Level of heavy metals and environmental pollution index in Ahvaz, Southwest Iran

This study was designed to evaluate the concentration of heavy metals (HMs) in the north of Ahvaz, southwest Iran. The soil samples were collected from the agricultural farm and riverside in Karun, for the investigation of the environmental impacts of the selected HMs in the soil of the Weiss and Arab Assad regions. For soil sampling in a period, nine farms were selected from each region, and 10 samples were taken from each agricultural farm. Zoning was done using GIS. The highest of Contamination Factor, Enrichment factor and geo-accumulation index of HMs for Cd (7.84, 73.92 and 2.38), and the lowest value of this index for Cr (0.21, 1.98 and − 2.82), respectively. Furthermore of the farm soil showed that the most toxic effect is related to Cd. The HMs contamination indices of the soil samples showed that the studied HMs had contaminated the agricultural fields. Moreover, the zoning maps of the Co, Cu, Pb and Cr showed that they had not contaminated the soil of wheat fields, but Cd and Zn revealed high contamination levels. The zoning of Ni concentration distribution showed that this metal contamination came from both anthropogenic aspects and geological activities in the region. According to our findings, the EF illustrated high levels of pollution for Cd, Cu, Cr, Pb, Ni, Fe, Mn, Co, and Zn, which seems to be in accordance with the accumulation of agricultural fertilizers (phosphate and nitrate), industrial and human activities in the region.


Study area
Ahvaz is the capital of the Khuzestan Province, southwest of Iran.Ahvaz, with an area of 185 square kilometers as the center of Khuzestan Province, is located in the southwest of Iran and in the north of the Persian Gulf.It is located 31° 19′ 13″ N, 48° 40′ 9″ E. Ahvaz is one of the largest cities in Iran 19 .The dominant wind direction is northwest to southeast.The population of this province is estimated to be 1.3 million people.It borders Iraq on the west and the Persian Gulf on the south 20 .According to the research objectives, this area was first divided into two parts: the first part, including agricultural lands downstream of Shushtar city, begins and ends at the bitumen dam.These lands are characterized by having medium to semi-heavy textured soil with relatively good drainage and a deep-water table in summer and semi-deep to shallow in autumn and winter.These lands are located between the Shatit and Gargar rivers.The second part starts from the bitumen dam and covers the north of Ahvaz city.The characteristics of these lands have soil with a medium-to-heavy texture and with moderateto-poor drainage conditions, with a semi-deep water table in summer and shallow in autumn and winter.The soil of the studied area consists of alkaline and calcareous, so the copper absorption efficiency in this soil is very low.A location map showing the sampled site with sampling points in the agricultural lands downstream of Shushtar and north of Ahvaz is shown in Fig. 1.According to the characteristics of the soil of this region based on the standard classification (based on the USDA soil taxonomy), it was determined that the soil of this region consists of entisols and aridsols types 21 .

Soil sampling
In this study, soil sampling and the measurement of HMs were performed in the surface soil of the Arab Asad region and, in some parts, from downstream of Shushtar city to the place of Qir dam using standard methods 22 .The selected soil sampling in this region was performed according to the soil quality and geographical coverage.Soil Samples were taken based on pollutant inputs, sources of pollutants, the agricultural farms, and the direction of the river in Ahvaz, southwest, Iran.A network with a dimension of 6 km (in terms of longitude) at 5 km along the river was selected to collect soil samples in each of the dual regions.In the longitude direction (perpendicular to the river flow), by taking a distance from the river, three soil samples were taken at a distance of 2.5 km from each other (with a vertical distance from it).In the latitude direction (along the river route), three axes with a distance of 5 km each (a total of three points in each latitude) soil sampling was performed.In this way, a hypothetical geographical network was created with three lengths and four latitudes (12 points or intersections for sampling) (Fig. 2).The spatial variations of HMs were measured to an average depth of 0-30 cm.For soil sampling in a period, nine farms were selected from each region, and 10 samples were taken from each agricultural farm.

