3.1 Concentration and distribution of heavy metals in soil
This study found that the average concentration of heavy metals in the landfill soil of Shiraz City and the nearby residential areas was Ni > Cr > Cu > Co > As > Cd (Fig S1, S2). It shows that the patterns of heavy metal change in the two research areas are similar (Fig. 2). The share of these heavy metals in landfill soil was equal to 42.20%, 29.06%, 17.42%, 7.24%, 3.95%, and 0.13 for Ni, Cr, Cu, Co, As, and Cd metals, respectively; while in a similar study by Thongyuan et al. (2021) in Central, Thailand, the average concentration of heavy metals in the form of Al > Fe > Mg > Mn > Zn > Cu > bi > Cr > Pb > la > Ni > Co > Ga > Cd (36), and in a similar study by Adamcová et al. (2017) in the Czech Republic, the average concentration of heavy metals was as Fe > Mn > Cr > Ni > Co > Zn > Co > Pb > Cd > Hg (37). The soils of stone-cutting plant contained comparatively low amounts of metals, except for cadmium, copper, and cobalt. This was probably due to the area's low exposure to waste and little traffic. Furthermore, the soils in the waste separation plant region exhibited the highest concentration of heavy metals for all elements when compared to other areas. The higher concentration of heavy metals surrounding the recycling plant can be attributed to several factors, including improper handling of hazardous waste, illegal disposal of mixed trash, inadequate management practices, and the absence of leachate collection systems (38). In a study comparing two landfills, Closed and Active, Adelopo et al. concluded that the concentration of metals is higher in the Closed landfill (39). In our study, however, the daily soil coverage of the old and active landfill sites likely contributed to lower concentrations compared to the recycling plant. Among the different sites, the old landfill site exhibited the highest concentration of Ni metal, while the Active (current) landfill site showed the highest concentration of Co and As metals. The compost and vermicompost site had the highest concentration of Cd metal, and the site of Barm Shur-e Olya village had the highest concentration of Cu metal. Cadmium, a heavy metal, is commonly found in plastic waste, batteries, construction materials, and discarded tires that accumulate in landfills. Copper metal has applications as anti-erosion material in landfill sites and is found in electric parts, wires, and oils. Arsenic and its related compounds are found in pesticide, insecticide, and herbicide residues, as well as in lead, copper, and steel alloys manufactured for use in the electronics industry (40). Nickel is released into the environment by some human activities, such as burning fossil fuels, applying synthetic and biological fertilizers, extracting and smelting metal, disposing of garbage from homes, businesses, and cities, and using machine fuel (41). A comparison of elements concentration with the geochemical background concentration of heavy metals in the earth's crust and the world soil average standard shows that the average concentration of all elements, except for Cr, is higher than the average concentration of world soil and the average concentration of all metals is higher than the concentration of the geochemical background of heavy metals in the earth's crust. When comparing the average concentration of heavy metals in the adjacent residential areas to the municipal landfill in Shiraz, all the elements in the residential area have a lower concentration. Our results were consistent with those of studies conducted by Klinsawathom et al. in Thailand, Rinklebe et al. in Germany, and Karimian et al. in Iran. The soils surrounding the landfill were impacted, according to research, which is consistent with our results (37, 42, 43). In general, factors like rainfall, the amount of heavy metal deposition in the soil, the concentration of heavy metals in the leachate, and the length of time that heavy metals are absorbed determine how different the concentration of heavy metals is in landfill soil (44). The results of the Kolmogorov-Smirnov and Shapiro-Wilk tests show that the concentration of heavy metals, except for Cd, does not follow a normal distribution. (P-value < 0.05) (Fig S3). Non-parametric Mann-Whitney the U test was used to compare the concentrations of heavy metals in two seasons, with the exception of Cd. This test showed a statistically significant difference in the arsenic heavy metal between the two seasons (P-value = 009) (table S1). Independent t-test analysis was used to compare the Cd heavy metal, which is normal, in summer and winter (table S2). This test shows that there is no statistically significant difference in the amounts of Cd metal between summer and winter. (P-value < 0.05).
