Risk assessment of potentially toxic elements in street dust from Mahd Ad Dhahab gold mine, Saudi Arabia

ABSTRACT

The current study evaluated the contamination level of potentially toxic elements (PTEs) and their associated risk in street dust in the vicinity of Mahd Ad Dhahab gold mine, KSA. 25 street dust samples were collected and then subjected to magnetic susceptibility (χ) measurements and analysed for Al, As, Cd, Co, Cr, Cu, Fe, Hg, Mg, Mn, Pb and Zn contents. The results revealed that the study area is contaminated by Cd, Zn, Hg, Cu, Pb and As, since their concentrations were higher than the background and displayed high values of coefficient of variation (41.86, 14.9, 11.05, 10.1, 8 and 5.3, respectively) combined with high enrichment factor (181.7, 167.5, 183.6, 122.8, 151.7 and 33.95, respectively). The higher concentrations of these elements, associated with higher χ values, were recorded close to the mining area. The correlation coefficient and heatmap categorized the studied PTEs into two groups; The first group comprised Al, Co, Cr, Fe, Mg and Mn of geogenic origin; the second included As, Cd, Cu, Hg, Pb and Zn, along with χ, which controlled by anthropogenic sources. Ecological risk assessment values indicated that 28% of the samples have very strong ecological risk. Regarding human health risk, among different pathways, ingestion is a primary pathway of PTEs harming human health. The non-carcinogenic risks from Cd, Cu and Pb and carcinogenic risk from As exceeded the acceptable level to local children. The study demonstrated that mining activities are the main source of street dust contamination in the area.

KEYWORDS:

• Mining contamination
• magnetic susceptibility
• potential ecological risk
• human health risk
• toxic elements
• enrichment factor

1. Introduction

Potentially toxic elements (PTEs) refer to metals, metalloids and non-metals that are categorized among the greatest threatening groups of environmental pollutants because of their severity and toxicity to different life forms [1]. They are naturally occurring and have densities greater than 5 g cm⁻³ and atomic numbers greater than 20 [2]. According to their health risk, PTEs are usually classified as non-carcinogenic (e.g. Fe, Mn, Zn) and carcinogenic (e.g. As, Cd, Pb) to humans [3–5]. Although PTEs are naturally occurring, they could be redistributed and reconcentrated throughout environmental system (soil, water, air) by different human activities such as agriculture, industry, mining, urbanization, smelting and sewage disposal [1,6–8]. These elements are non-biodegradable elements, and their toxicity depends on the absorbed dose, route of exposure, durability and chemical forms (speciation), along with the gender, age, genetics and nutritional status of persons who are exposed [9,10]. Even though some of these elements are essential to preserving various physiological and biochemical functions, above specific allowable concentrations they often lead to noxious effects for living organisms [5]. Despite the mining sector contributing to economic growth, it enriches the environment with potentially toxic substances which affect human health. It was found that environmental media in the vicinity of mining activities is always contaminated by PTEs, which are dispersed from mining dust [11–14]. Even worse, these dispersed elements can also lead to health risks for humans, animals and the ecosystem [15–17]. Street dust represents one of the main carriers of PTEs whether from natural (e.g. soil weathering) or anthropogenic sources [18]. Due to its large surface area, dust particles tend to adsorb a considerable amount of PTEs emitted from different anthropogenic activities. Subsequently, PTEs bearing road dust can be returned to the air via the resuspension and enter the body by different routes of exposure (e.g. ingestion, inhalation and skin absorption). In these circumstances, recognizing the spatial distribution and assessment of such PTEs in the street dust is essential for environmental risk management and sustainability. There are numerous methods to assess and evaluate the level of PTEs in environmental system. Enrichment factor (EF) is a traditional method used to assess the level of PTEs contamination in an environmental system (soil, sediments, dust and water) [19–22]. It determines the value of enrichment of an element in the environmental media. Moreover, Potential ecological risk index (PERI) and Human health risk assessment (HRA) models have been successfully employed to investigate the threat of PTEs to the environment and human health respectively. PERI is used to evaluate the potential ecological risk of contaminants as PTEs on an ecological system [23], while HRA is an approach to estimate the effects of PTEs on the human health due to different exposure pathways [3–5,8,9]. Furthermore, statistical analysis procedures, (univariate and multivariate) have been widely used in environmental studies to determine the relationship among PTEs and their possible sources [9,24–26]. Besides chemical pollutants, anthropogenic activities have been exhibited to increase the magnetization of street dust [27–30]. Subsequently, along with geochemical investigations, the magnetic susceptibility method is widely applied as an effective indirect method for delineating PTEs soil, sediment and road dust pollution [29–33]. It is an easily measurable, cheap, non-destructive geophysical technique that measures the capability of earth materials to gain magnetization by the application of external magnetic field. Its efficiency is coming from the significant correlation between its values and PTEs content in polluted soil/sediments where the dust emitted from industrial and mining activities generally carries out magnetic particles accompanied with PTEs [31–34].

Saudi Arabia is endowed with mineral resources that include, phosphate, iron, copper, gold, lead, silver, zinc and other mineral deposits. Although these minerals are dotted all over the country, Mahd Ad Dhahab gold mine is the oldest and largest mining site in Saudi Arabia and is considered a major producer of gold and silver in this region and production back to the 1980s. The ore is mined by an underground mine and then treated by grinding, floatation and then cyanide for metal extraction [35]. Unfortunately, the residential area (with a population of 64,000) is in the immediate vicinity downwind of the mining site [35,36] on its eastern side. No doubt the inhabitants of the downwind side are subjected to higher health risks since the wind carries considerable amounts of PTEs and other toxic emissions in their direction [15]. Although some researchers have investigated the PTEs contamination around Mahd Ad Dhahab mine whether in flora [35,37,38] groundwater [39], soil [35,37,40] and air dust [35], no studies so far have been investigated the ecological and health risk of PTEs in street dust in the mining residential area. Therefore, the present research was conducted to identify and quantify the concentrations of PTEs in the vicinity of the Mahd Ad Dhahab gold mine and evaluate the potential ecological and human health risks caused by these elements. Also, it demonstrates the potentiality of magnetic susceptibility measurements as an indirect method to delineate the contaminated dust that is dispersed from mining activities and associated potentially with toxic elements. In the current study, the term “potentially toxic elements” refers to metals and metalloids Al, As, Cd, Co, Cr, Cu, Fe, Hg, Mg, Mn, Pb and Zn. Results obtained from this research provide a scientific basis regarding the ecological and human health risks to prevent and control PTEs pollution and improve the environmental conditions in the Mahd Ad Dhahab residential area.

