Heavy Metal Pollution in Surface Soils of Ahvaz , Iran , Using Pollution Indicators and Health Risk Assessment

Article Notes: Received: Aug 1, 2020 Received in revised form: Sep 14, 2020 Accepted: Sep 26, 2020 Available Online: Nov 15, 2020 Background & Aims of the Study: Heavy metals are among the most dangerous environmental pollutants entering the air, water, and soil through human and natural activities with the potential of poisoning, pathogenicity, and carcinogenicity to living organisms and humans. The current study aimed to determine the contamination of heavy metals, including lead, zinc, copper, arsenic, chromium, and cadmium, in the surface soils of Ahvaz, Iran, using pollution indicators and health risk assessment. Materials and Methods: Sampling of surface soils in Ahvaz with 227 soil samples was performed in August 2017 based on the regular networks of 1 × 1 km. The measurement of heavy metals was carried out by inductively coupled plasma optical emission spectrometry. Results: The mean concentrations of Pb, Zn, Cu, Cr, Cd, and As in the surface soils of Ahvaz were reported as 180.99, 88.61, 185.13, 101.59, 5.63, and 8.97 mg/kg, respectively. Based on the contamination factor (CF), pollution index (PI), geoaccumulation index (Igeo), and ecological risk, the highest levels of pollution were related to Cd and Pb, and the lowest level was associated with Cr. The enrichment factor (EF) of heavy metals showed moderate contamination of Cr, high contamination levels of Cu, Cd, and As, and very high contamination levels of Pb and Zn. According to the Nemro Integrated Pollution Index (NIPI), 3% and 55% of the surface soils of Ahvaz showed high and low levels of pollution, respectively. The daily absorption of heavy metals through ingestion in children and adults was higher than that through respiration and dermal contact. The highest risk index was related to Cr and As through ingestion for children, and the lowest risk index was associated with Zn through respiration in adults. Conclusion: The levels of As, Cr, and Cd in the surface soil of Ahvaz were higher than the Iran soil standard values; however, the levels of Pb, Zn, and Cu were lower. Based on the CF, PI, Igeo, and ecological risk, the surface soils were highly contaminated with Cd and Pb. According to the EF, Pb and Zn showed the highest levels of contamination. Based on the NIPI, the pollution of heavy metals was reported at low levels. The highest and lowest values of metals carcinogenicity were related to Cr in children and Pb in adults, respectively.

Among the most important soil contaminants are heavy metals reducing soil quality, thereby decreasing its optimal performance. Heavy metals are a group of metals and quasi-metals the small amounts of which are toxic and dangerous (1,2). Heavy metals in the soil can directly enter the human body through ingestion and respiration after reaching the earth surface through precipitation, polluted water and soil sources, and plant structures (3,4).
Arsenic, lead, and cadmium are toxic elements and the most dangerous environmental pollutants not playing a role in biological interactions in the human body and causing hemoglobin biosynthesis, anemia disorders, hypertension, kidney damage, abortion, nervous system disorder, brain damage, and reduction of learning ability in children (5). Zinc and copper play a role in biological processes based on their amounts and are among the essential elements for biological reactions; however, the increased bioaccumulation of these two metals in body tissues causes their high toxicity (6). High levels of chromium cause stomach ulcers, respiratory problems, weakened immune systems, kidney and liver damage, lung cancer, and mortality in humans (7,8).
In a study carried out in Zahedan, Iran, Cd, Cr, Cu, Ni, Pb, and Zn were reported with the average concentrations of higher than the background concentrations (in suburbs). The highest metal concentrations were obtained in commercial and high-traffic areas (9). The average concentrations of Pb, Zn, Ni, Cr, except Cu and V, in the soil of parks in Ahvaz, Iran, are reported as several times higher than the background levels. According to the average enrichment factor (EF) and pollution index (PI), Ni, Zn, and Pb are highly polluted. The highest average geoaccumulation index (Igeo) is related to Ni, Zn, and Pb. Based on the evaluation of the Nemro Integrated Pollution Index (NIPI), 100% of the samples had a high level of contamination (7).
Based on the evidence, the pollution of heavy metals in the street dust of Ahvaz was reported at a very high level, and areas with high population density, traffic congestion, and industrial activities are severely contaminated with heavy metals (10). The examination of Cd, Pb, Cr, Ni, Zn, Cu, Hg, and Co in the soil of the northern plateau in Spain showed that 54.61% of the soil in the study area has moderate to low pollution. In addition, 22.31% of the soil had moderate pollution, and 21.54% of the samples were free from pollution (11). The concentrations of heavy metals in the surface soils of Ibadan Industrial City in Nigeria have been reported as Mg > Cr > Pb > Cu > Cd > Co > Ni. All the samples were heavily contaminated based on the contamination factor (CF), pollution load index (PLI), EF, and PI (12). In the soils of Ijebu-Ode, Nigeria, the carcinogenic risk of Cu, Mn, Pb, and Zn for children and Mn, Pb, and Zn for adults was higher than their acceptable level (13).
The calculations of the CF and Igeo determined that the soil samples collected from Urabi fields in El Obour, Egypt, are not contaminated with Cr, Cu, and Pb; however, they are significantly contaminated with Cd and highly contaminated with As. Integrated pollution calculation index, PLI, pollution degree, and potential environmental risk index showed that soil samples have been contaminated with the studied heavy metals. High CF values of As and Cd demonstrated that these two elements play a major role in the high pollution of the soil (14).
Ahvaz has special conditions due to the rich resources of oil and gas, large metallic and nonmetallic industries, cellulose and electricity, and hot and humid weather conditions. In addition, high consumption of fossil fuels in industry, automobiles, and miscellaneous sources, such as seasonal dust, relatively high population density with urban traffic, and lack of green space in the city and suburbs are the reasons for heavy metal pollution in the soil (15). Large industrial factories, such as oil and gas companies, and metal industries are the major sources of soil pollution in Ahvaz (16).

