Background Content of Polycyclic Aromatic Hydrocarbons during Monitoring of Natural and Anthropogenically Transformed Landscapes in the Coastal Area Soils

Abstract

The large-scale studies of the Lower Don and the Taganrog Bay coastal zone were carried out to determine the background concentrations of polycyclic aromatic hydrocarbons (PAHs) in soils. The content of 15 priority PAHs was determined by saponification method with HPLC detection and varied widely from 77 µg kg⁻¹ to 16,017 µg kg⁻¹ with mean and median values of 1040 µg kg⁻¹ and 406 µg kg⁻¹, respectively. PAHs’ highest concentration level was observed in the soils of the Don River delta and industrial areas of Taganrog city affected by pyrogenic contamination sources. Background monitoring sites were mostly represented with hydromorphic (Fluvisols Salic and Tidalic Fluvisols) and automorphic (Rendzic Leptosols, Mollic Leptosols) soil types in the study area. The PAHs content in the soils of the background plots did not exceed 600 µg kg⁻¹, with a share of low-molecular-weight PAHs: about 50%. Benzo(a)pyrene content did not exceed the maximum permissible concentration (20 µg kg⁻¹), and low-molecular-weight compounds did not exceed the threshold level for the impact of polyarenes. In automorphic soils, naphthalene and phenanthrene (35–54%) dominated in the composition of the low-molecular-weight PAHs compounds, and only phenanthrene (59–70%) dominated in hydromorphic soils.

1. Introduction

The soil is the main depositing component of landscapes, acting as a biogeochemical barrier, preventing the migration of pollutants into adjacent environments, and living organisms. Soil buffering capacity in relation to pollutants is strictly limited and depends on soil type and biodiversity. As pollutants accumulate in the soil, its ecological functions are disturbed, which leads to destabilization of the ecosystem and the formation of secondary pollution sources. In this regard, an important task is to monitor the ecological state of soils, especially in relation to the most dangerous chemical pollutants. These substances include polycyclic aromatic hydrocarbons (PAHs) are assigned to the class of widespread carcinogens [1], 16 of which are listed as priority pollutants by the US Environmental Agency [2]. The massive appearance of PAHs in environmental objects is mainly associated with the pyrolysis of hydrocarbon materials, emissions from vehicles, oil spills and oil products [3]. Due to the diversity and ubiquitous presence of PAHs sources, their accumulation in soils is inevitable. PAHs in the aerosols form are able to spread to the long distances through the atmosphere, covering polar and tropical zones [4,5,6]. In China, the annual atmospheric emissions of 16 priority PAHs reach approximately 100 thousand tons [7]. At the same time, by weight, approximately 97% of PAHs are concentrated in soils and only 3% belong to the aquatic and air objects [8].

PAHs are hydrophobic substances characterized by a low migratory activity with accumulation ability in the upper soil horizons firstly [9]. PAHs migration in soils occurs in combination with water-soluble organic matter [10]. Under the action of physical forces, up to 70% of hydrocarbons are washed out of contaminated soils within a few months after pollution, which leads to the secondary accumulation of these substances in the surface and ground waters [11]. About 19% of total annual PAHs emission are released to the sea through soil erosion [7]. Compared to land areas, coastal areas are a dynamic system with an active soil and water exchange, which enhances the migratory capacity of PAHs in the environment [12]. Among the integral factors contributing to the mass transfer of PAHs in aboveground-aquatic ecosystems are currents, wave processes and upwelling, leading to erosion of the coastline or re-suspension of coastal sediments that leads, accordingly, the return of pollutants to the water column with subsequent redeposition [13]. In coastal PAHs-contaminated areas where seafood is a staple food, their consumption is a key contributor to overall cancer risk [14]. PAHs’ transfer from soil to the marine environment is critical for environmental monitoring and research into the global biochemical cycle of pollutants [7]. As a result, monitoring the ecological state of soils in coastal areas is especially relevant in relation to multifunctional zones, with active economic and industrial activities, near areas with recreational and environmental status.

The study of the ecological state of coastal territories is complicated by the lack of generally accepted background values for the 16 priority PAHs content in soils. In general, the problem of determining background values concerns all soil types without exception but is more significant for the coastal zone. Conduction of the monitoring studies of automorphic soils at the local or global level is sufficient to study the quantitative and qualitative PAHs composition in zonal soils of territories with a minimum technogenic load or a special protected status. For hydromorphic soils, especially with local monitoring, this methodological approach is ineffective. Due to the high dynamics of mass transfer processes, under conditions of heterogeneous soil cover, variations in the total PAHs content can exceed 100% within one reserve [15,16]. Despite the variability, the reviews present the results of the PAHs content in the soils of natural zones (

Table 1. PAHs total content in soils of various regions of the world.