2. Materials and methods

2.1. Study area

Mahd Adh Dhahab, meaning “Cradle of Gold”, (23°30′ N, 40°51′ E) lies in the middle western part of the late Proterozoic Arabian shield. It belongs to the Madinah province of Saudi Arabia about 230 km southeast of Al-Madinah Al-Munawara and 380 km northeast of Jeddah. From the geological point of view, the host rock of Mahd Ad Dhahab gold deposit is the late Proterozoic Mahd Group (>500 m thickness); a bimodal basalt-rhyolite volcanic-volcaniclastic sequence that unconformably underlined by the basement of metamorphosed tonalite of Dhukhur batholith [41]. According to El-Shafei et al. [42], this group is divided into older Lahuf formation (felsic pyroclastics, tuffaceous sandstone, and siltstone and subareal mafic volcanic) unconformably overlain by the Tuwal formation (subaqueous felsic pyroclastic rocks, with minor dolostone and chert and subaerial ignimbrites). The ore predominantly consists of massive sulphides whereas the minerals in the gold ore consist mainly of sphalerite, chalcopyrite, sylvanite, pyrite, hessite, galena, petzite, tetradymite, altaite while chlorite, epidote and quartz occur as gangue mineral [41,42]. Currently, the mine is producing about 3 tons of gold per annum while the total gold production has reached 180 t [42]. Gold occurs in the quartz veins associated with Ag, Zn, Cu and other metals. The average ore composition is 24 ppm Au, 92 ppm Ag, 0.65% Cu and 3.11% Zn [42]. The climatic conditions in the study area are affected by the geographic and topographic characteristics of the region and its rapprochement to the Red Sea in the western part. Like most parts of Saudi Arabia, Mahd Ad Dhahab area is characterized by arid conditions. Based on Al-Jarash [43], Faisal and Osama, [44] and Al-Amri et.al. [45], the area has a low rainfall rate (about 53 mm/year) that occurs in the winter (January- April) with little or no rainfall in the other seasons. The air temperature is high, and the mean monthly temperature is 28°C which ranges between 16 and 49°C. The mean annual relative humidity is 24% and the annual average wind speed is approximately 6.3 km/h with prevailing wind direction west-southwest to north-northeast. No significant groundwater aquifer in Mahd Ad Dhahab area, since the water resources are restricted to Wadi sediments and rock fractures which are recharged via infiltration of the rare rainfall [39].

2.2. Sampling and analytical techniques

25 samples of outdoor street dust were collected in January- February 2020 from the residential area of Mahd Ad Dhahab city (Figure 1) using clean polyethylene brushes and a dustpan. Each sampled site was recorded by a portable GPS and represented by of composite sample (300–500 g) taken from four to five points from an area of about 10–20 m². The samples were stored in a clean polyethylene plastic bag after removing impurities (e.g. coarse debris and plant remains) and transported to the lab. In the lab, the samples were air-dried at 25°C for 2 weeks to get rid of the moisture. Dried samples were ground using an agate mortar and pestle passed through a 63 µm sieve and stored in small polyethylene plastic bags for analyses.

Figure 1. Location map of the study area and spatial distribution of street dust samples in Mahd Ad Dhahab, KSA, showing the mining site.

The prepared samples were subjected to magnetic susceptibility measurement. The magnetic susceptibility (χ) was measured in the lab at the Geology Department, Taibah University using the Bartington MS2 meter (Bartington Ltd., UK) with a dual frequency MS2B sensor. The calibrated instrument operates with a frequency of 875 Hz and a sensitivity of 39 × 10⁻⁸ SI. The street dust sample was paced in a plastic cubic holder and measured in a low field (300 A/m). For accuracy, the average value of three runs for each sample was considered. Magnetic enhancement (ME) of the street dust sample is calculated as; ME = χ _sample/χ _background [46]. If ME > 1 indicates enhancement due to anthropogenic input. For PTEs, the processed dust samples were submitted to a certified lab (Australian Laboratory Services, ALS Arabia Co. Ltd. Jeddah, KSA), for chemical analysis as per ME-ICP4- US EPA 6010 ICP/AES for Al, As, Cd, Co, Cr, Cu, Fe, Hg, Mg, Mn, Pb, Zn. 0.5 g of each powder sample was digested in HNO₃-HCl aqua regia for 45 min in a graphite heating block. After cooling the digestion solution was diluted to 12.5 mL using deionized water and analysed by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES). The quality assurance (QA) and quality control (QC) control measures included laboratory duplicate, method blank, control and matrix spikes, recovery, limits of detection (LODs) and limits of quantification (LOQs) were performed. All digestions and elements analyses were achieved in triplicate and precision was confirmed through using a standard analytical batch including a reagent blank to measure background and certified reference material (CRM) for the elements. The relative difference for replicate analysis was ±5% and the recovery of the reference material ranged between 80–120% for all metals. LOD and LOQ of the observed PTEs (Al, As, Cd, Co, Cr, Cu, Fe, Hg, Mg, Mn, Pb, Zn) were 0.01–25%; 2–10,000; 0.5–1000; 1–10,000; 1–10,000; 1–10,000; 0.01–50%; 1–10,000; 0.01–25%; 5–50,000; 2–10,000; 2–10,000, respectively.

2.3. Data analysis

The raw data were subjected Kolmogorov–Smirnov (K-S) method to check its normality. The non-normal data were transferred to achieve normality by using natural logarithm transformation. The statistical analyses were conducted after transforming the raw data to the lognormal distribution of concentrations for all studied variables. Descriptive statistics, including minimum, maximum, average, standard deviation (SD) and coefficient of variation (CV), were performed for the different variables in the dust samples using Microsoft Excel 365 version. CV was computed to investigate the variability or the homogeneity of PTEs concentration. It is frequently used to describe the spatial variation degree of variables. A low CV value indicates low variability, while a value greater than 30% indicates influence could come from the anthropogenic factor [24–26]. Multivariate statistical analysis involving correlation coefficient and Hierarchical cluster analysis (HCA) were carried out to measure the strength of inter-relationship between each studied PTEs and identify their potential sources to differentiate natural versus anthropogenic contribution. Correlation coefficient was carried out using Microsoft Excel 365 while Hierarchical cluster analysis (HCA), represented by heatmap, was produced using https://biit.cs.ut.ee/clustvis/ online. The Two- way HCA heatmap was prepared according to Ward’s method and squared Euclidean measure, being shown in dendrograms. Data were visualized using a false colour scale. The spatial distribution of magnetic susceptibility was interpolated based on 25 sampling points using inverse distance weighting (IDW) method using ArcGIS (version 10.1) software. This method is widely applied for interpolating the spatial distribution of contaminants in environmental studies [47–49].

