Carcinogenic risk assessment, health endpoint and source identification of heavy metals in Mahshahr, Iran

Abstract Heavy metals (HM) are one of the main agents that threaten human health. The most important source that release heavy metals (As, Cd, Cu, Cr, Pb, Ni, V, and Zn) in the environment is Petrochemical industries. This study was investigated the carcinogenic risk assessment, health endpoint and source identification of heavy metals. Samples were taken from the different regions: parks, residential, industrial, administrative and central and high traffic areas in Mahshahr, southwest, Iran and investigate the environmental impacts of the studied petrochemical industry. Measurement of heavy metals performed by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Results of the PCA showed that As had a different source than other heavy metals, which can be owing to numerous human and industrial activities in the region. Based on the results, the Igeo factor was assessed at high levels for Ni, Pb, and Cd metals. Moreover, the results demonstrated high intensities of pollution for Pb, Zn, Cd, and Ni metals under the application of PI and IPI indices, and for Ni, Pb, and Cd metals under the application of the EF index. The index of the risk index (RI) average of the studied metals represented a significant risk for Ni, Cr and As metals on children and adults. In addition, Pb isotopic results showed high levels of 208Pb/206Pb to 207Pb/206Pb, which indicated that the increase of the Pb isotope in the studied region was concerned with petrochemical activities of human origin. In this study, it was found that the soils of the Mahshahr port industrial zone have high anomalies of some metals, especially Ni, Pb, Zn and Cd, which were due to the growth of traffic and economic and industrial activities in the region. However, heavy metals such as As and V exhibited the lowest risk and very low pollution in all indicators.


Introduction
Heavy metals (HM) (Arsenic (As), Cadmium (Cd), Lead (Pb), Copper (Cu), Chromium (Cr), Nickel (Ni), Zinc (Zn), and Mercury (Hg)) have been considered as one of the main factors influencing the increase of carcinogenic risk and other health endpoint in those who are exposed to these pollutants , Saba et al. 2020, Sabir et al. 2019, Irshad et al. 2021.
Pollutions and adverse effects of heavy metals on, plant animals, humans, the marine life and ecosystem have attracted the attention of numerous social and regulatory organizations (Irshad et al. 2021, Sabir et al. 2019. The risk of soil pollution is not lower than the risk of air pollution, but it is less considered by irresponsible people because of not tangible and not understood this threaten (Goudarzi et al. 2018, Idani et al. 2020, Jia et al. 2020. Soil is the third most important component of the environment (Kabata-Pendias 2004). Soil can transfer many contaminants such as heavy metals on environmental, animals and humans (Miri et al. 2016).
Nowadays, isotopic materials have been studied as key detectors in identifying and determining the source and origin of heavy metal pollutions (Liu et al. 2014). Each of these stable and radioactive isotopes can help to identify the source of heavy metals. For example, it was reported that the combinations of oxygen isotopes, as a stable isotope, have been used in many fields (Nazarpour et al. 2019).
There are four isotopes for lead, including 204 Pb, 206 Pb, Various anthropogenic sources and ore minerals have different isotopic characteristics (Cheng and Hu 2010). Pb isotopic ratios do not change through industrial, environmental and anthropogenic processes, and their ore ratios are constant. Therefore, isotopic ratios are more useful for the source identification and pathway of Pb in pollution studies (Han et al. 2016). Soil pollution source identification by using Pb isotopes is based on theory that natural materials (unpolluted soils and rocks) and pollutions containing Pb from anthropogenic sources have different isotopic characteristics (Nazarpour et al. 2019). Disturb body's metabolic functions, toxicity potential, non-biodegradable, disturbing functioning organs (kidney, lungs, brain, liver, and heart), block activities the biological systems, mutagenicity, prolix biological half-lives, endocrine-disrupting chemicals (EDCs), metabolic disorder (Diabetes mellitus), Alzheimer's, Parkinson's, Multiple Sclerosis (MS), disruption of balance and levels of hormones and enzymes in the body (glucocorticoids, insulin), oxidative stress, DNA damage, increase risk of cardiovascular diseases and cancer are the main outstanding properties of heavy metals , Irshad et al. 2021, Rehman et al. 2018, Saba et al. 2020, Sabir et al. 2019.
Water, some vapor form in the air, food, dermal contact and soil the main sources that in this way, heavy metals can enter the food cycle or enter the human body directly and due to high density and can cause toxicity are very dangerous for humans (Gholizadeh et al. 2019, Irshad et al. 2021, Rehman et al. 2018. Naturally in earth's crust, geochemical cycles, farming, fertilizers, wastewater, sewage agricultural, pesticides, herbicides, construction, dust storm, traffic exhausts, petroleum refining, gasoline, paints, chemical manufacturing, pesticides and industrial processes are the most human activities release produces heavy metals (Al-Saleh and Abduljabbar 2017, Miri et al. 2016, Momtazan et al. 2019, Rehman et al. 2018, Zhang and Wang 2020.
In the recent century, the main reasons for increasing soil pollution by heavy metals were excessive use of agricultural fertilizers (herbicides, pesticides), industrialization process, and population growth (CHEN et al. 2018, Hu et al. 2013.
Protection of soil resources and soil stability is very vital because from the soil heavy metals can enter into human food chains, accumulation in plants and vegetables may cause metabolic, cardiovascular diseases, skin allergy, thrombosis symptoms, geno-toxicity, neurological disease and enhancement of carcinogenic risk (Kabata-Pendias and Mukherjee 2007, Kumar et al. 2016, Zwolak et al. 2019. The most important factors that highlight the risk of heavy metals including persisted for years after the occurrence and posed environmental hazards (Baldacchini et al. 2017).
Bandar Imam region has acted as a strategic and influential base in Iran's oil and gas regions. The development of industrial activities in southwest of Iran has always caused numerous problems in terms of pollution of surface soils in the industrial. Therefore, quality management of surface soils is an essential task for good planning and precision managing resources.
The main objective in this study was spatial distribution and health risk assessment of heavy metals on human health in surface soils of different areas of Mahshahr city. Also we in this study tried investigation of the concentration of heavy elements (Cu, Zn, Ni, Pb, Cd, V, As, Cr), the distribution of heavy metals, originating and determining the natural and manmade contribution of heavy elements, assessing health risk assessment, drawing distribution maps of heavy element concentrations in GIS environment and determining the severity of pollution in the surface soil of different areas of Mahshahr (petrochemical industries, port administration and export port) with industrial use.
The aimed this study was evaluation of effects of health risk assessment of heavy metals and source identification in Mahshahr port industrial zone, southwest, Iran. In this study, the study of pollution and heavy metals in drinking water in the Mahshahr region was not considered and the focus of this study was on soil pollution in the study area.