Georeferencing	Land Use Type	Number of PAHs	PAH Concentration, µg kg−1			Source
			Mean/Median	Minimum	Maximum
Land
Global	All types of land use	15	328/44	-	-	[18]
Africa			30/33	-	-
Asia			65/21	-	-
Europe			714/136	-	-
North America			347/91	-	-
South America			21/5.8	-	-
Oceania			60/13	-	-
Great Britain	Forest background	15	Average—580	-	-	[19]
	Pasture background		Average—700	-	-
Norway	Forest background		Average—350	-	-
	Pasture background		Average—110	-	-
China	All types of land use	16	-	31	136	[20]
China	All types of land use	16	Average—700	-	-	[21]
Russia, taiga zone	Forest	eleven	<60	-	-	[22]
Eastern Europe	Agricultural	13	-	160	260	[23]
China, south	Agricultural	16	318/265	22	1557	[24]
	Suburb	16	98/52	22	132
Italy, south	Countryside	16	336/1881	2	11,353	[25]
Poland	All types of land use	16	Median—395	80	7284	[26]
China	Suburb	16	-	52	888	[20]
	Agricultural	16	-	28	554
China, east	Countryside	16	Average—1060	22	3350	[27]
Poland	Agricultural	13	Average—264	28	2447	[28]
Poland	Countryside	13	Average—387	-	-	[29]
Germany	Forest	16	Average—551	20	9038	[30]
Northern Europe	Background	16	<100	-	-	[19]
Tropical zone	Background	16	-	12	380	[31]
Temperate zone	Background	12	65	-	-	[4]
Cuba	Background	16	<120	-	-	[32]
Russia, south	Background	16	<300			[33,34]
Near water bodies
China, east	Coastal zone	16	-	91	2311	[35]
China, north mouth of the river. Liaohe	Background	16	<100	-	-	[36]
Nigeria	Delta River Niger	28	Average—80	24	120	[17]
China, r. Pearl	River mouth	16	Average—427	-	-	[37]
China	River mouths	16	<500	-	-	[35]
China, r. Jinjiang and Quanzhou Bay	Coastal zone	16	-	19	282	[38]
China, Bohai and Yellow Sea	Coast	16	Average—234	-	-	[39]
China, Tianjin	Coast	16	Median—385	58	9160	[40]
South korea, west coast	Coast	16	Average—394	70	1175	[41]
China, Delta Liao	Background	16	Average—550	106	3148	[42]
China, r. Huanghei	Delta	16	Average—132	27	753	[43]
Spain, northwest	Coast	16	-	thirty	800	[16]
Italy, Palermo	Background, coast, nature reserve	16	-	35	545	[15]

). Table 1 presents some information about the soils involved in agricultural turnover, as well as rural and suburban settlements located remotely from point emission sources. In the soils of natural territories and in soils with a low total content of PAHs (<500), naphthalene and phenanthrene accumulate mainly, the share of which in the priority PAHs sum can reach 80% [4,17]. PAHs are one kind of widespread carcinogens, approximately 97% of which by weight are concentrated in soils. About 19% of total annual PAHs emission are released to the sea through soil erosion. Coastal areas are a dynamic system with an active soil and water exchange. In coastal PAHs-contaminated areas where seafood is a staple food, their consumption is a key contributor to overall cancer risk. PAHs transfer from soil to the marine environment is critical for environmental monitoring [3,5,10,17].

The purpose of this work was to determine the background content of PAHs during monitoring of coastal soils of natural and anthropogenically transformed landscapes.

2. Materials and Methods

2.1. Study Area

The Taganrog Bay, the Lower Don and the Don Delta are key areas on the transit route of large-tonnage long-distance tankers from the Azov and Black Seas deep into Russia. More than 200 thousand water transport units pass through the Don Delta every year, which corresponds to the traffic of the Finland Gulf and contributes to the accumulation of PAHs group carcinogenic substances in the soils of coastal areas [44]. The danger of this process is due to the concentration within the coastal zone of the Taganrog Bay, residential, agricultural, recreational and natural areas. The high functionality of coastal zones is a common feature for the southern regions of Russia. The study area soils are affected by a number of industrial factories working in the coal mining, ferrous metallurgy, chemical and energy areas, including the largest power plant in the south of Russia Novocherkasska Power Station, as the main pollution source by PAHs and heavy metals in the south of Russia, in addition [45,46].

For the purposes of soil monitoring in coastal areas, 97 monitoring sites were established (Figure 1). The location of the monitoring sites makes it possible to analyze the sources of entry, migration flows, and accumulation zones of pollutants in the eluvial, trans super aquatic, and super aquatic landscapes of the Lower Don–Taganrog Bay system.

The valley of the Lower Don is characterized by the presence of a wide floodplain with an abundance of emersed and meadow vegetation. The Don Delta is represented by several sandy islands, densely indented by gently sloping depressions of dried-up old riverbeds. The branches and channels of the delta have natural channel shafts at a height of up to 1.5 m above the low water level. The total area of the modern river Don delta is about 540 km². The northern coast of the Taganrog Bay is characterized by the predominance of abrasion and erosion processes, the southern part, from the Dolgaya Spit, is characterized by relatively more intense accumulative processes. The floodplain and coastal landscapes of the Lower Don and the Taganrog Bay are represented by alternating reservoirs, overgrown willows, floodplain meadows, sand dunes, beaches and spits, parks, gardens, and other tree plantations.

The sources of PAHs origin in the soil were determined on the basis of diagnostic ratios: (1) fluoranthene/(fluoranthene + pyrene) (<0.4—petrogenic, 0.4–0.5—pyrolysis of coal, wood, >0.6 spills of gasoline, diesel fuel); (2) benz(a)anthracene/(benz(a)anthracene + chrysene) (<0.2—petrogenic, 0.2–0.45—coal pyrolysis, >0.45 automobile emissions) [3]. According to this scale, the studied landscapes soils were classified as natural or anthropogenically transformed natural landscapes.

