Variability of Concentrations of Potentially Toxic Metals in the Topsoil of Urban Forest Parks (Southern Poland)

Abstract

Forest ecosystems and urban parks are an integral part of the natural environment and the natural system of a city, where they form a mosaic of habitats resulting from the variation in soil cover due to human activities. The study was conducted in urban forests in five urban parks in Upper Silesia (southern Poland) and investigated the chemical properties and content of potentially toxic metals (PTMs) in the topsoil, which had an average thickness of 15 cm for all samples. The soil reaction ranged from acidic (pH = 4.7–5.1, in KCl) to slightly acidic (pH = 5.6, to neutral—6.6–7.2) at most sites. The organic carbon (OC) content was relatively high at all sites, ranging from 1.19 to 14.3%, with the highest total nitrogen (Nt) content being 0.481%. The average total phosphorus (Pt) content in the parks ranged from 310 mg kg⁻¹ to 684 mg kg⁻¹, while the highest values were 1840 mg kg⁻¹. The total Cr, Cu, Co and Ni content was within acceptable limits, while the Zn, Pb, Cd, As, Sr and Ba exceeded acceptable standards. In terms of content, Zn dominated the PTMs at each site (Zn > Pb > Ba > Sr > Cu > Cr > As > Ni > Cd > Co), while Ba and Pb alternated in taking second place (Zn > Ba > Pb > Sr > Cu > Cr > Ni > As > Co > Cd). Environmental indicators, such as the geoaccumulation index (Igeo), enrichment factor (EF) and potential ecological risk index (RI), showed that the analyzed soils are highly contaminated with PTMs. Among the sources of pollution in the urban forest are low emissions from coal combustion, industrial activities, water runoff from streets and proximity to transportation routes. Identification of PTM levels in urban parks will provide valuable information on the behavior of these metals, which is important in sustainable development and can help evaluate the local spatial development plans of urbanized areas.

1. Introduction

Industrial development, particularly intense in Europe at the turn of the 20th century, brought significant changes to cities, mainly in areas rich in natural resources. More and more areas are now being developed for industrial purposes without programs to improve the environment and spatial order of newly built cities, apart from the fact that city parks have been created in some places [1]. Forest parks are an essential component of urban green spaces and serve various purposes, such as recreation, health and social functions and improving the urban environment [2,3,4].

Comprehending the complicated nature of how soil systems function and their interaction with human activities is particularly important in urban forest areas, which are most often urban parks. Due to continuous human existence and human activity, these soils are often cut down or buried under transported material and highly compacted and contaminated. As a result, their hydrological properties, degree of compaction and disturbance and displacement of material from the original levels are very different from natural soil types.

The city has become a new environment of anthropogenic soils due to transformed natural soils or their complete destruction due to human activities [5]. The formation of urban soils and their properties are linked to the history of the city and the numerous transformations in the course of urbanization [6], as well as the associated forests and urban greenery. Some authors call the evolution of urban soils ‘anthropogenic gene transformation’ [7]. Thus, each new land use changes the properties of the soil, and the soil starts to take on other functions.

In recent centuries, and particularly in the last few decades, human activity has continuously increased the level of heavy metals (HMs) circulating in the environment [8,9,10]. The mobility of heavy metals in the urban soil environment has become an essential process in the geochemical cycle in the environment [11,12,13]. Environmental contamination with trace elements is now an essential topic for analysis. This mainly concerns urban areas, which are characterized by the increased content of potentially toxic elements, especially in the surface layer of the ground [14,15,16]. There is a significant spatial variation in metal content mainly arising from the way urban environments develop and function [17,18,19,20,21].

Studies on soil contamination in urban parks and squares have been carried out in different regions of the world using different approaches and taking into account potential toxic metals [2,21,22,23,24,25,26,27,28,29,30]. Studies of urban soils in Poland included the assessment of heavy metal contamination, physicochemical properties and their classification [9,10,18,31,32,33,34,35,36,37].

In order to determine the occurrence of topsoil pollution with potentially toxic metals, their levels are compared with their content from the local geochemical background using the calculations of the geoaccumulation index (I_geo), enrichment factor (EF), contamination factor (CF) or pollution load index (PLI) [38,39]. Luo et al. [40] and Madrid et al. [22] confirm that the main factors influencing the distribution of heavy metals in the soil profile are the content of humus and iron and manganese oxides, as well as texture, soil pH and the rinsing processes, accompanied by the movement of water within the soil profile.

The concentration of potentially toxic metals in the soil is determined by lithogenesis and pedogenesis. It depends on the mineralogical composition, the nature of the parent rock and the direction and rate of soil formation, which determines the distribution of toxic elements in the soil profile [8,9,10,40,41,42]. Such metals are more mobile than those of pedogenic origin. In parks and urban areas, sources of potentially toxic metals are of anthropogenic origin. In urban areas, the nature of metal accumulation and their genesis in soils varies, and this is often due to the way urban areas are managed (mowing, fertilization, proximity to transport networks). Thus, their occurrence differs from that in natural soils [43]. This study aims to determine the content of potentially toxic metals in the topsoil of urban parks in a post-mining city.

2. Materials and Methods

2.1. Study Area and Soil Sampling

The study was conducted in urban parks in Sosnowiec, which is located in southern Poland (Figure 1). The city is considered a typical example of those in the Katowice agglomeration. It developed dynamically during the industrialization of the second half of the 19th century and in the post-war period—the 1970s and 1980s [44].

The condition of the natural environment and the spatial development of the part of the city analyzed here were influenced primarily by the exploitation of hard coal deposits as well as the metallurgy industry [45]. As a result of past intensive industrial development, the devastation of the natural environment is a problem for Sosnowiec. The first parks in Sosnowiec were created on the initiative of foreign industrialists living within 19th century Sosnowiec [46]. The construction of urban parks in this region contributed to the attractiveness of the area and resulted in accelerated urbanization, which undoubtedly had an impact on how the ecosystem functioned.

Table 1. General features of the urban parks studied in Sosnowiec.

Names of Park	Park Area [ha]	Year of Creation	Geographical Coordinates	Ecological Function	Soil Types	Number of Samples	History
Park Sielecki (PS)	10.4	1835	50°17′6″ N 19°8′28″ E	Ecological, sport and recreation, economic, culture and entertainment	Fluvic Cambisol, Hortic Anthrosol	24	Established as a private palace park, open to the public since 1945.
Park Dietla (PD)	6.13	1890	50°17′8″ N 19°8′10″ E	Ecological, sport and recreation	Hortic Anthrosol, Technic Anthrosol	21	Established as a palace walk park in a former deciduous forest, the sports infrastructure was built in the 1970s.
Park Schöena (PSch)	5.5	1885	50°17′59″ N 19°8′35″ E	Palace and park complex, culture and entertainment	Fluvic Cambisol, Hortic Anthrosol	21	Neo-Romanesque palace−garden park
Park Kruczkowski (PK)	4.8	1935	50°16′53″ N 19°6′14″ E	Ecological and recreation, biodiversity park	Urbic Technosol	21	Established as a dendrological garden, now an ecological center with an Exotarium.
Park Leśna (PL)	48	1958	50°18′5″ N 19°14′26″ E	Ecological, sport and recreation, culture and entertainment	Technic Anthrosol, Fluvic Cambisol	33	Created on the site of a former sand quarry and later reclaimed as a forest park.

shows the main characteristics of the parks studied. The plant cover of the parks is predominantly made up of numerous trees and shrubs. Most of these, however, are artificial plantings of various ages. They are further formed into decorative groups of tree-lined alleys, rows, hedges, lawns and many other spatial compositions [4]. Native trees in the parks include such species as Carpinus betulus, Tila cordata, Acer platanoides, A. pseudoplatanus, A. campestre, Alnus glutinosa, Quercus robur, Populus alba, P. tremula, P. nigra, Pinus sylvestris and Abies alba. The herbs/lawns are mown regularly.