3.2 Determining the relationship between heavy metals and soil physical and chemical parameters
High correlation coefficients (r) between different heavy metals can indicate that the metals were contaminated from the same source or that they went through similar chemical and physical processes. Statistically, if low (r ≤ 0.1), medium (0.1 < r ≤ 0.3), high (0.3 < r ≤ 0.6), and very strong interrelationship (r > 0.6) (45). The results showed that there is a very strong and significant positive relationship between heavy metals Cr-Ni (r = 0.86**p < 0.01), and Ni-Co (r = 0.766**p < 0.01). and between heavy metals Cr-Co (r = 0.561** p < 0.01), Cu-As (r = 0.475*p < 0.05), Co-As (r = 0.413*p < 0.05), had a strong and significant positive relationship. There was a strong and significant negative correlation between EC-Cr (r =-0.412* p < 0.05) and EC-pH (r =-0.519** p < 0.01) and no significant relationship was found with other metals. No significant relationship was found between heavy metals Cr, Ni, As and pH. Temperature has a strong and significant negative correlation with As (r =-0.481* p < 0.05), and pH (r =-0.582**p < 0.01). As-OC (r = 0.550*p < 0.01) has a strong and significant positive relationship, but with the temperature parameter (r =-0.694**p < 0.01) it has a very strong and significant, negative correlation. Cr-CEC (r = 0.721**p < 0.01), Ni-CEC (r = 0.683*p < 0.05), Co-CEC, (r = 0.707*p < 0.05) It has a very strong and significant positive relationship (Table 6).
Table 6
The Spearman correlation analysis of metal concentration in the soil
Variables | Cr | Cu | Ni | Co | As | Cd | pH | EC | Temperature | OC | CEC |
Cr | 1.0 | | | | | | | | | | |
Cu | 0.121 | 1.0 | | | | | | | | | |
Ni | 0.860** | 0.071 | 1.0 | | | | | | | | |
Co | 0.561** | 0.171 | 0.766** | 1.0 | | | | | | | |
As | 0.297 | 0.475* | 0.342 | 0.413* | 1.0 | | | | | | |
Cd | 0.046 | 0.339 | -0.230 | -0.395 | -0.168 | 1.0 | | | | | |
pH | 0.342 | -0.096 | 0.384 | 0.162 | 0.378 | -0.152 | 1.0 | | | | |
EC | -0.412* | 0.190 | -0.335 | -0.148 | -0.017 | -0.047 | -0.519** | 1.0 | | | |
Temperature | -0.197 | -0.216 | -0.309 | -0.076 | -0.481* | 0.163 | -0.582** | 0.061 | 1.0 | | |
OC | 0.121 | 0.278 | 0.164 | 0.070 | 0.550** | -0.246 | 0.306 | -0.072 | -0.694** | 1.0 | |
CEC | 0.721** | 0.159 | 0.683* | 0.707* | 0.049 | -0.262 | 0.273 | -0.322 | 0.175 | -0.315 | 1.0 |
** Correlation is significant at the 0.01 level (2-tailed). EC: electrical conductivity pH: power Hydrogen
* Correlation is significant at the 0.05 level (2-tailed). CEC: Cation exchange capacity OC: organic carbon
.Correlation is significant at the 0.05 level (p < 0.05) *
There are different methods for interpolation in GIS. In this study, the IDW method was used. The result is shown in Fig. 3 for the landfill site and its surrounding residential areas. The interpolation results showed that station No.10 of the recycling plant site, as an anthropogenic source in the landfill site, has the most pollution. On the other hand, station No.8 of the future landfill site also shows low pollution and violet color due to the digging that took place in this station. Villages and residential areas (stations 5, 7, 11, 4, and 1) also showed a lower degree of pollution in the interpolation than the landfill site.