2.2. Sampling and Pretreatment

The basis of the soil cover in the study area is formed by hydromorphic and semi-hydromorphic soils on alluvial deposits in the floodplains of the river. Don and small rivers, alluvial-marine deposits of the Don delta, leman deposits at the boundary of estuaries of large rivers, and marine deposits of the Taganrog Bay, represented by an extensive group of Fluvisols. The soil sampling sites were located in the territory of the Don river delta and their location is presented at the map (Figure 1). The most common soil type in the studied area is Fluvisols Salic. For this soil type, it was set at 97 monitoring sites. These soils are located mainly on the territory of the Lower Don and its delta, as well as in the coastal zone of the Kagalnik River. A distinctive feature of these soils was the high intensity of soil formation under conditions of regular moistening by surface and ground waters, constant modern sedimentation, and variability in the morphological structure and physicochemical properties presented in the

Table 2. Soil properties of the coastal territory of the Lower Don and the coast of the Taganrog Bay.

Soil Properties		Mean	Median	Minimum	Maximum	Standard Deviation	Coefficient of Variation, %
pH		7.88	7.90	7.24	8.91	0.33	4.23
CaCO3	%	2.36	1.80	0.19	8.30	1.91	80.63
Particle < 0.001		13.84	11.70	0.10	33.10	9.78	70.66
Particle < 0.01		29.42	26.00	1.30	67.76	17.22	58.55
Organic carbon		1.32	1.10	0.12	3.94	0.83	62.62
Ca2+	cmol (+)	16.06	15.80	3.20	36.30	8.40	52.28
Mg2+		3.02	2.10	0.20	13.20	2.61	86.44

. The floodplain and coastal landscapes of the study area are dominated by: Fluvisols Tidalic were found on the northern coast of the Taganrog bay. For this soil type, it was set at 26 monitoring sites. Fluvisols Calcaric were located on the south coast and on the territory of the Miussky Estuary. For this soil type, it was set at 12 monitoring sites. Fluvisols Humic were represented in the southern part of the Don River Delta and in the mouth area of the Kagalnik River. For this soil type, it was set at 5 monitoring sites. Under the conditions of pulsating water, as well as a result of surge phenomena, Tidalic Fluvisols prevail within the Don River delta. For this soil type, it was set at 1 monitoring site. On the southern coast of the bay, Solonchaks Gleyic are common in depressions on marine and alluvial deposits. For this soil type, it was set at 3 monitoring sites. Under automorphic conditions, soil-forming rocks are loess-like deposits and shell rock outcrops, on which Calcic Chernozems (for this soil type, it was set at 4 monitoring sites), Mollic Leptosols (for this soil type, it was set at 2 monitoring sites), Rendzic Leptosols (for this soil type, it was set at 1 monitoring site) were formed. Technosols were formed on technogenic deposits within urban areas, as a special anthropogenically modified soils. For this soil type, it was set at 9 monitoring sites. The total number of soil samples in the current study was 97 soil sites.

Soil sampling was carried out to a depth of 0–20 cm by the envelope method. From each monitoring site with an area of 10 m², 5 soil samples weighing 1 kg were taken from each corner of the monitoring site, as well as in the middle. Subsequently, the soil was mixed to create one average sample. The selected soil was dried in air, cleaned of plant residues, crushed in a mortar, and sifted through a sieve with a diameter of 1 mm.

Physicochemical properties of the studied soil samples were analyzed according to Theory and Practice of the Chemical Analysis of Soils [47]: pH was determined by potentiometry in suspension of soil/water ratio of 1:2.5; exchangeable bases were determined with 1 M NH4OAc. Total organic carbon (Corg) was determined by wet combustion with potassium dichromate and concentrated sulfuric acid according to the Tyurin method (which is very similar to the WalkleyBlack method [4]). Carbonates were determined by acid neutralization [47]. Particle size distribution was determined by the pipette method with pyrophosphate preparation [48].

The soil properties of the study area vary widely (CV > 50%), which is a consequence of the soil cover heterogeneity. However, the studied soils are predominantly characterized by a weakly alkaline reaction of the medium (CV < 5%), and are generally characterized by a medium loamy granulometric composition (Table 2).

2.3. Chemical Analysis

For the extraction of PAHs, soil samples were weighed by 1 g. To remove the interfering lipid fraction, the soil sample was subjected to saponification by boiling for 3 h in a 2% KOH (Aquatest, Rostov-on-Don, Russia) solution (30 mL) in a water bath with a reflux condenser. Extraction of PAHs was carried out with n-hexane (Aquatest) [49]. For that purpose, a 15 mL of hexane was poured into the sample and placed on a shaker. After 10 min, the hexane supernatant was carefully poured into a separating funnel. The operation was repeated three times. After that step, on a separating funnel, the layers of hexane were separated from the residual fraction of the alcohol solution of alkali. For mechanical purification and removal of residual liquid, the extract was filtered through a paper filter with anhydrous sodium sulfate. Next, the hexane’s extract was evaporated on a rotary evaporator. After evaporation, the precipitate was dissolved in 1 mL of acetonitrile [50]. The dry residue was dissolved in 1 mL of acetonitrile.