3. PTEs pollution assessment

3.1. Enrichment factor

The enrichment factor (EF) is a tool to estimate the enriched element in different environmental media. It is used to evaluate the PTEs input from the different anthropogenic activities relative to the natural background. EF is the normalization of the concentration of the intended element in a dust sample by the reference element. EF was calculated according to Zoller et al. [50], given in Equation (1): (1) $E{F}_x=\left(X/Eref\right)sample/\left(X/Eref\right)background$(1) where X is the concentration of the intended element, E_ref refers to the reference element for normalization. Generally, E_ref is a conservative element, with low variability relative to other elements, displays homogeneous spatial distributions, neither likely to be affected by anthropogenic activities nor correlated with PTEs pollutants [20,21,51]. In this study, Fe was selected as a reference element, which offers these conditions, and frequently selected as a reference element for evaluating the street dust contamination [22,52,53]. The average upper continental crust values (UCC) after Wedepohl [54] were selected as a background value. When the value of 0 < EF ≤ 1, indicates a natural or anthropogenic source and no enrichment, the value of EF was classified as a deficiency to no enrichment 1< EF ≤ 2, minimal enrichment, 2 < EF ≤ 5, moderate enrichment 5< EF ≤ 20, significant enrichment 20 < EF ≤ 40, very high enrichment and EF >40, extremely high enrichment [55,56].

3.2. Ecological risk assessment

To evaluate the levels of studied PTEs in the street dust and their association with ecological and environmental effects with toxicology, PERI method after Hakanson [57] was applied according to Equation (2): (2) $RI=\sum _{i=1}^{n}E{r}^i=\sum _{i=1}^{n}T{r}^i.C{F}^i=\sum _{i=1}^{n}T{r}^i.C_m/C_b$(2) where Erⁱ refers to the ecological risk factor, Trⁱ represents the toxic response factor for intended metal (As = 10, Cd  = 30, Co = 5, Cr = 2, Cu = 5, Hg = 40, Mn = 1, Pb = 5 and Zn = 1) [57,58], CFⁱ is the contamination factor of PTEs as the ratio of analysed concentration C_m to background value (UCC) after Wedepohl [54]. Erⁱ < 40 indicates Low, 40 ≤ Erⁱ< 80 moderate, 80 ≤ Erⁱ < 160 considerable, 160 ≤ Erⁱ < 320 high and Erⁱ ≥ 320 very high, while RI value is classified as Low (RI < 150), moderate (150 ≤ RI < 300), considerable (300 ≤ RI < 600), very high (RI ≥ 600) [57].

3.3. Health risk assessment

To assess non-carcinogenic and carcinogenic health risks to adults and children via these three exposure pathways, the recognized model of Environmental Protection Agency [59,60] was applied. Accordingly, four basic steps should be required to quantify HRA: identifying the hazard, estimating the exposure, evaluating the dose–response, and characterizing the risk [59–61]. The doses through three main exposure pathways for adults and children are calculated as defined in Equations (3), (4) and (5). (3) $\begin{array}{rl}{D}_{ing}& =\frac{C\times IngR\times EF\times ED}{BWCAT}\times {10}^{-6}\end{array}$(3) (4) $\begin{array}{rl}{D}_{inh}& =\frac{C\times InhR\times EF\times ED}{PEF\times BW\times AT}\end{array}$(4) (5) $\begin{array}{rl}{D}_{Dermal}& =\frac{C\times SA\times SL\times ABS\times EF\times ED}{BW\times AT}\times {10}^{-6}\end{array}$(5) where D_ing refers to the average daily intake from street dust ingestion; D_inh, inhalation; D_dermal, dermal absorption (in mg kg⁻¹ day^-1), C is the concentration of the intended element in the sample (mg/kg) and all the other parameters were showed in Table 1, as described by [56]. The dose quotients of each element are divided by the corresponding reference dose (RfD) in mg kg⁻¹ day^-1 to acquire the hazard quotient (HQ) for non-cancer risk, and then, the hazard index (HI) is yielded by the sum of HQ. If HI <1, it is indicated that there is no significant risk of non-carcinogenic effects while the risk magnitude increases with HI increases [63]. To get the level of cancer risk (CR), the dose quotients are multiplied by the corresponding slope factor (SF) in (mg kg⁻¹ day^-1)⁻¹ of cancer. Lifetime cancer risk (LCR) ΣCR (ingestion + inhalation + dermal) If CR, LCR ≤ 1.0E-06 indicates very low risk, 1.0E-06 < CR ≤1.0E-04 low risk, 1.0E-04 < CR ≤1.0E-03 medium risk, 1.0E-03 < CR ≤1.0E-01 high risk, CR ≥ 1.0E-01 very high risk [59,64,65].

4. Results and discussion

4.1. Magnetic susceptibility (χ)

As listed in Table 2, χ values vary from 9.1E-08 to 57.43E-08 m³kg⁻¹ with an average of 21.7E-08 m³kg⁻¹. Spatially, the high values of magnetic susceptibility were recorded for the samples that were collected close to the mining site in the northeastern part of the studied area (Figure 2). The high CV value (58.12%) of magnetic susceptibility is most probably attributed to an anthropogenic factor rather than the geogenic one. Unfortunately, no previous studies have been conducted with magnetic susceptibility in the study area to comparing the current values with former ones. Therefore, the average of the three minimum values (11.4E-08) of magnetic susceptibility among all analysed samples was selected to be local background for recognizing the anthropogenic contributions [47,66]. Applying such background allows detecting even low anthropogenic input [67]. The calculated magnetic susceptibility enhancement values of the street dust range from 0.8 to 5.03. The highest values (3.7, 3.3, 4.03 and 5.03) were registered for the samples that were close to the mining area. Enhancing the magnetic susceptibility of the dust samples is mainly due to the presence of major pollution sources. The enhancement of susceptibility is most probably due to the accumulation of heavy and opaque mineral assemblages associated with gold and iron oxide particles (technogenic particles) that are dispersed from different mining activities [30–33,66].

Figure 2. The spatial distribution of magnetic susceptibility in the study area.

Table 1. Parameters used to evaluate exposure risk to children and adults (modified after Ma et al., 2020 [56]).

