areas of the dwellings. The fourth factor FIV (MgO, MnO, Fe2O3, TiO2/P2O5, and Ctot) is associated with areas of accumulation of organic matter and bone residues (P2O5, Ctot), which could be concentrated in areas of animal cutting and food preparation. Sherds from pottery were also found in these zones. Figure 3 shows geochemical maps on the base of indicators-factor load data, giving an idea of the position of various sites associated with anthropogenic activity and reflecting the spatial arrangement of various functional zones in the area of the settlement. *Minerals* **2022**, *12*, x FOR PEER REVIEW 9 of 23

**Figure 3.** (**a**) Spatial distribution of functional zones on the surface of the 1st cultural horizon of the Serteya 3-3 site according to geochemical mapping adapted from [37]; (**b**) surface of the 1st cultural **Figure 3.** (**a**) Spatial distribution of functional zones on the surface of the 1st cultural horizon of the Serteya 3-3 site according to geochemical mapping adapted from [37]; (**b**) surface of the 1st cultural layer [37], designed by M.Kulkova.

In deposits of the 2nd upper cultural horizon at the Serteya 3-3 site, the remains of temporary structures are recorded. According to the geochemical data of deposits from the 2nd cultural horizon, the factors influencing the formation of these deposits were also established (*n* = 28). FI (SiO2/Al2O3, Fe2O3, MgO, CaO) reflects the features of the microrelief. In depressions in this case, sand deposits were accumulated, which may indicate small ancient river channels. This matches well with the 3D reconstruction of the ancient In deposits of the 2nd upper cultural horizon at the Serteya 3-3 site, the remains of temporary structures are recorded. According to the geochemical data of deposits from the 2nd cultural horizon, the factors influencing the formation of these deposits were also established (*n* = 28). FI (SiO2/Al2O3, Fe2O3, MgO, CaO) reflects the features of the microrelief. In depressions in this case, sand deposits were accumulated, which may indicate small ancient river channels. This matches well with the 3D reconstruction of

surface. The areas of the fireplaces were determined by the indicator ratio K2Oantr = K2O/(K2O + Al2O3). Comparison of the anomalous K2Oantr values with charcoal accumula-

the CaO/SiO2 ratio. The abnormal values match the locations where calcified bones were found. The distribution map of the positive values of the second factor shows the zones of humus content (Ba, MnO, TiO2). Negative values of the third factor FIII (MgO, K2O/P2O5, Ctot) indicate habitat areas (P2O5, Ctot). The integrated application of the geochemical method and the geomagnetic method [37], as well as the use of more accurate methods for measuring magnetic susceptibility with a kappameter, makes it possible to detect traces of unreadable structures during excavations and obtain data for determination of zones associated with human activity. Such sites have been recorded in the cultural

layer [37], designed by M.Kulkova.

the ancient surface. The areas of the fireplaces were determined by the indicator ratio K2Oantr = K2O/(K2O + Al2O3). Comparison of the anomalous K2Oantr values with charcoal accumulations in these zones shows a good correlation. It allows us to trace the change in the location of the hearths in space. Areas of concentration of bone residues were determined by the CaO/SiO<sup>2</sup> ratio. The abnormal values match the locations where calcified bones were found. The distribution map of the positive values of the second factor shows the zones of humus content (Ba, MnO, TiO2). Negative values of the third factor FIII (MgO, K2O/P2O5, Ctot) indicate habitat areas (P2O5, Ctot). The integrated application of the geochemical method and the geomagnetic method [37], as well as the use of more accurate methods for measuring magnetic susceptibility with a kappameter, makes it possible to detect traces of unreadable structures during excavations and obtain data for determination of zones associated with human activity. Such sites have been recorded in the cultural layer at the multilayer site of the Neolithic period Serteya XIV [18,37]. In addition to the main indicators of anthropogenic activity, the comparison of geochemical data and geophysical anomalies also made it possible to obtain additional information on other indicator elements that may be associated with biogenic processes and human activities. Elevated concentrations of iron oxides Fe2O<sup>3</sup> and FeO appear around plant roots and may characterize the remains of decayed stumps [37]. High anomalous concentrations of iron oxides (Fe2O<sup>3</sup> and FeO) also correspond to the localization of long-term fires. Accumulation of lead (Pb) may be associated with hearths, or with deposits of humus under reducing conditions [24–29]. In this archaeological context, the distribution of Pb correlates with the accumulation of K2Oantr in wood ash. The accumulation of copper (Cu) can also be associated with reducing conditions and concentrates in the areas of the fireplaces. According to geochemical indicators, high values of such indicators as K2Oantr, Cu, and Pb were established, which can be explained by the accumulation of ash or the localization of a small hearth that was not visually detected during excavations.

#### *3.2. Sredny Peninsula, the Cult Place "Bratja"*

The method of geochemical indication makes it possible to establish several main factors influencing the process of sedimentation at the archaeological sites and to identify anthropogenic components, if they were present in the deposits (*n* = 40). The formula of the first factor (FI): CaO, P2O<sup>5</sup> (Sr)/Al2O3, SiO2, TiO<sup>2</sup> (Zr) shows antagonism based on negative and positive correlation between the groups of components associated with anthropogenic activity; the association of components (CaO, P2O<sup>5</sup> (Sr)), which are characterized by high correlations and rock-forming and accessory components (Al2O3, SiO2; TiO2; (Zr)), was associated with the minerals of the sandy deposits [39]. The contents of anthropogenic components CaO, P2O5; (Sr; Rb) were also determined for sediments at two sites. In a comparison of the concentration of these elements with the background samples taken outside the site, anomalous concentrations in the sediments at the site stand out. Two anomalous zones are distinguished on both sites of the monument investigated by the complex of anthropogenic components. These zones can be considered places of ritual actions. In such zones, anomalous contents of anthropogenic potassium and rubidium have been recorded. Elevated concentrations of K2Oantr are associated with wood ash in the cultural layer and are a marker of the fireplace zone. This is also noted for the Rbantr values, which correlate well with K2Oantr. In the areas of fireplaces, which are enriched with ash, increased concentrations of zinc (Zn) and copper (Cu) are also registered. Thus, basic data obtained on the zones of fireplaces, as well as areas of ash emissions from them, located nearby, were reconstructed. It can be assumed that zones with elevated contents of a complex of elements such as P, Ca, K, Sr, Rb, Zn, and Cu mark areas of ancient ritual practice where sacred offerings could be left, animal meat was butchered, and there were fireplaces. Thus, according to geochemical studies, it can be concluded that, in the space between the two stone columns of the "Bratja" ceremonial, ritual actions were performed, which can be considered pagan sacrifices associated with hunting and household magic. These rocks could be perceived as expressive natural objects, endowed with sacred properties, and the

areas near them were used for ritual practices. The ethnographical evidence of the ritual activity is known at this place [39].

#### *3.3. Ladoga Lake Basin (Sites Okhta 1 and Podolye 1)*

At the sites of Okhta 1 and Podolye 1, various types of functional zones were considered on the surface of cultural layers, which formed around 3300 calBC [35,38,40–42]. They are presented by soils and medium grain sands. At the Okhta 1 site, geochemical mapping data and the distribution of artifacts show the burial site (Figures 4–6). The burial deposits are characterized by anomalous values of a complex of chemical anthropogenic components, such as P2O5antr, Cantr and Fe, Srantr. The geochemical composition of the deposits at the Podolye 1 site shows a different character of the distribution of geochemical elements (Figures 7–10) [38]. On the terrace of the small river channel, there were seasonal fishing camps with fireplaces and places for cutting and cooking animals and fish. This is supported by the archaeological evidence [35,38]. A combination of such anthropogenic elements as K2Oanhtr, Rbantr, CaOantr, and Srantr stands out in the cultural layer of the sediments of this site. On the shore of the channel, it is possible to distinguish areas for the location of fireplaces (K2Oantr and Rbantr—elements that are accumulated in charcoal and wood ash) and areas for cutting and cooking animals and fish (CaOantr and Srantr—main elements of bone tissue). In the pit located on the terrace of the channel, another geochemical complex with anomalous values was characterized: K2Oantr, Rbantr, CaOantr, Srantr, Mnantr, Baantr, Fe, and P2O5anthr, which may indicate the accumulation of waste in the channel pit.: K2Oantr and Rbanhtr-components, which are part of ash and charcoal, P2O5 antr, CaOantr, Srantr are components of bone tissues, Mnantr, Baantr, and Fe are components that are part of decomposed organic matter. The distribution geochemistry of major sediment-forming elements such as alumina (Al2O3) and silica (SiO2) is an important factor in determining the ancient microrelief at this site (Figure 11). Associations of geochemical indicators from the same lithological context at the sites of Okhta 1 and Podolye 1 have different anthropogenic loads. The deposits from the burial are characterized by anomalous values of such bioindicators as P2O5anhtr and CaOantr, which are the main components of bones and tissues. Iron in the form of hematite (Fe2O3) is part of the ocher, which was used in the ritual ceremony. *Minerals* **2022**, *12*, x FOR PEER REVIEW 11 of 23 (Figures 7–10) [38]. On the terrace of the small river channel, there were seasonal fishing camps with fireplaces and places for cutting and cooking animals and fish. This is supported by the archaeological evidence [35,38]. A combination of such anthropogenic elements as K2Oanhtr, Rbantr, CaOantr, and Srantr stands out in the cultural layer of the sediments of this site. On the shore of the channel, it is possible to distinguish areas for the location of fireplaces (K2Oantr and Rbantr—elements that are accumulated in charcoal and wood ash) and areas for cutting and cooking animals and fish (CaOantr and Srantr—main elements of bone tissue). In the pit located on the terrace of the channel, another geochemical complex with anomalous values was characterized: K2Oantr, Rbantr, CaOantr, Srantr, Mnantr, Baantr, Fe, and P2O5anthr, which may indicate the accumulation of waste in the channel pit.: K2Oantr and Rbanhtr-components, which are part of ash and charcoal, P2O5 antr, CaOantr, Srantr are components of bone tissues, Mnantr, Baantr, and Fe are components that are part of decomposed organic matter. The distribution geochemistry of major sediment-forming elements such as alumina (Al2O3) and silica (SiO2) is an important factor in determining the ancient microrelief at this site (Figure 11). Associations of geochemical indicators from the same lithological context at the sites of Okhta 1 and Podolye 1 have different anthropogenic loads. The deposits from the burial are characterized by anomalous values of such bioindicators as P2O5anhtr and CaOantr, which are the main components of bones and tissues. Iron in the form of hematite (Fe2O3) is part of the ocher, which was used in the ritual ceremony.