Samples were analyzed for PAHs with an Agilent 1260 Infinity (Agilent Technologies, Santa Clara, CA, USA) high performance liquid chromatograph (HPLC) equipped with a fluorescence detector following the ISO 13877-2005 requirements [51]. The HPLC system was fitted with reversed phase column Hypersil BDS C18 (Agilent Technologies) (125 × 4.6 mm, 5 μm). A mixture of acetonitrile (Criochrome, Moscow, Russia) (75%) and bidistilled water (25%) was used as the liquid phase at a flow rate of 0.5 mL min⁻¹. The volume of injected extract was 20 µL. In the present study it was determined the content of 15 priority PAHs. Of these, low molecular weight: 2-ring naphthalene and 3-ringed (acenaphthylene, acenaphthene, biphenyl, fluorene, phenanthrene, anthracene), and high molecular weight: 4-ringed (fluoranthene, pyrene, benzo(a)anthracene, chrysene), 5-ringed (benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, dibenz(a,h)anthracene) and 6-ring benz(g,h,i)perylene.

The efficiency of target PAHs extraction from soils was determined using a matrix spike method by preparing calibration curves [52]. The fresh soil sample as well as air-dried soil sample (1 g) was placed into a round-bottom flask and benzo(a)pyrene standard solution in acetonitrile was added to give the target PAHs concentrations of 2, 4, 6, 8, 16 or 32 μkg kg⁻¹. After evaporating the solvent for 30 min under a hood under ambient conditions, the PAHs-spiked soil samples were incubated for 24 h at 4 °C. The samples were then analyzed by the saponification method described above with consequent HPLC analysis.

Quality control of every HPLC detection was performed according to Agilent Application Solution [52]. Individual standard solutions were purchased from the Sigma-Aldrich (Merch) (Burlington, MA, USA). A calibration standard of PAH mixture was injected after every six samples to correct for drift in retention time within a run. After preparation of calibration curve for every detected PAH the coefficient of detection was counted:

The content of PAH in the test samples was determined by the external standard method [51]. The content of PAH in the soil was calculated from the equation:

Cs = k Si × Cst × V/(Sst × m),

(1)

where Cs is the content of PAH in the soil sample (μg kg⁻¹); Sst and Si are the PAHs peak areas for the standard solution and the sample, respectively; Cst is the concentration of the standard PAH solution (μg kg⁻¹); k is the recovery factor of PAH from the sample; V is the volume of the acetonitrile extract (mL); and m is the mass of the sample (g).

The certified reference materials and calibration curves were used for calculation of the limits of detection (LOD) and limits of quantification (LOQ), which counted 2–200 μg kg⁻¹. For the developed methods of extracting the target PAH in the soil, a random component of the measurement error was estimated, which for the concentration range of 2–200 μg kg⁻¹ was 3.5–14%. Detection limits for individual PAH compounds are presented in the Supplementary Table S1 and Figure S1.

Solvents and reagents were HPLC grade and included ethanol (96%, analytical grade) (Aquatest,), n-hexane (99%, analytical grade) (Aquatest), potassium hydrate (98%, analytical grade) (Aquatest), acetonitrile (99.9%, analytical grade) (Cryochrome), NaOH (97%, analytical grade) (Aquatest), and anhydrous Na₂SO₄ (Aquatest). The total 15 priority PAHs standards in acetonitrile with concentration 200 µg cm⁻³ produced by Merch Burlington, MA, USA (Priority pollutant PAHs (in acetonitrile) NIST^® SRM^® 1647f) was used to prepare total PAHs standard solutions for HPLC analyses. For every target PAH the individual standard was used for determination (Supplementary Table S1 and Figures S1 and S2). The analytical standards were purchased from the Sigma-Aldrich (Merch) was used as the internal analytical standard.

The degree of soil contamination was assessed based on the gradation proposed by Maliszewska-Kordybach [26]: up to 200 µg kg⁻¹—not polluted, 200–600 µg kg⁻¹—slightly polluted, 600–1000 µg kg⁻¹—polluted, above 1000 µg kg⁻¹—heavily polluted. This gradation was based on the results of a large-scale monitoring of soils in agricultural areas of Poland. Due to the fact that PAHs content in arable soils is usually lower than in fallow soils [53], taking into account the PAHs average concentrations in the soils of natural areas (Table 1), soils with pollutant concentrations below 600 µg kg⁻¹ were taken as background in this study. The threshold level of PAHs exposure correspondent to the minimal carcinogenic risk, was taken into account: the sum of low-molecular-weight PAHs < 312 µg kg⁻¹, and high-molecular-weight < 655 µg kg⁻¹ [8]. Moreover, the hygienic standards for soil quality adopted in Russia for benzo(a)pyrene, as the most dangerous PAHs representative with level of maximum permissible concentration (MPC) in soil 20 µg kg⁻¹ [54]. Additionally, the results were compared with global data on the background content of PAHs in soils (Table 1).