Analyses of samples
The soil samples were collected from each of the two study areas and kept in polyethylene bags.Then, the particles smaller than 63 mm in diameter were separated and immediately transferred to a laboratory for the characterization of their physical and chemical properties.Soil texture (percentage of sand, silt, and clay), and pH levels were determined by the hydrometric method and pH meter.After that the soil samples were treated with a few drops of hydrochloric acid and 7 mL of hydrofluoric acid before being heated in a water bath at 100 °C until nearly dry.Then, they were ground using a mill and weighed one gram of each with a digital scale and were ashed for 5 h at 550 °C.After cooling the samples, 7 ml of 65% HNO 3 and 35% HCl were added and gently closed the cap to enable the sample to remain overnight for reactions to take place.Next, it was heated in a water bath to dry.After hydrochloric acid (HCl) and nitric acid (HNO 3 ) were added to soil samples in a 3:1 ratio (HNO 3 :HCl 23 .Then, the dissolved samples were passed through a paper strainer and were volumized in a 20 ml volumetric flask. In this study used Inductively Coupled Plasma Optical Emission Spectroscopy (ICP; model Varian 710-ES) for detection and measurement of HMs 24 .We dilute the 1 ml of diluted solution with 9 ml of deionized water.In accordance with standard procedures and protocols, the ICP-OES was used to measure the amount of HMs 1 .Dehumidified samples were ground in a rustproof steel grinder (< 0.25 mm), and the total intents of the abovementioned HMs were determined using ICP-OES.Samples were analyzed in triplicate for certifying the measurement precision and accuracy in the entire analytical procedure.Synchronously, blank reagent determinations were used to rectify the appliance readings.The obtained results of average recoveries from certified reference material analysis and the determined limit of detection (LOD) and recovery (%) for the measured HMs were a range 0.001-0.015mg kg −1 and 93.75-109.9%.After evaluating 20 samples using standard calibration solutions, the ICP-MS was tuned and calibrated.The number of HMs in each sample was determined to the extent of ppb detection.The samples were analyzed in triplicate for certify the measurement precision and accuracy of the entire analytical procedure.In addition, 90 sediment samples from the soil of the studied region were separately sent to the laboratory to measure Pb isotopic ratios.The collected samples were analyzed in the advanced analysis laboratory of the University of Ahvaz.Quality assurance (QA) and quality control (QC) were assessed by measuring reference material blanks and the NIST 2710 standard.This provided an accuracy of 100 ± 5% (n = 15) while the accuracy of duplicate samples was 6-4%.To ensure the data quality of the samples, the standard reference material (SRM) 2710 was applied simultaneously to 15% of the soil collected from the surface soil samples.

Contamination factor (CF)
For the assessment of the contamination of toxic metals, the CF was applied (Eq. 1) 8,25 .
Which Cn is the concentration of each element in the soil, and Co is the average concentration of each element in the natural background 8,26 .According to the classification provided by Huckanson, CF < 1 demonstrate low, 3 > CF ≥ 1 moderate pollution, 6 > CF ≥ 3 high pollutions, and 6 ≤ CF severe pollution 26 .

Pollination index (PI)
The PI was calculated for checking the quality of the experimental soil for each HMs 27 .
where C n refers to the measured concentration in the sample, and B n is the measured concentration of the HM S in the Earth's crust (Table 1) 27 .The pollution index was classified as follows: This index was classified into three levels, including; PI ≤ 1 is equivalent to low pollution level, 1 < PI ≤ 3 is equivalent to a moderate pollution level, and PI > 3 is equivalent to a high pollution level.

Integrated pollution index (IPI)
The IPI is the average value of the PI of each element 28 .This index is classified as the low amount of element IPI ≤ 1, medium amount of element IPI ≤ 2, and high amount of element IPI > 2 = High levels of pollution 28 .In many indicators of HMs pollution in the soil, a reference value is used and the severity of pollution is determined for a specific metal, but in the IPI index, the total HMS pollution in the studied soil is assessed cumulatively (Table 1).

Nemro integrated pollution index (NIPI)
The advantage of NIPI over other indicators is that in this index, the contamination risk for all metals studied in the region is determined 8,29 .This index was classified into five levels including NIPI < 0.7 without pollution, (1)

Enrichment factor (EF)
The EF for each metal was calculated from the ratio between the normalizing element and the background value of the HMs, according to the following equation.The measured HMs, according to the sample reference HM, is stable in the Earth's crust for an estimated EF index 30 .This index is obtained by Eq. ( 4): where C ref is the concentration of the reference element in the sample, and C x is the concentration of the element considered in the sample.A geological origin is the most essential factor in determining the reference element.
The reference element in determining the enrich HM S sent factor is an element with a purely geological origin.
In this research, the iron (Fe) element was used as a reference HM S .According to the classification, EF < 2 low pollution, 5 > EF ≥ 2 moderate pollution, 20 > EF ≥ 5 high pollution, 40 > EF ≥ 20 very high pollution and EF ≤ 40 indicate extremely high pollution 30 .

