The physicochemical components of soil can have a significant mathematical and statistical relationship with soil pollution levels. In this study, in addition to aromatic and aliphatic compounds, soil parameters such as pH, organic carbon, and soil texture were also measured. The results of the average sampling points in five zones (four polluted zones and one control zone) are presented in Table 1. The results showed that the average amount of organic carbon in the exploitation unit area (zone 1) and oil sludge pit (sample 4) was higher than in other areas. The average amount of organic carbon in the control sample was significantly lower than other samples. This is due to the presence of organic compounds in oil that produce organic carbon during their decomposition process.
Table 1
The results of measuring soil physicochemical parameters in 5 sampling areas
| pH | %OC | %Clay | %Silt | %Sand |
1(Oil exploitation) | 7.31 | 0.19 | 16 | 9 | 75 |
2(Oil desalination) | 7.25 | 0.15 | 15 | 11 | 74 |
3(Oil Rig) | 7.29 | 0.1 | 17 | 9 | 74 |
4(Oil Pump Unit) | 7.3 | 0.18 | 16 | 11 | 73 |
5(Control Area) | 7.47 | 0.06 | 18 | 12 | 70 |
The results of determining the aromatic compounds in soil samples
The results of measuring the amounts of aromatic compounds (16 main compounds) in 5 sampling zones in the Ahvaz oil field area are presented in Table 2. In this study, 15 main compounds of environmental concern were measured. The values presented are the average of 3 repetitions. The comparison of the amounts of multi-ring aromatic compounds in soil samples in the Ahvaz oil field area is shown in Fig. 2.
Table 2
The results of measuring aromatic compounds in soil samples in the Ahvaz oil field (ug/kg)
Row | PAH Components | 1(exploitation) | 2(desalination) | 3(Rig) | 4(Pump Unit) | 5(Control) |
1 | Naphthalene | 2135.84 | 1178.98 | 785.86 | 1611.98 | 402.6 |
2 | Acenaphthylene | 1704.54 | 965.316 | 609.29 | 1915.56 | 496.7 |
3 | Acenaphthene | 1908.5 | 771.68 | 868.25 | 2263.13 | 554.1 |
4 | Fluorene | 2421.04 | 1524.18 | 1294.13 | 2029.49 | 536.5 |
5 | Phenanthrene | 2046.22 | 379.4 | 311.42 | 4032.363 | 457.4 |
6 | Anthracene | 3819.35 | 4034.47 | 2757.793 | 12123.06 | 502.12 |
7 | Fluoranthene | 2070.61 | 756.178 | 938.63 | 692.12 | 65.07 |
8 | Benzo.a.anthracene | 5705.02 | 2680.95 | 2214.05 | 7329.48 | 919.4 |
9 | Chrysene | 3651.73 | 627.67 | 345.41 | 1947.87 | 463.6 |
10 | Benzo.b.fluoranthene | 8048.57 | 4132.7 | 3107.29 | 11113.506 | 1876.5 |
11 | Benzo.k.fluoranthene | 4957.73 | 2761.73 | 1674.67 | 6841.347 | 1502.56 |
12 | Benzo.a.pyrene | 1946.8 | 755.63 | 479.9936 | 2866.75 | 514.68 |
13 | Indenopyrene | 1496.495 | 516.55 | 342.85 | 1166.95 | 365.23 |
14 | Dibenz.a.h.anthracene | 1570.4 | 767.8 | 839.39 | 2226.1 | 482.79 |
15 | Benzo. ghi.perylene | 2137.634 | 566.92 | 358.86 | 1992.91 | 533.89 |
The results of determining the amounts of aromatic compounds in soil samples in the Ahvaz oil field area showed that Benzo.b.fluoranthene had the highest concentration level with an average of 5667.7 ug/kg in the samples. The highest average was in the samples from the oil pump unit pit with an average of 7329.48 ug/kg, and the lowest was in the control samples with an average of 1919.4 ug/kg. The average amounts of Anthracene and Benzo.a.anthracene were 4647.3 and 5657.7 ug/kg. The lowest measured amounts of aromatic compounds were of 777.6 ug/kg. The average amounts of PAHs in the soil samples from the 5 sampling areas in the Ahvaz oil field area are presented in Figure 3.