Symbol	Parameter	Unit	Value
C	Concentration of an element	mg/g
IngR	Soil ingestion rate	mg/day	200 (for child), 100 (for adult)
EF	Exposure frequency	day/year	350
InhR	Air inhalation rate	m^3/day	7.6 (for child), 20 (for adult)
ED	Exposure duration	year	6 (for child), 24 (for adult)
SA	Skin area	cm^2	2800 (for child), 5700 (for adult)
SL	Skin adherence factor	mg/cm^2	0.2 (for child), 0.07(for adult)
BW	Body weight	kg	15.9(for child), 56.9(for adult)
AT	Mean Time	day	ED × 365 days for non-carcinogens 70 × 365 days for carcinogens
ABS	Dermal absorption factor	Unitless	0.006 (Pb), 0.14 (Cd), 0.1 (Cu), 0.02 (Zn), 0.05 (Hg), 0.03 (As), 0.04 (Cr), 0.001 for rest
PEF	Particle emission factor	m^3/kg	1.36E9
RfD	Chronic reference dose	mgkg^−1day^−1	RfDing RfDinh RfDderm
			5.0E-03 2.9E-05 2.5E-04 (Cr)
			3.7E-02 4.0E-02 1.9E-03 (Cu)
			3.0E-01 3.0E-01 6.0E-02 (Zn)
			3.0E-04 3.0E-04 1.2E-04 (As)
			1.0E-03 1.0E-03 5.0E-05 (Cd)
			3.5E-03 3.5E-03 5.3E-04 (Pb)
			3.0E-04 8.6E-05 2.1E-05 (Hg)
SF	Carcinogenic slope factor		Ing. SF: 1.5 (As), 6.3 (Cd), 0.0085 (Pb) Inh. SF: 15.1 (As), 0.38 (Cd), 42 (Cr), 0.042 (Pb) Derm. SF: 3.66 (As)

Carcinogenic slope factor (SF) [62].

Table 2. Descriptive statistics for potentially toxic element concentrations and magnetic susceptibility (χ^E-08) in street dust samples.

Parameters	Min	Max	Average	SD	CV (%)	Background^a
Al (%)	1.69	3.55	2.22	0.37	16.68	7.96
As (ppm)	3.00	12.00	6.59	2.24	33.95	1.5
Cd (ppm)	0.50	34.30	3.83	6.89	180.02	0.1
Co (ppm)	14.00	23.00	17.15	2.26	13.20	24
Cr (ppm)	31.00	83.00	50.44	10.30	20.42	126
Cu (ppm)	46.00	1320.00	222.89	273.73	122.81	25
Fe (%)	2.88	4.37	3.62	0.38	10.43	4.32
Hg (ppm)	1.00	3.00	0.44	0.73	167.41	0.04
Mg (%)	1.41	2.01	1.74	0.17	9.87	2.2
Mn (ppm)	480.00	953.00	651.93	105.43	16.17	716
Pb (ppm)	11.00	766.00	106.41	161.48	151.76	14.8
Zn (ppm)	83.00	7320.00	871.63	1460.08	167.51	65
χ (m^3kg^−1)	9.07	57.43	21.67	12.59	58.12	–

Note: SD: standard deviation; CV: coefficient of variation.

^a background values of Upper Continental Crust values (UCC) after Wedepohl (1995).

4.2. Potentially toxic elements

The results of analysed street dust samples (Table 2) indicated that the average concentrations of PTEs were found in the order of Fe (36,159 ppm)> Al (22,174 ppm)> Mg (17,359 ppm)> Zn (871 ppm)> Mn (651 ppm)> Cu (222.8 ppm)> Pb (106.4 ppm)> Cr (50.4 ppm)> Co (17.14 ppm)> As (6.59 ppm)> Cd (3.8 ppm)> Hg (0.4 ppm). Al, Fe, Mn and Mg occur with high concentrations since they are major constituents in many sediments, soil and rock-forming minerals. Comparatively, the mean concentrations of these elements are lower than UCC. On the other hand, the average concentration of As, Cd, Cu, Hg, Pb and Zn were higher than their respective average values of UCC. The average concentrations of these elements were 4.4–38.3 times higher than the corresponding background values which indicated that these elements have accumulated with various degrees [65]. Relatively, Cd (38.3) represents the major contributor to PTEs contents of street dust samples. Spatially, the highest contents of these elements were recorded in the samples that were collected from the northeastern part (almost the same spatial pattern of magnetic susceptibility) in the vicinity of the mining area. This specific distribution indicates that the street dust in the mining area comprises more severe PTEs than the other areas. Besides, As and Pb were found with high contents at traffic circles and rotaries within the residential area which may represent additional anthropogenic source to the gold mine. The average PTE concentration was compared with similar research (Table 3) conducted in other cities around the world (Jeddah in Saudi Arabia; Rafsanjan in Iran; Mongolia, Suzhou, Huainan and Suzhou in China; Singhbhum, Delhi and Jharia in India; Sonora in Mexico and Northern Spain). The average concentration of Al in Mahd Ad Dhahab street dust is higher than Northern Spain (261.2%) and lower than Delhi (49.2%). As is almost similar to Huainan (99.7%), higher than Jharna (161.8%) and lower than the other cities compared (6.3–54.9%). The Cd concentrations are lower than Northern Spain (17.2%) and Jeddah (51.3%) but higher than the other cities compared (123.5–1519%). The Co concentration is close to Jharna (98.5%), lower than Delhi (119.3%), Inghbhum (122.4) and Mongolia, (157.2%), but higher than Northern Spain (41.0%), Huainan (54.2%) and Jeddah (68.0%). The Cu concentration is close to Delhi (103.4%), lower than Sonora, (80.2%), Rafsanjan, (28.2%) and Singhbhum, (19.5%), but higher than the other cities compared (121.8–810.5%). The Hg concentrations in the analysed samples are lower than Northern Spain (17.2%), but higher than both Huainan and Suzhou, (223.4 and 244.4%, respectively). For Fe, its average concentration is lower than Delhi (60.1%), Singhbhum (61.7%) and Northern Spain (85.8%), but higher than Sonora (118.7%), Suzhou (181.5) and Jeddah (290.8%). The average concentration of Mn is almost similar to Jharia (99.2%), higher than Jeddah (118.4%), Sonora (153.5%) and Suzhou (159%), but lower than other cities compared (19.1–78.6%). The Pb concentrations are higher than Jharia (157%), Delhi (194.4%), Suzho (235.4%) and Huainan (249.8%), but lower than the other cities compared (20.7–95.2%). The concentrations of Zn in the analysed samples are lower than Northern Spain (17.8%) and Singhbhum (92%), but higher than the other cities compared (110–534.7%). The comparison revealed that almost the concentrations of PTEs in studied street dust were not quite high. Moreover, each city has its own geochemical characteristics and the similarity or variation of element contents among the cities compared did not differentiate between the geogenic and anthropogenic sources.