2.4. Data Analysis

Statistical data processing was carried out in the programs STATISTICA 7.0 and Sigmaplot 12.5. Cartographic material received using the QGIS 3.10 program. To determine the features of the spatial distribution for individual PAHs compounds, considering landscape features, soil properties, and typical differences in the emission source, a factor analysis was carried out. Due to anomalously high concentrations of PAHs, the monitoring site No. 70 was excluded from the calculation. In the future, this site was allocated to a separate cluster. Removal of factors was carried out by the method of principal components with the redistribution of variance using the Varimax tool row.

3. Results and Discussion

3.1. PAHs Concentration

The content of individual PAHs compounds varied widely in the studied area, which indicated the anthropogenic origin of pollutants in soils. At the same time, high-molecular PAHs compounds formed as a result of technogenic processes have been characterized by variability above 190%. As the PAHs variability increased, as an increase in the difference between the mean value and the median has been established, which indicates the presence of the pollutants point sources in the Don River delta (

Table 3. Content of individual PAHs compounds in the soils of the study area.

PAHs	Mean	Median	Minimum	Maximum	Standard Deviation	Coefficient of Variation	Average Content of PAHs in Temperate Soils
	µg kg−1					%	µg kg−1
Naphthalene	SSS	13.7	1.8	103.2	19.4	101.6	4.0
Fluorene	18.4	10.3	1.3	114.7	21.3	116.0	1.9
Phenanthrene	94.9	62.0	14.8	623.3	92.1	97.1	19.0
Anthracene	1.4	0.3	0.02	21.0	3.6	266.1	1.6
Acenaphthene	8.0	6.0	0.7	34.4	6.9	85.9	-
Acenaphthylene	9.6	6.1	0.7	57.3	9.4	97.78	-
Fluoranthene	155.0	54.0	3.9	2070.2	302.6	195.2	51.0
Pyrene	85.5	39.1	4.6	1811.0	194.4	227.4	25.0
Chryzene	77.4	31.9	4.1	1448.8	155.2	200.5	-
Benzo[a]anthracene	80.7	21.4	0.7	1619.4	212.8	263.6	26.0
Benzo[b]fluoranthene	106.9	31.5	1.1	2191.4	250.2	234.2	34.0
Benzo[k]fluoranthene	76.5	16.4	0.4	1289.3	179.7	235.0	20.0
Benzo[a]pyrene	125.1	22.7	0.5	2013.3	297.5	237.8	19
Dibenzo[a,h]anthracene	27.3	10.5	0.3	408.5	53.0	194.3	10
Benzo[g,h,i]perylene	154.6	45.5	1.8	3767.6	413.7	267.5	28
∑15PAHs	1040.1	406.4	76.6	16,017.0	2001.8	192.5	64.5

).

The median values of the pollutants concentration in the soil characterized the content of PAHs in the soils of natural landscapes since most of the monitoring sites are located in the native landscapes. According to the median values, phenanthrene and fluoranthene predominated in the soils of the study area. The concentrations of most PAHs corresponded to the average content of polyarenes in temperate soils according to Wilcke [4]. Nevertheless, there was an excess of the median concentration for low-molecular-weight naphthalene, fluorene, phenanthrene by 3.2–5.4 times, and high-molecular pyrene and benzo[g,h,i]perylene by 1.5–1.6 times, which is most likely a specific feature of highly productive coastal areas. Under natural conditions, low-molecular-weight and four-ring PAHs can be formed due to the mineralization of soil organic matter [54,55]. The genesis of high-molecular-weight PAHs in soils of native landscapes was similar to that of low-molecular-weight ones. At the same time, the pedogenic pathway for the formation of five- and six-ring compounds was more likely under reducing conditions, watered soil horizons, which is typical for coastal landscape soils [56]. The productivity of plant communities, the composition of stagnant biomass, the position of the site in the relief, the granulometric composition of the soil, the process of cryogenesis and pyrogenesis are the main factors regulating the pedogenic type of PAHs formation [4,57,58]. The average PAHs concentrations in the soils of the studied coastal areas often exceeded those for the soils of the temperate zone [4].

3.2. PAHs Spatial Distribution

The total content of 15 priority PAHs in the soils of the study area varied from 77 µg kg⁻¹ to 16,017 µg kg⁻¹. The soils of the Don River Delta and the south coast of the Taganrog Bay had the lowest content of PAHs in soils. The most polluted site was confined to the northern coast of the Taganrog Bay (no. 70) (Figure 2). Here, the total content of PAHs exceeded 16,000 µg kg⁻¹, of which more than half was accounted for by five- and six-ringed compounds. The high content of PAHs was noted in the southeastern part of the Don River Delta (up to 8862 µg kg⁻¹). In both cases, the coastal zone was characterized by the activity of water transport. The monitoring site was located on the territory of the spit, and, most likely, high concentrations of pollutants were accumulated due to the impact of small-scale shipping within the coastal zone [59]. At the same time, a long-distance tanker channel operates within the Don River Delta. In general, PAHs content in the soils of the Don River Delta was higher than in the soils of the river’s coastal areas and the Taganrog Bay, which is consistent with the studies of Andrade et al. [60]. The soils of the deltas are characterized by an increased ability to accumulate toxic organic compounds and a reduced ability to degrade them (mainly due to constant changes in the redox conditions in the soil profile) [11]. About 70% of the studied monitoring sites soils were close to the natural territories of the world various regions in terms of the PAHs total content (Table 1) and were classified as uncontaminated (<600 µg kg⁻¹). Of these, in a quarter of soil samples, the pollutants content was less than 200 µg kg⁻¹. These concentrations were less than in the background soils of the eastern and central [26], western, and northern Europe [19], but exceeded the average values for PAHs content in soils of the temperate zone [4].