Geo-accumulation index (Igeo)
The Igeo can determine the degree of soil pollution.The Igeo is a common method for assessing soil pollution by HMs.Therefore, the Igeo can be used to determine the severity of the pollution 31 .This index is estimated by Eq. ( 5) 25 : In this regard, Igeo is the geo-accumulation index, Log 2 is the logarit HM S sic base 2, Cn is the concentration of HMs in the soil, and Bn is the concentration of the field background (average shale).To correct the effects of soil parent materials and natural fluctuations in the content of the given material in the environment and the very little change caused by human activities, a coefficient of 1.5 was used.Seven classes of pollution, Igeo < 0 unpolluted, 0-1 unpolluted to slightly polluted, 1-2 slightly polluted, 2-3 slightly polluted to very polluted, 3-4 very polluted, 4-5 very polluted to heavily polluted and 5 < Ige It is classified as heavily contaminated 31 .

Statistical analysis
Excel and SPSS were used for the analyses.Sampling and data collection were done by the researcher.Data analysis was used to produce descriptive statistics for the soil pollution indexes.The concentration of HMs was analyzed using SPSS version 25 and Sigma Plot statistical software.Mean data were used to compare significant differences with 95% confidence limits (P = 0.05) using the t-test and one-way analysis of variance (one-way ANOVA).The normality of the data was checked using a Kolmograph-Smirnov test.Excel software was also utilized to draw tables and calculate contamination indices.

Ethical considerations
The Research Ethics Committee of the Islamic Azad University of Ahvaz Branch approved the study protocol.All research experiments were done in the Islamic Azad University of Ahvaz Branch laboratory.The authors are grateful to the Department of Environment, Ahvaz Branch Islamic Azad University (with code IR.AZA.REC.1399.587)for providing the necessary facilities to perform this research.Considering the fact that the data collection method was observation and there were no human participants in the current study, obtaining informed consent is deemed unnecessary according to regulations.

HMs concentration
The statistical description of the concentration of HMs in the surface soil of Weiss and Arab Assad regions is given in Table 2.In this study, the highest of HMs in the soil of Weiss and Arab Assad regions (9013.545± 70.77 and 9208.69 ± 93.63 mg/kg) belonged to Fe, respectively.The concentration of Cd had the lowest amount among HMs in the soil of agricultural fields of Weiss and Arab Assad regions, 1 ± 58.10 and 1 ± 56.08 mg/kg, respectively.The statistical analysis of HMs showed that the level of Cd, Pb and Zn in the soil of the fields of the Weiss region was higher than the level in Arab Assad region (P > 0.05), but the amounts of Ni and Fe (P < 0.05) and Cr, Cu, Co and Mn obtained in the soils of the fields of the Arab Asad region were higher than in the Weiss region (P > 0.05) (Table 2).
The results of soil evaluation indicated that the average levels of Fe and Cd were the highest and lowest in the Weiss and Arab Assad regions (north of Ahvaz).Fe is the principal constituent of the Earth's outer and inner nuclei and is the sixth most common element on Earth 33 .The results of the study regarding the high levels of Fe in the soil can be explained by the fact that the probable reason for the high levels of this metal in the studied surface soil is that iron is the most abundant constituent of the earth's crust.Various researchers have reported a high concentration of Fe in the surface soil of different regions 34 .In general, the previous results showed that the Weiss and Arab Assad regions (north of Ahvaz) and their pollution played crucial roles in changing the soil of the studied area and its natural state.The primary sources of Cd in the environment include fossil fuels, waste disposal in plastic bags, insecticides, and sewage sludge in agriculture 35 .The main source of Cd in Iran is phosphorus fertilizers and industries activities.The soil of farms in the Weiss and Arab Asad regions (north of Ahvaz) was used by the available standards introduced by Iran, China, Canada and the United States of America (USQG) for comparing and investigating the level of HMs (Table 3).In Shenyang, northeast of China, Li et al. 36 studied the HM S contamination of urban soil in an old industrial city and reported that the concentrations of Pb, Cd, and Cu were beyond the study area 36 .While the high concentrations of these HM S , along with Zn and Hg, were the same as the concentrations observed in this study, the main reason for this similarity is industries city around the study area including (large metallic and nonmetallic industries, HMsrochemical, steel, pipeline, gas, oil, sugarcane industries), the existence of many vehicles and human activities.In another study, another research team evaluated the level of HMs in the surrounding soils of the Khuzestan steel industry 37 .In agreement with the findings of this study, they reported an increased number of HMs 37 .Furthermore, another research group investigating contaminated soil samples in northern Serbia showed that emission HMs from distance and proximity to the road, industrial, and agricultural activities can play the most important role in the concentration of pollutants entering soil sources 38 .The findings of this study revealed that pollution caused by human activities, especially agriculture and pollutants entering from industries, are the most important cases of HMs entering the soil and pollution of resources.