Table 3. PTEs concentrations (ppm) in street dust of the study area and various cities of the world.

City, Country	Al	As	Cd	Co	Cr	Cu	Fe	Hg	Mg	Mn	Pb	Zn	Reference
Jeddah, KSA	–	21.5	7.4	11.6	65.4	139.1	1.2	–	–	550.6	140.7	–	Shabbaj et al., 2018 [67]
Rafsanjan, Iran,	–	105.3	3.1	–	18.4	791.4	–	–	–	–	123.1	252.6	Aminiyan et al., 2018 [68]
Mongolia, China	–	12.02	2.2	26.9	141.2	36.4	–	–	–	3407.3	183.93	299.4	Kexin et al., 2015 [69]
Huainan China	–	6.62	0.25	9.3	61.1	36.3	–	0.197	–	–	42.6	–	Tang et al., 2017 [70]
Suzhou, China	–	13.4	2.45	–	25.7	104.8	–	0.18	–	–	262.2	376.9	Ma et al., 2016 [71]
Singhbhum, India	–	75.3	1.7	24.6	181.3	2217	6.2	–	–	964	159.7	1558	Mahato et al., 2023 [72]
Delhi, India	4.46		1.3	20.4	757	216	6.0	–	6725	1384	54.7	707	Pathak et al., 2013 [73]
Jharia (India)	–	4.08	0.78	16.9	75.4	56.8	–	–	–	658	67.8	163	Rout et al., 2013 [74]
Sonora, Mexico	–	15	–	–	–	278	3.0	–	–	424.6	–	184.6	Meza-Figueroa et al., 2009 [50]
Northern, Spain	0.85	17.5	22.3	7.03	41.6	183	4.2	2.56	1.1	1,661	514	4,892	Ordonez et al., 2003 [75]
This Study	2.22	6.59	3.83	17.15	50.4	222.8	3.62	0.44	1.74	651.9	106.	871.63

4.3. Statistical evaluation

The studied metals were arranged according to CV value in order of Hg (183.6)> Cd (181.7)> Zn (167.5)> Pb (151.7)> Cu (122.8)> As (34%)> Cr (20.4)> Al (16.6)> Mn (16.2)> Co (13.2)> Fe (10.4)> Mg (9.9). High CV values of elements (e.g. Hg, Cd, Zn, Pb, Cu and As) indicate fluctuation distribution and distinctive anthropogenic disturbance. The other metals (e.g. Cr, Al, Mn, Co, Fe, Mg) are most probably of geogenic origin from weathered rocks and sediments. As mentioned before, the ore deposits consist predominantly of massive sulphides associated with minerals. So, the emitted dust from the mining activities (particularly crushing and grinding processes) with considerable amounts of PTEs is the main source of these high-CV elements. It can be seen from Figure 3 that there are strong positive correlations (p < 0.01) found between Cd, Cu, Hg, Pb and Zn (r = 0.88–0.98). These elements as well, display a moderate positive correlation with As (r = 0.52–0.63). These indicate that in the studied area, Cd, Cu, Hg, Pb and Zn might be from one main source, while As is most probably affected by further source. The occurrence of As with high concentration in the traffic circles and rotaries in the residential area indicated that traffic emissions represent another source of As concentration. It is noted however that, magnetic susceptibility is positively correlated with As (r = 0.49), Cd (r = 0.69), Cu (r = 0.73), Hg (r = 0.63), Pb (r = 0.76) and Zn (r = 0.68) which indicates an association among the magnetic susceptibility and these metals. On the other hand, these variables displayed weak, negative, or no relation with the other studied metals (e.g. Al, Co, Cr, Fe, Mg and Mn).

Figure 3. Correlations among the analysed elements and magnetic susceptibility in the studied street dust samples; line colours indicate the strength of correlation.

Hierarchical cluster analysis (HCA), as a heatmap, was performed to recognize the associations between elements and their potential source using relative class correlation. Figure 4 showed that HCA categorized the elements and samples into different clusters using dendrograms. The studied elements were clustered in two main clusters. First cluster comprises Al, Co, Cr, Fe, Mg and Mn. Since these elements display low CV values and their concentrations are lower than the background and correlate to each other, these elements are of the geogenic source. Contrarily to the first cluster, the second cluster’s variables (e.g. As, As, Cd, Cu, Hg, Pb, Zn) display high CV values and their concentration violates the concentration of background which refers to anthropogenic source mainly due to gold mining activity. Traffic emissions may be representing another anthropogenic source. The occurrence of Magnetic susceptibility (Sus.) in this cluster indicates that the source(s) that enhanced the magnetic susceptibility is (are) the same source(s) that are responsible for increasing the PTEs contents in the study area. No doubt that the magnetic particulates that are emitted from different mining activities represent the main holder of such PTEs due to their high specific surface area [29]. Consequently, the magnetic susceptibility method provides a helpful qualitative and semi-quantitative approach for delineating the spatial distribution of pollutants in mining sites. On the other hand, HCA categorize the sampling sites into three main clusters; cluster 1 comprised 8 samples (S1, S5, S4, S3, S10, S22, S2 and S17) which represent 32% of samples; cluster 2 also contained 8 samples (S8, S14, S7, S13, S9, S25, S20 and S15) which represent 32% of samples; cluster 3 covered the remaining 36% of the samples (S11, S12, S19, S18, S23, S6, S16, S21 and S24). Cluster 1 displays the samples that were affected by the anthropogenic activity where samples S1, S5, S4, S3 and S2 are distinguished by the high amounts of As, Hg, Cd, Zn, Cu and Pb which is most probably due to mining activities. Cluster 2 displays a somewhat low amount of other anthropogenic activities rather than mining which may be attributed to traffic emissions. Cluster 3 demonstrates samples of geogenic sources and is not influenced by anthropogenic activities.

Figure 4. Heatmap obtained based on hierarchical cluster analysis (HCA) for relationships among different variables and street dust samples (produced based on https://biit.cs.ut.ee/clustvis/ online).