Study area
Mahshahr port industrial zone is one of the cities Khuzestan Province, Iran. Geographically, the Bandar Imam region has access to international open waters through the Musa Estuary and access to Turkey, Europe and Central Asia via national railways. Access to oil and gas resources will further facilitate the raw materials and feed of industrial units. At present, there is approximately 40% of the production capacity of Iranian petrochemical products in this region. Owing to the presence of several petrochemical complexes and metal and oil industries, Mahshahr port industrial zone enjoys a strategic position and is an important industrial hub in Iran. Mahshahr is located in latitude 49.4 0 33.837 00 and latitude 30.27 51.963 00 and with a population of 155 000 inhabitants and an area of 27.13 km 2 in the southwest of Iran. It borders Iraq on the west and the Persian Gulf on the south (Dastoorpoor et al. 2019Khaefi et al. 2016).
Mahshahr has an arid climate with an annual average temperature of 25.5 C and annual average precipitation of 213.4 mm (Rastegari Mehr et al. 2016). The dominant wind direction is northwest to southeast with a maximum speed of 19 m/s. The study area is covered with quaternary sediments and alluvial deposits, and pedagogical maps indicate that entisols and aridisols (according to the USDA soil taxonomy these soils defined: soils that do not show any profile development other than an horizon, a very low concentration of organic matter and reflecting the paucity of vegetative production on these dry soils) are the only soil types in this region (Moore and Keshavarzi 2014).

Sampling
To study the concentration of heavy metals in the surface soils of the Mahshahr port industrial zone, sampling and measurement were performed using standard methods provided in the published literature (Keshavarzi et al. 2019, Nazarpour et al. 2019. In this study, sampling sites were selected based on geographical coverage to provide an accurate understanding of soil quality. Sampling locations were selected according to pollutant inputs (hot spots) and based on proximity to sources of pollutants and proximity to industries. According to the area of study range, 47 surface soil samples were collected in the Mahshahr port industrial zone, which covers the whole of the urban area ( Figure 1). Samples were taken from the different regions: parks, residential areas, industrial areas, administrative areas and central areas and high traffic areas in Mahshahr.
In the present study, we choose 47 stations in four geographical directions in Mahshahr port industrial zone, southwest, Iran. Sampling method were randomly and considering three replications, a total of 47 soil samples and the same number of soil samples were taken from the Mahshahr Petrochemical Special Economic Zone, Imam Khomeini Port, Imam Khomeini port authority, and export port.