Zoning of HMs
Zoning investigated HMs in 180 samples (nine agricultural farms were selected from each region, and 10 samples were taken from each farm) from different zones of the soil of the fields in the Weiss and Arab Assad regions.Zoning was done using GIS. Figure 3 shows the HMs concentration distribution map in surface soils in the Weiss and Arab Asad regions.According to the Iranian National Standard for Human Health (the level of standard is less than 5 mg/kg), the soil cadmium contamination status in the Weiss and Arab Assad regions was classified as non-contaminated (1.38-1.75 and 1.32-1.74mg/kg, respectively) (Fig. 3).However, according to the level of international standards, including the United States, Japan, Russia, and Canada, the permissible limits are 0.48, 0.4, 0.76, and 1.4 mg/kg, respectively; based on these standards, Cd pollution in the soil was classified in the category 'soil infected' 40,41 .The Cd contamination in the soil of the studied regions according to international standards showed that farmers in this region had not tested the soil for using chemical fertilizers, especially phosphorus fertilizers.It is worth mentioning that the most important source of the Cd contamination of the soil is the non-optimal consumption of chemical fertilizers (especially superphosphate).Unfortunately, most farmers use this fertilizer every year on their fields where they grow wheat or summer crops.This is a major reason why the soil is contaminated by Cd.
The soil zoning map in the Weiss and Arab Asad regions in terms of chromium element showed that the concentration of this element (19.23-23.9 and 18.6-24.1 mg/kg, respectively) was within the permissible range according to the standards of Iran and Canada, with a permissible limit of 165 and 87 mg/kg, and hence not considered polluted (Fig. 3).In the past decade, soil and groundwater pollution due to the use of chromium in various artificial activities has become a severe concern for plant and animal health 42 .According to the results of many environmental studies, high levels of chromium have been reported in areas near landfills, chromate industries, and highways 43 .
The study of the soil zoning map in the Weiss and Arab Asad regions in terms of cobalt showed that the concentration level of this element (10.6-11.8 and 7.5-10.25 mg/kg, respectively)was within the permissible range based on the standards of Iran and the United States of America (40 and 20 mg/kg, respectively) 41 .It is within the permissible range and thus not considered polluted (Fig. 3).Cu is an essential element of the bodies of living organisms and is widely found in the environment 44 .The human resources of Cu include chemical industries, mineral fertilizers, industrial and mineral effluents and wastes; its natural resources are the minerals and geology of the native soil of the region 45 .The distribution of Cu in the soil depends on human activities, the climate, and the geology of the region.In general, it has less influence in mountainous areas and heights that do not have dense vegetation, so these areas have a lower concentration of Cu in the soil than in the plain areas 46 .The study area in the present study is located in the Khuzestan plain, an area with plain conditions.The evaluation of Fig. 3 showed that the level of Co revealed that the concentration of Cu was (6.95-10.5 and 6.35-11.75mg/kg, respectively), which according to the standards of Iran, the United States, and Canada with permissible limits of 400, 30, and 360 mg/kg, respectively 40 , this amount was within the allowable range and hence not considered contaminated (Fig. 3).Cu is one of the essential HMs in the bodies of living organisms that is widely distributed in the environment 47 .Copper metal and alloy are used in electric wire and rod, various metal alloys such as silver, jewelry, in computer hardware, coins, thermometers, and thermocouples 48 .The distribution of nickel concentration in the soil of the Weiss and Arab Asad regions shows that the concentration of Ni was (36.5-45.6 and 41.2-53.1 mg/kg, respectively).According to Iranian standards, the soil was not contaminated (155 mg/kg), but according to the standards of America (40 mg/kg) and Canada (50 mg/kg), it was contaminated (69) (Fig. 3).By interpreting zoning maps, it can be inferred that the source of Ni contamination is two significant cases: materials from parent rocks and soils and human activity.In terms of the origin of the parent material, the distribution of Ni is usually uniform in the depth of the soil profilebut is higher on the soil surface, so that it has the highest accumulation at higher levels 49 .In terms of human origin, it also depends on two factors: agricultural and industrial activity 49,50 .The results of this study showed that Ni was distributed in the soil of the region; it seems that in the case of the Arab Asad region, the source of nickel contamination is the native material.The investigation of the soil zoning map of the Weiss and Arab Asad regions in terms of lead element demonstrated that the concentration of Pb (0-12.2 and 7.96-10.7 mg/kg, respectively) in terms of Iranian and Canadian standards, with allowable limits of 80 and 91 mg/kg, respectively, was within the allowed range and thus not considered polluted 51 (Fig. 3).
Pb is naturally present in all environments and ecosystems.However, most of the lead concentrations found in the environment results from anthropogenic activity.The increase in lead in the environment from human activities stems from the burning of fossil fuels, mining activities, and industry 52 .This metal is a powerful toxin and is harmful even in minimal amounts 17 .Due to the known toxicity of Pb and its damage and effects on humans and other living organisms, it is one of the most critical environmental pollutants 17 .The most important and significant sources of pollutants related to Pb are industries and vehicles 35 .Industries such as plating, battery manufacturing, and the production of electronic components are among the most pollutant industries 53 .
The concentration of zinc according to the zoning maps was 20.25-37.70mg/kg in the Weiss region and 23.90-33.10mg/kg in the Arab Asad region (Fig. 3).In this study, the distribution of zinc in the studied regions was within the permissible range (500 mg/kg) according to the standards of Iran and Canada, but according to US standards with a level of 50 mg/kg, it can be considered contaminated 40,41  www.nature.com/scientificreports/Regarding the zoning of the distribution of Fe and Mn, the critical levels of Fe and Mn are 8 and 6 mg/kg 54 .The result of this study regarding the concentration of Fe and Mn in the soil of the Weiss and Arab Asad regions showed that the soil was severely contaminated (Fig. 3).The findings showed that the most important source of contamination for these two HMs in the soil was the use of iron and manganese sulfate fertilizers and organic fertilizers.According to interviews with farmers, it was found that these fertilizers are used during the summer growing season, which farmers say is without soil testing and is often used much more than recommended.Due to the high intensity of Fe and Mn contamination in the soil of agricultural fields in the Weiss and Arab Assad regions, causing disturbance and contamination in the soil, it can be concluded that many divalent metal cations such as manganese, iron, cobalt, nickel, copper, and zinc are structurally very similar and replaced by each other.