**Figure 4.** Distribution of P2O5antr (%) on the surface of the burial layer at the Okhta 1 site [38], designed by M.Kulkova, a, b, c, d—burial area. **Figure 4.** Distribution of P2O5antr (%) on the surface of the burial layer at the Okhta 1 site [38], designed by M.Kulkova, a, b, c, d—burial area.

signed by M.Kulkova.

by M.Kulkova.

**Figure 5.** Distribution of Caantr (ppm) on the surface of the burial layer at the Okhta 1 site [38], de-**Figure 5.** Distribution of Caantr (ppm) on the surface of the burial layer at the Okhta 1 site [38], designed by M.Kulkova. **Figure 5.** Distribution of Caantr (ppm) on the surface of the burial layer at the Okhta 1 site [38], designed by M.Kulkova.

**Figure 6.** Distribution of Fe (%) on the surface of the burial layer at the Okhta 1 site [38], designed by M.Kulkova. **Figure 6.** Distribution of Fe (%) on the surface of the burial layer at the Okhta 1 site [38], designed by M.Kulkova.

**Figure 6.** Distribution of Fe (%) on the surface of the burial layer at the Okhta 1 site [38], designed

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**Figure 7.** Geochemical maps of P2O5anthr distributions in sediments on the surface of the cultural layer at the Podolye 1 site [38], designed by M.Kulkova. **Figure 7.** Geochemical maps of P2O5anthr distributions in sediments on the surface of the cultural layer at the Podolye 1 site [38], designed by M.Kulkova. **Figure 7.** Geochemical maps of P2O5anthr distributions in sediments on the surface of the cultural layer at the Podolye 1 site [38], designed by M.Kulkova.

layer at the Podolye 1 site [38], designed by M.Kulkova.

**Figure 8.** Geochemical maps of CaOanthr distributions in sediments on the surface of the cultural **Figure 8.** Geochemical maps of CaOanthr distributions in sediments on the surface of the cultural layer at the Podolye 1 site [38], designed by M.Kulkova. **Figure 8.** Geochemical maps of CaOanthr distributions in sediments on the surface of the cultural layer at the Podolye 1 site [38], designed by M.Kulkova.

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**Figure 9.** Geochemical map of K2Oantr distribution in sediments on the surface of the cultural layer at the Podolye 1 site [38], designed by M.Kulkova. **Figure 9.** Geochemical map of K2Oantr distribution in sediments on the surface of the cultural layer at the Podolye 1 site [38], designed by M.Kulkova. **Figure 9.** Geochemical map of K2Oantr distribution in sediments on the surface of the cultural layer at the Podolye 1 site [38], designed by M.Kulkova.

**Figure 10.** Geochemical map of Rbantr distributions in sediments on the surface of the cultural layer at the Podolye 1 site [38], designed by M.Kulkova. **Figure 10.** Geochemical map of Rbantr distributions in sediments on the surface of the cultural layer at the Podolye 1 site [38], designed by M.Kulkova. **Figure 10.** Geochemical map of Rbantr distributions in sediments on the surface of the cultural layer at the Podolye 1 site [38], designed by M.Kulkova.

**Figure 11.** Features of relief on the base of SiO2 distribution at the site surface and location of different types of functional zones based on geochemical reconstructions at the Podolye site [38], designed by M.Kulkova. **Figure 11.** Features of relief on the base of SiO<sup>2</sup> distribution at the site surface and location of different types of functional zones based on geochemical reconstructions at the Podolye site [38], designed by M.Kulkova.

#### *3.4. Tarkhankut Peninsula (Crimea), Bronze Age Cattle Pens*

*3.4. Tarkhankut Peninsula (Crimea), Bronze Age Cattle Pens*  Maps of the distribution of components (SiO2, Al2O3) associated with the lithological composition of the deposits show the features of the microrelief in the excavation area (Figure 12) [43]. Elevated areas are clearly distinguished, on which the stone walls of the corral were built, and low areas, in which cattle were kept inside stone structures. The sediments in these areas are heavily "trampled" and reworked. Elevated values of anthropogenic components (CaOantr, K2Oantr, P2O5antr) in sediments in the areas of depression inside the paddocks indicate intense anthropogenic activity associated with livestock management (Figures 13). These sites are also characterized by high values of the main anthropogenic components, compared with the background site, outside the paddocks. The aggressive chemical environment, which changed the chemical composition of the sediments and led to the almost complete dissolution of carbonates, was formed in the conditions of the cattle stable, which is also confirmed by archaeological evidence. The content of CaO in the paddocks is much lower than in the background deposits (Figure 13). Anom-Maps of the distribution of components (SiO2, Al2O3) associated with the lithological composition of the deposits show the features of the microrelief in the excavation area (Figure 12) [43]. Elevated areas are clearly distinguished, on which the stone walls of the corral were built, and low areas, in which cattle were kept inside stone structures. The sediments in these areas are heavily "trampled" and reworked. Elevated values of anthropogenic components (CaOantr, K2Oantr, P2O5antr) in sediments in the areas of depression inside the paddocks indicate intense anthropogenic activity associated with livestock management (Figure 13). These sites are also characterized by high values of the main anthropogenic components, compared with the background site, outside the paddocks. The aggressive chemical environment, which changed the chemical composition of the sediments and led to the almost complete dissolution of carbonates, was formed in the conditions of the cattle stable, which is also confirmed by archaeological evidence. The content of CaO in the paddocks is much lower than in the background deposits (Figure 13). Anomalies of anthropogenic K2Oantr and Ba, which are associated with the processes of formation of humus, manure, etc. are also noted in paddock areas. Thus, according to the data of geochemical elements, it is possible to determine the main function of this area as a pen-stall for keeping animals.

alies of anthropogenic K2Oantr and Ba, which are associated with the processes of formation of humus, manure, etc. are also noted in paddock areas. Thus, according to the data of geochemical elements, it is possible to determine the main function of this area as a pen-

stall for keeping animals.

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**Figure 12.** The Tarkhankhut 18 site. Geochemical maps of SiO2 distribution in the sediments of (**a**) inside cattle pen; (**b**) background pit; (**c**) 3D reconstruction surface based on SiO2 distribution [43], designed by M.Kulkova. **Figure 12.** The Tarkhankhut 18 site. Geochemical maps of SiO<sup>2</sup> distribution in the sediments of (**a**) inside cattle pen; (**b**) background pit; (**c**) 3D reconstruction surface based on SiO<sup>2</sup> distribution [43], designed by M.Kulkova. **Figure 12.** The Tarkhankhut 18 site. Geochemical maps of SiO2 distribution in the sediments of (**a**) inside cattle pen; (**b**) background pit; (**c**) 3D reconstruction surface based on SiO2 distribution [43], designed by M.Kulkova.

**Figure 13.** The Tarkhankhut 18 site. Geochemical maps of P2O5 distribution in the sediments of (**a**) inside cattle pen; (**b**) background pit; geochemical maps of CaO distribution in the sediments of (**c**) **Figure 13.** The Tarkhankhut 18 site. Geochemical maps of P2O<sup>5</sup> distribution in the sediments of (**a**) inside cattle pen; (**b**) background pit; geochemical maps of CaO distribution in the sediments of (**c**) inside cattle pen; (**d**) background pit; (**e**) archaeological plan of cattle pen [43], designed by M.Kulkova.

#### *3.5. The Upper Paleolithic Site Yudinovo 3.5. The Upper Paleolithic Site Yudinovo*

M.Kulkova.