3.3 Physical and chemical characteristics of the soils
Soil pH is one of the important factors that affect physical and chemical properties, biological pathways, and soil properties, as well as plant growth and biomass performance. Soil pH can control the dynamics bio, availability, and solubility of heavy metals. Also, soil pH affects the solubility of organic matter and the activity of soil microorganisms (46). The results of the measured pH in the soil showed that this characteristic has changed in the range of 7.29–8.21, its average is 7.8, and the soil of the region is alkaline. Meanwhile, the acidity of the leachate at the location of the leachate lagoon has been measured at 8.04 and the acidity of the leachate from the machinery entering the landfill has been measured at 3.35. The first sign of the methanogenic phase or mature leachate is a slightly high pH in the leachate analysis. According to Tchobanoglous et al. (1993), in new landfills, the pH is in the range of 4.5 to 7.5 and in the mature landfill, the pH is variable in the range of 6.6 to 7.5 (47). Zhou et al. stated that the results of pH ranged from 7.54 to 10.90 with an average of 8.59 (48). The high concentration of calcium in the analyzed samples confirms that the soil of the landfill site has carbonated compounds and causes the alkalinity of the soil created from them.
EC values ranged from 242 to 5020 (µs/cm), and the highest EC value was found in the sampling station of Barm Shur-e Olya, with a value of 5020(µs/cm) in the summer season; when the EC of the soil reaches less than 200, it indicates a decrease in soil nutrients and as a result, a lack of nutrients for plant growth which causes a decrease in soil fertility. If the EC value reaches above 1600, it indicates high soil salinity (49). In Obiri-Nyarko et al., EC values were expressed in the range of 510–1454 (µs/cm) at the Kpone (17). Mirsal et al. concluded that the accumulation of heavy metals in plants occurs less in soil with lower EC (50). According to Rattan et al., one of the most important parameters in the soil nutrient cycle is soil organic carbon (51). Soil organic matter is one of the important factors in agriculture due to its effect on nutrient mineralization, soil microbial community structure, soil porosity, and water penetration, as well as water retention capacity, reduction of soil crust, and apparent density (52, 53). OC values ranged from 0.24 to 2.58. Fig S4 and the average value was 1.09 and the highest OC value was found in the sampling station of the medical landfill in the winter season. Ideriah and (2001) Ibitoye mentioned that the increase of OM and OC in landfill soil is high due to high amounts of degradable waste materials (54, 55). In the summer season, due to the stimulation of biological activity due to the increase in temperature, the CEC of the soil increases, and this increase in the cation exchange capacity can increase the precipitation and complexation of metals. Also, soil CEC plays a role in absorption and retention processes (56, 57). CEC values ranged from 2.05 to 29.97 (meq/100g), and the average was 13.11(meq/100g), and the highest CEC value was found in the sampling station of the old landfill in the summer season (Fig. 4). In Fonge, B. A. et al., CEC values ranged from 9.61 to 16.20 in Cameroon (58). The average soil temperature during the two seasons was 27.5 ◦C; its highest value was 48 ◦C in the summer season, and its lowest value was 13 ◦C in the winter season. AMT-300 device qualitatively showed soil moisture as DRY in summer and Wet in winter.
The results of the soil pollution index of the studied area show that the landfill site in Barm Shur-e Olya, Shiraz, has a low to very high degree of pollution. The findings indicated that the soils in the sampling station of the recycling plant show very high levels of pollution for all elements, and for copper metal in most of the stations, except for the station of the waste separation plant, the pollution index shows a low degree of pollution. The highest value of PI is related to As and Ni metal, and the lowest one is related to Cu metal (Table 7); among the reasons for these metals can be the leachate from waste and materials used in agriculture, which has a high concentration of heavy metals such as Cu, Cd, Ni, and Zn. Other uses of these metals can be mentioned as raw materials and products used in homes and fungicides, pesticides, as well as fertilizers and herbicides (58). In Mavakala et al., the results of the pollution index showed that the study site has low to very high pollution. For the elements of Co, Cu, Zn, Cd, Pb, Hg, and Cu, the value of the pollution index was greater than 1. The highest PI value was related to Zn metal, and the lowest value was related to Cr metal (16). In Hosseini Beinabaj et al., the value of the pollution index for all elements Pb, Cd, Mn, Ni, Cu, and Fe was less than 1, which shows a low-degree pollution index (45). Penetration of heavy metals in soil is expressed using the Pollution Load Index (PLI) parameter (59). The results of the pollution index of the sampling stations showed that the active landfill site, the old landfill site, and the sampling station of Tayūn Village had a high degree of pollution (2 < PLI < 3), and the rest of the stations had a medium degree of pollution.