5. Pollution quantification

5.1. Enrichment factor

Table 4 shows that, among the studied elements, the dust samples are enriched with As, Cd, Cu, Hg, Pb and Zn with different degrees ranging from moderate to extremely high enrichment (Figure 5). The order of element EF (in average) is Cd (41.86)> Zn (14.9)> Hg (11.05)> Cu (10.1)> Pb (8)> As (5.3). It is observed that the EF values of elements in the vicinity of mining area are obviously higher than the other sampling sites. Meanwhile, the result of EF is consistent with CV where the enriched elements (As, Cd, Cu, Hg, Pb and Zn) displayed high CV values. which confirmed that these enriched PTEs were significantly affected by different mining activities. On the other hand, Al, Co, Cr, Fe, Mg, Mn, which have low CV values, showed no enrichment in the collected street dust samples, meaning that they were likely to derive from geogenic source. This was coincident with the research results of Wang et al., 2021 [76] and Huang et al., 2022 [77].

Figure 5. Calculated enrichment factors of studied elements in the street dust samples.

Table 4. Calculated enrichment factor (EF) for potentially toxic elements in street dust samples.

	Al	As	Cd	Co	Cr	Cu	Fe	Hg	Mg	Mn	Pb	Zn
Min	0.24	2.14	1.00	0.71	0.33	2.01	1.00	1.00	0.77	0.87	0.86	1.43
Max	0.46	8.55	366.55	1.08	0.76	56.43	1.00	80.15	1.30	1.52	55.31	120.35
Average	0.34	5.31	41.86	0.87	0.49	10.12	1.00	11.05	0.97	1.11	8.04	14.90

5.2. The potential ecological risk index

Table 5 tabulates the individual ecological risk indices (E_r) and total ecological risk index (RI) for As, Cd, Co, Cr, Cu, Mn, Hg, Pb and Zn. The contributions of these elements were all expressed as Cd>> Hg > Cu > As > Pb > Zn > Co > Mn > Cr. From the perspective of E_r, the average value of Cd (1148.9) displayed the highest value followed by Hg (438.5). These two elements were categorized as extremely high ecological risk levels. The average values of E_r for As, Cu and Pb were 38, 44 and 35.9, respectively, indicating these elements are at the level of medium ecological risk. Zn has E_r of 13.4 which represents a low ecological risk. Mining activities most probably are the main contributor to PTEs, and both Cd and Hg are the most significant pollutant elements in the street dust in the studied area. The average value of the RI was found to be 1719 which is categorized as a very high ecological risk which indicates severe impacts on ecosystems in study area. According to RI values, 28% of the samples are classified as having low ecological risk; 24% as having medium ecological risk; 20% as having high ecological risk and 28% as having very high ecological risk. It is seen that raising RI values is mainly due to the contributions from Hg and Cd (Figure 6(a)). The radar graph representation for RI (Figure 6(b)) shows that the supreme ecological risk was registered for the northwestern part of the study area due to the mining activities. The radar graph pointed out however that other samples displayed very high ecological risk (S14 and S15). High RI values of these samples are most probably due to traffic sources which increased, along with mining activities, the concentration of As and Pb. The spatial distribution of RI denoted that local ecological system could deteriorate to some extent by mining activities followed by traffic emissions [78]. Therefore, more attention should be given to the study area from the As, Cu, Pb and Zn particularly Cd and Hg which are the most harmful elements.

Figure 6. a. The contribution of different elements to total ecological risk index (RI); b. Radar graph of RI of the respective PTEs of the street dust in the Mahd Ad Dhahab area.

Table 5. Calculated ecological risk index for potentially toxic elements in street dust samples.

	Ecological risk indices for single PTE (Er^i)
	As	Cd	Co	Cr	Cu	Mn	Hg	Pb	Zn	RI
Min	17.6	30.0	2.9	0.5	9.2	0.7	40.0	3.7	1.3	101
Max	70.6	10,290.0	4.8	1.3	264.0	1.3	3000.0	258.8	112.6	13,996
Average	38.8	1148.9	3.6	0.8	44.6	0.9	438.5	35.9	13.4	1719

5.3. Health risk assessment

The results of the health risk assessment of PTEs from the three exposure pathways for adults and children are provided in Tables 6 and in the street dust of the study area are listed in Table 6 for adults and Table 7 for children, respectively. Due to CSF values are not available for the other studied PTEs, only the carcinogenic risk of As, Cd, Co, Cr and Pb was estimated. Hazard Quotient values for single elements for both children and adults were in order of ingestion > dermal > inhalation for As, Pb and Zn whereas in order of dermal ≥ ingestion > inhalation for Cd, Co, Cr, Cu and Hg. HQ values indicated that the risk through inhalation was negligible compared to ingestion and dermal exposures for all elements. Regarding hazard index for non-carcinogenic risk, the contribution of HQ_derm to HI was the highest (59.3–50.9%) then HQ_ing (41.2–48.9%) for adults and children respectively while the HQ_inh was the lowest. This means that dermal contact and ingestion are the primary pathway of PTEs harming human health in the study area while inhalation is negligible. Many studies have also found ingestion [56,64,71,77–80] and/or dermal contact [56,81,82] as the main daily dose exposure pathways posing health risks related to contaminated street dust. The average HI values of children (7.9E-01) were in order of significance higher compared to adults (1.0E-01) as illustrated in Figure 7(a). Regardless of the adults or children, the average HI values of the studied elements were found in the order of Co > Cu > Cd > Pb ≥ Cr > As > Hg > Zn. Based on HI values (Tables 6 and 7), all examined elements, except Cd, Cu and Pb for children, did not have a non-carcinogenic risk via three exposure pathways. It was observed that for children, HI >1 for Cd (3.88) and Cu (2.94) via dermal contact and Pb (3.11) via ingestion close to mining area which indicates potential non-carcinogenic toxic effects of these metals. This means that adverse impacts are likely to threaten the health and safety of children since they may liaison much more the street dust during their outdoor play activities via skin contact or ingestion by hand-to-mouth actions [83,84]. Several studies have linked the accumulation of these metals to several health diseases and abnormalities. Cu toxicity in humans is relatively rare but long-term exposure to it can lead to serious diseases, such as irritation, headaches, nausea and stomachaches [85,86], and at high intake, it can lead to Wilson’s disease [86]. Although acute Cd toxicity is rare at present, at high concentrations, it can cause serious problems to human health, such as Itai-Itai disease, Pulmonary disorders, osteomalacia and osteoporosis, Kidney damage, Liver Disturbances, anaemia. Long-term intake of high doses of cadmium sometimes culminating in death [87,88]. The toxicity symptoms of Pb poisoning include brain damage [89], general fatigue, tremor, kidney malfunction and increasing blood pressure [90,91]. Thence, it is necessary to conduct children to develop good hygiene behaviours [77]. Regarding the carcinogenic risk, CR values ranged between 2.0E-11-3.19E-05 (average, 5.08E-06) for adults and 5.1E-11-2.77E-04 (average, 6.2E-05) for children which indicated that, like noncarcinogenic risk, children are more susceptible to the carcinogenic risk than the adults (Figure 7(b). PTEs based on CR values, are arranged in order As > Cd > Pb > Cr > Co. Regarding the long-life carcinogenic risk (LCR), except As for children, all the carcinogens pose very low restricted carcinogenic risk (LCR = 6.25E-11-1.86E-05) while As (LCR = 1.52E-04) was categorized as low carcinogenic risk [59,64,65,76]. Like other studies around mining activities [90–93], the CR for As, is relatively higher than the other elements. Arsenic is a common toxic element which Induced carcinogenesis and Immune dysregulation [94]. Numerous studies indicated that exposure to arsenic was associated with dermatitis, keratosis and hepatopathy [91]. Long-term intake contributes to several malignancies, in the integumentary, respiratory, hepatobiliary and urinary systems [88,92]. So, based on the health risk assessment, ingestion followed by dermal contact is the most important exposure pathway for the PTEs, in the study area. The children are more vulnerable to risks associated with the PTES-contaminated street dust than the adults. This may be due to differences in body weights [95]. Moreover, exposure to street dust may cause cancer risk for a lifetime, so monitoring this element in street dust is essential.