Samples analysis
The soil samples were collected using a shovel from depths of 0-10 cm and were sieved by a 2-mm mesh. Then, the samples were placed into polyethylene sacs and transferred to a location away from direct sunlight for future tests. After this process, the plant residues and excesses were first separated from soil samples, and the samples were placed in an oven for 48 h at 60 C. Then, the soil samples were sifted, and particles smaller than 63 mm in diameter were separated and immediately transported to a laboratory to measure their physical and chemical properties. Soil texture (percentage of sand, silt, and clay) and pH value were determined by the hydrometric method and pH meter, respectively. The collected soil samples were dried in air and winnow through a 2-mm rustproof steel mesh to dispose of plant roots and rocks. Following digestion of soil samples with hydrochloric acid (HCl) and nitric acid (HNO 3 ) in a ratio of 3:1 (HNO 3 : HCl), the total concentrations of As, Cd, Cu, Cr, Pb, Ni, V, and Zn were defined using inductively coupled plasma (ICP) optical emission spectroscopy (ICP-OES). Dehumidified samples were ground in a rustproof steel grinder (<0.25 mm), and the total intents of the above-mentioned heavy metals were determined using ICP-OES (Nickson et al. 2000). For certifying the measurement precision and accuracy in the entire analytical procedure, samples were analyzed in triplicate. Synchronously, blank reagent determinations were used to rectify the appliance readings. In addition, 12 sediment samples from the soils of the studied region were separately sent to the laboratory to measure Pb isotopic ratios. Collected samples were analyzed in the advanced analysis laboratory of the university of Ahvaz.

Geo-accumulation index (I geo )
Comparison of concentrations of each heavy metal per sample to its geochemical background concentration in the soil using the Geo-accumulation index is a common method to assess soil pollution by heavy metals. Therefore, the Geo-accumulation index can be used to determine the severity of the pollution (Muller 1969). This index is estimated by Equation (1): , where I geo is the Geo-accumulation index or pollution intensity index, log 2 is the logarithm base 2, C n is the concentration of metals in the analyzed sediments/soils, and B n represents the background geochemical concentration of the metal in the Earth's crust. Furthermore, 1.5 coefficient is the studied background matrix correction factor determining the impacts of natural fluctuations and influences of anthropic sources. Based on this index and according to Table S1, soils are divided into seven degrees of pollution (Muller 1969).

Pollination index (PI)
To calculate the quality of the experimental soil, the pollution index (PI) was calculated for each metal. The pollution index is the ratio of the concentration of heavy metals in the soil to the amount of the background concentration of the same metal (Lu et al. 2009). This index is obtained with the following equation (Lu et al. 2009).
where C n refers to the measured concentration in the sample, and B n is the measured concentration of the heavy metal in the Earth's crust (Lu et al. 2009). The pollution index is classified as follows: PI 1 is equivalent to low pollution level 1 < PI 3 is equivalent to moderate pollution level PI > 3 is equivalent to high pollution level 2.7. Enrichment factor (EF) One of the common ways to assess the impacts of anthropogenic activities on the soil is the enrichment coefficient calculation for higher background concentrations of metals. To estimate the EF index, it must normalize the amount of the measured metal according to the sample reference metal, such as Fe, Al and Sr, which are stable in the Earth's crust (Adamo et al. 2005). The enrichment factor index of a particular element in a given sample is ratio of the concentration of each element in a sample to the background concentration of the same element in its community (Deely andFergusson 1994, Zonta et al. 2007). This index is obtained by Equation (3): where Cx is the concentration of the element considered in the sample, Cref is the concentration of the reference element in the sample, Cx is the concentration of the element in the background, and Cref is the concentration of the reference element in the background. The reference element is an element with a geological origin (Sutherland et al. 2000). The reference element in determining the enrichment factor is defined as an element with a geological origin that is particularly stable in the soil, such as Al, Sr, Zr, Ti, and Fe. According to Table S2, the element Fe was used as the reference element in this study (Adamo et al. 2005). Table S2 also presents the enrichment factor classification in samples.

Integrated pollution index (IPI)
Integrated pollution index is the average value of the pollution index (PI) of each element, which has been classified as low, medium, and high level of pollution as follows (Liu et al. 2016): IPI 1 ¼ Low levels of pollution 1 < IPI 2 ¼ Medium levels of pollution IP > 2 ¼ High levels of pollution; this index is shown for the studied metals.