Sampling
The samples (n=227) of the surface soil were collected from a depth of 0-10 cm below the ground surface in June 2017 to avoid potentially toxic elements washed by rainwater ( Figure 1). The map of Ahvaz was divided into 1 × 1 km networks, and a total of 227 regular networks were created using ArcGIS software (version 10.5). Each network in the city was then tracked using the Global Positioning System. The soil samples were collected and mixed from three scattered points in each network, and the composite samples were subsequently passed through a 2-mm mesh. In addition, eight soil samples as the control soils were collected from eight pits from a depth of 120 cm at four main directions and four subdirections of the city in the uninhabited areas with no traffic (17). After the removal of surface contaminants and separation of pebbles, plant roots, and other wastes, the collected samples were sieved through a 2-mm nylon net with a wooden spoon and dried in the air. In order to chemically digest the samples, they were placed in a polyethylene beaker, and a few drops of hydrochloric acid and 7 cc of hydrofluoric acid were added to the beaker. The samples were heated in a water bath at a temperature of 100°C until drying. After cooling the samples, 7 cc of nitric acid and hydrochloric acid were added to each sample, and they were heated on a water bath until drying. After the chemical digestion of all the samples, a completely clear solution was obtained by adding distilled water to each of them and gently heating them. All the samples were brought to a volume of 50 cc by 1N hydrochloric acid and then injected into the device (17). Zn, Cu, Pb, Cd, Cr, and As were measured by inductively coupled plasma optical emission spectrometry. Varian 710-ES (Varian, USA) was used for the pollution measurement of the heavy metals.

Contamination Factor
The CF is the ratio of the concentration of each metal to the amount of the natural background concentration of that metal (Equation 1). Accordingly, CF < 1, 3 > CF ≥ 1, 6 > CF ≥ 3, and 6 ≤ CF demonstrate low, moderate, high, and severe pollution, respectively (18).

Pollution Index
In the PI equation (Equation 2), C is the concentration of the studied element in soil samples; Xa is the heavy metal enrichment concentration threshold; Xc is the pollution low-level concentration threshold; Xp is the pollution high-level concentration threshold  (19).

Enrichment Factor
This factor indicates the intensity of anthropogenic impact (Equation 4). In this equation, EF is the enrichment factor; Cx is the measured element concentration in soil samples; CRef is the reference element concentration. The reference element in the determination of the EF is an element, with a purely geological origin. According to the presented classification, EF < 2, 2 ≤ EF < 5, 5 ≤ EF < 20, 20 ≤ EF < 40, and 40 ≤ EF show low, moderate, high, very high, and extremely high pollution, respectively (20):