Figure 7. a. non-carcinogenic and b. carcinogenic cumulative potential risk of PTEs for adults and children in street dust in the Mahd Ad Dhahab area.

Table 6. Calculated health risk assessment of non-carcinogenic (HI) and carcinogenic (LCR) PTEs for adults in the in street dust samples.

		Ding	Dinh	Dderm	HQing	HQinh	HQderm	HI	CRing	CRinh	CRderm	Cancer Risk LCR
	Min	4.1E-06	6.0E-10	4.9E-07	1.4E-02	2.0E-06	4.1E-03	1.8E-02	6.2E-06	9.1E-09	1.8E-06	8.0E-06
As	Max	1.6E-05	2.4E-09	2.0E-06	5.5E-02	8.1E-06	1.6E-02	7.1E-02	2.5E-05	3.7E-08	7.2E-06	3.2E-05
	Average	9.0E-06	1.3E-09	1.1E-06	3.0E-02	4.4E-06	9.0E-03	3.9E-02	1.4E-05	2.0E-08	4.0E-06	1.8E-05
	Min	1.4E-07	6.0E-10	7.7E-08	1.4E-04	2.0E-08	1.5E-03	1.7E-03	5.2E-08	1.3E-10	–	5.2E-08
Cd	Max	4.7E-05	2.4E-09	2.6E-05	4.7E-02	6.9E-06	5.2E-01	5.7E-01	1.8E-05	4.4E-08	–	1.8E-05
	Average	5.2E-06	1.3E-09	2.9E-06	5.2E-03	7.7E-07	5.9E-02	6.4E-02	2.0E-06	4.9E-09	–	2.0E-06
	Min	1.9E-05	2.8E-09	2.3E-06	6.4E-02	1.7E-03	7.7E-02	1.4E-01	–	2.9E-11	–	2.9E-11
Co	Max	3.2E-05	4.6E-09	3.8E-06	1.1E-01	2.7E-03	1.3E-01	2.3E-01	–	4.7E-11	–	4.7E-11
	Average	2.3E-05	3.5E-09	2.8E-06	7.8E-02	2.0E-03	9.4E-02	1.7E-01	–	3.5E-11	–	3.5E-11
	Min	4.2E-05	6.2E-09	5.1E-06	8.5E-03	2.2E-04	2.0E-02	2.9E-02	–	1.5E-10	–	1.5E-10
Cr	Max	1.1E-04	1.7E-08	1.4E-05	2.3E-02	5.8E-04	5.4E-02	7.8E-02	–	4.0E-10	–	4.0E-10
	Average	6.9E-05	1.0E-08	8.3E-06	1.4E-02	3.5E-04	3.3E-02	4.7E-02	–	2.4E-10	–	2.4E-10
	Min	6.3E-05	9.3E-09	2.5E-05	1.7E-03	2.3E-07	1.3E-02	1.5E-02	–	–	–	–
Cu	Max	1.8E-03	2.7E-07	7.2E-04	4.9E-02	6.6E-06	3.8E-01	4.3E-01	–	–	–	–
	Average	3.1E-04	4.5E-08	1.2E-04	8.3E-03	1.1E-06	6.4E-02	7.2E-02	–	–	–	–
	Min	5.5E-07	8.1E-12	1.1E-08	1.8E-03	9.4E-08	5.2E-04	2.3E-03	–	–	–	–
Hg	Max	4.1E-06	6.0E-10	8.2E-07	1.4E-02	7.0E-06	3.9E-02	5.3E-02	–	–	–	–
	Average	9.8E-07	8.8E-11	1.2E-07	3.3E-03	1.0E-06	5.7E-03	9.0E-03	–	–	–	–
	Min	1.5E-05	2.2E-09	3.6E-07	4.3E-03	6.3E-07	6.8E-04	5.0E-03	1.3E-07	9.3E-11	–	1.3E-07
Pb	Max	1.0E-03	1.5E-07	2.5E-05	3.0E-01	4.4E-05	4.7E-02	3.5E-01	8.9E-06	6.5E-09	–	8.9E-06
	Average	1.5E-04	2.1E-08	3.5E-06	4.2E-02	6.1E-06	6.6E-03	4.8E-02	1.2E-06	9.0E-10	–	1.2E-06
	Min	1.1E-04	1.7E-08	9.1E-06	3.8E-04	5.6E-08	1.5E-04	5.3E-04	–	–	–	–
Zn	Max	1.0E-02	1.5E-06	8.0E-04	3.3E-02	4.9E-06	1.3E-02	4.7E-02	–	–	–	–
	Average	1.2E-03	1.8E-07	9.5E-05	4.0E-03	5.9E-07	1.6E-03	5.6E-03	–	–	–	–
	Average							1.0E-01	–	–	–	5.8E-06

Table 7. Calculated health risk assessment of non-carcinogenic (HI) and carcinogenic (LCR) PTEs for children in the in street dust samples.