Evaluation of the environmental index
The CF of Cd, Ni, Co, and Mn in the soils of agricultural fields in the Arab Assad region was higher than in the Weiss region, but it was lower in the Arab Assad region for Pb, Zn, and Cu.The highest of CF to Cd (7.84) and the lowest value of this index to Cr (0.21).The pattern of HMs CF values in the soil of wheat fields of the Weiss and Arab Assad regions was in the form of Cd > Mn > Ni > Zn > Cu > Pb > Co > Cr.The enrich HM S sent factor of Cd, Pb, Cr, Zn, Cu, and Mn was also higher in the soil of Weiss than in the soil of Arab Assad, and the EF of Ni and Co in the soil of the Weiss was lower than in Arab Assad.The highest amount of EF was related to Cd (73.92), and the lowest value of this index was related to Cr (1.98).The pattern of EF values in the soils of wheat fields of Weiss and Arab Assad regions was in the form of Cd > Mn > Zn > Cu > Pb > Ni > Co > Cr.The Igeo index of HMs of Ni, Cr, Co, and Mn in the soil of the Assad Arab region was higher than in Weiss, but the geo-accumulation index for Pb, Ni, and Co was higher in the soil of Weiss region than in the of Arab Assad region.The highest value of the HMs Igeo index was related to Cd (2.38), and the lowest value of the earth geo-accumulation index was related to Cr (− 2.82).The pattern of geo-accumulation index values of HMs in the soil of the Weiss and Arab Assad regions for cadmium chrome was in the form of Cd > Mn > Ni > Zn > Cu > Pb > Co > Cr.Regarding HMs in soils, the values of environmental indicators, including the PI and NIPI, in the Arab Asad region were higher than in the Weiss region (Table 4).Cd is one of the HMs that has no role and activity in the body of living organisms and if it enters the body of animals and humans, it causes toxicity and causes disease and carcinogenesis in the long term 15 .In the past years and recent researches and studies, researchers conducted many studies on the evaluation of Cd at different levels of the environment, which reports indicated the contamination of HMs.In other researches, contamination of Cd has been reported on rice and wheat agricultural products in the agricultural fields of Khuzestan province and around the cities of Ahvaz and Bavi 8,22,32,37,55 .Forest fires and volcanoes are important sources of Cd entry into the environment, but the main sources of cadmium metal entry are through anthropogenic activities and industrial waste production 15 .This metal is used in the industry as an anti-friction material, catalyst, anti-rust, compound of alloys.Cadmium is also used in rod protection semiconductors in nuclear reactors, metal plating, ceramic manufacturing, PVC factories and plastic industries, battery production, fungicide compounds, motor oil, rubber manufacturing and photography 17,35 .The main use of this element is as a stabilizer and pigment in plastic and electrolysis industries, but its main part is used in soldering and as an alloy in nickel-cadmium batteries.Phosphate fertilizers are also emitters of cadmium metal in agricultural lands 34 .
Due to the growing population, the expansion of urbanization, industrialization, oil and gas extraction, production of steel mills, cultivating more land for food supply, more use of agricultural fertilizers, wastewater, and the existence of many vehicles, urban traffic in Ahvaz in the past decades, this region is heavily affected by a large volume of environmental pollutants.In the same study conducted by Rastmanesh et al. 55 in Abadan, Iran, on the impact of Abadan Petrochemical complex and petroleum refinery on heavy soil metals, it was found that the main sources of HMs production was agricultural fertilizers, industries (oil, gas, petrochemical) and vehicles (high traffic),which is in agreement with the findings of this study.The high values of 208Pb/206Pb and 207Pb/206Pb in the soil of the studied region indicates that the pollutants were of human origins, like industrial activities for the Pb metal.According to the results of the present study, the excessive use of agricultural fertilizers and the location of many industries in this region are among the most important HMs contamination factors in the region.NIPI is calculated for each sample point.The advantage of this index compared to other www.nature.com/scientificreports/indices is that in this index, the pollution risk for all the HMs that are studied in the region is determined.The results obtained from Table 4 show the contamination index of all samples ranges in agricultural soils of Weiss (5.67) and Arab Assad (5.69) regions.The average value of NIPI index in studied performed by Halil et al. 56 was 7.18.The reference element in determining the enrichment factor is an element that has a completely natural origin.Zn, Ti, Fe, Al and Sr are usually used as reference HMs in environmental research 56 .The average EF, Cd and Mn had high levels of contamination and Zn contamination was high.Halil et al. 56 reported in their study that average EF, Ni, Cu and Pb had high levels of contamination and Zn contamination was high that same with the result of this study.Considering that the EF the increase in the concentration of an element compared to its natural concentration in the soil 4,31 , also in environmental analysis, one of the important factors for evaluating the concentration of elements under the influence of anthropogenic factors and it is natural 22,32 , so it can be concluded that the agricultural soils of Arab Asad and Weis regions are affected by heavy metals from man-made sources in the region.In numerous researches and studies, the entry of heavy metals through anthropogenic means such as industrial and agricultural activities has been reported 1, 12,23,28 .Based on the results of study by Kamalu et al. in 2011, the oil extraction process has resulted in the spilling of toxic drilling by-products into local and international soil and water bodies surrounding the study area which agrees with the findings of the current study 57 .In a study on HMs in Huixian wetlands in southern China, Huang et al. reported that there are moderate to high ecological hazards due to HMs in the coastal soils of this wetland 58 .The contamination factor for Pb, Cu, Co, and Cr was less than 1, indicating the low contamination of these metals in the soil.The C f for Se was in the range of 1-3, indicating moderate contamination for this metal 59 .The C f for Cd and Mn at the third station was higher than 3, which showed the high contamination of this element.The contamination factor for HMs implies the concentration of these HMs is higher than that of the earth's crust and shale.In some countries, such as New Zealand, Finland, and particular areas of China, the available HMs in the is naturally low 60 .

