Investigating the level of pollution and assessing the environmental risk of soil to polycyclic aromatic compounds in Ahvaz oil field using ERL, ERM, and TEF index





Introduction



Polycyclic aromatic hydrocarbons (PAHs) are of greater concern among petroleum hydrocarbons due to their toxicity, low evaporation, resistance to microbial degradation, and high tendency to form deposits. Oil spills usually hurt soil biodiversity. Most soils containing bacteria that can absorb high amounts of hydrocarbons may have the ability to reduce aliphatic and aromatic hydrocarbons. Researchers have concluded that soil containing 2-3% crude oil has the highest number of microorganisms compared to typical soils. It is also reported in research that 8 months after soil contamination with oil, oil-degrading bacteria grew 10 times and accounted for about 5% of the soil microbial population. The presence of petroleum hydrocarbons in the soil leads to major changes in its physicochemical and biological properties. For example, with the entry of sewage, the TDS of the soil usually increases, which leads to a decrease in soil fertility. According to Aniagor et al., oil pollution remains in the soil for more than a hundred years. Soil pollution with petroleum substances is considered one of the world's environmental problems. The accumulation of oil pollutants in the environment has caused a lot of challenges because the soil contaminated with these compounds is unfavorable for agricultural, industrial, or recreational purposes, and it is also considered a potential source for polluting surface and underground water. Therefore, this research was conducted to investigate the level of contamination and assess the biological risk of soil to polycyclic aromatic compounds in the Ahvaz oil field using ERL, ERM, and TEF indices.



Materials and methods

The current descriptive-applied research was conducted in 1402. The studied area includes the soil of 4 oil centers (an oil exploitation, a desalination unit, an oil rig, and an oil pump house) in the southwest of Iran. Samples were taken from 20 locations within 4 oil centers (5 samples within each oil center) with three repetitions. For this purpose, topsoil was removed from 20 stations in the study area using a grab van with a cross-sectional area of 0.1 square meters using a winch tool from a depth of 10 to 14 cm.



Results and discussion

Results of Concentration of PAH in soil samples



The total amount of PAHs among the sampling areas ranges from 9.87 to 2632.688 μg/kg, with an average of 2373.44 μg/kg. The average concentration of compounds with low molecular weight (LMW two and three rings) is 347.152 μg/kg and the average concentration of compounds with high molecular weight (HMW four, five, and six rings) is 60.24 μg/kg. In general, the classification based on the total concentration for PAHs in sediments includes 3 groups: if the total concentration is more than 500 μg/kg as highly polluted, the total concentration is between 250 and 500 μg/kg is moderately polluted and if the concentration is less than 250 μg/kg If it is a kilogram, the pollution is insignificant. Qualitative effects guidelines are also used to correlate effects and total and individual PAH chemical concentrations. Based on this, 30.4% of the stations had very high pollution, 15.17% of the stations had moderate pollution and the rest had low pollution levels.

It was determined that the cyclic aromatic Indene[1,2,3-cd]pyrene (IndPy) with 13.47 has the lowest average PAH and the cyclic aromatic Phenanthrene (Phen) has the highest average PAH with 721.54 μg/kg in the studied area. Also, it was found that Fluoranthene (Flu) and Acenaphthene (Ace) have the lowest ERM. It should be noted that the ERM index could not be calculated for the rest of the cyclic aromatics. Based on the results, Anthracene (Ant) and Acenaphthene (Ace) have the lowest ERL. It should be noted that the ERL index could not be calculated for the rest of the cyclic aromatics. High concentrations of PAHs in soil can have harmful effects on the environment and human health. Due to their lipophilic properties, these compounds are easily accumulated in fat tissues and are transferred to living organisms through the food chain. Studies have shown that some PAHs can be carcinogenic and increase the risk of various types of cancer by causing genetic changes in cells. In addition, the presence of PAHs in aquatic environments can lead to a decrease in water and soil quality and affect aquatic life by affecting biological processes such as photosynthesis and cellular respiration.



Conclusion

Continuous and accurate monitoring of PAH concentration in soils and waters near oil centers is of great importance. This monitoring can help identify sources of pollution and assess environmental risks. Also, the development and implementation of pollution management programs that include the use of cleaner technologies, improvement of wastewater treatment processes, and leakage control can help reduce the pollution load of PAHs. Education and increasing public awareness about the dangers of PAHs and the importance of protecting the environment are also other necessary measures in this field.





Keywords: Soil Pollution, Petroleum Compounds, Polycyclic Aromatic Hydrocarbons

(PAHs)