The cultural deposits at the Yudinovo site consist of Pleistocene loess and sandy loam. The geochemical results were considered in the article [44]. Important indicators for assessment of the ancient microrelief are changes in the content of the main rockforming components (Al2O3)-alumina and (SiO2)-silica. 3D geochemical maps of Al2O<sup>3</sup> and SiO<sup>2</sup> distributions can be used for reconstruction of ancient microrelief in the area of the excavation (Figure 14). Elevated concentrations of alumina (Al2O3) are associated with depressions in the relief, where loam deposits accumulate. Low concentrations of SiO<sup>2</sup> also mark areas of depressions in the relief. Silica, which is associated with the sandy component, is confined to elevated areas and is associated with areas of gray sandy loam in the excavation area. Elevated areas are composed of gray sandy loam enriched in silica and depleted in alumina; they mark the layer in squares t-28 and t-29, and are also noted in the middle part of square t-30, on the border with the pit-depression. It is possible that the sand could have been specially brought by ancient people to this place. The cultural deposits at the Yudinovo site consist of Pleistocene loess and sandy loam. The geochemical results were considered in the article [44]. Important indicators for assessment of the ancient microrelief are changes in the content of the main rock-forming components (Al2O3)-alumina and (SiO2)-silica. 3D geochemical maps of Al2O3 and SiO2 distributions can be used for reconstruction of ancient microrelief in the area of the excavation (Figure 14). Elevated concentrations of alumina (Al2O3) are associated with depressions in the relief, where loam deposits accumulate. Low concentrations of SiO2 also mark areas of depressions in the relief. Silica, which is associated with the sandy component, is confined to elevated areas and is associated with areas of gray sandy loam in the excavation area. Elevated areas are composed of gray sandy loam enriched in silica and depleted in alumina; they mark the layer in squares t-28 and t-29, and are also noted in the middle part of square t-30, on the border with the pit-depression. It is possible that the sand could have been specially brought by ancient people to this place.

*Minerals* **2022**, *12*, x FOR PEER REVIEW 17 of 23

inside cattle pen; (**d**) background pit; (**e**) archaeological plan of cattle pen [43], designed by

**Figure 14.** The Yudinovo site. Geochemical maps of (**a**) Al2O3 (**b**) 3D distribution of SiO2 in the sediments [44], designed by M.Kulkova. **Figure 14.** The Yudinovo site. Geochemical maps of (**a**) Al2O<sup>3</sup> (**b**) 3D distribution of SiO<sup>2</sup> in the sediments [44], designed by M.Kulkova.

Geochemical maps of the distribution of complex of anthropogenic components, such as phosphorus (P2O5antr), calcium (CaOantr), potassium (K2Oantr), and rubidium (Rb) (Figures 15 and 16) show areas associated with burnt bone remains. The largest accumulations are associated with depressions in the relief. Such zones are observed on the surface of the depression in squares t-28 and t-29 and form ring structures. A zone with high contents of the complex of these anthropogenic elements is also recorded in the T-30 square on the border of the "ash pit" and the pit-depression. The second group of chemical components (Fe2O3, MnO, Ba) may be connected with the location of organic residues (skins, wood, etc.), which were subject to decomposition and decay. High concentrations of these elements are noted in the ash pan deposits. The zone of high concentrations of these elements is confined to the border of the squares t-29 and t-30, which are planigraphically connected with the edge of the "ash pan". The layer with high concentrations of these elements Geochemical maps of the distribution of complex of anthropogenic components, such as phosphorus (P2O5antr), calcium (CaOantr), potassium (K2Oantr), and rubidium (Rb) (Figures 15 and 16) show areas associated with burnt bone remains. The largest accumulations are associated with depressions in the relief. Such zones are observed on the surface of the depression in squares t-28 and t-29 and form ring structures. A zone with high contents of the complex of these anthropogenic elements is also recorded in the T-30 square on the border of the "ash pit" and the pit-depression. The second group of chemical components (Fe2O3, MnO, Ba) may be connected with the location of organic residues (skins, wood, etc.), which were subject to decomposition and decay. High concentrations of these elements are noted in the ash pan deposits. The zone of high concentrations of these elements is confined to the border of the squares t-29 and t-30, which are planigraphically connected with the edge of the "ash pan". The layer with high concentrations of these elements "flows" further into the "pit-depression" zone. Anomalous arsenic (As) concentrations are recorded in the area of square t-30, in its middle part. Some contents of arsenic (As), lead (Pb), and zinc (Zn) are also noted in the deposits of the "ash pan".

"flows" further into the "pit-depression" zone. Anomalous arsenic (As) concentrations are recorded in the area of square t-30, in its middle part. Some contents of arsenic (As),

"flows" further into the "pit-depression" zone. Anomalous arsenic (As) concentrations are recorded in the area of square t-30, in its middle part. Some contents of arsenic (As),

lead (Pb), and zinc (Zn) are also noted in the deposits of the "ash pan".

lead (Pb), and zinc (Zn) are also noted in the deposits of the "ash pan".

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**Figure 15.** The Yudinovo site. Geochemical maps of (**a**) P2O5anthr and (**b**) CaOantr distributions in the sediments [44], designed by M.Kulkova. **Figure 15.** The Yudinovo site. Geochemical maps of (**a**) P2O5anthr and (**b**) CaOantr distributions in the sediments [44], designed by M.Kulkova. **Figure 15.** The Yudinovo site. Geochemical maps of (**a**) P2O5anthr and (**b**) CaOantr distributions in the sediments [44], designed by M.Kulkova.

**Figure 16.** The Yudinovo site. Geochemical maps of (**a**) K2Oantr and (**b**) Rb distributions in the sedi-**Figure 16.** The Yudinovo site. Geochemical maps of (**a**) K2Oantr and (**b**) Rb distributions in the sediments [44], designed by M.Kulkova. **Figure 16.** The Yudinovo site. Geochemical maps of (**a**) K2Oantr and (**b**) Rb distributions in the sediments [44], designed by M.Kulkova.

Data on anthropogenic chemical elements make it possible to reconstruct the following structures on the surface of the studied area of the Yudinovo site: firstly, the upper cultural horizon above the pit-depression is characterized by increased contents of a complex of components that are associated with burnt bones: phosphorus (P2O5anthr), calcium (CaOantr), potassium (K2Oantr), and rubidium (Rb), which allows us to consider it as a

ments [44], designed by M.Kulkova.

separate cultural layer with bone remains; secondly, in the layer of the ash pit, which belongs to the main, lower cultural layer, there are high concentration (Fe2O3, MnO, Ba), elements associated with decomposed organic residues, such as skins, wood, etc. The totality of these chemical elements is also concentrated in deposits associated with deposits in the filling of the pit-depression, especially increased concentrations of elements in the deposits on the sides of the pit-depression, into which the ash pan layer falls. Along with this complex of elements, elevated contents of arsenic (As), lead (Pb), and zinc (Zn) are recorded in the ash pan deposits at the edge of the depression. It is known that the concentrations of these elements increase in a reducing environment, during the decomposition of organic matter. In addition, a complex of such elements as arsenic (As), zinc (Zn), and lead (Pb) can be associated with various components that make up the substances, the sources of which were located far from the Yudinovo site. For example, realgar is a red arsenic sulfide mineral, which, according to geological maps, is not found in the region where this archaeological site is located. The nearest sources of that mineral could be regions of Ukraine and Caucasus.

#### **4. Discussion**

Multicomponent statistical analysis of the geochemical composition of loose Quaternary deposits at archaeological sites located in Eastern Europe makes it possible to identify geochemical elements associated with both the lithological component of deposits and anthropogenic elements that accumulate in deposits as a result of the activities of prehistoric people. Such an approach allows identification of insoluble mineral and organo-mineral compounds that are formed as a result of the ancient anthropogenic activity at the sites. On the other hand, the distribution of the main rock-forming geochemical elements, such as Si (silicon) and Al (aluminum) in sediments on the area of settlements, allows us to establish the features of the microrelief on the site that existed during the period of occupation of the ancient population. This is important for multi-layer sites. At the Serteya 3-3 site, the upper and lower Neolithic cultural layers were formed as a result of the transformation of dune sand deposits under the influence of various anthropogenic activities. According to geochemical data, it is possible to distinguish zones in the settlement enriched in the sandy component in elevated areas, characterized by an association of elements (SiO2, Zr) and areas of depressions in the relief enriched in the clay-mica component (Al2O3, TiO2, Fe2O3, MgO, Na2O).

Comparison of the 3D surface of this paleorelief with archaeological data [37] makes it possible to reconstruct the contours of residential structures, pits from tree roots, and natural depressions in the relief that are not associated with an anthropogenic activity. Features of the microrelief reconstructed on the data of lithological geochemical elements were analyzed on the studied sites. At the Paleolithic site Yudinovo on the surface of loess cultural deposits, there are elevated parts with sediments enriched by SiO<sup>2</sup> and lower areas with high Al2O<sup>3</sup> concentrations of loess. Analysis of the distribution of SiO<sup>2</sup> in loess deposits in different parts of the site and comparison with archaeological data on the distribution of finds and structural features also makes it possible to identify artificial sand beds [44]. Geochemical mapping of the coastal zone of the ancient channel at the Podilye 1 site, composed of sandy-silty deposits, also makes it possible to identify microrelief features according to the data of the distribution of alumina (Al2O3) and silica (SiO2). Comparison with anthropogenic geochemical indicators (K, Rb) determined the lowland zones-pits associated with fireplaces. According to the geochemical data of sand deposits at the Tarkhankut 18 site, the distribution of components (SiO2, Al2O3) in the deposits also fixes the features of the microrelief in the paddock zones and artificial sand bedding, which was used in the construction of the stone fences of the paddocks.