In addition to native species, there are also alien species such as Acer negundo, Acer saccharinum, Robinia pseudoacacia, Ailanthus altissima, Catalpa bignonioides, Q. rubra, Pinus nigra and Picea pungens, as well as herbaceous species of alien origin, such as Impatiens parviflora, Solidago gigantea or Reynoutria japonica, apart from cultivars [13,26].

The main criterion for the number of soil samples taken from individual parks was their size. When selecting places, the principle was that the points should be distributed relatively evenly throughout the park. A total of 120 soil samples with thicknesses ranging from 0–10 to 25 cm were taken from the humus horizon, mainly in the root zones. The largest number of samples—33 samples (11 points)—were taken from the biggest park (48 ha)—Park Leśna (PL), 24 samples (8 points) were taken from Park Sielecki (PS), which is the second in order (10.4 ha) and definitely bigger than the other three, while 21 samples for analysis were taken from the other three parks—Park Dietla (PD, 7 sampling points), Park Schöena (PSch, 7 sampling points) and Park Kruczkowski (PK, 7 sampling points). Three 1 m² soil samples were taken from each point, which, after drying, were mixed and analyzed in the laboratory as one sample.

2.2. Chemical and Physical Analysis of the Soil

In the laboratory, air-dried samples were sieved (<1 mm) and analyzed following the standard procedures described by Bednarek et al. [47], i.e., the pH was measured potentiometrically in H₂O, the 1N KCl was measured using the potentiometric method in 1:2.5 soil/solution suspensions, while the total organic carbon (OC_tot) was measured according to Tyurin’s method, the total nitrogen (N_t) was measured using the Kjeldahl method and the hydrolithic acidity (H_h) was measured according to the Kappen method [47].

The total content of potential toxic elements (Pb, Cd, Ba, Zn, Fe, Mn, Cr, Cu, Co, Ni, As, and Sr) in the soil was determined using the ICP-OES (inductively coupled plasma optical emission spectrometry) technique after wet mineralization in aqua regia (3HCl + HNO₃). The analysis was performed in the ACME Laboratory (Vancouver, Canada) using AQ250_EXT (soils). All soil samples were analyzed in triplicate for all the parameters being investigated, and the mean values were then calculated.

2.3. Data Analyses—Environmental Indices

In the estimation of the geoaccumulation index (I_geo), the regional geochemical background values set by Lis and Pasieczna [48] were used. For individual elements, this background was determined as follows: As-6.0 mg·kg⁻¹, Ba-59 mg·kg⁻¹, Cd-2.0 mg·kg⁻¹, Co-2.0 mg·kg⁻¹, Cr-8.0 mg·kg⁻¹, Cu-9.0 mg·kg⁻¹, Ni-6.0 mg·kg⁻¹, Pb-77.0 mg·kg⁻¹, Sr-9.0 mg·kg⁻¹ and Zn-184.0 mg·kg⁻¹. Regional geochemical background values were determined (for particular elements) by means of the specific geological structure, the intensity of anthropopressure and the underground and exterior mining and associated infrastructure [48,49].

The geoaccumulation index (I_geo) was originally defined and used for assessing metal concentration in sediments [38,50], but it is also used here to determine the condition of the soil.

This index is calculated according to the formula:

$$Igeo=lo{g}_2\left(\frac{C_n}{1.5B_n}\right),$$

(1)

where C_n is the measured concentration of the element in the sample and B is the geochemical background of elements (for the Upper Silesia regional background level). A coefficient of 1.5 in the formula was used to reduce the variability of the geochemical background associated with lithology.

According to Müller [38], Hakanson [51], Loska et al. [52] and Barbieri [53], the I_geo for each metal is calculated and classified as uncontaminated (I_geo ≤ 0); uncontaminated to moderately contaminated (0 < I_geo ≤ 1); moderately contaminated (1 < I_geo ≤ 2); moderately to heavily contaminated (2 < I_geo ≤ 3); heavily contaminated (3 < I_geo ≤ 4); heavily to extremely contaminated (4 < I_geo ≤ 5); and extremely contaminated (I_geo ≥ 5).

2.3.1. Enrichment Factors

Heavy metal enrichment factors were calculated for averaged soil samples from each park (as mean) in relation to the background values of chemical elements in the local parent rock, selecting Fe as the reference element [43,54]:

$$EF=\frac{\left[\raisebox{1ex}{$C_{metal}$}\!\left/ \!\raisebox{-1ex}{$C_{normalizer}$}\right.\right]soil}{\left[\raisebox{1ex}{$C_{metal}$}\!\left/ \!\raisebox{-1ex}{$C_{normalizer}$}\right.\right]control},$$

(2)

where C_metal is the concentration of the element of interest and C_ref is the concentration of reference element for normalization.

The five categories are recognized on the basis of the enrichment factor [55,56]:

EF < 2—states deficiency to minimal enrichment. EF = 2–5—moderate enrichment. EF = 5–20—significant enrichment, EF = 20–40—very high enrichment and EF > 40—extremely high enrichment.

2.3.2. Contamination Factor

The contamination factor (CF) was calculated using the equation:

$$CF=\frac{C_n}{B_n},$$

(3)

where C_n is the element content in the soil and B_n is the background concentration same element concentration in the element.

2.3.3. Pollution Load Index

The pollution load index (PLI) was estimated based on the contamination factors according to the following formula:

$$PLI=\sqrt[n]{C{F}_1\times C{F}_2\times C{F}_3\times \dots \times C{F}_n}$$

(4)

where CF (contamination factor) is the contamination factor obtained by calculating between each metal’s concentration and its background value. To calculate the PLI, we used the five highest contamination factors suggested by Tomlinson et al. [39]. This index shows heavy metal contamination, and it can assume values < 1 (absence of pollution) or 1 < (existence of pollution) [39].

2.3.4. Potential Ecological Risk Index

The potential ecological risk index (RI) allows for the evaluation of the heavy metal’s impact on the environment. It was calculated using the formula:

$$RI={\displaystyle \sum }{E}_r^i$$

(5)

where $E_r^i$ is the potential ecological risk factor of the specific element [57]. The $E_r^i$ was calculated using the equation:

$$E_r^i=T_r^i\times CF$$

(6)

where $T_r^i$ is the toxic response factor of the metal given by Zhu et al. [58]. The ecological risk can be divided into the following five classes [59]: <40—low, 40 ≤ $E_r^i$ < 80—moderate, 80 ≤ $E_r^i$ < 160—considerable, 160 ≤ $E_r^i$< 320—high, 320 ≤ $E_r^i$—very high. The risk index can be classified as follows [59]:

• RI < 150—low risk
• 150 ≤ RI < 300—moderate risk
• 300 ≤ RI < 600—considerable risk
• 600 ≤ RI—very high risk.