Soil pollution was typical only for industrial areas for the Taganrog city territory and reached up to 4309 µg kg⁻¹ of PAHs total amount. In the soils of the coast recreational area, which are part of the city, it has been determined the low concentrations of PAHs. This indicated the possibility of using the soils of recreational areas as a background [6].

The relationship between the PAHs total content and the amount of individual polyarenes in soils has been established. As the total content of PAHs in the soils of the study area increased, an interconnected increase in the concentration of almost all polyarenes has been observed, except for naphthalene, which is most likely due to its pedogenic origin. For other low-molecular-weight PAHs such as fluorene, anthracene, acenaphthene and acenaphthylene, the correlation coefficient values indicated the duality of their origin from natural and technogenic sources (r < 0.7). The highest values of the correlation coefficient are characteristic of high-molecular-weight PAHs (r > 0.84), especially fluoranthene and benzo(a)anthracene (r > 0.95), indicating their technogenic origin (

Table 4. Relationship between (r) the PAHs total content and the amount of individual polyarenes in the soils of the study area.

Naphthalene	Fluorene	Phenanthrene	Anthracene	Acenaphthene
0.04	0.68 *	0.81 *	0.42 *	0.60 *
Acenaphthylene	Fluoranthene	Pyrene	Chryzene	Benzo[a]anthracene
0.66 *	0.96 *	0.87 *	0.88 *	0.95 *
Benzo[b]fluoranthene	Benzo[k]fluoranthene	Benzo[a]pyrene	Dibenzo[a,h]anthracene	Benzo[g,h,i]perylene
0.89 *	0.84 *	0.86 *	0.91 *	0.88 *

Note: * Results obtained using Spearman’s correlation coefficient at <0.0001.

).

3.3. Source Analysis

The results of calculating the PAHs diagnostic ratios showed that the soils of the studied coastal area are subjected to the technogenic effect to one degree or another. The accumulation of pollutants in soils often occurs under the several sources effect. At the same time, the soils of the monitoring sites are grouped depending on the predominant routes of PAHs entry (Figure 3). The first group provides information about merged sites with natural source of PAHs origin was represented by soils within the delta of the river Don, characterized by the influx of pollutants mainly with fuel spills. Fuel spills within the territory of the delta were accumulated due to the presence of a shipping channel through which the transit of large-tonnage long-distance tankers are carried out [44]. The second provides information about merged sites with fuel pyrolysis source of PAHs origin was the most extensive group. For this group of soils, the input of PAHs was mainly due to the combustion of fuels (Figure 3 and Figure 4). The third group provides information about merged sites with fuels spills source of PAHs origin was represented by slightly polluted soils located near the Tsimlyansk reservoir, river Kagalnik and the southern coast of the Taganrog Bay.

The calculation of diagnostic ratios was based on the fact that PAHs always enter the soil as part of a mixture. The ratio of individual (marker) pollutants to each other in the initial hydrocarbon feedstock before and after its combustion, as well as the form of substance intake indicates the most probable PAHs genesis [3,11,61]. Soils are capable of self-purification from PAHs pollution mainly under the microorganism’s activity [46,62]. The rate of this process mainly depends on soil properties. In this regard, the identification of pollutants origin sources is possible either with a real long-term continuous technogenic impact, or with a one-time exposure to a source with subsequent immediate sampling for further chemical analysis. To clarify the PAHs origin sources, the methods of multivariate statistical analysis have been often used [29,63,64,65].

Taking into account the Kaiser criterion, two factors with eigenvalues greater than 1 were identified. Together, the obtained factors describe 77.1% of the total variance. The contribution of Factor 1 to the resulting model was 63.6%, factor 2–13.5%. Factor 1 was loaded with three-rings, except for anthracene, as well as four-ring fluoranthene, benzo(a)anthracene, and five- and six-ring PAHs. Pyrene and chrysene made a significant contribution to the second Factor 2 (Figure 5).

The relationship between the coordinates of Factor 1 and the PAHs content in soils, as well as Factor 2 mainly with soil properties, was established (

Table 5. Correlation (r) between the new coordinates of Factor 1 and Factor 2 obtained from the results of Factor analysis with soil properties and PAHs accumulation.

Parameter	Factor 1	Factor 2	Parameter	Factor 1	Factor 2
pH	−0.12	0.16	Acenaphthene	−0.81 *	−0.25
CaCO3	0.05	0.31	Acenaphthylene	−0.85 *	−0.22
<0.001	−0.30	0.36	Fluoranthene	−0.92 *	0.23
<0.01	−0.30	0.40 *	Pyrene	−0.69 *	0.68 *
organic carbon	−0.25	0.56 *	Chryzene	−0.70 *	0.61 *
Ca++	−0.34	0.40 *	Benzo[a]anthracene	−0.89 *	0.18
Mg++	−0.33	0.33	Benzo[b]fluoranthene	−0.78 *	0.27
Naphthalene	0.01	−0.06	Benzo[k]fluoranthene	−0.82 *	−0.06
Fluorene	−0.85 *	−0.21	Benzo[a]pyrene	−0.83 *	0.01
Phenanthrene	−0.91 *	0.23	Dibenzo[a,h]anthracene	−0.75 *	0.39
Anthracene	−0.56 *	0.23	Benzo[g,h,i]perylene	−0.74 *	0.31

Note: * Results obtained using Spearman’s correlation coefficient at <0.001.