		Ding	Dinh	Dderm	HQing	HQinh	HQderm	HI	CRing	CRinh	CRderm	Cancer Risk LCR
	Min	3.8E-05	1.1E-09	3.2E-06	1.3E-01	3.6E-06	2.7E-02	1.5E-01	5.8E-05	1.6E-08	1.2E-05	6.9E-05
As	Max	1.5E-04	4.3E-09	1.3E-05	5.1E-01	1.4E-05	1.1E-01	6.2E-01	2.3E-04	6.5E-08	4.7E-05	2.8E-04
	Average	8.4E-05	2.4E-09	7.1E-06	2.8E-01	7.9E-06	5.9E-02	3.4E-01	1.3E-04	3.6E-08	2.6E-05	1.5E-04
	Min	1.3E-06	3.6E-11	5.0E-07	1.3E-03	3.6E-08	1.0E-02	1.1E-02	4.9E-07	2.3E-10	–	4.9E-07
Cd	Max	4.4E-04	1.2E-08	1.7E-04	4.4E-01	1.2E-05	3.4E + 00	3.9E + 00	1.7E-04	7.7E-08	–	1.7E-04
	Average	4.9E-05	1.4E-09	1.9E-05	4.9E-02	1.4E-06	3.8E-01	4.3E-01	1.9E-05	8.6E-09	–	1.9E-05
	Min	1.8E-04	5.0E-09	1.5E-05	6.0E-01	2.9E-03	5.0E-01	1.1E + 00	–	5.1E-11	–	5.1E-11
Co	Max	2.9E-04	8.2E-09	2.5E-05	9.8E-01	4.8E-03	8.2E-01	1.8E + 00	–	8.4E-11	–	8.4E-11
	Average	2.2E-04	6.1E-09	1.8E-05	7.3E-01	3.6E-03	6.1E-01	1.3E + 00	–	6.3E-11	–	6.3E-11
	Min	4.0E-04	1.1E-08	3.3E-05	7.9E-02	3.8E-04	1.3E-01	2.1E-01	–	2.6E-10	–	2.6E-10
Cr	Max	1.1E-03	3.0E-08	8.9E-05	2.1E-01	1.0E-03	3.6E-01	5.7E-01	–	7.1E-10	–	7.1E-10
	Average	6.4E-04	1.8E-08	5.4E-05	1.3E-01	6.2E-04	2.2E-01	3.5E-01	–	4.3E-10	–	4.3E-10
	Min	5.9E-04	1.6E-08	1.6E-04	1.6E-02	4.1E-07	8.7E-02	1.0E-01	–	–	–	–
Cu	Max	1.7E-02	4.7E-07	4.7E-03	4.6E-01	1.2E-05	2.5E + 00	2.9E + 00	–	–	–	–
	Average	2.8E-03	8.0E-08	8.0E-04	7.7E-02	2.0E-06	4.2E-01	5.0E-01	–	–	–	–
	Min	5.1E-07	1.4E-11	7.2E-08	1.7E-03	1.7E-07	3.4E-03	5.1E-03	–	–	–	–
Hg	Max	3.8E-05	1.1E-09	5.4E-06	1.3E-01	1.2E-05	2.6E-01	3.8E-01	–	–	–	–
	Average	5.6E-06	1.6E-10	7.8E-07	1.9E-02	1.8E-06	3.7E-02	5.6E-02	–	–	–	–
	Min	1.4E-04	3.9E-09	2.4E-06	4.0E-02	1.1E-06	4.5E-03	4.5E-02	1.2E-06	1.7E-10	–	1.2E-06
Pb	Max	9.8E-03	2.7E-07	1.6E-04	2.8E + 00	7.8E-05	3.1E-01	3.1E + 00	8.3E-05	1.1E-08	–	8.3E-05
	Average	1.4E-03	3.8E-08	2.3E-05	3.9E-01	1.1E-05	4.3E-02	4.3E-01	1.2E-05	1.6E-09	–	1.2E-05
	Min	1.1E-03	3.0E-08	5.9E-05	3.5E-03	9.9E-08	9.9E-04	4.5E-03	–	–	–	–
Zn	Max	9.4E-02	2.6E-06	5.2E-03	3.1E-01	8.7E-06	8.7E-02	4.0E-01	–	–	–	–
	Average	1.1E-02	3.1E-07	6.2E-04	3.7E-02	1.0E-06	1.0E-02	4.8E-02	–	–	–	–
Average								7.9E-01				5.2E-05

6. Conclusions

In the present study, the street dust from Mahd Ad Dhahab city, KSA were investigated deeply with the aim of assessing their PTEs contents, possible pollution sources, potential ecological and human health risks. The results exhibited that, the average concentrations of PTEs were found in the order of Fe > Al > Mg (17,359 ppm)> Zn (871 ppm)> Mn (651 ppm)> Cu (222.8 ppm)> Pb (106.4 ppm)> Cr (50.4 ppm)> Co (17.14 ppm)> As (6.59 ppm)> Cd (3.8 ppm)> Hg (0.4 ppm). Compared to other cities there was no specific trend, and almost the concentrations of PTEs in studied street dust were not quite high. The concentration of As, Cd, Cu, Hg, Pb and Zn in the studied street dust was higher than in the background. Spatially, the highest concentration of these elements along with highest magnetic susceptibility values were recorded in the vicinity of the mining area indicating that these elements have accumulated with various degrees most probably due to gold mining activities. Statistically, As, Cd, Cu, Hg, Pb and Zn were found to have high CV values and significantly positively correlated with each other and with magnetic susceptibility values. Furthermore, the results of HCA revealed that Al, Co, Cr, Fe, Mg and Mn originated from geogenic sources. On the other hand, As, As, Cd, Cu, Hg, Pb and Zn were of anthropogenic sources mainly due to gold mining activity. Traffic emissions may be representing another anthropogenic source. It is important to mention that the magnetic susceptibility method provides a helpful qualitative and semi-quantitative approach for delineating the spatial distribution of pollutants in mining sites. The elements which accumulated due to anthropogenic sources were enriched in the street dust with different degrees ranging from moderate to extremely high enrichment and pose very high ecological risk (RI = 1729). Ingestion is a primary pathway of PTEs harming human health in the study area followed by dermal contact and children are more exposed than the adult population. Except Cd, Cu and Pb for children in one sample close to mining area, all examined elements, did not have a non-carcinogenic risk via exposure pathways. Regarding the long-life carcinogenic risk (LCR), except As for children, all the carcinogens pose very low risk (LCR = 6.25E-11-1.86E-05) while As was categorized as low risk.

Acknowledgments

The author is grateful to anonymous reviewers whose critical and constructive comments helped to improve the quality of the manuscript. Special thanks to Prof. Dr Abdullah O. Bamousa for his help with this paper, assisting with sampling, data collection and revision. The author also acknowledges Dr Emad Nagm and Dr Ibrahim Babikir for their contributions to the sampling process, and Dr Said Shetaia for his mapping support.

Disclosure statement

No potential conflict of interest was reported by the author.