Conclusion
The contamination level of soil was evaluated on the basis of indices, namely contamination factor, pollution index, enrichment factor, geo-accumulation index, and Nemro integrated pollution.Considering that these two regions are located in the plain of Khuzestan and this region is one of the most fertile regions of Iran in terms of agriculture and the planting of wheat and other crops, it is of great importance and plays a strategic role in providing food to the people.So the investigation of HMs in the soil of this Areas and using ways to reduce these harmful substances and enter the food cycle play a very important and effective role in increasing people's health.This study was to investigate the pollution of HMs (Cd, Cu, Cr, Pb, Ni, Fe, Mn, Co, and Zn) in the soil caused by the activity of agricultural fertilizers, vehicles, and industries in the north of Ahvaz, southwest of Iran.According to our findings, the enrichment factor (EF) illustrated high levels of pollution for Cd, Cu, Cr, Pb, Ni, Fe, Mn, Co, and Zn, which seems to be in accordance with the accumulation of agricultural fertilizers (phosphate and nitrate), industrial, and human activities in the region.The Average level of analyzed HMs in the soil was greater than the average standard levels in soil.According to the results of the enrichment factor, it seems that HMs in the soil was affected by anthropogenic activities.The findings of this study revealed that human activities are the main factor that significantly affects the concentration of HMs.HMs contamination indices in the soil showed that the studied HMs were contaminated in the soil of agricultural fields.According to the zoning maps of Co, Cu, lead, and Cr, there was no contamination in the soil of wheat fields, but Cd and Zn were highly contaminated and also, the Assessment of soil pollution of wheat farms showed that the best effect of toxicity was related to Cd.Moreover, the zoning of nickel-metal concentration distribution showed that the source of this metal contamination had geological activities and anthropogenic aspects in the region.The major limitations this study include the large number of tables and figure in this manuscript, the results related to heavy metals in water and cultivated wheat are not presented and will be published in future articles.It is suggested to investigate the ecological risk assessment of heavy metals in wider areas of agricultural fields in the Karun River catchment area.Also, the assessment of the risk of heavy metals in other agricultural products cultivated in the northern areas of Ahvaz city should also be investigated.On the other hand, the risk assessment of health hazards and ecological hazard of arsenic and mercury metals in agricultural soils and cultivated crops in fields in important areas of Khuzestan province should be investigated.