At all sites under studies, despite the deposits having a different genesis, it is possible to reliably identify groups of anthropogenic geochemical elements in cultural layers, which reflect various functional zones of ancient anthropogenic activity. At the Serteya 3-3 site, a group of geochemical elements is distinguished in the cultural deposits of the lower

horizon. P2O5, CaO, MnO, and Ctot are components of carbonate-apatite of bone tissues and teeth, with organic residues. In the sediments of the upper cultural horizon, an association of elements (P2O5, Ctot) was determined. Anomalous concentrations of these elements in the sediments at the site are associated with animal butchering zones. This is confirmed by archaeological material and the remains of calcined bones and ceramic fragments accumulated inside these areas. At the Okhta 1 site, high concentrations of the group of elements P2O5antr, CaOantr and Fe2O3, Sranthr are associated with the burial zone. In the sediments, in addition to P2O5antr, CaOantr, and Sranthr, a high content of Fe2O<sup>3</sup> is also recorded, which is associated with the ocher component. Geochemical maps of individual element distributions show the same contours, which coincide with the stonework in which the amber adornments were found. On the shore zone of the Podolye 1 site, a different pattern of distribution of elements P2O5antr, CaOantr, and Srantr is observed. They are concentrated in depressions of the relief in separate spots. These zones were probably used for butchering animals. Pottery sherds were found in these zones. At the Yudinovo site, an association of elements (P2O5antr), calcium (CaOantr) and, additionally, potassium (K2Oantr) and rubidium (Rb) is distinguished, the anomalous values of which are associated with depressions in the relief, in which burnt bones were found. Pits could be both utilitarian and ritual purposes. At the cult site "Bratja", a complex of anthropogenic components CaO, P2O5, (Sr, Rb) is distinguished in the sediments, which can be associated with remains of bones and ashes. Along with these elements, increased concentrations of other elements, such as K2Oanthr, as well as microelements Zn, Cu, are recorded. Such an association of elements in sediments may characterize ritual activity. An interesting situation is observed at the Tarkhankut 18 site, which is a cattle pen. An association of anthropogenic components (CaOantr, K2Oantr, P2O5antr) is distinguished, the anomalous values of which are typical for deposits inside stone pens. At the same time, CaOantr concentrations are an order of magnitude lower than in sediments outside the paddocks. This is due to the dissolution of the carbonate component of sediments as a result of the vital activity of animals.

The complex of elements (K2Oantr, Rbantr) in cultural deposits at all sites is associated with the remains of charcoal and wood ash. At the sites of Serteya 3-3, Okhta 1, and Podolye 1, anomalous concentrations of these elements are connected with fireplaces. The sediments also contained charcoal inclusions. The association of elements (K2Oantr, Ba, MnO, Copr) can characterize zones of decomposed organics (skins, wood, food residues) and humus rich sediment zones.

For the reconstruction of various factors influencing on the cultural deposit formation, an approach has been developed using multivariate factorial and correlation matrix analyses. It possible in this case to identify individual groups of geochemical elements and their associations for characteristics of anthropogenic or natural factors influencing sediment composition [1,18,45]. Another important criterion for detailed interpretation of geochemical data is archaeological information about the site. Using complex research methods, such as geochemical, geophysical, and archaeological analysis in the assessment of an archaeological site, in detail for all the features of life, including household activity, pottery manufacturing, ancient metallurgy, etc. can be reconstructed [18,36,38]. An assessment of the distribution of anomalous concentration of certain elements or associations of elements at archaeological sites has shown that the interpretation of the data obtained cannot always be unambiguous and depends on many factors [46,47]. For example, the technogenic impact on the distribution of the chemical elements at the archaeological site located in the industrial zone has been considered by authors in the Central Greece region [48]. The authors introduce the term "equifinality" for chemical elements which are characterized by multiple sources. An important question in the interpretation of geochemical data is also the geochemical explanation that the accumulation of one or another chemical element is of an anthropogenic nature and is not related to the mineral composition of deposits at a given site and excluded other factors of contamination [49].

#### **5. Conclusions**

The assessment of the impact of anthropogenic activity on the transformation of sediments and the formation of cultural layers on the ancient settlements was carried out using the method of geochemical indication of functional zones on settlements. Thus, it is possible to note the main associations of anthropogenic elements in sediments at archaeological sites that characterize certain functional zones. Abnormal concentrations of such complex association of components (P2O5antr, CaOantr and Srantr) in sediments are attributed to zones of accumulation of bone remains. A more precise and detailed interpretation of functional zones (animal dressing zone, burial, ritual zone, waste pit) can be possible using other additional geochemical markers and archaeological context. Anomalous concentrations of a group of elements (K2Oantr, Rbantr) at archaeological sites are associated with wood ash residues and fireplaces, ash residues from ritual activities, and fires. The group of components (Ba, MnO, Corg) reflects the accumulation of humus and organic residues, and can characterize areas with food residues, remains of skins, and rotten wood. Combinations of the main groups of elements may also indicate various other types of functional zones, living zones, household areas, and places of ritual purposes. 3D geochemical maps of Al2O<sup>3</sup> and SiO<sup>2</sup> distributions can be used for the reconstruction of ancient microrelief at the sites. Data from the archaeological context should be studied to carry out more detailed reconstructions. Using a geochemical multicomponent approach, even in complex geo-morphological contexts, anthropogenic geochemical indicators can be used to reconstruct functional zones at ancient settlements.

**Funding:** This research was funded by RSF, project No. 22-18-00065 "Cultural and historical processes and paleoenvironment in the Late Bronze-Early Iron Age of the North-Western Black Sea region: interdisciplinary approach" and the project with financial support from the Ministry of Education of the Russian Federation under program No. FSZN-2020-0016.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The author declares no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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**Tatiana Karavaeva , Elena Menshikova , Pavel Belkin \* and Vyacheslav Zhdakaev**

Natural Science Institute, Perm State University, Genkel st.4, Perm 614068, Russia **\*** Correspondence: pashabelkin@mail.ru

**Abstract:** The purpose of the present study is to analyse the distribution of arsenic in the soils of the Verkhnekamskoe potassium salt deposit (Perm Krai, Russia). The danger of arsenic pollution is determined by its high toxicity and carcinogenic hazard. Being a technophilic element, arsenic enters the environment primarily as a result of mining activities. Mining and processing sites for arsenic-containing ores are the most prone to technophilic arsenic accumulation. Solid wastes from potash production also contain elevated concentrations of arsenic. The content of arsenic in soils was determined by inductively coupled plasma mass spectrometry (ICP-MS). Statistical methods were used to analyse the features of arsenic distribution in soils of background areas and potash mining areas near production facilities. Three types of landscapes were studied within each territory, which were each distinguished by the leading processes of substance migration. Arsenic concentrations in both the background areas and the potash mining territories vary considerably, ranging from <sup>n</sup> <sup>×</sup> <sup>10</sup>−<sup>1</sup> to n × 10. The study found no statistically significant differences in arsenic concentrations in soils of potash mining areas and background areas. Arsenic concentrations in soils from various types of landscapes also do not differ statistically. Arsenic concentrations in soils of saline areas were found to be higher than in the rest of the territories. Outside of saline areas, the identified patterns of arsenic distribution in the soils of the Verkhnekamskoe potassium salt deposit indicate that potash operations are not a determinant in the technophilic accumulation of arsenic.

**Keywords:** geoecology; pollution; arsenic; soil contamination; condition assessment; trace elements; potassium salt deposits; Verkhnekamskoe deposit

## **1. Introduction**

Mining areas have an increased geochemical technogenic load due to mining and processing. In the hypergenesis zone, primary minerals are transformed, and toxic elements and their compounds are released into the environment [1]. Studies of natural environment components' contamination with toxic trace elements in mining-affected areas are of particular interest.

Arsenic is a chemical element of the first class of environmental hazard with high toxicity and carcinogenic properties that pose a serious risk to humans [2,3]. Arsenic can enter the human body through the consumption of arsenic-contaminated water or agricultural products grown on arsenic-contaminated soils. In India, Bangladesh, Nepal, China, Taiwan, Thailand, Argentina, Mexico, and other countries, arsenic contamination of groundwater used for drinking is a concern [3,4]. In this regard, effective methods for detecting [5] and removing arsenic from drinking water are being actively developed [6–8].