The reason for the low PLI in the medical landfill, despite the waste containing more metals, can be explained by the existence of a landfill based on the principles and compliance with scientific rules and the daily lime coating to reduce the spread of dust particles (38). In contrast, the site of the recycling plant shows a very high degree of pollution (PLI > 3). The reason for the high level of pollution in the recycling plant site can be attributed to the breakdown of various wastes in the landfill (waste separation plant) and heavy inputs of plastic, iron and glass (38). The average PLI in the analyzed soils is 2.58, which shows the high pollution of these soils. In a similar study by Sadeghi Poor Sheijany et al., the PLI for all stations had a pollution-low degree (PLI < 1) (60), which is not consistent with the results of the present study. Dirisu et al. (2019) reported PLI > 1 for soil from the Ewhere landfill in Nigeria, which was consistent with the results of the present study at many stations (61).
Table 7
The values of Pl and PLI for various soil sampling points
| Pl | |
Sampling points | Cr | Cu | Ni | Co | As | Cd | PLI | Level |
1 | 1.80 | 1.65 | 3.74 | 1.29 | 2.35 | 2.31 | 2.06 | High level |
2 | 2.03 | 0.67 | 4.4 | 1.65 | 2.40 | 1.74 | 1.85 | Moderate |
3 | 1.85 | 1.22 | 4.47 | 2.15 | 3.16 | 1.58 | 2.18 | High level |
4 | 2 | 0.72 | 4.27 | 1.5 | 2.22 | 1.89 | 1.83 | Moderate |
5 | 1.96 | 0.87 | 3.74 | 1.43 | 2.19 | 1.97 | 1.84 | Moderate |
6 | 2.23 | 0.62 | 4.87 | 1.5 | 2.08 | 1.85 | 1.84 | Moderate |
7 | 1.71 | 0.7 | 3.5 | 1.29 | 2.43 | 1.97 | 1.71 | Moderate |
8 | 1.91 | 0.62 | 4.3 | 1.65 | 1.98 | 1.77 | 1.52 | Moderate |
9 | 2.30 | 0.79 | 4.94 | 1.93 | 2.40 | 1.77 | 2.04 | High level |
10 | 11 | 5.29 | 22.8 | 8.08 | 22.82 | 6.35 | 10.76 | Extremely high |
11 | 1.37 | 0.77 | 2.6 | 1.43 | 1.69 | 1.97 | 1.53 | Moderate |
12 | 1.98 | 1.05 | 3.8 | 1.29 | 1.79 | 2.66 | 1.90 | Moderate |
3.4 Ecological Risks
The results of the ecological risk index (Eri) are shown in Table 8. The assessment of the ecological risk of heavy metals in the area showed Cd and As have an important role in determining the ecological risk in the landfill site in Shiraz and the surrounding areas. The heavy metals As, Co, Cr, Cu, Ni, and Cr have low ecological risk potential (Eri < 40), while Cd has a medium ecological risk index (80 ≤ Eri < 40). The ERI index for residential areas and all other landfill investigation sites are in the low pollution class, except for the location of the recycling plant, which has extremely high pollution. These results are consistent with those of Sadeghi Poor Sheijany et al., who reported that the soil at the Saravan landfill site in Gilan, Iran, has an Eri of less than 40 for Pb, Zn, Cu, and Cr. The assessment of the ecological risk of metals in the area showed that As, Hg, and Cd were important metals in determining the ecological risk in the areas surrounding the Saravan landfill site. The average value of the ERI indicated a Moderate level of pollution (60). In Wang et al. (2020) study, the ERIs elements varied from 46.72 to 482.43. It was found that 42% of the landfill site and 58% of the landfill site had high risk and moderate risk, respectively. It was also shown that the enhanced environmental risk is due to a higher concentration of As and Hg elements (4).