Based on the results of comparing the total amount of aromatic compounds in the 5 sampling areas (3A) in the Ahvaz oil field, the highest pollution level was related to the oil pump unit area. The total amount of aromatic compounds in this area was 60152.6 ug/kg. In the exploitation area, the total amount of compounds was 45620.4 ug/kg, indicating a high level of pollution with aromatic compounds. The lowest pollution level was related to the control and oil well zones with amounts of 10738 ug/kg and 16927.8 ug/kg, respectively. According to the research results, there was a significant difference between the amounts of different aromatic compounds and also between the average amounts of these compounds in the 5 sampling areas (p < 0.05). Considering the obtained results and the relatively significant difference between the measured amounts of aromatic compounds in different areas and control samples, it can be concluded that oil activities in the Ahvaz oil field area have had an impact on soil pollution with harmful petroleum compounds.
Aliphatic compounds assay results
Aliphatic compounds are naturally present in soil due to the existence of oil and gas sources. Additionally, these compounds are transferred to the soil as waste from oil transportation and refining processes. The components of petroleum aliphatic compounds include long-chain linear hydrocarbons, with various mixtures of paraffins, olefins, and naphthenes. Some of these compounds are highly volatile and therefore have very low stability in soil. In this study, only compounds with medium and high stability were measured. The results of measuring the amounts of aliphatic compounds in 5 sampling areas in the Ahvaz oil field area are presented in Table 3. The values presented are the average of 3 repetitions. The comparison of the amounts of aliphatic compounds in soil samples in the Ahvaz oil field area is shown in Fig. 4.
Table 3
The results of measuring aliphatic compounds in soil samples in Ahvaz oil field (mg/kg)
| 1(exploitation) | 2(desalination) | 3(Rig) | 4(Pump Unit) | 5(Control Area) |
n-C14 | 370.764 | 341.7 | 275.2 | 368.5 | 31.6 |
n-C15 | 328.74 | 311.6 | 304.1 | 357.8 | 24.5 |
n-C16 | 258.888 | 204.9 | 178.5 | 267.1 | 65.2 |
n-C17 | 225.672 | 204.1 | 192.5 | 242.6 | 25.1 |
Pristan | 92.004 | 90.7 | 74.1 | 91.6 | < 10 |
n-C18 | 207.816 | 189.5 | 142.8 | 214.5 | 35.6 |
Phitan | 288.84 | 155.6 | 140 | 307.3 | 28 |
n-C19 | 186.72 | 180.3 | 172.3 | 185.3 | < 10 |
n-C20 | 198.576 | 189.4 | 165.6 | 205.1 | < 10 |
n-C21 | 184.14 | 174.5 | 171.2 | 194.2 | 52.5 |
n-C22 | 173.34 | 171.6 | 156.7 | 171.5 | < 10 |
n-C23 | 167.112 | 156.7 | 151.5 | 173 | 35 |
n-C24 | 139.92 | 135.6 | 126.4 | 140.4 | < 10 |
n-C25 | 124.92 | 123.6 | 111.7 | 129.3 | < 10 |
n-C26 | 122.46 | 106.7 | 94.2 | 121.9 | < 10 |
n-C27 | 20.04 | 21.5 | 18.5 | 22.5 | < 10 |
n-C28 | 106.2 | 105.6 | 85.3 | 111.8 | 11.5 |
n-C29 | 86.76 | 52.1 | 34.5 | 89.5 | < 10 |
n-C30 | 76.92 | 73.8 | 61.4 | 82.4 | < 10 |
n-C31 | 63 | 61.5 | 53.6 | 65 | < 10 |
n-C32 | 53.64 | 54.9 | 45.6 | 56.3 | < 10 |
n-C33 | 50.52 | 38.3 | 29.8 | 51.4 | < 10 |
Total | 3526.992 | 3144.2 | 2785.5 | 3649 | 309 |