Figure 1 .
Figure 1.Location map of the sampled site with sampling points ((a): Arab Asad region (first part from downstream of Shushtar city to Qir dam; (b): Weiss region (second part sampling from Bandaqir to the north of Ahvaz city).

Figure 2 .
Figure 2. Hypothetical geographical network with three lengths and four latitudes for sampling in each of the two study areas (the intersection of the two axes X and Y are the sampling areas, and the numbers above each location are the sampling location code).

Figure 3 .
Figure 3. HMs concentration distribution map in surface soils in Weiss and Arab Asad regions (a: zoning of HMs Arab Asad; b: zoning of HMs Weiss).

Table 1 .
Maximum and minimum values of the PI and IPI in the samples.

Table 2 .
Concentration of HMs (Mean ± SD) in the soil of farms in the Weiss and Arab Asad regions.Different letters (a and b) in each row show a significant difference (P < 0.05).

Table 3 .
Average 32,39ntrations of HMs in the Weiss and Arab Asad regions and different countries.Iranian Soil Quality Guidelines (ISQG); China Soil Quality Guidelines (ChSQG); Canadian Soil Quality Guidelines (CSQG); the United States of America Soil Quality Guidelines (USQG)32,39.HM S (mg.

Table 4 .
Results of environmental index in agricultural soils of Weiss and Arab Assad regions.