2.3.5. Statistical Data

Spearman’s rank correlation coefficient was applied to check for any association between the metals in the soil. This is the coefficient used for samples that do not meet the assumptions of normality. The statistical significance of the Spearman correlation coefficient data was determined using the Spearman rank correlation test. The exact values of the correlation coefficient were calculated for alpha = 0.001, 0.01 and 0.05 [13,60]. All statistical analyses were performed using SPSS Statistics software (version 18). In addition, cluster analysis using Euclidean distance was applied.

3. Results

3.1. Physiochemical Properties of Soil

The soil analyzed in terms of physicochemical properties varied in individual anthropogenic urban forests (

Table 2. Physical and chemical properties of urban soils in the Sosnowiec area.

Urban Parks Names	Sampling Points	pH		Hh	H+	Al3+	Loss on Ignation	Corg	Nt	C/N	Pt
		H2O	KCl	[cmol (+) kg−1]			[%]				[mg kg−1]
Park Sielecki (PS)	S-1	6.8	6.3	2.76	0.72	0.16	3.35	1.85	0.181	10	272.4
	S-2	7.4	7.2	3.6	1.02	0.1	7.61	3.11	0.312	10	551
	S-3	6.2	5.7	3.12	0.58	0.8	10.69	4.82	0.184	26	946
	S-4	5.7	5.1	6.16	0.54	0.07	11.99	5.56	0.481	12	1840
	S-5	6.6	6.1	3.48	1.96	2.04	7.21	3.75	0.125	30	250.8
	S-6	7.3	6.6	2.72	0.54	0.42	3.12	2.29	0.322	7	978
	S-7	7.1	6.4	2.52	0.94	0.48	11.79	6.49	0.091	71	105.2
	S-8	6.9	6.3	1.84	0.69	0.04	8.81	4.53	0.223	20	535
Park Schöena (PSch)	PSch-1	6.7	6.1	0.6	0.4	0.4	31.08	14.3	0.131	109	320.1
	PSch-2	6.5	6.1	19.6	4.1	5.6	13.03	5.82	0.172	34	281.6
	PSch-3	6.8	6.7	7.5	3.6	4.7	5.23	3.29	0.132	25	197.6
	PSch-4	7.2	6.6	1.7	0.61	0.4	7.67	2.52	0.081	31	114.8
	PSch-5	5.8	5.3	5.4	1.7	0.9	9.56	3.48	0.211	16	514.1
	PSch-6	6.6	6.1	7.8	0.31	0.1	18.29	6.45	0.301	21	390.1
	PSch-7	7.1	6.6	2.2	0.72	0.2	15.63	6.54	0.124	53	356
Park Kruczkowski (PK)	PK-1	7.1	6.4	9	6.4	12.24	7.39	4.22	0.142	30	96.8
	PK-2	6.9	6.2	4	2.12	1.72	7.18	5.17	0.11	47	107.2
	PK-3	6.4	5.8	1.68	0.61	0.16	13.21	8.47	0.184	46	360
	PK-4	6.4	5.9	1.16	0.28	0.04	10.65	5.07	0.071	71	209.6
	PK-5	5.8	5.1	3.04	0.68	0.09	5.79	5.52	0.131	42	538
	PK-6	7.1	6.6	1.04	0.14	0.11	9.18	4.66	0.042	111	260.8
	PK-7	6.5	5.4	8.04	0.84	0.41	3.83	1.19	0.281	4	1252
Park Dietla (PD)	PD-1	6.3	5.5	2.72	0.32	0.02	21.72	11.12	0.271	41	575
	PD-2	6.6	6.1	7.48	4.6	2.96	9.42	5.29	0.142	37	157
	PD-3	7.3	6.9	2.6	1.18	0.86	3.54	1.77	0.031	57	165.2
	PD-4	5.9	5.3	3.88	1.66	0.98	4.62	4.11	0.012	343	218.8
	PD-5	6.3	5.5	5.08	1.64	2.1	2.7	0.99	0.132	8	232.2
	PD-6	7.0	6.4	1.12	0.06	0.02	9.27	5.79	0.101	57	529
	PD-7	5.8	5.3	8.24	2.92	3.08	7.31	3.76	0.21	18	347
Park Leśna (PL)	PL-1	5.5	5.0	2	0.55	0.58	4.61	4.33	0.161	27	294.4
	PL-2	5.3	4.7	1.64	0.53	0.09	9.13	5.9	0.132	45	353
	PL-3	5.6	5.0	2.64	0.78	0.6	5.73	3.87	0.271	14	800
	PL-4	7.2	6.7	2.28	0.51	0.07	8.68	4.98	0.182	27	882
	PL-5	5.4	5.1	3.48	1.31	1.03	12.94	8.66	0.091	95	162.4
	PL-6	5.3	4.8	4.44	1.6	1.24	8.45	2.59	0.152	17	404
	PL-7	4.7	4.2	1.08	0.32	0.26	8.01	4.17	0.084	50	233.6
	PL-8	5.9	5.3	5.8	2.56	2.44	4.61	1.55	0.021	74	178.8
	PL-9	6.4	5.8	5.28	3.28	0.6	6.08	3.51	0.121	29	209.6
	PL-10	5.1	4.7	3.32	0.6	0.17	7.75	4.59	0.261	18	432
	PL-11	6.9	6.4	4.56	1.08	0.64	7.74	3.61	0.201	18	878

and Table S1). The granulometric composition in all the samples analyzed was dominated by sand, and its average value was 83% in PSch, PD, 76% in PL and 90% in Sielecki Park (PS). A significant share of the gravel fraction (10%–16%) was found in most of the parks, with individual sites being different in terms of this fraction. The share of silt and clay in the soil samples studied varied from 2.34%–4.23% to 2.5%–2.8%, respectively (Table S1).

Soil pH varied from site to site across the parks (Table 1). The soil was characterized by a general reaction ranging from acidic (4.7-PL-2, 5.1-PK-5, PS-4 in KCl) to slightly acidic and neutral (in most sites) and rarely alkaline (7.4-PS-2.6 samples). High pH values affected the concentration of acid cations, such as H⁺ and Al³⁺. Hydrogen ranged from 0.83 to 0.76, while aluminum values varied between 0.51 and 2.11 cmol (+) kg⁻¹. The highest contents (6.4-H⁺ and 12.24 Al³⁺) were found in PK-1. Hydrolytic acidity (H_h) varied in all studied sites, ranging from 3.27 to 6.4 cmol (+) kg⁻¹. As with the other parameters, it varied within a given park, where different samples were analyzed (Table 2).

Analysis of the loss of ignition indicated a significant content of organic matter in the topsoil, which was undoubtedly related to the development of the park and partly fertilized materials during the organization of the park. The results were similar in all the parks studied except for Park Schöena, where the highest value was recorded (14.35%, Table 2). Organic carbon values were similar in all study points in each park, except for single samples that stood out with high levels compared to other samples in the park, e.g., PSch-1 (14.3%) and PD-1 (11.12%). The highest levels of total nitrogen (Nt) were found at sites PS-4 (0.481%), PSch-6 (0.301%) and PD-1 and PL-3 (0.271%. Table 2), respectively. The C/N ratio varied, ranging from 4 to 109. This range indicates that biological activity varied from site to site in the parks and conveys the influence of anthropogenic factors. The average content of total phosphorus (Pt) as an indicator of anthropogenic pollution of soils in the parks varied from 310 mg kg⁻¹ (PSch) to 684 mg kg⁻¹ (PS). Its highest values were found at site PS-4 (1840 mg kg⁻¹) and the lowest at PK-1-96.8 mg kg⁻¹.