Human health risk assessment
Assessment of health risks of heavy metals is a multistep process, which was evaluated in two parts with carcinogenic and non-carcinogenic risks and presented based on the health risk assessment method of the United State Environmental Protection Agency (USEPA) (Qing et al. 2015). In investigating of carcinogenic and non-carcinogenic risks, human exposure to heavy metals through all three main routes of ingestion, inhalation, and dermal absorption was considered. Then, the daily adsorption values of metals (ADD) in each route were calculated by the following equations (Ravankhah et al. 2016): where ADD ingestion, ADD inhalation, and ADD dermal, are the mean values of daily absorbtion of metals (mg kg À1 day À1 ) through ingestion, inhalation, and dermal, respectively. C soil , IngR and InhR, are the concentrations of metals in the soil (mg kh À1 ), ingestion rate (mg.day À1 ) and soil inhalation rate (m 3 day À1 ), respectively. EF is the frequency of exposure to metals (day per year); BW is the body weight of a person exposed to metals (kg); AT is the average duration of exposure to each value of metals (day); PEF is the pollutant emissions factors from the soil to air (m 3 kg À1 ); SA is the area of dermal that has been exposed to metals (cm À2 ); AF is the soil to dermal adherence factor (mg cm À1 day À1 ). and ABS is the dermal absorption factor (no unit) (Davtalabnezam et al. 2017). Definition and standard unit for calculating chronic daily intake during normal and abnormal operation simulation as shown in Table S3 (Abd Wahil et al. 2020).
After calculating the average daily dose value of the metals (ADD) via the three routes of ingestion, inhalation, and dermal contact, hazard quotient (HQ) was calculated based on the reference daily intake (R f D j ) by Equation (7): where HQ i is the hazard quotient in each intake path, ADDi is the average daily dose value of metal intake by each of the three mentioned routes (mg/kg/day) and R f D j is the reference daily intake estimating the maximum risk in the human population daily exposed to heavy metals in sensitive groups (i.e., adults and children) . The values of R f D j were obtained from the United State Department of Energy's Risk Assessment Information System (USRAIS) (Ravankhah et al. 2016). If the average acceptable daily intake (ADD i ) is less than the reference daily intake, there will be no adverse effects on human health; otherwise, if ADDi is higher than R f D j , it is likely to have an adverse effect on human health. When the HQ value is 1, there will not be an adverse effect, but when HQ > 1, it is expected to have an adverse effect on human health (Man et al. 2010). By summarizing the hazard quotient in each intake path (HQ i ), the HI was estimated as the aggregate risk of all contaminated metals according to Equation (8): All of the studied metals had hazard index (HI) effects, while some studies reported that Cd, Pb, and Ni metals had both risk index (RI) and hazard index (HI) effects on human health (Yousefi et al. 2021). Thus, assessment of the carcinogenic and risk index (RI) risks for all routes was performed for Cd, Pb, and Ni, and their values were obtained by Equation (9) (Ravankhah et al. 2016).

Statistical analysis
Data analysis was used to descriptive statistics for the soil pollution indexes. The level of the heavy metal were analyzed using SPSS version 18. Principal component analysis (PCA) with Varimax rotation was employed to see the difference of target compounds in three different sampling sites.

Ethical considerations
Excel and SPSS were used for analysis, sampling and data collection were done by researcher. The Ethics Committee of Islamic Azad University of Ahvaz Branch approved the study protocol. All experiments of this research were done in the Islamic Azad University of Ahvaz Branch laboratory.