Table 8
The values of \({\varvec{E}}_{\varvec{r}}^{\varvec{i}}\) and ERI for various soil sampling points
Sampling points | As | Cd | Co | Cr | Cu | Ni | ERI | Level |
1 | 23.5 | 69.3 | 2.58 | 3.6 | 8.25 | 18.7 | 125.93 | Low |
2 | 24 | 52.2 | 3.3 | 4.06 | 3.35 | 22 | 108.91 | Low |
3 | 31.6 | 48 | 4.3 | 3.7 | 6.1 | 22.35 | 116.05 | Low |
4 | 22.1 | 56.7 | 3 | 4 | 3.6 | 21.35 | 110.85 | Low |
5 | 21.9 | 59.1 | 2.86 | 3.92 | 4.35 | 18.7 | 110.83 | Low |
6 | 20.8 | 5.55 | 3 | 4.46 | 3.1 | 24.35 | 61.26 | Low |
7 | 24.3 | 59.1 | 2.58 | 3.42 | 3.5 | 17.5 | 110.4 | Low |
8 | 19.8 | 53.1 | 3.3 | 5.88 | 3.1 | 21.5 | 106.68 | Low |
9 | 24 | 53.1 | 3.86 | 4.6 | 3.95 | 24.7 | 114.21 | Low |
10 | 228.2 | 190.5 | 16.16 | 22 | 26.45 | 114 | 597.31 | Very high |
11 | 16.9 | 59.1 | 2.86 | 2.74 | 3.85 | 13 | 98.45 | Low |
12 | 17.9 | 79.8 | 2.58 | 3.96 | 5.3 | 19 | 128.54 | Low |
3.5 Health Risks
3.5.1 Non-carcinogenic Risk
The non-carcinogenic risk for adults and children in the study area is calculated using all three ways of ingestion, inhalation, and dermal contact in Tables S3, S4. Acceptable values were found for both HQ and HI (HQs < 1, HI) in residential areas and landfill sites. Therefore, there is no non-carcinogenic risk of the investigated metals for children and adults (Fig. 5). The non-carcinogenic risk assessment indicates that ingestion is the main source of exposure; for all metals in both age groups, the highest and lowest HQ values are HQ ingestion > HQ dermal > HQ inhalation. The total HI values for heavy metals in the soil samples of the study area were calculated as 0.3933 and 0.0854 for adults and children, respectively.
These results indicate that children have a four times higher non-cancer risk than adults. This is because children are more likely to be exposed to heavy metals due to their physiological and behavioral characteristics, such as running, bouncing, playing in the soil, sucking on fingers, toys, and heavy breathing (62).
3.5.2 Carcinogenesis Risks
In Tables S5 and S6, the daily exposure to the carcinogenic risk of heavy metals studied are shown; also, in Tables 9 and 10, the risk of carcinogenicity of As, Cd, Cr, and Ni heavy metals through the three routes of ingestion, inhalation, and dermal contact for children and adults in landfill soil and surrounding residential areas is stated. Ni metal and the children age group showed the highest risk of carcinogenesis in the soil of the landfill site and its residential areas. For nickel metal, the value of risk carcinogenesis is 1.98×10− 4 in landfill soil and 1.005×10− 4 in residential areas, respectively, for children.
, in the pathway of ingestion. The carcinogenic risk of heavy metals studied both in Shiraz landfill and its residential areas was as follows: Ni > Cr > As > Cd. The TCR values for heavy metals in landfill site soil were 2.57×10− 4 for children and 1.60×10− 4 for adults. which is more than 1×10− 4 and is unacceptable and dangerous for human health. The TCR values of heavy metals in the soil surrounding the landfill and residential areas were 1.30×10− 4 for children and 9.17×10− 5 for adults. For children, this is higher than 1×10− 4, which is unsuitable for human health. It isn't safe.