3.2. Concentration of Potential Toxic Elements

The concentrations of potentially toxic metals found in the topsoil under urban forests at the different study sites vary (Table 2). The same metal was present in different contents in the same park at different study sites, which is true for almost all elements. The topsoil in all samples studied was characterized by high levels of contamination with Pb, Zn, Ba, Cd and As. Their average value for all samples from each city park is shown in

Table 3. The concentration of potential toxic elements in the parks analyzed.

Urban Parks	Sampling Points	Limit Values [61]
		200	2	200	150	200	500	50	25	400	10–30 [43]
		Pb	Cd	Cr	Ni	Cu	Zn	Co	As	Ba	Sr
		mg kg−1
Park Sielecki (PS)	S-1	410.7	2.0	31	14.1	15.0	997	5.4	14	377	55
	S-2	669.1	7.2	71	46.9	97.1	2338	17.0	33	606	147
	S-3	624.5	19.3	57	32.3	81.3	2237	12.4	34	797	105
	S-4	399.5	19.1	40	18.7	45.3	981	7.1	20	378	70
	S-5	338.1	10.6	40	19.6	80.8	1315	7.2	24	367	73
	S-6	910.8	9.3	39	19.4	49.4	3526	6.7	32	362	75
	S-7	717.8	27.7	50	30.1	63.9	2727	11.4	52	541	96
	S-8	255.8	17.3	54	26.4	38.9	1077	11.3	18	486	67
Mean values		540.8	14.1	48	25.9	59.0	1900	9.8	28	489	86
Park Schöena (PSh)	PSch-1	387.7	14.1	78	96.7	186.5	2048	36.7	32	1137	391
	PSch-2	213.3	4.7	88	47.3	63.2	863	16.6	15	550	106
	PSch-3	285.4	10.8	43	22.0	53.1	1056	7.6	19	352	68
	PSch-4	538.2	12.6	72	45.6	117.7	1781	16.7	34	635	111
	PSch-5	865.0	21.0	71	42.4	82.0	1880	16.4	41	660	104
	PSch-6	585.1	17.2	59	43.1	107.1	2112	16.2	38	572	117
	PSch-7	565.1	16.2	66	39.8	112.8	2118	14.9	27	580	136
Mean values		491.4	13.8	68	48.1	103.2	1694	17.9	29	641	148
Park Kruczkowski (PK)	PK-1	426.4	15.0	51	35.4	65.6	1757	13.9	25	492	99
	PK-2	2040.3	37.3	69	44.1	105.2	3103	14.6	68	643	129
	PK-3	2019.1	24.4	70	32.5	89.0	1827	11.3	40	513	109
	PK-4	1913.4	29.0	59	33.1	79.2	2625	10.9	50	484	99
	PK-5	894.5	18.8	41	22.7	51.9	1429	8.2	37	446	76
	PK-6	343.7	9.3	54	23.5	71.5	997	7.7	11	329	77
	PK-7	156.8	3.8	47	20.2	28.9	418	5.6	10	238	44
Mean values		1113.5	19.7	56	30.2	70.2	1737	10.3	34	449	90
Park Dietla (PD)	PD-1	1102.4	25.6	88	75.2	209.0	2759	28.3	37	878	210
	PD-2	796.1	22.4	72	40.0	113.0	2153	12.2	25	610	120
	PD-3	280.3	9.4	37	19.5	48.2	1161	6.9	22	386	77
	PD-4	352.2	10.1	41	25.2	46.4	1243	8.7	29	439	71
	PD-5	323.5	11.1	38	18.3	37.3	1044	5.9	21	382	64
	PD-6	905.4	29.7	85	53.9	126.8	3351	20.0	58	670	139
	PD-7	493.3	13.4	49	27.9	50.7	1276	9.8	19	477	91
Mean values		607.6	17.4	59	37.1	90.2	1855	13.1	30	549	110
Park Leśna (PL)	PL-1	150.3	3.0	27	131.1	17.4	249	4.5	7	248	47
	PL-2	510.5	14.5	46	22.3	41.0	1035	8.0	23	414	75
	PL-3	255.1	8.2	50	18.8	37.4	616	6.9	16	311	58
	PL-4	128.8	2.8	61	30.1	34.7	507	10.0	8	390	87
	PL-5	659.3	18.4	53	26.4	53.1	1287	9.0	24	450	84
	PL-6	264.2	6.2	37	13.8	27.9	494	4.8	13	375	60
	PL-7	228.7	5.5	40	14.8	21.8	352	4.7	14	390	62
	PL-8	449.2	11.7	32	16.1	31.9	877	7.0	16	345	66
	PL-9	245.0	8.5	67	23.8	38.9	900	9.6	20	486	83
	PL-10	541.8	13.7	46	22.5	42.7	994	9.5	25	483	87
	PL-11	208.0	10.4	30	16.3	30.5	680	6.6	13	351	75
Mean values		331.0	9.4	44	30.6	34.3	726	7.3	16	386	71

. These are values that exceeded the permissible limit for this type of site. Relatively low arsenic and barium contents were recorded in Park Leśna compared to the other research sites.

The total content of Cr, Cu, Co and Ni are within the permissible limit (Table 3), and their average content for each study point is, respectively, Cr-44-68 mg kg⁻¹, Cu-34-103 mg kg⁻¹, Co-7-18 mg kg⁻¹ and Ni-26-48 mg kg⁻¹. The metals analyzed are present in different concentrations at particular sites and can be ranked as follows:

Park Sielecki (PS):	Zn > Pb > Ba > Sr > Cu > Cr > As > Ni > Cd > Co
Park Schöena (PSch):	Zn > Ba > Pb > Sr > Cu > Cr > Ni > As > Co > Cd
Park Kruczkowski (PK):	Zn > Pb > Ba > Sr > Cu > Cr > As > Ni > Cd > Co
Park Dietla (PD):	Zn > Pb > Ba > Sr > Cu > Cr > Ni > As > Cd > Co
Park Leśna (PL):	Zn > Pb > Ba > Sr > Cr > Cu > Ni > As > Cd > Co

3.3. Correlation between Metals

The Spearman rank correlation tests showed correlations of heavy metal content in soils in almost all the urban parks analyzed (

Table 4. Correlation between metals within the urban parks analyzed.