Heavy metals concentration
In this study, the percentages of CV for the soil samples of heavy metals of the Mahshahr port industrial zone were for metals of Pb (99.14%) < Zn (87.67%) < Cu (82.1%) < Cd (41.34%) < V (37.73%) < Cr (28.97%) < Ni (15.7%) < As (8.64%), respectively ( Table 1). The highest variations of heavy metals concentrations were observed for Pb, Cu, and Zn, moderate variations were obtained for Cd, V and Cr, and low variations were recorded for Ni and As. The coefficient of variation (CV) indicates the relative amount of changes in the concentrations of the heavy metals in industrial soil samples per area. In this regard, CV 20%, 21% < CV 50%, 51% < CV 100% and 100% < CV were considered low, moderate, high, and very high variations, respectively (Keshavarzi et al. 2019).
The CV (change coefficient) values for heavy metals in the study area showed that Pb, Zn, and Cu had the highest concentrations. Table 1 summarizes the descriptive statistics of the concentration of heavy metals in the soil of the present study. The highest and lowest concentrations of metals in the soil of the study area were obtained for Ni (70.89 mg kg À1 ) and As (0.37 mg kg À1 ), respectively (Table 1).
In this study, the global reference standards (Alloway 2012), Iran (IDOE 2014), China (NEPAC 1995), Canada (CCME 2007) and the Netherlands (VROM 2000), were used to evaluate the concentrations of heavy metals in the study area ( Table 2). The results of soil evaluation of the Mahshahr Petrochemical Industrial Zone indicated that the average concentrations of Ni and Cd metals, completely and partially, were higher than other global soils'. In general, the previous results showed that the petrochemical industry in the Mahshahr Port Industrial Zone and its pollution played crucial roles in changing the soils of the studied area and its natural state.
In Shenyang, Northeast China Li et al. in 2013 studied heavy metal contamination of urban soil in an old industrial city (Li et al. 2013). Their evaluated showed that the concentrations of Pb, Cd, and Cu were beyond the study area (Li et al. 2013). High  concentrations of these metals, along with Zn and mercury were same to in this study that the main reason this similarity was human activities. In another study, Rastmanesh et al. evaluated the level of heavy metal on Khuzestan steel industry in soil pollution around it (Rastmanesh et al. 2013). They demonstrated that steel industry activated increased the amount HM especially the concentration of As and Pb (Rastmanesh et al. 2013). Result our study resembling in compare to Rastmanesh study.
Skrbi c and -Duri si c-Mladenovi c in 2010 in northern Serbia and Bosnia and Herzegovina investigated levels of six heavy metals (Pb, Cu, Zn, Cd, Ni, Cr) in soil pollution ( Skrbi c and -Duri si c-Mladenovi c 2010). they the overall set with all data gathered in this study containing 264 samples ( Skrbi c and -Duri si c-Mladenovi c 2010). Their result showed that soil pollution with Ni and Cr caused by human and agricultural activities as well as a significant reduction and elements such as Pb, Ni, and Cr with an increasing distance from the edge of the road according to existing standards ( Skrbi c and -Duri si c-Mladenovi c 2010). This paper revealed it was seen that anthropogenic and background sources had different impact on the data variability in the case of polluted and unpolluted soils that is similar to result of our study.

Source identification
The pollution index (PI) was employed to investigate the metals of the present study. Table S4 presents the maximum, minimum, and average values of the metals.
The data sampling of As showed that there were 23 samples of low levels of pollution (49%) and 24 samples of moderate levels of pollution (51%) for this metal. Additionally, the results showed that 35 samples of Cd data had a high pollution level (74.4%), 11 samples had a moderate pollution level (23.4%), and one sample had a low pollution level (3%). Furthermore, the results revealed 20 samples with a low pollution level (42%), 13 samples with a moderate pollution level (27%), and 14 samples with a high pollution level (31%) for Cr, two samples with a high pollution level and 45 samples with a low pollution level for Cu. In other words, there were low pollution levels equal to 95.74% and high pollution levels equal to 4.26% for Cu. The sampling results also indicated that there were 38 samples with a high pollution level (80.8%) and 9 samples with a moderate pollution level (19.2%) for Ni, 33 samples with a high pollution level (70.2%), 13 samples with a moderate pollution level (27.6%), and one sample with a low pollution level (3%) for Pb. Data collections were equal to 23 samples with a low pollution level (49%) and 24 samples with a moderate pollution level (51%) for V. Finally, the results of Table S4 indicated that pollution degrees for Zn metal were 1 sample (2%) with a low pollution level, five samples (10.63%) with a moderate pollution level, and 41 samples (87.23%) with a high pollution level for Zn.
According to Muller's (1969) classification for the Ioe index, the heavy metals of Cd, Pb, and Ni were among the heaviest metals investigated in the present study and had the highest values of pollution in the Igeo index. It appears that a large amount of the Igeo index for Cd, Pb and Ni metals is the result of abundant petrochemical and industrial activities in the study area. Other heavy metals are free of contaminants.
The results showed that the EF index pollution levels of heavy metals in the study area were calculated based on the average of this parameter. The results indicated no enrichment levels for As, Cu, and V, low enrichment levels for Cr and Zn, and relatively rich enrichment levels for Cd, Ni and Pb. The amount of EF obtained for Cd, Ni and Pb indicates that the source of these heavy metals in the soils of the region was formed by humans.
The value IPI for heavy metals in the study area was obtained according to Table S5. The results of the IPI index show that human activities significantly affect the soil of the study area.
The IPI index is the average of PI. Therefore, it can be stated that the studied elements exhibited different pollution levels as follows ( Figure S1): As, Cu, and V with low levels of pollution; Cr with a medium level of pollution; and Pb, Zn, Cd, and Ni with high levels of pollution.
Accordingly, this study aimed to undertake (1) investigate the pollution level of the studied heavy metals, geo-accumulation index (Igeo), pollination index (PI), enrichment factor (EF), integrated pollution index (IPI) (2) apportion source of metal pollution (natural or anthropogenic), by using multivariate analysis, assess geochemical fractionation of the studied heavy metals, Pb isotopic ratio as well as an end-member model of isotopic ratios, (3) evaluate human health risk assessment of heavy metals according to the carcinogenicity risk index (RI) and the non-cancer index (HI). In another research, Garcia et al. (2018) investigated Sonora Rivers and Bacanuchi of Mexico to evaluate mobility, bioavailability, and pollution of heavy metal using the enrichment factor (EF), potential ecological risk (PER), and integrated pollution index (IPI). They showed that the EF and PER of Cu, Cr, Mn, Ni, Pb, and Zn in both rivers were high and showed extreme pollution of studied heavy metals, which were mainly derived from mining activities in the upper stream. In addition, the IPI values were classified as non-contaminated to moderate pollution levels.