Based on the results of comparing the total amount of aliphatic compounds in the 5 sampling areas (3A) in the Ahvaz oil field, the highest pollution level was related to the oil pump unit area, similar to aromatics. The total amount of aliphatic compounds in this zone was 3649 mg/kg. The amounts of these compounds in the exploitation unit and oil desalination areas were 3144.2 and 3526.9 mg/kg, respectively. The lowest pollution level was related to the control samples with a total amount of 309 mg/kg. Some of the aliphatic compounds in the control samples were below the limit of detection (LOD). According to the research results, there was a significant difference between the amounts of different aliphatic compounds and also between the average amounts of these compounds in the 5 sampling areas (p < 0.05). Considering the obtained results and the relatively significant difference between the measured amounts of aliphatic compounds in different areas and control samples, it can be concluded that oil activities in the Ahvaz oil field area have had an impact on soil pollution with harmful petroleum compounds.
The results of the source determination of petroleum compounds in soil samples based on the PMF model Determining the source of pollution is essential to identify the role of humans in causing pollution or the natural causes of pollution. In human-caused pollution, environmental management can be considered to combat pollution. To determine the contribution of different sources to soil pollution with aromatic compounds, the PMF model was used in this study. In some studies, researchers use three to seven factors to examine the sources of compounds. In the present study, based on the existing sources of petroleum pollution in the Ahvaz oil field area, four factors were analyzed as potential sources of multi-ring aromatic compounds in soil samples. The results of distribution of the factors obtained in the PMF model for aromatic compounds in soil samples are presented in Figure 6.
Determining the source of pollution is essential to identify the role of humans in causing pollution or the natural causes of pollution. In human-caused pollution, environmental management can be considered to combat pollution. To determine the contribution of different sources to soil pollution with aromatic compounds, the PMF model was used in this study. In some studies, researchers use three to seven factors to examine the sources of compounds. In the present study, based on the existing sources of petroleum pollution in the Ahvaz oil field area, four factors were analyzed as potential sources of multi-ring aromatic compounds in soil samples. The results of distribution of the factors obtained in the PMF model for aromatic compounds in soil samples are presented in Fig. 6.
Based on the analysis of the PMF model results, factor 1 (Fig. 5A) had a moderate weight in terms of its role in the origin of aromatic compounds. It accounted for 26.18% of the total variance of the factors. In factor 1, which was related to vehicles, Dibenz. a.h.a (62%), Benzi. a.p (52%), Benzo. ghi. p (46%), Indenopyrene (36%), and Benzo. b.f (35%) were indicative of its weight. These compounds are mainly produced from vehicle exhaust emissions.
The results of the model showed that factor 2 (Fig. 5B) was the most important source of aromatic compounds in soil, accounting for 51.52% of the total variance of the factors. In this factor, Phenanthre (83%), Benzo, a, a (81%), Benzo. k.f (79%), Acenaphthene (75%), and Fluorene (64%) formed their weight. These compounds are mainly derived from petroleum compounds and their derivatives in the soil. Phenanthrene is a compound that is mostly found in crude oil. These results are justifiable considering the oil activities in this area.
Factor 3 (Fig. 5C) had the least weight in measuring aromatic compounds in soil samples, accounting for 10.18% of the total variance of the factors. The highest amount of this factor was related to Anthracene (42%). This compound is an important indicator of wood burning, so its source can be attributed to biomass.