As a technophilic element, arsenic enters the environment primarily as a result of the mining and processing of minerals, where it is a major component in the ore or is present as an impurity as well as in the composition of pesticides used in agriculture. Arsenic compounds are found in small quantities in ores, hydrocarbon feedstocks, industrial clays, etc. During the mining and processing of minerals, arsenic is emitted into the atmosphere with inorganic pollutants, discharged into sewage, deposited in solid waste dumps, and washed out by atmospheric precipitation, polluting surface water and groundwater. Localised

**Citation:** Karavaeva, T.; Menshikova, E.; Belkin, P.; Zhdakaev, V. Features of Arsenic Distribution in the Soils of Potash Mines. *Minerals* **2022**, *12*, 1029. https://doi.org/10.3390/min12081029

Academic Editor: Juan Antelo

Received: 15 July 2022 Accepted: 13 August 2022 Published: 16 August 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

areas of ecologically hazardous pollution are formed as a result of arsenic distribution in the natural environment [9].

The assessment of arsenic contamination of soils in mining areas has received a lot of attention in China, which holds 70% of the world's arsenic reserves [10]. Arsenic concentrations of up to 76,400 mg/kg with an average concentration of 1205.97 mg/kg have been found in soils near waste sites of mining and processing of arsenic-containing ore in Yunnan province, southwest China [10].

Negative environmental impacts, such as increased concentrations of a variety of trace elements in the natural environment, can also occur after mining has ceased, e.g., as a result of tailings material dispersion into the natural environment [11], or a lack of mothballing of abandoned mining sites [12]. The following elements dominate in the geochemical series in the surface element concentrations within the areas affected by the Dalnegorsk and Krasnorechensk tailings (Dalnegorsk district, Primorsky Krai, Russia): manganese, zinc, and arsenic; zinc, manganese, lead, and arsenic, respectively [11]. Arsenic and other trace element concentrations in mining wastes reach levels that classify tailings sites as technogenic deposits. The arsenic content in tailings of tin ore deposits in the Kavalerovsky region (Primorsky Krai, Russia) reaches 0.01–0.05% [12]. Arsenic has the highest concentrations in technogenic soils of tailings among toxic elements and exceeds average concentrations in the earth's crust by 20–886 times in gold ore tailings, 152–5340 times in polymetallic tailings, and 1.2–172 times in rare-metal tailings [13].

Pollutants are carried out of the technogenic system via air and water streams, causing changes in the geochemical background of adjacent territories. The mining plant "Khrustalnensky GOK" (Primorsky Krai, Russia) ceased its operations at the beginning of the 21st century. In different soil horizons up to the depth of 45 cm within the area affected by tailing dumps of the mining plant, arsenic concentrations are 317.27–377.86 mg/kg [1], while the established approximate allowable concentrations are 2–10 mg/kg, depending on the grain size composition and acidity of soils. The arsenic content in tin ore processing dust ranges from 16.04 to 28.3 mg/kg [14]. In soils near an arsenic-containing ore (As4S4) mining facility that closed in 2011 (Hunan Province, China), arsenic is the main pollutant, with average concentrations of 394 mg/kg, exceeding background values by 23 times. Arsenic contamination of soil was detected at a relatively close distance—about 500 m from the sources of exposure [15].

According to [16], the majority of the As found in soils at abandoned mine sites (Rita Mine, Tres Amigos Mine, Las Viescas Mine (northern part of Castilla-León, Spain))is in the so-called "residual fraction", i.e., in grains of specific As minerals that come from wastes and are later integrated within the soil mineral fraction. Mechanical dispersion is thus quantitatively greater than chemical dispersion.

Researchers consider arsenic to be the most dangerous of all mining waste pollutants due to the prolonged activity (chemical transformation and migration) of arsenic technogenic formations in the natural environment [12,17] and the high bioavailability of arsenic [11,18].

The geochemical stress caused by the potassium industry (extraction and processing of fossil salts) is seen in an increase in the content of potassium and sodium chlorides in soils and natural waters near potash enterprises and waste disposal sites [19,20]. In addition to easily soluble compounds, fossil salts contain impurities of high-risk elements such as zinc, lead, copper, nickel, cadmium, and arsenic [21], which create areas of technogenic dispersion when extracted on the earth's surface.

Several studies [22–26] explore the mineral and trace element compositions of salt rocks, insoluble salt residues, and potassium production wastes, which determine the man-made transformation of the geochemical spectra of natural environment components.

Halite, sylvite, and carnallite dominate the mineral composition of the Verkhnekamskoe salt deposit (Perm Krai, Russia) [24] with varying percentages of their content in different beds. The main components of the chemical composition of productive formations of the deposit are NaCl (45.31–76.11 wt%), KCl (15.25–31.04 wt%), MgCl<sup>2</sup> (0.24–0.34 wt% in sylvi-

nite formations; 5.97–12.48 wt% in carnallite formations), and CaSO<sup>4</sup> (1.15–2.68 wt%) [27], which determine the predominance of Na<sup>+</sup> , K<sup>+</sup> , Mg<sup>+</sup> , and Ca2+ cations and Cl<sup>−</sup> and SO<sup>4</sup> 2− anions in salt compositions. At the same time, approximately 30 trace elements were found in the ores of the deposit [22], with the metals of greatest ecological interest being zinc, lead, copper, nickel, cadmium, and arsenic, which is related to metalloids. These elements can be found as minerals on their own or as impurities that isomorphically replace the main cation. The mineral composition is very diverse, containing both soluble and slightly soluble compounds. Most researchers believe that the majority of these elements are found in the insoluble ore residue [22], which is mainly represented by carbonates (10–20%), sulphates (5–30%), and aluminosilicates (42–51%), the mineral composition of which is dominated by dolomite and magnesite, anhydrite and gypsum, hydroslides and feldspars, respectively [27].

The content of insoluble residue in different strata of the potassium deposit as well as potassium production wastes is as follows (in wt%): 2.6–6 (in sylvinite stratum), up to 6 (in carnallite rock), up to 10–12 (in interstitial rock salt), 15–40 (in clay and anhydrite interlayers), up to 4 (in solid potash waste), 15–70 (in the solid phase of clay-salt pulp), and 5–28 (in slimes) [24,27]. Cu-As-Sb is present in the composition of minerals–micro impurities of insoluble residue of sylvinite, carnallite, and rock salt [28]. The content of arsenic in the insoluble salt rock residue reaches 6.1 µg/g [24], while solid wastes of potassium production contain 0.48–30.7 mg/kg of arsenic [26].

The results of determining the background content of a number of macrocomponents and trace elements, including arsenic, in soils of six natural zones of Perm Krai (Russia) are presented in [29]. In general, Perm Krai has an average regional content of arsenic, which is 7.52 ± 0.25 mg/kg. In the soils of the middle and southern taiga, in the contact zone of which the potash industry facilities are located, the average arsenic content is 5.77 ± 0.62 and 7.83 ± 0.62 mg/kg, respectively. According to A.P. Vinogradov, arsenic accumulation relative to its average content in the earth's crust was noted in all natural areas of Perm Krai. The following concentration factors were found: 4.42 for Perm Krai, 3.39 for the Middle Taiga natural area, and 4.61 for the Southern Taiga natural area. Relative to the calculated regional average content [29], the Middle Taiga area is characterised by element dispersion (the dispersion factor is 1.3). The Southern Taiga area is distinguished by insignificant accumulation (the concentration factor is 1.04). It should be noted that in [29], the distribution of arsenic and other determined elements is estimated by natural areas without regard to soil type, composition, or physical and chemical properties.

The goal of this research is to analyse the specifics of arsenic distribution in soils near potash mines in Perm Krai (Russia), where the Verkhnekamskoe salt deposit is being developed. The studies included: (1) an analysis of the landscape structure of the study area; (2) a comparative analysis of arsenic content in soils of background territories and potash mining territories near production facilities, with a detailed range of arsenic concentrations determined by saline soil studies. The obtained results provide useful information on the arsenic content in the mining-affected area. The findings can be used to assess the role of salt mining in the formation of territorial environmental situations.

#### **2. Materials and Methods**

The study considers the soils of the Verkhnekamskoe potassium salt deposit (Perm Krai, Russia). The potash industry in Perm Krai is associated with the development of one of the world's largest deposits—the Verkhnekamskoe salt deposit, whose development started in the 1930s. In addition to the potash facilities, other major regional industrial enterprises in metallurgy, chemistry, and oil production, as well as residential and agricultural development, have all greatly contributed to the transformation of the natural environment in the area.

The confinement of the territory to the taiga zone in humid climate conditions has determined the dominance of typical taiga soils with a clear morphogenetic differentiation of the profile. A clarified and silt-lightened eluvial horizon forms under the humus horizon. This horizon is underlain by a median horizon with morphologically and analytically pronounced illuvial accumulation. The local territorial differentiation of soil formation factors leads to the development of intrazonal soil processes in river valleys under constant moistening. It determines the development of regenerative conditions in the environment. Economic activity alters the profile structure, acidity range, and chemical composition of natural soils.

Methods of system analysis and generalisation of theoretical and experimental research, statistical processing of empirical data, and modern instrumental and chemical methods are used in this work. The findings of geo-ecological soil studies from 2012 to 2021 are summarised and analysed.