Park Sielecki (PS)	Pb	Cd	Cr	Ni	Cu	Zn	Co	As	Ba	Sr
Pb	1.000
Cd	0.000	1.000
Cr	−0.012	0.359	1.000
Ni	0.238	0.333	0.934 ***	1.000
Cu	0.357	0.190	0.683 *	0.810 **	1.000
Zn	0.810 **	0.095	0.228	0.524	0.548	1.000
Co	0.119	0.405	0.970 ***	0.976 ***	0.786 *	0.357	1.000
As	0.667 *	0.548	0.539	0.738 *	0.786 *	0.762 *	0.690 *	1.000
Ba	0.071	0.452	0.862 **	0.786 *	0.500	0.071	0.857 **	0.524	1.000
Sr	0.619	0.262	0.683	0.833 **	0.929 ***	0.714	0.786	0.905 **	0.571	1.000
Park Schöena (PSch)
Pb	1.000
Cd	0.619	1.000
Cr	−0.095	−0.333	1.000
Ni	−0.024	−0.286	0.905 **	1.000
Cu	0.452	−0.071	0.548	0.667 *	1.000
Zn	0.762 *	0.524	−0.048	0.095	0.643 *	1.000
Co	0.095	−0.190	0.905 **	0.952 ***	0.738 *	0.119	1.000
As	0.929 ***	0.548	0.024	0.167	0.524	0.619	0.310	1.000
Ba	0.571	0.286	0.619	0.643 *	0.833 **	0.571	0.786 *	0.667 *	1.000
Sr	0.357	−0.119	0.500	0.643 *	0.905 **	0.714 *	0.595	0.333	0.667	1.000
Park Kruczkowski (PK)
Pb	1.000
Cd	0.964	1.000
Cr	0.714 *	0.643	1.000
Ni	0.679 *	0.714 *	0.607	1.000
Cu	0.857 **	0.821 *	0.929 **	0.750 *	1.000
Zn	0.929 **	0.964 ***	0.714 *	0.857 **	0.857 **	1.000
Co	0.786 *	0.750 *	0.571	0.929 **	0.714 *	0.857 **	1.000
As	0.964 ***	1.000	0.643	0.714 *	0.821 *	0.964 ***	0.750 *	1.000
Ba	0.893 **	0.821 *	0.714 *	0.857 **	0.821 *	0.893 **	0.964 ***	0.821 *	1.000
Sr	0.865 **	0.811 *	0.865 **	0.883 **	0.937 ***	0.901 **	0.901 **	0.811 *	0.955 ***	1.000
Park Dietla (PD)
Pb	1.000
Cd	0.929 **	1.000
Cr	1.000	0.929 **	1.000
Ni	0.964 ***	0.857 **	0.964 ***	1.000
Cu	0.893 **	0.821 *	0.893 **	0.964 ***	1.000
Zn	0.929 **	0.893 **	0.929 **	0.964 ***	0.929 **	1.000
Co	0.964 ***	0.857 **	0.964 ***	1.000	0.964 ***	0.964 ***	1.000
As	0.643	0.571	0.643	0.679 *	0.607	0.714 *	0.679 *	1.000
Ba	0.964 ***	0.857 **	0.964 ***	1.000	0.964 ***	0.964 ***	1.000	0.679 *	1.000
Sr	0.893 **	0.821 *	0.893 **	0.964 ***	1.000	0.929 **	0.964 ***	0.607	0.964 ***	1.000
Park Leśna (Pl)
Pb	1.000
Cd	0.864 ***	1.000
Cr	0.178	0.123	1.000
Ni	−0.182	−0.064	0.360	1.000
Cu	0.700 **	0.745 **	0.697 **	0.318	1.000
Zn	0.764 **	0.918 ***	0.460	0.164	0.918 ***	1.000
Co	0.245	0.318	0.811 **	0.436	0.782 **	0.655 *	1.000
As	0.868 ***	0.849 ***	0.462	−0.005	0.863 ***	0.872 ***	0.502	1.000
Ba	0.433	0.446	0.680 *	0.182	0.692 **	0.647 **	0.697 **	0.682 *	1.000
Sr	0.251	0.388	0.600 *	0.361	0.699 **	0.644 **	0.872 ***	0.484	0.780 **	1.000

*** p < 0.001; ** p < 0.01, * p < 0.05.

). Park Sielecki showed an almost complete (>0.9) statistically significant positive correlation between Ni-Cr (r = 0.934, p = <0.001), Co-Cr (r = 0.970, p = <0.001), Co-Ni (r = 0.976, p = <0.001), Sr-Cu (r = 0.929, p = <0.001), Sr-As (r = 0.905, p = <0.01). A very strong (0.8–0.9) statistically significant correlation occurred between Cu-Ni (r = 0.810, p = <0.01), Zn-Pb (r = 0.810, p = <0.01), Ba-Cr (r = 0.862, p = <0.01), Ba-Co (r = 0.857, p = <0.01), and Sr-Ni (r = 0.833, p = <0.01) (Table 4).

Correlation studies of the dependence of heavy metal content in the topsoil in PSch showed an almost complete (>0.9) statistically significant positive correlation between Ni-Cr (r = 0.905, p = <0.01), Co-Cr (r = 0.905, p = <0.01), Co-Ni (r = 0.952, p = <0.001), As-Pb (r = 0.929, p = <0.001) and Sr-Cu (r = 0.905, p = <0.01). In PK, similar patterns were also found, where correlations showed an almost complete, statistically significant positive relationship between Cu-Cr (r = 0.929, p = <0.01), Zn-Pb (r = 0.929, p = <0.01), Zn-Cd (r = 0.964, p = <0.001), Co-Ni (r = 0.929, p = <0.01), As-Pb (r = 0.964, p = <0.001), As-Zn (r = 0.964, p = <0.001), Ba-Co (r = 0.964, p = <0.001), Sr-Cu (r = 0.937, p = <0.001), Sr-Zn (r = 0.901, p = <0.01), Sr-Co (r = 0.901, p = <0.01) and Sr-Ba (r = 0.955, p = <0.001).

In Park Dietla, an almost full statistically significant positive correlation was revealed between Cd-Pb (r = 0.929, p = <0.01), Cr-Cd (r = 0.929, p = <0.01), Ni-Pb (r = 0.964, p = <0.001), Ni-Cr (r = 0.964, p = <0.001), Cu-Ni (r = 0.964, p = <0.001), Zn-Ni (r = 0.964, p = <0.001), Zn-Cu (r = 0.929, p = <0.001), Co-Pb (r = 0.964, p = <0.001), Co-Cr (r = 0.929, p = <0.001), Co-Cu (r = 0.964, p = <0.001), Co-Zn (r = 0.964, p = <0.001), Ba-Pb (r = 0.964, p = <0.001), Ba-Cr (r = 0.964, p = <0.001), Ba-Cu (r = 0.964, p = <0.001), Ba-Zn (r = 0.964, p = <0.001), Sr-Ni (r = 0.964, p = <0.001), Sr-Zn (r = 0.929, p = <0.001), Sr-Co (r = 0.964, p = <0.001) and Sr-Ba (r = 0.964, p = <0.001). In PL, a statistically significant positive relationship was found only between Zn-Cd (r = 0.918, p = <0.001) and Zn-Cu (r = 0.918, p = <0.001, Table 4).

3.4. Similarity of Metal Content in the Parks

The cluster analysis carried out using the Ward method for heavy metal contents in the chemical composition of soils in PS, PSch and Dietla. Crossing the clusters at the 3000 level showed the following three main groups of chemical elements: Group 1 (Sr, Cu, As, Ni, Cr, Co, Cd)—these elements are very similar to each other, as they form a single cluster on the dendrogram. Their contents in the soil chemical composition and properties are similar. Group 2 (Pb, Ba)—this group forms a separate cluster. This means that Pb and Ba are different from the other elements in the sample. The third group is Zn, which has the highest content in all the study sites.

For PK and PL, intersecting the clusters at the 3000 level show two main groups of elements. In the first of these, we can include PK Ba, Sr, Cu, Cr, Co, As, Ni and Cd, while we can include Zn and Pb in the second. In Park Leśna, the first group includes Ni, Sr, Cu, Cr, As, Co and Cd and the second group includes Zn, Ba and Pb.