Health Risk Assessment
Heavy metal health risk assessment was performed based on the health risk assessment method provided by the US Environmental Protection Agency (EPA). This assessment is considered in two parts of carcinogenic and noncarcinogenic risks and risks of human exposure to metals from all three pathways of ingestion, respiration, and dermal contact. An average daily dose of metal through each pathway was calculated using equations 8 to 10 as follows (22): where ADDing, ADDinh, and ADDdermal are the average daily doses of metals (mg/kg-day) through ingestion, respiration, and dermal contact, respectively; C is the concentration of metals in soil (mg/kg); IngR and InhR are ingestion rate and soil respiration rate (mg/day and m 3 /day), respectively; EF is the exposure frequency of metal (day/year); ED is exposure duration of metals (year); BW is bodyweight of the individual exposed to metals (kg); AT is the average time for exposure to each metal (day); EF is a factor of emission of metals from soil to air (m 3 /kg); SA is the skin surface area exposed to metals (cm 2 ); AF is an adhesion factor of soil to the skin (mg/cm 2 -day); ABF is the dermal absorption factor (without units).
The noncarcinogenic hazard of all the pathways of ingestion, respiration, and dermal contact for children and adults was determined using the ratio of total daily absorption of heavy metals in each pathway to the reference value of the toxicity of that metal according to Equation 11 as follows (22): Equation 11 HQ where HQ is the noncarcinogenic hazard quotient of metals in each pathway; ADDi is the average daily dose of metal in each pathway of exposure to metal (mg/kg-day). If HQ is less than 1, it is not incompatible with human health. Moreover, if HQ is higher than 1, it has adverse effects on human health. The values of cumulative noncarcinogenic hazard index of total metals for both groups of adults and children were obtained according to Equation 12 as follows (22): Equation 12 HI = ∑ HQ i The carcinogenic assessment of each of the three pathways for these metals was performed using Equation 13 as follows (22): where RI is the carcinogenic risk; ADDi is the value of average daily metal uptake in each of the metal exposure pathways (mg/kg-day); SFi is the cancer risk factor per unit of metal exposure (mg/kg/day).