Human health risks and carcinogenic risk
RI is the average of the risk index (cancer risk); ADD i is the daily value of metal uptake by routes exposed to heavy metals (mg kg À1 day À1 ), and SFi is the factor, the inhalation slope factor or the risk of cancer per unit exposed to metals (mg kg À1 day À1 ) (Esmaeili et al. 2014, Ravankhah et al. 2016. The World Health Organization (WHO) has determined daily intake levels of these HMs for people weighing 60 kg as Cd: 0.06; As: 0.26; Cu: 30; Zn: 45; Ni: 0.2; Cr: 0.2; Pb: 0.214 (mg kg À1 ) (Eskola et al. 2020;Kwon et al. 2017). The WHO then introduced these suggested PTDI values for the studied HMs: Cr: 0.0033; Pb: 0.0035; Cd: 0.001; As: 0.002; Cu: 0.5; Zn: 0.75; Ni: 0.005 (mg kg À1 day À1 ).
Some heavy metals, such as Cd and Pb, are unnecessary and carcinogenic elements. Some other metals, such as Ni, Zn and Cu, are essential for the growth of the body and are essential elements for low consumption, but have toxic effects at high concentrations (Table 3). Table 3 showed the daily absorption rates of metals (ADD) and their non-carcinogenic risk ratio in each of the routes (HQ) that are listed separately for children and adults.
The highest and lowest daily adsorption rates were for As with a value of 0.000000478 (mg kg À1 ) through the ingestion route for both age groups. The daily absorption rates of all studied heavy metals were higher through the ingestion route than through respiratory and dermal absorption routes in both groups. Daily absorption of metals in the ingestion and inhalation routes was higher for children than for adults, and it was higher in the dermal uptake route for adults than for children According to the US Environmental Protection Agency, if the daily adsorption rate of metals (ADD) is higher than the reference level of metal toxicity in each route (RfD), the non-carcinogenic risk of metals in each route is higher than the allowable level (noncarcinogenic risk > 1) will be. In addition, if the noncarcinogenic risk of heavy metals in each path is more than one, the toxicity of that metal may have adverse effects on human health. The examination of the hazard quotient (HQ) of heavy metals in each route showed that all studied metals in the present study had non-carcinogenic risks of less than HQ ¼ 1. HQ value was high for all elements in the ingestion route, so that there was a higher value for chromium (0.165) (mg kg À1 ) than for the rest of the elements.
In 2015 Rezaei et al in Birjand, Iran studied the variation and probabilistic risk assessment of exposure to HMs (Fallahzadeh et al. 2018). Based on the result, the Hazard Index (HI) values for the children and teens groups were 1.02 and 2.02, respectively, which was more than 1 (Fallahzadeh et al. 2018).
The hazard index (HI) values for all routes in the metals of Cd, Pb, Ni, Zn, Cu, As, V and Cr were recorded as 25, 68.8, 21.4, 0.127, 0.315, 0.751, 0.000319, and 84.1 (mg kg À1 ) for children and as 11.7, 10.9, 0.318, 0.195, 0.0463, 0.0000428, and 26.3 (mg kg À1 ) for adults in the studied area, respectively. Figure 2(a) shows the results of the hazard quotient (HQ) of all three routes for all metals, separately for children and adults. The collective hazard index of all metals was obtained as 0.0196 for children and equal to 0.00319 for adults in the surface soils of the studied zone (Table 3).