Field studies included a route reconnaissance survey of the territory, the laying and description of soil trenches with soil taxonomic identification, and soil sampling from the upper humus horizon (0–10 to 0–20 cm) for subsequent laboratory tests. The soil was sampled from the background areas remote from economic activities (the planned development area of the Verkhnekamskoe salt deposit), developed areas of the deposit (adjacent to production facilities), and saline soil areas affected by potash industry facilities. Within the background area, sampling was conducted at three sites: (1) eluvial and transit landscapes occupied by typical forest communities on zonal soils; (2) eluvial and transit former agricultural landscapes; (3) transaccumulative and accumulative landscapes within wetland ecotopes and small river floodplains. A total of 81 soil samples were collected within the background areas. In addition, 64 soil samples were also collected within potash mining areas from (1) eluvial and transit landscapes occupied by forest vegetation on conditionally natural zonal soils; (2) eluvial and transit former agricultural landscapes; (3) transaccumulative landscapes within small river valleys. Furthermore, soils were sampled at sites with high concentrations of water-soluble salts and chloride-ion concentrations in soils ranging from 1.49 to 36.35 g/kg. Salinisation areas are distributed locally, located near salt waste disposal sites, and have no direct impact on the soil structure of the area. Seven soil samples were collected at the salinisation sites. The granulometric composition of soils was determined in the field using the rolling cord method according to N.A. Kachinsky [30]. Three to four grams of soil were moistened until they formed a thick paste (no water was squeezed out of the soil). The soil was well kneaded and mixed by hand before being rolled out in the palms into a cord (about 3 mm thick) and then rolled up into a ring (about 3 cm in diameter). When rolled, the cord takes on a different appearance depending on the granulometric composition of the soil. If no cord is formed, the soil has sandy composition (sand); rudiments of the cord are formed—the soil has loamy–sandy composition (loamy sands); the cord crumbles when rolled—the soil has light loamy composition (light loam); the cord is continuous, the ring is breaking up when rolled—the soil has medium loamy composition (medium loamy loam); the cord is continuous, the ring has cracks—the soil has heavy loamy composition (heavy loamy loam); the cord is continuous, the ring is continuous without cracks—the soil has clayey composition (clay).

Laboratory tests were conducted in the Nanomineralogy Sector of the Perm State University's Collaborative Use Centre and the Hydrochemical Analysis Laboratory of the Geology Department of Perm State University. Analytical studies were conducted using unified methods.

The As concentrations were measured using the Aurora M90 ICP-MS spectrometer (Bruker, Fremont, CA, USA). Autoclave digestion was used to dissolve the sample prior to ICP-MS measurements. To achieve an efficient digestion, sediment was used with various acids or mixtures, such as concentrated HNO<sup>3</sup> or other acids (HCl, HClO4, and H2SO4) or H3BO<sup>3</sup> solution diluted with deionized water. For the analysis, 0.1 g sample weights were used. Control samples (blank samples) and one standard sample were decomposed together with the analysed samples. To ensure the accuracy of the sample analysis, standard samples from the Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences (Irkutsk, Russia) were used. The validity of the analytical methods was confirmed

by the analysis of the standard reference material Gabbro Essexit STD-2A (GSO 8670- 2005).The following are the typical measurement errors for the method used, depending on chemical element concentrations: <0.001 µg/dm3—RSD >25%; 0.001–0.1 µg/dm3—RSD 25–10%; 0.1–1 µg/dm3—RSD 10–5%; >1 µg/dm3—RSD 5%.

The pH was determined using the national standard method (GOST 26483-85 [31], Russia) by extracting soil samples with a potassium chloride solution prepared at 75 g of potassium chloride per 1000 cm<sup>3</sup> of solution, which was followed by a pH-meter measurement (ANION 4100, Infraspak-Analit, Novosibirsk, Russia). The measurement error was less than 0.1 pH.

The obtained results were statistically processed using STATISTICA 12 software (Stat-Soft. Inc., Tulsa, OK, USA) and Microsoft Office Excel 2019 (Microsoft, Redmond, DC, USA). The Cheddock scale was used to assess the correlation between arsenic content and soil pHKCl with Spearman's rank correlation coefficient. The statistical significance of the Spearman's correlation coefficient was determined using the Student's t-criterion. The Mann–Whitney U-criterion with a 95% confidence probability was used to assess the reliability of differences in arsenic content in soils from background areas and potash mining territories. The Kruskal–Wallis H-criterion with a 95% confidence probability was used to assess the reliability of differences in arsenic content in soils from selected types of landscapes. The Kruskal–Wallis criterion is used to compare three or more samples; thus, it was used to evaluate the differences between the three studied landscape types within the background areas and the three types of landscapes within potash mining areas. The FactoMineR package in R was used to perform principal component analysis (PCA) to visualise the correlations [32]. Figures were made using the package "ggplot2" [33].

The contamination factor (*CF*) was calculated as the ratio between the metal concentrations and its background values:

$$
\mathbb{C}F = \mathbb{C}^i / \mathbb{C}^i\_B \tag{1}
$$

*CF* is the contamination factor;

*C i* (mg/kg) is the concentration of a target element in a sampled soil;

*C i B* (mg/kg) is the background value of the element.

The criteria adopted to determine the extent of the contamination were as follows: no contamination/low contamination (*CF* < 1), moderate (1 ≤ *CF* < 3), high (3 ≤ *CF* < 6), and very high (6 ≤ *CF*) [34].

Potential Ecological Risk Index, proposed by Hakanson [35], is a standard and widely used method in modern research [36] for assessing the ecological risk posed by potentially toxic elements in soils. The ecological risk factor (*E i r* ) for individual elements, e.g., arsenic, was calculated using its toxicity factor (*T i r* ) according to the formula:

$$E\_r^i = T\_r^i \left(\mathbf{C}^i / \mathbf{C}\_B^i\right) \tag{2}$$

*E i r* is the ecological risk factor for individual elements;

*T i r* is the toxicity response factor. The toxicity coefficient of arsenic is 10 [36];

*C i* (mg/kg) is the concentration of a target element in a sampled soil;

*C i B* (mg/kg) is the background value of the element.

For risk assessments, we adopted the following classification: *E i <sup>r</sup>* ≤ 40 represented low risk; 40 < *E i <sup>r</sup>* ≤ 80 moderate risk; 80 < *E i <sup>r</sup>* ≤ 160 considerable risk; 160 < *E i <sup>r</sup>* ≤ 320 high risk; 320 < *E i <sup>r</sup>* very high risk [37].

#### **3. Results and Discussion**

The soils of the taiga zone, which include the studied soils, are characterised by the active development of the oxidogenesis processes in conditions of free access of oxidants to weathering products, resulting in a decrease in the migration activity of chemical elements, especially oxidised forms of iron and manganese. As a result, weathering products and soils of humid taiga landscapes become saturated with these elements.

Under oxidising conditions, the predominant form of arsenic among the dissolved forms is As(V), which is present in the form of arsenic acid oxyanions. Arsenic adsorption in soils occurs mainly on the surfaces of colloidal soil particles. These particles can be represented by clay, oxides or hydroxides of aluminium, iron and manganese, calcium carbonates, and organic matter. Because iron oxide and hydroxide are the best adsorbents, iron arsenates are the most common arsenic compounds in acidic soils [38]. Studies of the correlations between clay and arsenic content in the top layer of soils also show that oxygen iron compounds play a determining role in clay fractions. The absorption of arsenic by clays is determined by the content of oxide and hydroxide forms of iron. Purified quartz sand without clay fractions, for example, showed minimal adsorption of arsenic oxyanions [38]. The low arsenic content is typical of soils in the State of Pará (Brazilian Amazon) with a predominant sand fraction [39].

Researchers noted that arsenic adsorption has a strong pH dependence due to the variable charge of the adsorbent surface (iron oxides and hydroxides). The maximum adsorption of arsenic oxyanions is observed in acidic conditions, at pH values close to 3. In the pH range from 3 to 7, arsenic adsorption is reduced up to 95–85%. A sharp reduction in arsenic adsorption is observed at pH 7–10 with an average of about 8.5. In this pH range, iron oxides and hydroxides have a zero charge. A higher pH value promotes the formation of a total negative charge on the adsorbent surface, preventing the adsorption of arsenic oxyanions from the solution. At pH values of 9–10, arsenic adsorption is reduced to 40–50% [38]. Soils generally have a pH below 8.6, at which most iron oxide and hydroxide surfaces should be positively charged, facilitating the adsorption of arsenic oxyanions [38]. The results of experiments evaluating the adsorption of arsenic by various clay minerals indicate that kaolinite, montmorillonite, illite, halloysite, and chlorite have the highest adsorption of As(V) at pH around 7, and that it decreases with increasing pH [38].