Statistical Analysis
Analyzing the data, drawing tables, calculating the equations of pollution indices, and assessing health hazards were performed using SPSS software (version 16.0) and Excel software (version 2007).
The accumulation amount of the heavy metals in the surface soils of Ahvaz was obtained as Cu > Pb > Cr > Zn > As > Cd. Statistical results showed an abnormal distribution in this regard. The amplitude of the coefficient of variation corresponding to the samples of the study area demonstrated high changes in this parameter (the coefficient of variation>20%). In addition, the skewness coefficient of the studied metals was higher than 0, indicating the skew distribution to the right ( Table 2).
The average CF and heavy metal PI in the surface soils of Ahvaz were obtained as Cd > Pb > Cu > As > Zn > Cr. The CF for Pb, Cd, and Cu showed that 55.27%, 72.37%, and 34.21% of the soil samples had severe pollution, respectively. The Zn, Cr, and As soil samples were reported with moderate (38.6%), low (59.65%), and high (41.23%) pollution, respectively ( Table 3). The average PI of the heavy metals showed that the surface soils of Ahvaz were polluted with heavy metals. The PI for Pb demonstrated that 46.49% of the soil samples had low pollution. Zn, Cr, Cd, and As had no pollution in 55.7%, 55.7%, 89.47%, and 87.28% of the soil samples, respectively. Regarding Cu, 70.61% of the soil samples had moderate pollution ( Table 3).
The average NIPI of the heavy metals of surface soils of Ahvaz was calculated at 1.8 ( Table 4). The obtained values of the EF were reported as Zn > Pb > Cu > Cd > As > Cr. The EF of Pb, Cu, Cd, and As showed that 52%, 84%, 100%, and 96% of the soil samples were highly polluted. Moreover, the EF of Cr demonstrated that 96% of the soil samples were moderately polluted. In addition, the EF of Zn indicated that 48% of soil samples were severely polluted ( Table 5).
The average Igeo in the form of Cd > Cr > Cu > As > Zn > Pb was obtained indicating that 26.32% and 21.49% of the soil samples were strongly polluted with Pb and Cd, respectively. In addition, 35.96%, 41.67%, and 23.68% of the soil samples were moderately polluted with Zn, As, and Cu, respectively. Moreover, 75.88% of the soil samples were not polluted with Cr ( Table 6). The risk index of heavy metals in the surface soils of Ahvaz was 157.42      (Table 7), which was at the average level. The highest and the lowest daily doses of heavy metals were 0.00310469 and 9.08956e -10 mg/kg-day related to Pb in adults through ingestion and Cd in adults through respiration, respectively ( Table 8). The highest risk indices were reported for Cr and As with the values of 0.42958686 and 0.382283105 in children through ingestion, respectively. In addition, the lowest risk index was reported as 4.76866e -8 related to Zn in adults through respiration ( Table 9). The highest and lowest values of carcinogenic risk index were related to Cr in children with a value of 4.01121e -5 and Pb in adults with a value of 9.58801e -9 , respectively. In case of the noncarcinogenic index, the   (Table 10).
The average amounts of Pb, Zn, Cu, Cr, Cd, and As in the surface soils of Ahvaz were higher than the background values probably indicating the impact of activities, such as oil, gas, and steel industries, and light and heavy traffic in the study area. The levels of As, Cr, and Cd in the surface soils of Ahvaz were higher than the standard values of Iran soil; however, the levels of Pb, Zn, and Cu were lower. The average levels of As, Pb, Cr, Cu, and Cd in the surface soils of Ahvaz were higher than the global average values (i.e., 6, 35, 70, 30, and 0.35 mg/kg, respectively); nevertheless, the amount of Zn was equal to the world average value (i.e., 90 mg/kg) (23). Other studies carried out on the heavy metals in the surface soils of Aran and Bidgol, Isfahan, Iran, (6) and soils adjacent to Kerman Steel Industry in Kerman, Iran, (24) have confirmed the effects of industrial pollution on surface soils, which is consistent with the results of the present study.
The average CF of Cd, Pb, and Cu indicated that the surface soils were severely polluted with these metals. Zn, Cr, and As had moderate, low, and high pollution, respectively. The urban environment has undergone many changes by humans to provide habitats suitable for human living conditions. Some distinctive features of such environments are the dense population and relatively high levels of change resulting from industrial activities (25). In fact, the soils of urban areas are significantly damaged due to intense human activities and may even be transported from other areas (26).
The PI of Pb, Zn, Cu, Cr, Cd, and As showed that the surface soils of Ahvaz were highly polluted, because the PI of all the studied metals was higher than 1, indicating the frequency of contamination in comparison to that of the noncontaminated site (27). The EF demonstrated that Pb, Zn, Cu, Cr, Cd, and As in the surface soils of Ahvaz were affected by human activities. The heavy metals in the soil, such as Pb, Zn, Cu, and Cd, can originate from automobile components, tire friction, grease and lubricants, industrial exhaust (i.e., smoke), and furnaces (15). The results of a study conducted on heavy metal pollution of road dust at Vellore, India, using EF and CF were calculated for 10 trace elements, namely Cu, Co, Cr, Ga, Mn, Ni, Pb, Rb, Sr, and Zn. Ga and Zn were reported with the highest EF and CF values (28). The average NIPI indicated low levels of metal contamination. The most important advantage of the NIPI, compared to other indices, is the determination of the pollution risk of all metals studied in the region using this index (29).