Factor 4 (Fig. 5D) had a relatively low weight among the factors, accounting for 12.12% of the total variance of the factors. Chrysene (72%) and Fluorene (27%) had the highest amounts in this factor, which was related to natural gas combustion. These compounds are likely to originate from the gases emitted from gas flares. The percentage distribution of PAH concentrations in each factor is presented in Fig. 7.
Biological risk assessment results
In the present study, the index of the range of effects with low range (ERL) and the range of effects with medium range (ERM) have been used to evaluate the biological risk caused by aromatic compounds in soil samples in 5 sampling areas, the results of which are shown in Table 4 is presented.
Table 4Comparison of measured amounts of aromatic compounds in Ahvaz oil field and its comparison with ERM and ERL indices.
PAH
Component
|
SQGs
|
exploitation
|
desalination
|
Rig
|
Pump Unit
|
Control Area
|
ERL(ppb)
|
ERM (ppb)
|
1
|
2
|
3
|
4
|
5
|
Naphthalene
|
160
|
2100
|
X>ERM
|
ERL<X<ERM
|
ERL<X<ERM
|
ERL<X<ERM
|
ERL<X<ERM
|
Acenaphthylene
|
44
|
640
|
X>ERM
|
X>ERM
|
ERL<X<ERM
|
X>ERM
|
ERL<X<ERM
|
Acenaphthene
|
16
|
600
|
X>ERM
|
X>ERM
|
X>ERM
|
X>ERM
|
ERL<X<ERM
|
Fluorene
|
19
|
640
|
X>ERM
|
X>ERM
|
X>ERM
|
X>ERM
|
ERL<X<ERM
|
Phenanthrene
|
240
|
1500
|
X>ERM
|
ERL<X<ERM
|
ERL<X<ERM
|
X>ERM
|
ERL<X<ERM
|
Anthracene
|
85.3
|
1100
|
X>ERM
|
X>ERM
|
X>ERM
|
X>ERM
|
ERL<X<ERM
|
Fluoranthene
|
600
|
5100
|
ERL<X<ERM
|
ERL<X<ERM
|
ERL<X<ERM
|
ERL<X<ERM
|
X<ERL
|
Benzo.a.anthracene
|
261
|
1600
|
X>ERM
|
X>ERM
|
X>ERM
|
X>ERM
|
ERL<X<ERM
|
Chrysene
|
384
|
2800
|
X>ERM
|
ERL<X<ERM
|
X<ERL
|
ERL<X<ERM
|
ERL<X<ERM
|
Benzo.b.fluoranthene
|
320
|
1880
|
X>ERM
|
X>ERM
|
X>ERM
|
X>ERM
|
ERL<X<ERM
|
Benzo.k.fluoranthene
|
280
|
1620
|
X>ERM
|
X>ERM
|
X>ERM
|
X>ERM
|
ERL<X<ERM
|
Benzo.a.pyrene
|
430
|
1600
|
X>ERM
|
ERL<X<ERM
|
ERL<X<ERM
|
X>ERM
|
ERL<X<ERM
|
Indenopyrene
|
NA
|
NA
|
NA
|
NA
|
NA
|
NA
|
NA
|
Dibenz.a.h.anthracene
|
NA
|
NA
|
NA
|
NA
|
NA
|
NA
|
NA
|
Benzo. ghi.perylene
|
NA
|
NA
|
NA
|
NA
|
NA
|
NA
|
NA
|
The results of the ecological risk index analysis due to the release of aromatic compounds in the environment showed that in the crude oil exploitation center, except for Fluoranthene, all other 12 aromatic compounds with defined levels of the environmental risk slope factor (ERL and ERM) were above the ERM level (medium risk range) and at a high-risk level. In the desalination zone, 7 compounds, in the oil rig area, 6 compounds, and in the oil pumping station area, 10 aromatic compounds in soil samples were at a high-risk level for the environment. In the control area, none of the measured compound levels were above the average level (ERM). These results indicate a high level of environmental risk resulting from the release of petroleum compounds in the soil in the oil field facilities of Ahvaz.