). Consequently, Factor 1 characterized the technogenic component in the model (the source of pollutants and its intensity), and Factor 2 characterized the natural components (soil properties, landscape characteristics, etc.). At the same time, despite the greater contribution of Factor 1 to the model, the new coordinates were differentiated with respect to Factor 2 that indicated the soil properties’ effect on the PAHs spatial distribution.

Based on the results of the factor analysis, the soils of the monitoring sites were grouped in space depending on the emission source type, its intensity, and soil properties. The distribution of soil clusters obtained in the course of factor analysis (Figure 6) was consistent with the grouping of soils in space relative to the pollutant’s sources (Figure 4). At the same time, the soil group affected by the pyrogenic source of PAHs (group 2 in Figure 4) was subdivided into two subgroups (clusters 1 and 3 in Figure 6). The total PAHs content in the soils of these subgroups varied greatly (Figure 2). However, higher concentrations of pollutants were noted in the soils of cluster 3, which is associated with the proximity of the monitoring sites to the impact zones of the industrial territory of Taganrog city (sites no. 78–84) (Figure 6) and Novocherkassk Power Station (sites no. 1–4), as well as large, populated areas located at sites no. 5–8, 66 [45,46].

Considering the spatial distribution of the pollutants content in soils, as well as the sources of their probable input, it can be concluded that with a similar genesis of polyarenes in terms of the 15 PAHs total content, soils are often heterogeneous in space. At a relatively close location (up to 2 km), the concentrations of pollutants in the soils of monitoring sites can differ by tens or even hundreds of times (Figure 7). This is primarily due to the typical features of the soil cover, mainly the content of the finely dispersed fraction, organic carbon, and exchangeable Ca²⁺. The specificity of coastal areas background monitoring lies in the difficultly predictable processes of soil particles mass transfer with subsequent redeposition of pollutants in soils or their removal outside the studied landscapes [13].

3.4. Background Content of PAHs in Soils

Acceptance of the monitoring site soils as a local background is possible provided that the criteria for the ecological state of soil standards in relation to PAHs have been satisfied. Based on the data obtained, as well as the literature data, the following indicators were proposed for determining the soils of background areas: (1) the total content of priority PAHs should not exceed the upper threshold of uncontaminated soils (600 µg kg⁻¹) according to international standards [8,26,53] and the background level of various countries of the world [15,16,42,43,63]; (2) the ratio of low-molecular-weight PAHs to high-molecular-weight should be higher or close to 1, due to the fact that 2- and 3-ring compounds are formed under the natural conditions [4,17]; (3) the soil of the background monitoring site should be located at a distance from the pollution source, predicting a mass-transfer of the pollutant particles [6,21,43,65,66]; (4) the results of the diagnostic ratios calculation should indicate a pedogenic or biogenic source of PAHs intake [65].

For the main soil types of the Lower Don and the Taganrog Bay, background monitoring sites have been identified that can serve as a standard for priority PAHs content: Fluvisols Salic (site no. 64), Tidalic Fluvisols (site no. 55). According to the criteria presented, rare Rendzic Leptosols (site no. 90) and Mollic Leptosols (site no. 91). Soil properties are shown in

Table 6. Physical and chemical properties of background soils of monitoring sites.

No.	Type of Soil	pH	CaCO3	Organic Carbon	<0.001	<0.01	Ca2+	Mg2+
			%				cmol (+) kg
55	Tidalic Fluvisols	8.1 ± 0.1	2.2 ± 0.1	2.5 ± 0.2	29.6 ± 0.5	49.1 ± 2.0	28.7 ± 1.7	4.0 ± 0.3
64	Fluvisols Salic	7.9 ± 0.2	0.9 ± 0.1	1.0 ± 0.1	4.9 ± 0.1	26.6 ± 1.7	12.0 ± 0.5	1.0 ± 0.2
90	Rendzic Leptosols	8.0 ± 0.2	6.1 ± 0.3	1.0 ± 0.1	26.8 ± 1.0	49.6 ± 2.1	30.6 ± 0.6	2.4 ± 0.1
91	Mollic Leptosols	8.0 ± 0.1	5.6 ± 0.4	2.4 ± 0.2	5.3 ± 0.1	23.8 ± 1.0	10.3 ± 0.6	5.7 ± 0.3

.

The total PAHs content in the reference soils did not exceed 600 µg kg⁻¹ and corresponded to the category of uncontaminated areas [26,28,67]. For Fluvisols Salic, Leptosols Rendzic Leptosols Mollic the PAHs content was at the level of the zonal Haplic Chernozem soil [33]. It was not detected as exceeding the maximum permissible concentration of BaP [53] (

Table 7. Content of individual PAHs compounds in background soils, µg kg⁻¹.