3.5. The Risk of Potential Toxic Element Contamination

The results of the environmental geo-indicators were tabulated (

Table 5. The geoaccumulation index (*I_geo) of heavy metals within the urban soil studied.

Sites		Ba	Sr	Cr	Zn	Ni	Cu	Co	Pb	As	Cd
PS	Mean	2.46	2.67	1.99	2.78	1.52	2.12	1.70	2.32	1.65	2.22
	Range	2.03–3.17	2.02–3.44	1.36–2.56	1.82–3.67	0.64–2.38	0.15–2.84	0.84–2.5	1.24–3.07	0.64–2.53	−0.58–3.2
PSch	Mean	2.85	3.45	2.50	2.61	2.41	2.93	2.57	2.18	1.70	2.20
	Range	1.99–3.68	2.33–30.3	1.84–2.87	1.64–2.93	1.28–3.42	1.97–3.78	1.34–3.61	0.98–3	0.73–2.18	−0.64–2.51
PK	Mean	2.34	2.74	2.21	2.65	1.74	2.37	1.78	3.36	1.93	2.71
	Range	1.42–2.86	1.7–3.25	1.77–2.54	0.59–3.49	1.16–2.29	1.09–2.96	0.91–2.28	0.53–4.23	0.15–2.91	0.34–3.63
PD	Mean	2.63	3.03	2.87	2.74	2.04	2.74	2.12	2.49	1.74	2.53
	Range	2.1–3.31	2.24–3.59	1.62–2.87	1.91–3.6	1.02–3.06	1.46–3.95	0.97–3.27	1.37–3.35	1.07–2.68	1.64–3.3
PL	Mean	2.12	2.40	1.88	1.39	1.76	1.34	1.28	1.61	0.85	1.64
	Range	1.48–2.45	1.79–2.68	1.32–2.48	−0.14–2.22	0.61–3.86	0.36–1.97	0.58–1.73	0.25–2.6	−0.36–1.47	−0.09–2.73

* 0—uncontaminated, 0–1—uncontaminated to moderately, 1–2—moderately, 2–3—moderately to strongly, 3–4—strongly, 4–5—strongly to extremely strongly, >5—extremely contaminated.

) as mean values for all samples from the park and their range for individual samples. Analysis of the geoaccumulation index (I_geo) indicated that the levels of the elements studied in the soil exceeded the geochemical background limit. It can be concluded that the soil investigated was moderately contaminated, moderately to highly contaminated and strongly contaminated (with Sr and Pb). In one case, the Igeo indication for Sr was 30 (extremely contaminated).

The enrichment factor (EF) was used to assess the degree of anthropogenic influence. An increasing EF indicates an increasing proportion of anthropogenic sources. Analysis of the EF index showed that the topsoil (humus horizon) developing under the urban forest was more strongly influenced by anthropopressure. The results show that the soil was contaminated with metals to varying degrees and can be described with significant enrichment, very high enrichment and extremely high enrichment. Only sample PS-2 showed a deficiency to minimal enrichment for Cd (1.26) (

Table 6. The enrichment factor (*EF) of heavy metals within the urban soil studied.

Sites		Ba	Sr	Cr	Zn	Ni	Cu	Co	Pb	As	Cd
PS	Mean	37.12	42.77	26.72	46.22	19.34	29.32	21.95	33.62	4.72	31.47
	Range	27.46–60.47	27.35–73.12	17.34–39.73	23.86–85.78	10.52–34.99	7.46–48.29	12.08–38.05	15.9–56.63	10.52–38.79	1.26–62
PSch	Mean	48.62	73.4	38.13	41.21	35.9	51.33	40	30.55	29.95	30.89
	Range	26.7–86.27	33.82–194.49	24.06–49.24	20.99–51.53	16.41–72.15	26.41–92.76	17.01–82.14	13.26–53.78	11.19–30.59	10.52–47
PK	Mean	34.9	44.97	31.25	42.25	22.54	34.9	23.07	69.23	25.68	43.98
	Range	18.05–48.78	21.88–64.16	22.94–39.17	10.17–75.49	15.07–32.9	14.37–52.32	12.53–32.68	9.74–126.86	7.46–50.73	8.5–83.49
PD	Mean	41.64	54.85	32.77	45.13	27.71	44.86	29.34	37.77	22.48	38.9
	Range	28.98–66.62	31.83–104.45	20.7–49.24	25.4–81.53	13.65–56.1	18.55–103.96	13.2–63.34	17.42–68.54	14.17–43.27	21.04–66.48
PL	Mean	29.26	35.45	24.87	17.67	22.78	17.06	16.38	20.58	12.13	20.92
	Range	18.81–36.84	23.37–43.27	15.1–37.49	6.05–31.31	10.29–97.81	8.65–26.41	10.07–22.38	8–40.99	5.22–18.65	6.26–41.18

*EF < 2—deficiency to minimal enrichment. EF > 2–5—moderate, EF > 5–20—significant, EF> 20–40—very high and EF > 40—extremely high enrichment.

).

The CF analysis further confirmed the EF results that the samples tested were not free of contamination. High levels of contamination were found for almost all elements involved. Based on the CF results, this anthropogenic ecosystem belongs to the class of soils with significant contamination to very high contamination factors. The pollutant load the index (PLI) results showed that pollutants were present at all the analyzed sites (

Table 7. * Contamination factor (CF) and pollution load index for study sites.

Sites		Ba	Sr	Cr	Zn	Ni	Cu	Co	Pb	As	Cd	PLI
PS	Mean	8.29	9.55	5.96	10.32	4.32	6.55	4.9	7.51	4.72	7.03	8.45
PSch	Mean	10.86	16.39	8.51	9.2	8.02	11.46	8.93	6.82	4.9	6.9	11.09
PK	Mean	7.61	10.04	6.98	9.43	5.03	7.79	5.15	15.46	5.73	9.82	10.23
PD	Mean	9.03	12.25	7.32	10.8	6.19	10.02	6.55	8.43	5.02	8.69	10
PL	Mean	6.53	7.91	5.55	3.94	5.09	3.81	3.66	4.59	2.71	4.67	5.84

* CF < 1—low contamination, 1–3—moderate contamination, 3–6—considerable contamination <6—very high contamination factor.

).

The potential ecological risk for most elements was low (<40). All sites found high indications for Cd (

Table 8. Potential ecological risk factor and potential ecological risk index for the heavy metals analyzed (mean values).

	Cr	Zn	Ni	Cu	Pb	As	Cd	RI **
$T_r^i$ *	2	1	5	5	5	10	30
PS	11.93	10.32	21.61	32.75	37.55	47.28	210.9	372.59
PSch	17.03	9.21	40.1	57.33	34.12	49.03	207	414.06
PK	13.96	9.43	25.17	38.98	77.32	57.36	294.75	517.23
PD	14.64	10.08	30.95	50.10	42.18	50.23	260.7	459.2
PL	11.11	3.94	25.45	19.05	22.98	27.11	140.25	250.14

* Toxic response factor given by Zhu et al. [58]. ** RI < 150—low risk, 150 ≤ RI < 300—moderate risk, 300 ≤ RI < 600—considerable risk, 600 ≤ RI—very high risk.