Principal component analysis (PCA)
Principal component analysis of the main components is an effective method to determine the roles of humans on spatial scales (Keshavarzi et al. 2019). PCA has been used extensively in receptor modeling to classify the main potential source categories influencing a provided receptor site. In general, a high correlation coefficient between Zn and Cd indicates pollution emissions associated with imperfect combustion of hydrocarbon fuels, Steel factory, petroleum, human activities and incineration (Pongpiachan and Iijima 2016). In this study, Cu, Zn, Pb, Cd, Cr, Ni, As, and V were used in the analysis of the main components to investigate the differences and precision origins. The loading values of heavy metals indicated a relationship between metals and their components, so that loading values were equal to Pb < Zn < Cd < Cu < As < Ni < V <Cr for the first component, equal to Ni < Cr < As < Zn < Cu < Pb < V <Cd for the second component, and equal to V < Pb < Zn < Ni < Cd < As < Pb < Cr for the third component. These arrangements were particularly important, and they were reflected in the degree of influence and control of each component in each element. The analysis results of the main components of the studied heavy elements showed that the first component with the highest loading value played a crucial role in determining the source of pollution in the studied region.
According to the results, three main combinations were identified for all points by PCA. In the first component, it was observed that Pb, Zn, and Cd had the same dependencies and origins. Furthermore, the results revealed different sources and low dependency for Cd and V in the second component and for Ni and Cr in the third component. Similar studies have shown that metals with smaller distances have higher dependencies and the same origins (Abdi et al. 2014). The results of our study showed that As, compared to Pb, Zn, and Cd in three components, had greater distance, less dependence, and different geochemical properties and origin. The finding our study showed that the petrochemical activities of the Mahshahr port industrial zone is the most important agent related to entry HM in this regions. In general, the average concentrations of heavy metals in study area for Cu, Zn, Cr, Ni, As, and V metals were lower than the mean concentrations of the mentioned metals in the Earth's crust. Furthermore, these values for Pb and Cd metals in the studied area were higher than the concentrations of the mentioned metals in the Earth's crust. Higher concentrations of these metals can reflect the role of human or petrochemical activities in soil pollution in the study area. Based on correlation coefficients and the principal component analysis (PCA), there was a similar relationship between heavy metals such as Pb, Zn and Cd in terms of sources of pollution.