Monitoring Site Number		55	64	90	91
Soil Type		Tidalic Fluvisols	Fluvisols Salic	Rendzic Leptosols	Mollic Leptosols
PAHs Type
Low molecular weight	Naphthalene	19.7 ± 2.0	3.8 ± 0.1	68.2 ± 3.6	58.5 ± 2.5
	Fluorene	17.0 ± 2.1	2.4 ± 0.2	6.8 ± 0.2	7.6 ± 0.3
	Phenanthrene	165.3 ± 7.2	22.8 ± 0.5	45.0 ± 4.0	62.0 ± 3.1
	Anthracene	2.3 ± 0.1	3.7 ± 0.1	0.5 ± 0.1	0.5 ± 0.1
	Acenaphthene	16.5 ± 0.8	4.2 ± 0.1	3.0 ± 0.1	3.9 ± 0.2
	Acenaphthylene	15.9 ± 0.5	4.9 ± 0.3	4.0 ± 0.2	5.2 ± 0.2
High molecular weight	Fluoranthene	37.5 ± 2.1	3.9 ± 0.2	27.0 ± 1.2	31.1 ± 1.0
	Pyrene	88.4 ± 4.2	8.1 ± 0.5	30.4 ± 1.5	33.8 ± 1.5
	Chryzene	70.8 ± 4.3	20.0 ± 1.2	22.0 ± 1.0	28.0 ± 1.5
	Benzo[a]anthracene	5.0 ± 0.2	0.7 ± 0.1	7.0 ± 0.2	8.4 ± 0.2
	Benzo[b]fluoranthene	10.2 ± 0.3	1.1 ± 0.1	11.5 ± 0.5	13.0 ± 0.6
	Benzo[k]fluoranthene	2.3 ± 0.1	0.4 ± 0.1	3.4 ± 0.1	3.8 ± 0.1
	Benzo[a]pyrene	4.7 ± 0.1	0.5 ± 0.1	7.0 ± 0.3	8.5 ± 0.3
	Dibenzo[a,h]anthracene	6.7 ± 0.2	0.3 ± 0.1	0.9 ± 0.1	1.4 ± 0.1
	Benzo[ g,h,i]perylene	17.6 ± 0.5	1.8 ± 0.1	13.0 ± 0.4	15.0 ± 0.8
Σ PAH		478.1	75.6	249.3	280.2
Σ low-molecular-weight PAHs/Σ high-molecular-weight PAHs		0.97	1.05	1.04	0.96

). At the same time, the concentration of low-molecular-weight compounds was below 312 µg kg⁻¹, the threshold level for the most bioavailable PAHs [68].

The ratio of low-molecular PAHs groups to high molecular PAHs and their constituent compounds, in the absence of changes over time, indicates the stability of the soil system [61]. The group composition of individual PAH compounds can serve as a qualitative characteristic of soils in the background plots, performing a marker function in monitoring natural and impact zones confined to the coast. Typical representatives of PAHs of pedogenic origin are naphthalene and phenanthrene [57]. The features of the group composition of PAH compounds in reference soils depending on their position in the landscape were revealed. For automorphic soils Rendzic Leptosols and Mollic Leptosols, naphthalene and phenanthrene dominated in the composition of low-molecular-weight soils, the share of which reached 35–54%. A distinctive feature of the hydromorphic soils Tidalic Fluvisols, Fluvisols Salic was the pronounced dominance of phenanthrene, whose share in the composition of low-molecular-weight compounds reached 59–70%. At the same time, the share of naphthalene did not exceed 10%, which is most likely due to its lability and instability in the hydromorphic soils of the coast. Regardless of the position in the landscape, among the high-molecular-weight PAHs, the four-ring compounds dominated in the background soils. At the same time, fluoranthene, pyrene, and chrysene predominated in Rendzic Leptosols and Mollic Leptosols (18–25%), while pyrene and chrysene predominated in Tidalic Fluvisols and Fluvisols Salic (22–54%) (Figure 8).

4. Conclusions

It has been established that the content of 15 priority PAHs varies widely from 77 µg kg⁻¹ to 16,017 µg kg⁻¹ with mean and median values of 1040 µg kg⁻¹ and 406 µg kg⁻¹, respectively, in the soils of the Lower Don and the Taganrog Bay. The content of PAHs in the soils of the coast of the Lower Don and the Taganrog Bay is caused by fuel spills, as well as coal combustion. In general, the most vulnerable areas are confined to the Don River delta (8862 µg kg⁻¹) and industrial areas of the Taganrog city (4309 µg kg⁻¹). Under the conditions of the multifunctionality of the studied coastal zones, with an increase in the technogenic load, an increase in the content of high-molecular PAHs, especially fluoranthene and benzo(a)anthracene, has been observed. The soils were often heterogeneous in space, which is due to the typical features of the soil cover with a similar genesis of polyarenes in terms of the total 15 PAHs content. It has been established the background territories for the determination of PAHs content for the main soil types located in the studied coastal area of the Taganrog Bay, the total content of priority PAHs in which does not exceed 600 µg kg⁻¹. The features of the PAHs compounds group composition in background soils depending on their position in the landscape were revealed. In automorphic soils, naphthalene, and phenanthrene (35–54%) dominated in the composition of the low-molecular-weight compounds, and only phenanthrene (59–70%) dominated in hydromorphic soils. The results of the study will serve as a basis for environmental monitoring of soils.