). Elements in the moderate and considerable risk range (in some parks) included Pb, As and Cu, while those in the low risk range included Cr and Zn. The risk index assessed the pollution level and showed that only soil from Park Leśna belonged to the moderate risk class and that all remaining sites belonged to the considerable risk class.

4. Discussion

Centuries-old and contemporary anthropogenic human activities in cities significantly impact the chemical composition and morphology of soils more than natural pedogenesis. Such activities in urban areas foster a significant transformation of the natural soil cover and lead to the formation of soils with specific properties, the parameters of which depend on the intensity, direction and duration of human impact [7,62] and, therefore, cannot always be compared with other areas. The topsoil, mainly the humus horizon in parks and greeneries, is often formed using artificial material transported from outside. The urban parks analyzed in the study were established in areas transformed by mining activities [63]. This material usually contains a significant amount of organic matter, which affects soil properties such as pH value, loss on ignition and content of various elements that differ from natural settings. Long-term maintenance of parks brings about permanent changes in the properties of urban soils [64].

4.1. Variability of Basic Soil Properties

The soil reaction of the parks studied is predominantly characterized as slightly acidic to neutral (in most sites) and even alkaline (Table 1). High values influence the concentration of H⁺, Al³⁺ and the nature of hydrolytic acidity (H_h). Changes and variations in the reaction in urban parks are influenced by dissolving concrete, walls made of limestone rock and their location along traffic routes. Similar results were obtained from other urban parks regarding this type of reaction [4,9,13,62,65]. The organic matter content of these anthropogenic systems is high, according to the analysis of the loss of ignition, which is undoubtedly related to park development. Park soils are often intensively organically fertilized soils that are fertilized cyclically by the application of organic materials, such as composts, peats, horticultural soils or sewage [17,62]. The OC_org values are similar at all study points in each park, so it may be indicative of the even application of humus material to the ground surfaces and their regular fertilization [17,21,62]. The contents of N_t revealed their relation to the values of OC_org (Table 2). The C/N ratio is diverse and ranges from four to 109, which indicates that the biological activity of the soil varies depending on the site in the parks as well as the influence of many factors. Such a ratio is typical for anthropogenic soils [5,66].

The average content of total phosphorus (P_t) as an indicator of anthropogenic soil pollution in the parks is relatively similarly distributed and ranges from 310 mg kg⁻¹ (PSch) to 1840 mg kg⁻¹ (PS). High concentrations of Pt. were found at all sites. Bednarek et al. [47] and Charzyński et al. [6] state that P contents above 300 mg-kg⁻¹ in soil indicate causation in various human activities. The source of this element (P_t) in urban parks is related to the fertilization and maintenance of urban greenery, organic waste accumulation, domestic sewage run-off, waste and pet feces, which are permanent features of urban parks. We can conclude that the occurrence of Pt at high concentrations in anthropogenic soils is related to human activity.

4.2. Factors Causing Spatial Variability of Potentially Toxic Elements

The amount of potentially toxic elements present in the soil environment should be considered in terms of their presence in the bedrock as a background, determining their possible accumulation in the soil [43] for natural systems. In contrast, the elevated content of potentially toxic elements in urban ecosystems is primarily related to the functioning of the city as a whole. In turn, the distribution of these elements may be due to their geochemical mobility and sorption and desorption on surface horizons [67] and in anthropogenic sources.

Emissions from fuel combustion increase lead and zinc in soils, as does heavy industry. In addition to emissions from fuel combustion, sources of trace elements include tire abrasion, oils, construction, waste disposal, metal-bearing dust, and surface runoff from surrounding streets, lubricants and leaks from batteries and damaged tanks [31] or industrial activities (mining, metallurgy and chemical engineering), construction and waste disposal, and combustion-contaminated soils and ecosystems [12,68]. Additionally, maintenance work and fertilization in parks, especially in areas planted with ornamental plants, also contribute to this phenomenon. Traffic is a significant source of aerosols containing not only toxic metals but also PAHs, i.e., polycyclic aromatic hydrocarbons or harmful PM10 and PM2.5 [5].

Concentrations of potentially toxic metals did not always vary from site to site in the parks studied. Detailed information on the content of the analyzed elements is presented in Table 3. In almost all the parks, the contents of Pb, Cd, Zn, As, Ba and Sr exceeded the limit values adopted in Poland [61]. Arsenic and barium were exceeded at sites near the main street (PS-7-As-52 mg kg⁻¹, PS-2.3-Ba-797 mg kg⁻¹), where maximum values were also found (Ps-7-Cd-27.7 mg kg⁻¹, PS-6-Pb-910 and PS-6-Zn-3526 mg kg⁻¹) (Table 3, Figure 1). In PSch, lead (PSch-5, four times), cadmium (PSch-5, five times) and zinc (PSch-7, four times) exceeded standards at all points, with maximum values being recorded in the center of the park and along the national road (Figure 1). In this park, maximum values of As (PSch-6-38 mg kg⁻¹) and Zn (PSch-7-2118 mg kg⁻¹) were found in the center of the park. For Ba and Sr, the maximum values were often found along transport routes in most parks (Table 3).

In the case of PK, maximum values for lead, cadmium, zinc, arsenic, barium and strontium were found at the PK-2 test site on a residential street (Figure 1) and in the center of the park. Cd (37.3 mg kg⁻¹, ×19; Table 3), and Pb (2040, mg kg⁻¹ ×10, Table 3) exceeded the standards most here. Park Dielta was the most polluted area compared to the other parks and had the highest excess level of potential toxic metals (Table 3). This park is surrounded by both road and rail transport routes (Figure 1), which also affected the distribution of metals in the soil. In the PL park, the contents of heavy metals were relatively lower in all sites than in the other parks, with the exception of samples from PL-5 (Pb-659 mg kg⁻¹, Zn-1287 mg kg⁻¹, Cd-18.8 mg kg⁻¹).

It should be emphasized that the parks were built in areas impacted by mining and that the subsoils of these parks contain post-mining waste. Hence, there are significant amounts of toxic elements in the topsoil compared to parks that were established on natural soils [21,69]. The variation in the occurrence of heavy metals in different regions of the world is shown in

Table 9. Total content of PTMs in some park soils in different cities (mean and range values, median in parentheses).