Isotope ratios
Lead ores (mostly Galen (PbSO 4 )) compared to other ores have higher Pb/Th and Pb/U ratios, and their isotopic ratios during age are constant (Hansmann and K€ oppel 2000). Therefore, lead ore minerals have a specific isotopic composition, which is independent of age with low 206 Pb/ 204 Pb values (Doe and Delevaux 1972). Comparison of the Pb isotope compositions of 28 surface soils of the Mahshahr Port Industrial Zone, including ten soil samples of the industrial zone, six soil samples of parks, six soil samples of roads, and six soil samples of residential areas, were designed in Table 4. Table 4 presents the isotopic ratios of Pb for 28 collected surface soil samples. The data indicate that samples from the industrial area have isotopic compositions distinct from other samples. Generally, polluted soil samples have lower 207 Pb/ 206 Pb and higher 208 Pb/ 206 Pb ratios, being consistent with the calculated ratios in the collected samples (Table 4). Isotopic ratios of Pb in unpolluted soils in the study area (such as S18A, S20A, and S22A) are more radiogenic, and their values are similar to the average of the continental crust (Galu skov a et al. 2014). Therefore, these samples can be considered natural soils with more geogenic isotopic ratios rather than anthropogenic sources.
Furthermore, the average compositions of Pb isotopes in Khuzestan regions (Nazarpour et al. 2019), Pb-Zn deposits of MVT type (Bao et al. 2017), Shimanto shale (Shinjo et al. 2000), Kosovo soils (Prathumratana et al. 2020), as well as Shanghai soils and Baoshan soils (Bi et al. 2018) were used to be compared to the present study data. The average concentration ratios of 208 Pb/ 206 Pb to 207 Pb/ 206 Pb in the surface soils of the Mahshahr Port Industrial Zone were recorded as 3.9 and 2.85, respectively, which were compared to the soils of other regions ( Figure  S2). In general, the high values of 208 Pb/ 206 Pb to 207 Pb/ 206 Pb in the surface soils of the Mahshahr Port Industrial Zone compared to other samples may show the anthropogenic origin of industrial activities (petrochemical) and their pollutants. In addition, the position of the isotope composition of soils in the Mahshahr Port Industrial Zone ( Figure S2) can indicate the combined (multiple) origins for Pb in the soils of the region. The Pb isotope ratios of 208 Pb/ 206 Pb to 207 Pb/ 206 Pb showed the highest values for Shimanto shales, Baoshan soils, Khuzestan soils, Kosovo soils, MVT deposits and Shanghai soils, respectively.
Mahshahr port industrial region in recent years suffered from environmental pollution produced by the petrochemical industries, wastewater, vehicles and agricultural fertilizer. Furthermore, it has been revealed that petrochemical industries and vehicle exhaust emissions are the main sources of heavy metals in the air and soil. In the same study conducted by Rastmanesh et al. (2016) in Abadan, Iran on the Impact of Abadan petrochemical complex and petroleum refinery on soil heavy metal (Rastmanesh et al. 2016). Also, their demonstrated that the main sources of heavy metals production were petrochemical, petroleum refinery and vehicle that was same to our finding in this study. The Mueller (Igeo) index showed high levels of pollution for Ni, Pb and Cd metals. The high values of 208Pb/206Pb and 207Pb/206Pb in different surface soils of the Mahshahr port industrial zone compared to isotope standards in other parts of the world (Shimanto shales, Baoshan soils, Khuzestan soils, Kosovo soils, MVT deposits and Shanghai soils) showed that the pollutants in the study area were of human origins, like industrial (petrochemical) activities for the Pb metal.
The areas investigated in this study were the special economic zone, export port, and Mahshahr port authority, which have been considered as important areas to assess the pollution levels in soils due to the high density of petrochemical industries. The activities of the petrochemical industry reduce the stability of the ecosystem and its biodiversity, thereby affecting the public environment of the regions. In 2011 in Nigeria, Kamalu et al. studied land resource inventory and ecological vulnerability (Kamalu and Wokocha 2011). Their result showed that the oil extraction process has resulted in the spilling of toxic drilling byproducts into local and international soil and water bodies surrounding the study area that matched in compare to our study.

Strength and limitations
The main strength of this research study was spatial distribution and health risk assessment of heavy metals on human health among people in the industrial area.
Sampling time limit was one of the limitations of the study. The most important limitation of this study was that the evaluation was performed only in the surface soil. For further studies, it is suggested that health risk assessment, spatial distribution the risk of carcinogenicity related to heavy metal be evaluated in drinking water, air and food among residents in different area (residential, industrial and high-traffic area).

Conclusion
The results of this study indicated that the mean of nickel metal was higher than the mean of other heavy metals in different soils. The aim of this study was to evaluate the human health effect and carcinogenic risk assessment of heavy metals (As, Cd, Cu, Cr, Pb, Ni, V, and Zn) caused by the activity of the petrochemical industry and source identification in the Mahshahr port industrial zone, southwest, Iran.
The surface soils of the Mahshahr Port Industrial Zone, in terms of Pb, Zn, Cd and Ni metals exhibit high pollution index (PI) values, and for other metals low to medium PI values. The IPI index for Pb, Zn, Cd and Ni metals indicated high levels of pollution. The amounts obtained from the enrichment factor index (EF) in the soil of the Mahshahr showed a low level pollution for As, Cu and V, a medium contamination level for Cr and Zn, and a high pollution level for Cd, Ni and Pb. The origin of the heavy metals studied in this study, especially for Pb, Zn and Cd metals, demonstrated that the accumulation of these metals in the soil of the region was through the activities of Bandar Imam Petrochemical Company. The highest (hazard quotient) HQ for both adults and children was allocated to Cr. Three metals of Cd < Pb < Cr and Pb < Cd < Cr had the highest average of the hazard index (HI) for children and adults, respectively. Three metals of As < Cr < Ni for children and adults, respectively, exhibited the highest average of the cancer risk index (RI). The results of the indicators used in this study revealed that the metals of Pb, Zn and Cd, and to some extent Ni, had high amounts of pollution caused by human activities, and other elements caused low amounts of pollution. Finally, it is suggested that in future studies, the amount of heavy metal pollution in drinking water supply areas of Mahshahr city be investigated.

Open access
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