Table 1 shows the statistical characteristics of the pHKCl of the studied soils. In 90% of cases, the soils in the background areas had pHKCl values ranging from 3 to 7. Acidic soils with pHKCl values < 3 were observed in 10% of cases. Soils of eluvial and transit former agricultural landscapes had relatively higher pHKCl values. In 100% of cases, pHKCl values ranged from 3 to 7, and in 96% of cases, pHKCl values were above 5. Zonal soils of eluvial and transit landscapes occupied by typical forest communities in 93% of cases had pHKCl values in the range from 3 to 7. In 7% of cases, more acidic soils were found. Within the areas of transaccumulative and accumulative landscapes of wetland ecotopes and small river floodplains, the proportion of acidic soils with pHKCl < 3 increased to 22%. In 78% of cases, the pHKCl values ranged from 3 to 7.

Soils in potash mining areas generally had higher pHKCl values. Soils with pHKCl < 3 were not found in these areas. In 95% of cases, soils had pHKCl values ranging from 3 to 7. pHKCl values > 7 were observed in 5% of cases. In 100% of cases, soils of eluvial and transit former agricultural landscapes had pHKCl values ranging from 3 to 7, as in background areas.

Zonal soils of the territories of eluvial and transit landscapes occupied by typical forest communities in 97% of cases had pHKCl values ranging from 3 to 7. In 3% of cases, more alkaline soils were found. Within the transaccumulative landscapes of small river valleys, the proportion of soils with pHKCl > 7 increased to 17%. In 83% of cases, pHKCl values ranged from 3 to 7.

Saline areas had even higher pHKCl values—all pHKCl values were >7, but they had a minimal spread compared to the rest of the study area.

No correlation was found between arsenic content and the soil pHKCl (Figures 1 and 2, Table 2). The lack of a strong correlation can be explained by the prevailing range of pHKCl values. Only 2% of the total number of soil samples fell within the pHKCl range from 7 to 10, which is characterised by a decrease in arsenic adsorption by soil colloids [38], while all values were close to its lower limit (pHKCl = 7.4). The observed correlation between


As-2

As

Scatterplot of As against pHKCl Spreadsheet2 10v\*64c As = 2.594+0.2057\*x; 0.95 Conf.Int.

345678 pHKCl

Scatterplot of As-2 against pHKCl Spreadsheet2 10v\*64c As-2 = 12.5452-1.5966\*x; 0.95 Conf.Int.

4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 pHKCl

arsenic content and pHKCl of soils in the saline areas cannot be considered reliable, as it is not statistically significant (Figure 3, Table 2). Eluvial and transit former agricultural landscapes (n = 27) 3.30 6.30 5.61 5.80 Transaccumulative and accumulative landscapes within wetland ecotopes and floodplains of small rivers (n = 27) 2.42 5.70 3.95 4.03

**Sampling Min. Value Max. Value Average Value Median** 

Entire territory (n = 81) 2.42 6.30 4.56 4.61

communities on zonal soils (n = 27) 2.60 5.54 4.10 4.03

**Table 1.** Variation of pHKCl in soils. Potash mining areas outside of soil salinisation areas:

Areas of eluvial and transit landscapes occupied by typical forest

**Table 1.** Variation of pHKCl in soils.

by landscape type:


*Minerals* **2022**, *12*, x FOR PEER REVIEW 7 of 16

Background area relative to potash mining area:

to potash mining area (As); (**b**) background areas of eluvial and transit landscapes occupied by typical forest communities on zonal soils (As-1); (**c**) background areas of eluvial and transit former agricultural landscapes (As-2); (**d**) background areas of transaccumulative and accumulative landscapes within wetland ecotopes and floodplains of small rivers (As-3). to potash mining area (As); (**b**) background areas of eluvial and transit landscapes occupied by typical forest communities on zonal soils (As-1); (**c**) background areas of eluvial and transit former agricultural landscapes (As-2); (**d**) background areas of transaccumulative and accumulative landscapes within wetland ecotopes and floodplains of small rivers (As-3).

Scatterplot of As-1 against pHKCl Spreadsheet2 10v\*64c As-1 = -0.3548+0.8165\*x; 0.95 Conf.Int.

2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 pHKCl

> Scatterplot of As-3 against pHKCl Spreadsheet2 10v\*64c As-3 = 7.218-0.5309\*x; 0.95 Conf.Int.

4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 pHKCl

(**a**) (**b**)

As-1

(**c**) (**d**)

areas within the transaccumulative landscapes of small river valleys (*As-3*).

As-3

**Figure 2.** Dependence of arsenic content in soils of potash mining areas on pHKCl: (**a**) potash mining areas outside of soil salinisation areas (*As*); (**b**) potash mining areas within eluvial and transit landscapes, covered with forest vegetation on conditionally natural zonal soils (*As-1*); (**c**) potash mining areas within eluvial and transit former agricultural landscapes(*As-2*); (**d**) potash mining

As-2

Scatterplot of As-2 against pHKCl Spreadsheet1 10v\*81c As-2 = 5.2462-0.3818\*x; 0.95 Conf.Int.

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 pHKCl

 (**c**) (**d**)

landscapes within wetland ecotopes and floodplains of small rivers (As-3).

As-3

**Figure 1.** Dependence of arsenic content in background soils on pHKCl: (**a**) background area relative to potash mining area (As); (**b**) background areas of eluvial and transit landscapes occupied by typical forest communities on zonal soils (As-1); (**c**) background areas of eluvial and transit former agricultural landscapes (As-2); (**d**) background areas of transaccumulative and accumulative

Scatterplot of As-3 against pHKCl Spreadsheet1 10v\*81c As-3 = 7.6896-0.9649\*x; 0.95 Conf.Int.

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 pHKCl

**Table 2.** Assessment of the correlation between arsenic content and soil pHKCl by Spearman's coefficient (ρ).


112


**Table 2.** *Cont.*

\* ρ—Spearman's correlation coefficient; ρcritical—Spearman's criterion critical value; t—Student *t*-test; tcritical—critical value of Student's *t*-test.

By analysing the features of arsenic distribution in different types of landscapes

) values for As regarding the toxicity response factor (

) is 61.22, indicating moderate risk.

 )

 ) for

trations in both background and potash mining territories vary considerably (from n × 10−1 to n × 10), with 97% of values falling within the range from n × 10−1 to n. A smaller range of variation and the most uniform distribution were found for areas of eluvial and transit landscapes occupied by typical forest communities on zonal soils, both in background areas and potash mining areas, and for background areas of eluvial and transit former agricultural landscapes. There were no statistically significant differences in arsenic content in the soils of potash mining areas outside of saline areas and the background areas. The concentration factors, calculated as the ratio of average arsenic concentrations in the soils of potash mining territories to the average concentrations in the background areas, are 1.04–1.26. The contamination factor () values for As are moder-

are 10.41–12.65, indicating low risk, both for the area as a whole and for each of the identified landscape types. There are also no statistically significant differences in arsenic

have little effect on arsenic adsorption [38]. Nonetheless, studies have found higher arsenic concentrations in saline soils than in the rest of the area. Arsenic concentrations in saline soils are comparable to those in solid wastes from potassium production, as shown in [26]. The concentration factors of average values are 6.12 in relation to background territories and 5.65 in relation to potash mining territories outside of saline areas. The

contamination factor () values for As are very high. The ecological risk factor (

potash mining territories is lower than the average regional contents shown in [29]. Relative to the calculated regional average content [29], soils are characterised by element dispersion: the factor of dispersion within background areas ranges from 1.94 to 2.46; within potash mining areas, it ranges from 1.86 to 2.24. Soils in saline areas relative to the calculated regional average content [29] are characterised by element accumulation: the

Outside of saline areas, the average arsenic content in soils of both background and

Dissolved sulphate, nitrate, and chloride in saline soil concentrations were shown to

**Figure 3.** Dependence of arsenic content in soils of saline areas. **Figure 3.** Dependence of arsenic content in soils of saline areas.

content in the soils of the examined landscape types.

As regarding the toxicity response factor (

concentration factor is 2.73.

ate. The ecological risk factor (

By analysing the features of arsenic distribution in different types of landscapes (Tables 3–5, Figures 4 and 5), the following patterns can be identified. Arsenic concentrations in both background and potash mining territories vary considerably (from n <sup>×</sup> <sup>10</sup>−<sup>1</sup> to n <sup>×</sup> 10), with 97% of values falling within the range from n <sup>×</sup> <sup>10</sup>−<sup>1</sup> to n. A smaller range of variation and the most uniform distribution were found for areas of eluvial and transit landscapes occupied by typical forest communities on zonal soils, both in background areas and potash mining areas, and for background areas of eluvial and transit former agricultural landscapes. There were no statistically significant differences in arsenic content in the soils of potash mining areas outside of saline areas and the background areas. The concentration factors, calculated as the ratio of average arsenic concentrations in the soils of potash mining territories to the average concentrations in the background areas, are 1.04–1.26. The contamination factor (*CF*) values for As are moderate. The ecological risk factor (*E i r* ) values for As regarding the toxicity response factor (*T i r* ) are 10.41–12.65, indicating low risk, both for the area as a whole and for each of the identified landscape types. There are also no statistically significant differences in arsenic content in the soils of the examined landscape types.

**Table 3.** Statistical characteristics of arsenic distribution in soils.


\* SD—Standard deviation; V—Coefficient of variation.