City, Parks	Pb	Zn	Cd	Cu	As	Co	Reference
Parks in Sosnowiec (5)	617	1582	15	71	28	58	Current study
Toruń urban area, Poland	25.23	23.75	0.15	11.8	n.a.	n.a.	[9]
Zielona Góra, Poland	85.2	293	0.58	33	n.a.	n.a.	[34]
Parks in Katowice, Poland	270	590	4.4	30	n.a.	7.0
Parks in Zabrze, Poland	67.5	250	1.98	13.5	n.a.	4.1	[37]
Parks in Dąbrowa Gornicza, Poland	270	660	9.2	21	n.a.	4.1	[37]
Parks in Tarnowskie Góry, Poland	930	1390	9.0	22.0	n.a.	6.0	[37]
New York, City Parks (Central, Pelham Bay Park, Van Cortland Park	40–730	19–300 (81)	0.1–3 (0.4)	14–138 (46)	1–46 (13)	3–13 (7)	[71]
Soundview Park, NYC	160–1049	184–792	-	48–529	-	-	[72]
City Park ins Hong Kong, China	7.7–496 (70)	23–930 (781)	0.11–1.36	1.3–277 (10.4)	-	0.6–10.9	[70]
Phoenix Park, Dublin, Ireland	39	94	-	25	--		[73]
Parks in Ostrava (9), Czech Republic	27–125 (49)	78–922 (151)	0.3–2.4	18–175 (38)	8–18	7–10	[64]
Prague urban parks (13), Czech Republic	22–213	63–285	0.2–0.1	16–88	11–31	10–17	[64]
Alexandra Park, Glasgow, United Kingdom	114–414 (179)	67–305 (104)	n.a.	33–113 (59)	n.a.	n.a.	[22]
Glasgow Green Park, Glasgow (Great Britain)	98–676 (279)	102–377 (174)	n.a	24–113 (88)	n.a.	n.a.	[22]
Tivoli Park, Ljubljana	39–225 (72)	84–300 (103)	n.a.	21–78 (31)	n.a.	n.a.	[22]
Valention Park, Turin, Italy	68–257 (137)	116 (317)	n.a.	44–123 (83)	n.a.	n.a.	[22]
Galitos Park, Aveiro, Portugal	7–38 (20)	18–82 (49)	n.a	8–61 (16)	n.a	n.a	[22]
Los Principes Park, Seville (Spain)	43–247 (100)	73–191 (99)	n.a.	30–72 (47)	n.a.	n.a.	[22]
Stadsträdgarden. Uppsala, Sweden	7–116 (36)	27–193 (106)	n.a.	8–90 (31)	n.a.	n.a.	[22]
Moscow, Russia	74.2	58.2	0.3	24.1	n.a	n.a	[24]
Hannover, Germany	172	186	1.2	52	n.a	n.a	[23]
Debrecen, Hungary	10.3	67.7	<1	7.1	n.a	n.a	[9]

[64,70,71,72,73], presenting several contrasting comparisons with the study area. In this area, the results obtained range from several to many times higher than in the other parks shown in the table. The potential contents of toxic substances in the soils of the parks analyzed show some similarities to the parks found in Upper Silesia (Table 9), which have a similar history of formation [37]. In all the parks, Cu, Ni, Co and Cr do not exceed the limit values and therefore do not have a negative impact on the surrounding ecosystems. Similar values for Cu have been obtained in other parks in both Europe [22,64] and the USA [71]. Numerous studies have documented also the substantial contamination of urban surface soils with Zn, Pb, Ni, Co, Cr and Cu [7,12,36,62,68,74].

The Spearman rank correlation coefficient showed the presence of a statistically significant positive correlation between the metals analyzed in all pairs (very strong correlation > 0.9: Ni-Cr, Co-Cr, Co-Ni, Sr-Cu, Sr-As, Table 4). Such a strong correlation between elements is related to their common origin and occurrence in the area. In this case, the strong and similar correlation in the parks studied is due to their location along highways, railways and historical conditions with the development of heavy industry (mining, metallurgy and chemical engineering). Excessive car traffic and low emissions are another significant source of pollution in the study region. The I_geo, EF and CF results also indirectly confirm the results indicating the occurrence and similarity of metal content in the parks obtained by the Ward method (Figure 2). Similarly, values in terms of correlation and environmental indicators (EF, CF, RI, PLI) were obtained by other authors from different regions [2,7,8,9,10,13,18,21]. Other authors [18,75,76,77] emphasize that the elemental content of potential toxic substances is influenced by the materials forming the topsoil layers. These are formed as a result of land reclamation, where the parent material is often rubble/post-industrial waste and usually covered by a 0.5 m layer of fertilized humic material from other sites. Such transported material has different physical properties.

The authors report that half of the pollution in Poland comes from transport [31], which significantly impacts the functioning of urban forests. Urban parks in Sosnowiec are located along heavily used district roads (e.g., PK, PL, PD and PS) and national roads (PK, PSch). Hence, this significantly impacts the heavy metal content of the soil in these ecosystems. The high distribution of heavy metals in the soils of this region is due to intensive mining and processing activities, especially of non-ferrous ores [45,74]. Coal, which is used in households, is also one of the pollutant carriers and contains few heavy metals. However, in the process of burning vast amounts of this raw material, especially during the heating season, a large proportion of these heavy metals is released into the atmosphere in the form of dust [78], which becomes one of the sources of heavy metals in urban ecosystems. This is related to the adsorption capacity of organic matter [67] and may also be related to potentially toxic elements from the atmosphere being accumulated by leaves [79]. The cause of lead and arsenic pollution in these and similar areas is metal-containing dust (metal-bearing dust) from the transportation of pollution from industrial and metallurgical plants [79,80].

Another source of pollution in terms of Cr, Cu, Ni, Pb and Zn in urban park soils is road dust, which consists of mineral and organic particles containing metals and comes from industrial emitters, vehicles and fuel combustion. Significant amounts of zinc, copper, nickel and chromium have been found along tram and trolleybus lines in various cities in Poland [6,10,16,81]. All the parks analyzed are located in such conditions, hence the high concentrations of the aforementioned elements.

The analysis of environmental indicators like I_geo, EF, and CF confirm the degree of pollution of the urban parks in Sosnowiec as being of an anthropogenic origin. Analyses of the pollutant load index (PLI) showed that contamination was present at all the sites. The EF results indicate the presence of metal pollution to varying degrees and can be described as significant enrichment, very high enrichment and extremely high enrichment. The I_geo, and EF results obtained in the study area for Zn, Pb, Cu and Cr were often several times higher than the values for other urban parks in the world [8,82,83]. As confirmed by Nadłonek et al. [45], the occurrence of many potentially hazardous elements in the Upper Silesian region was caused by the historical mining of Zn-Pb ores, as well as zinc and iron smelting. The results of the correlation analysis confirm convincingly the co-occurrence of these elements and their accompanying elements.

The risk index (RI) provides an indication of the degree of ecological risk [84]. The RI calculated for the topsoil studied here was characterized by values ranging from 250.14 (moderate risk) to 517 (significant risk). The potential ecological risk in these classes was higher than the RI calculated on the basis of heavy metal content in urban soils studied by other authors [8,82,83].

5. Conclusions

This study of the distribution and occurrence of potentially toxic metals in urban parks shows that the areas studied are significantly contaminated with these metals. This is related, on the one hand, to the operation of the urban mining industry and land use and, on the other, to their location in city centers and along road and rail transport routes. The soil at the sites in question was characterized by a reaction ranging from weakly acidic to neutral at most of the analyzed points and contained significant amounts of OC_tot., N_t and Pt, which is also related to the management and annual maintenance of the park areas. In almost all parks and study sites, the contents of Pb, Cd, Zn, As, Ba and Sr exceeded the acceptable standards for recreational areas, among which Zn, Pb and Ba showed the highest values, respectively. The geoaccumulation index (I_geo) of the soils indicates that the high concentrations of many potentially toxic metals were due to historical mining of Zn-Pb ores as well as zinc and iron smelting in the vicinity of the study area. The environmental indicators used facilitated the determination of the degree of heavy metal contamination: I_geo shows that Cd, Pb and Zn have had the highest impact on soil contamination in all the areas studied. Most of the samples studied have extremely high contents (Cd, Zn, Ba, Pb) based on EF analysis. The results of the geochemical survey and the analysis of environmental indicators show the current state of soils in selected parks in Sosnowiec and can be useful in evaluating local spatial development plans, decision-making regarding environmental conditions for the designation, as well as the use of green areas for social purposes.