In contrast, the range of risks for adults is 1×10− 6 to 1×10− 4, indicating an acceptable or tolerable risk. The elements under study in the landfill Shiraz do not present a non-carcinogenic risk, thus it is critical to concentrate on the long-term, chronic impacts of these metals and how they influence adults and children. Also, as the recycling plant workers come into direct touch with the heavy metal pollution there, particular consideration should be given to their well-being, including the creation of rest areas specifically designed for them. In the study by Karimian et al. (2021) in Tehran, Iran, the hazard index (HI) value was 6.5 times higher in children than in adults; nonetheless, this value was at a safe level for both landfill workers and residents of the target area (HI < 1), which was consistent with our results (38). Obiri-Nyarko et al. reported that for all routes, the ingestion route HI was 1.72, higher than the recommended threshold of 1. And the TCR is higher than the limit for both adults (8.54×10− 4) and children (6.19×10− 3). Also, the risk of cancer was higher in adults and children with arsenic than with lead (18). whereas, nickel in our study had the greatest impact on the risk of carcinogenesis in both the adult and child groups
Table 9
The carcinogenic risk index of heavy metals in the area surrounding the Shiraz landfill
| CR ingest | CR inhale | CR dermal | CR total |
| Adults | Children | Adults | Children | Adults | Children | Adults | Children |
As | 3.57E-06 | 6.37E-06 | 5.28E-09 | 1.79E-09 | 6.51E-08 | 4.35E-08 | 3.64E-06 | 6.41E-06 |
Cd | 9.21E-07 | 1.64E-06 | 1.44E-10 | 4,79E-11 | - | - | 9.21E-07 | 1.64E-06 |
Cr | 1.12E-05 | 2.005. E-05 | 1.38E-07 | 4.73E-08 | 3.36E-06 | 2.24E-06 | 1.46E-05 | 2.23E-05 |
Ni | 5.25E-05 | 9.40E-05 | 3.82E-09 | 1.30E-09 | 2.01E-05 | 6.54E-06 | 7.26E-05 | 1.005E-04 |
| Adults | Children | | | | | | |
Total | 9.17E-05 | 1.30 E-04 | | | | | | |
Table 10
The carcinogenic risk index of heavy metals in the soil from the Shiraz landfill
| CR ingest | CR inhale | CR dermal | CR total |
| Adults | Children | Adults | Children | Adults | Children | Adults | Children |
As | 8.61E-06 | 1.53E-05 | 1.27E-08 | 4.34E-09 | 1.57E-07 | 1.04E-07 | 8.77E-06 | 1.54E-05 |
Cd | 1.12E-06 | 2.01E-06 | 1.71E-10 | 5.85E-11 | - | - | 1.12E-06 | 2.01E-06 |
Cr | 2.11E-05 | 3.78E-05 | 2.61E-07 | 8.90E-08 | 6.34E-06 | 4.22E-06 | 2.77E-05 | 4.21E-05 |
Ni | 1.04E-04 | 1.85E-04 | 7.59E-09 | 2.58E-09 | 1.95E-05 | 1.30E-05 | 1.23E-04 | 1.98E-04 |
| Adults | Children | | | | | | |
Total | 1.60E-04 | 2.57 E-04 | | | | | | |
El Fadili et al.'s study in Morocco (2022) reported that the hazard index (HI) levels for elements were higher than the safe threshold (HI > 1) for children. Likewise, the total carcinogenic risks of Pb and Cd for each group are less than the EPA threshold, indicating an acceptable level of carcinogenic risk. Many researchers have reported that arsenic is the main cause of both carcinogenic and non-carcinogenic concerns in their previous research, which is inconsistent with our findings. Most likely, the causes are the relatively high arsenic concentration in the soil and/or low Rfd. Exposure to high arsenic concentrations can have negative effects on human health, including cancers of the skin, lungs, prostate, bladder, liver, and other organs, as well as diseases of the skin, reproductive system, circulatory system, neurological system, and heart (17). The results of this study indicated that Ni was the main source of carcinogenic risks. Children who work near the residential areas of the Shiraz City landfill and the workers at the landfill site may have negative effects such as coughing, lung cancer, headaches, nausea, vomiting, stomach issues, visual, discomfort, pain, and dizziness (64). Consistent with our findings, other studies have reported that ingesting soil exposure individuals to carcinogenic and noncarcinogenic risks (17, 38, 60, 65). In contrast to dermal contact, leading to allergic reactions and skin inflammation, ingesting metals damages the mucosal tissue of the gastrointestinal tract and can induce both acute and chronic liver disorders (65).