**Table 4.** Mann–Whitney U-test for significance of differences in arsenic content in soils, *p* < 0.05.

\* n—number of values in the sample; T—sum of ranks in the sample; U—criterion value; Ucritical—critical importance.

**Table 5.** Assessment of the significance of differences in arsenic content in soils by Kruskal–Wallis H-criterion, *p* < 0.05.


\* n—number of values in the sample; T—sum of ranks in the sample; H—criterion value.

**As, mg/kg**

**Figure 4.** Change in arsenic content of soils.

**Abscissa axis**: **a1**—background area relative to potash mining

**Abscissa axis**:

**a1**—background area relative to potash mining area; **a2**—potash mines outside of saline soil areas; **b1**—background areas of eluvial and transit landscapes occupied by typical forest communities on zonal soils; **b2**—potash mining areas within eluvial and transit landscapes, covered with forest vegetation on conditionally natural zonal soils; **c1**—background areas of eluvial and transit former agricultural landscapes; **c2**—potash mining areas within eluvial and transit former agricultural landscapes; **d1**—background areas of transaccumulative and accumulative landscapes within wetland ecotopes and floodplains of small rivers; **d2**—potash mining areas within the transaccumulative landscapes of small river valleys; **f**—chloride saline areas **Legend**:1—range; 2—mean; 3—median; 4—value (variant) of arsenic content area; **a2**—potash mines outside of saline soil areas; **b1**—background areas of eluvial and transit landscapes occupied by typical forest communities on zonal soils; **b2**—potash mining areas within eluvial and transit landscapes, covered with forest vegetation on conditionally natural zonal soils; **c1**—background areas of eluvial and transit former agricultural landscapes; **c2**—potash mining areas within eluvial and transit former agricultural landscapes; **d1**—background areas of transaccumulative and accumulative landscapes within wetland ecotopes and floodplains of small rivers; **d2**—potash mining areas within the transaccumulative landscapes of small river valleys; **f**—chloride saline areas **Legend**:1—range; 2—mean; 3—median; 4—value (variant) of arsenic content

**Figure 4.** Change in arsenic content of soils. **Figure 4.** Change in arsenic content of soils.

*Minerals* **2022**, *12*, x FOR PEER REVIEW 13 of 16

a b c d f

distribution.

distribution.

0

2

4

6

8

10

12

14

16

18

20

22

**Abscissa axis**: **a**—sandy soils;


**c**—light loamy soils; **d**—soils of medium loamy composition;

**d**—soils of medium loamy composition; **f**—heavy loamy soils

**f**—heavy loamy soils **Legend**:1—range; 2—mean; 3—median; **Legend**:1—range; 2—mean; 3—median; 4—value (variant) of arsenic content

Figure 6 shows the general distribution patterns of arsenic and pHKCl for Samples A

Figure 6 shows the general distribution patterns of arsenic and pHKCl for Samples A (background area) and B (potash mining areas outside of saline areas), with changes in

Dissolved sulphate, nitrate, and chloride in saline soil concentrations were shown to have little effect on arsenic adsorption [38]. Nonetheless, studies have found higher arsenic concentrations in saline soils than in the rest of the area. Arsenic concentrations in saline soils are comparable to those in solid wastes from potassium production, as shown in [26]. The concentration factors of average values are 6.12 in relation to background territories and 5.65 in relation to potash mining territories outside of saline areas. The contamination factor (*CF*) values for As are very high. The ecological risk factor (*E i r* ) for As regarding the toxicity response factor (*T i r* ) is 61.22, indicating moderate risk.

Outside of saline areas, the average arsenic content in soils of both background and potash mining territories is lower than the average regional contents shown in [29]. Relative to the calculated regional average content [29], soils are characterised by element dispersion: the factor of dispersion within background areas ranges from 1.94 to 2.46; within potash mining areas, it ranges from 1.86 to 2.24. Soils in saline areas relative to the calculated regional average content [29] are characterised by element accumulation: the concentration factor is 2.73.

Arsenic concentrations differ statistically in soils with different granulometric compositions (Tables 2 and 5, Figure 5). The average arsenic content in soils of heavy loam composition is 1.5–2.5 times higher than its average content in other soils. Minimum average values are typical of soils with prevailing sandy fractions in their granulometric composition. The identified characteristics support the role of clay particles as an adsorbent.

In environmental studies, permissible arsenic concentrations in soils are determined based on particle size distribution and pHKCl. In Russia, the permissible concentration is 2 mg/kg for sandy and sandy loam soils, 5 mg/kg for sour (pHKCl < 5.5) loamy and clayey soils, and 10 mg/kg for near neutral and neutral (pHKCl > 5.5) loamy and clayey soils.

Soils of potash mining territories outside of saline areas in 20% of cases exceed permissible concentrations by 1.05–3.98 times. The ranges of exceedances in soils of different granulometric composition are similar: 1.05–3.75 in soils of sandy and sandy loam composition, and 1.1–3.98 in loamy and clayey soils. Despite their lower adsorption capacity, sandy soils consistently exceed the permissible values (69% of cases). This is due to the lower permissible arsenic concentrations in sandy and sandy loam soils.

The principal component analysis (PCA) was used to detect patterns and analyse linear dependencies in samples of arsenic and pHKCl concentrations in background areas as well as non-saline and saline areas near potash enterprises(Figure 6). The first PC1 axis explains 70.05% of the total variance between samples and the second PC2 axis explains 29.95%.

Figure 6 shows the general distribution patterns of arsenic and pHKCl for Samples A (background area) and B (potash mining areas outside of saline areas), with changes in acid–alkaline conditions playing the most essential role. Sample C (saline areas near potash mines) has considerably more variation in arsenic content.

The world's largest potash mining operations (Russia, Belarus, and Germany) are located in temperate latitudes in a humid climate zone [40,41]. Therefore, it can be assumed that the patterns of arsenic distribution in soils of potash mining areas in other countries with similar environmental conditions will be similar to those identified in this study.

**Figure 6.** Analysis of the main components for samples A (background area), B (potash mining territories outside of salinity areas), and C (salinity areas). **Figure 6.** Analysis of the main components for samples A (background area), B (potash mining territories outside of salinity areas), and C (salinity areas).

acid–alkaline conditions playing the most essential role. Sample C (saline areas near

The world's largest potash mining operations (Russia, Belarus, and Germany) are located in temperate latitudes in a humid climate zone [40,41]. Therefore, it can be assumed that the patterns of arsenic distribution in soils of potash mining areas in other countries with similar environmental conditions will be similar to those identified in this

potash mines) has considerably more variation in arsenic content.

#### **4. Conclusions 4. Conclusions**

study.

The findings show a significant range of variation in arsenic concentrations in soils from both potash mining areas and background areas. In 97% of cases, arsenic concentrations range from n × 10−1 to n. No statistically significant differences in arsenic concentrations were found in the soils of potash mining territories, background territories, and soils of different types of landscapes. The soils of saline areas have higher arsenic concentrations than the rest of the territories. The arsenic concentrations in saline soils are comparable to those found in potash production solid waste. Saline areas are distributed locally, usually near salt waste disposal sites (salt dumps, sludge storages). The findings show a significant range of variation in arsenic concentrations in soils from both potash mining areas and background areas. In 97% of cases, arsenic concentrations range from n <sup>×</sup> <sup>10</sup>−<sup>1</sup> to n. No statistically significant differences in arsenic concentrations were found in the soils of potash mining territories, background territories, and soils of different types of landscapes. The soils of saline areas have higher arsenic concentrations than the rest of the territories. The arsenic concentrations in saline soils are comparable to those found in potash production solid waste. Saline areas are distributed locally, usually near salt waste disposal sites (salt dumps, sludge storages).

Despite high arsenic concentrations in the insoluble salt rock residue and solid waste from potash production, the identified patterns suggest that the activities of the potash companies operating in the Verkhnekamskoe potassium salt deposit are not determinants in the technophilic accumulation of arsenic in potash mining territories outside of saline areas. Arsenic concentrations in background soils can be used to adjust its regional background concentrations. Information on this toxic element is required by environmental studies for construction projects. Despite high arsenic concentrations in the insoluble salt rock residue and solid waste from potash production, the identified patterns suggest that the activities of the potash companies operating in the Verkhnekamskoe potassium salt deposit are not determinants in the technophilic accumulation of arsenic in potash mining territories outside of saline areas. Arsenic concentrations in background soils can be used to adjust its regional background concentrations. Information on this toxic element is required by environmental studies for construction projects.

**Author Contributions:** Conceptualization, T.K. and E.M.; data curation, V.Z.; formal analysis, V.Z.; funding acquisition, T.K. and E.M.; investigation, T.K. and P.B.; methodology, T.K., E.M. and P.B.; project administration, T.K. and E.M.; resources, E.M.; software, E.M.; supervision, E.M.; validation, E.M.; visualization, T.K. and P.B.; writing—original draft, T.K. and V.Z.; writing—review and editing, T.K., E.M. and P.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research was supported by the Perm Research and Education Centre for Rational Use of Subsoil, 2022.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