Discussion
The results of the Igeo showed that Pb, Cd, As, Zn, and Cu have entered the surface soils of Ahvaz as a result of human activities and had no natural or geological origin. The CF and Igeo of the studied metals in the surface soils around Ahvaz Industrial Town No. 2 confirmed the highest amount of contamination related to Pb (30), which is in line with the findings of the present study for these two indices regarding Pb. The results of the Igeo in the agricultural soils of Kamfiruz, Fars, Iran, showed that the soil was not contaminated with Cr, Cu, Ni, Pb, and Zn; however, it was moderately contaminated with Cd and As (31). Knowledge of the concentration of heavy metals in the soil helps to discover the source of contamination in the study area (24).
According to the calculations, the highest and lowest ecological risk values were related to Cd and Cr, respectively. Other studies performed on the surface soils of Abadan, Ahvaz, Kerman, Isfahan, Birjand, Qazvin, Rasht, and Aran and Bidgol, Iran, (5,6,32,10,24,33,34) considered the industrial activities, traffic, and urban transportation the factors increasing heavy metal pollution in the soil. The average concentrations of Cr, Zn, Ni, Pb, Cu, and V in the soil of Ahvaz parks have been reported as 167. 26, 131.92, 78.75, 51.83, 28.71, and 12.49 mg/Kg, respectively. The average concentrations of all heavy metals, except Cu and V, are several times higher than the background values. According to the average EF and PI in the study area, Ni, Zn, and Pb highly polluted the area. In addition, the highest average Igeo is related to Ni, Zn, and Pb. The contamination sources of the studied metals are anthropogenic activities, such as traffic and fossil fuels (35). The results of the aforementioned study confirmed the contamination associated with the heavy metals of the surface soils of Ahvaz.
The results of a study carried out on the heavy metal pollution of Urabi fields in El Obour, Egypt, using CFs, degree of pollution, PLI, environmental risk index, and Igeo showed that the soil samples were not contaminated with Cr, Cu, and Pb; however, they were significantly contaminated with Cd and highly contaminated with As (14). According to the results of the present study, the daily doses of Pb, Zn, Cu, As, Cr, and Cd were obtained in children through ingestion and adults through respiration and dermal contact. In addition, the highest risk index for Cr and As was obtained through ingestion in children, and the lowest risk index for Zn was obtained through respiration in adults.
In a study conducted on the surface soils of Arak, Iran, it was reported that the daily dose of Pb through ingestion is higher for children than adults (36), which can be considered a possible reason for more childrenʼs contact with soil (37,38). This finding is in line with the results of the present study. Ravankhah et al. (2016) also reported that the daily doses of Cd, Ni, Zn, and Cu in the soil of Aran and Bidgol through dermal contact and respiration were higher than that of Pb. In addition, carcinogenic and noncarcinogenic risks are higher in children than adults (6), which is consistent with the results of the current study.
The metal carcinogenic risk index demonstrated that the highest and lowest values of this index are observed for Cr in children and Pb in adults, respectively. The highest and lowest noncarcinogenic indices were reported for Pb in children and Zn in adults, respectively. According to the amounts of heavy metals in the soil, the risk indices of Pb, Zn, Cu, As, Cr, and Cd were less than 1. Based on the standard of the US EPA, if the daily metal uptake is greater than the reference value of metal toxicity in each pathway, the risk of noncarcinogenic metals in each pathway will be higher than the permitted limit which has adverse effects on human health (39,40). The reference daily intakes of Cd, Pb, Zn, and Cu are reported as 0.001, 0.3, 0.02, and 0.04 mg/kg/day, respectively (41,42). It should be noted that in the present study, the total daily intakes of Cr, Cu, Zn, As, Pb, and Cd in humans are less than the acceptable daily intakes provided by the US EPA.
Heavy metal risk assessment in the surface soils of Bojnourd, Iran, showed that the daily exposure of children to Cu, Zn, Pb, and Cd through soil ingestion in the sampling stations resulted in the maximum daily intakes of 6.64×10 -4 , 19.6×10 -4 , 1.03×10 -4 , and 4.45×10 -6 for Cu, Zn, Pb, and Cd, respectively (5). The heavy metal health risk assessment of agricultural soils in Tangail industrial areas of Bangladesh demonstrated that the total risk index for each heavy metal in the areas was reported as less than 1, and their carcinogenic risk values were less than 10 -6 (43). A study carried out on soil contamination by Cd, Cr, Cu, Mn, Ni, Pb, and Zn in Ijebu-Ode, Nigeria, reported the carcinogenic risk of Cu, Mn, Pb, and Zn in children and Mn, Pb, and Zn in adults as higher than the acceptable limits (13).
According to a formal risk assessment of human health using the US EPA framework, it was indicated that Pb in Kolkata, India, and Cr in Bengaluru, India, were mostly concerned among Cr, Ni, Cu, Zn, and Pb. Nevertheless, childrenʼs exposure to Pb in Kolkata was the only significant risk combination (i.e., hazard index). Contamination through ingestion dominating the risk pathways was significantly greater than that reported for dermal and inhalation routes (44).
The amounts of Cu, Cr, and Cd in the surface soils of Ahvaz were higher than those reported for other cities in the world. The amount of Zn in the surface soils of Ahvaz was lower, compared to those of other cities in Iran and around the world. The amounts of As, Cr, and Cd in the surface soils of Ahvaz were higher than the standard values of Iranian soil; however, the amounts of Pb, Zn, and Cu were lower. The CF, PI, and Igeo calculated in the present study showed the amounts of heavy metals in the surface soils of Ahvaz as Cr < Zn < As < Cu < Pb < Cd, respectively. Based on the NIPI and ecological risk, more than 50% of the surface soils of Ahvaz were reported with low pollution.
According to the obtained results, it is suggested to carry out further studies to obtain the sources of production and dispersion patterns of heavy metals in different components of the environment, especially on individuals who are directly affected. It is also recommended to perform studies on the pollution of heavy metals in other cities and alongside industrial towns and using various physical and chemical methods for the elimination of heavy metal pollution in the soil. Recently, phytoremediation has been used as a new method for the removal of soil pollution.