Abstract: Background, aim, and scope Northern Shaanxi is the birthplace of the oil industry. In the process of large-scale exploitation of oil, it will inevitably lead to leakage of oil spill, which causes a certain degree of pollution of soil and groundwater. In order to investigate the effect of oil pollution on soil water characteristics in northern Shaanxi, we conducted this study. Materials and methods In this study three different soils (Lou soil, Loess soil and Aeolian sandy soil) were tested for soil wettability and saturated hydraulic conductivity at four different pollution gradients (0.5%, 1%, 2% and 4%), with clean soil as a control. Results Soil moisture of Lou soil, Loess soil and Aeolian sandy soil infiltrated quickly at 0% and 0.5% oil contaminated concentration, wetting grade 0, no water repellent. The Lou soil began to show slight water repellency at 4% oil treatment. Starting from 2% oil content, the water repellency of Loess soil was enhanced. At 1% oil content, the Aeolian sandy soil exhibit strong water repellency and extreme water repellency at 4% oil content. The effect of oil pollution on soil wettability is: Aeolian sandy soil, Loess soil and Lou soil. The oil pollution has a significant effect on the saturated hydraulic conductivity of the three soils. The saturated hydraulic conductivity of the Lou soil and Loess soil was on the order of 10-5, the Aeolian sandy soil was on the order of 10-3. At the same pollution concentration, the saturated hydraulic conductivity of Loess soil is larger than that of Lou soil. The effect of pollution concentration on the saturated hydraulic conductivity of three soils was significant at low oil content. With the increase of oil content, the trend of saturated hydraulic conductivity decreased gradually, and this effect was more significant in Aeolian sandy soil. Discussion Clean soil usually has a certain degree of hydrophilicity, but when the soil is contaminated by oil organic matter, the surface wettability may change from the hydrophilic surface into a hydrophobic surface, resulting in varying degrees of water repellency, causing the Soil water holding capacity reduced. After the oil pollutes the soil, the residual oil in the soil contains a lot of polar molecules. In the case where the soil is relatively dry or the moisture content is very low, the polar molecules will have the characteristics of oil wetting on the surface, which will cause the soil hydrodynamic characteristics to change greatly. The change of soil hydrodynamic characteristics will seriously affect the in situ treatment of petroleum contaminated soil. Our study showed that oil pollution has a significant effect on soil wettability, which has the greatest effect on the wettability of sandy soil and the smallest soil. The content of clay particles and hydrophilic components in the sandy soil is low, so the hydrophobic oil pollutants can greatly weaken the soil surface tension and increase the soil contact angle. Soil water transports, surface water to groundwater recharge, the migration of pollutants are closely related to soil permeability. We found that with the increase of oil content, the saturated hydraulic conductivity of sandy soil gradually decreased and the decreasing trend gradient was gentle. The possible reason for the slowdown in the saturated hydraulic conductivity of sandy soil is that the oil pollution causes the soil surface to produce some hydrophobicity, reduces the resistance of water infiltration, and relieves the trend of decreasing the saturated hydraulic conductivity. On the one hand, oil pollution of the soil will plug the soil moisture migration channel, on the other hand will make the soil surface to produce a certain degree of hydrophobicity to reduce the water migration resistance, these two areas together, causing soil permeability changes. Conclusions Oil pollution has a significant effect on soil water characteristics. When the soil is polluted by oil, it caused the wettability to change, resulting in a certain degree of water repellency. This change is particularly evident in sandy soils. The oil pollution of the three soils showed a decreasing trend with the increase of the pollution concentration. Although the saturated hydraulic conductivity of oil-contaminated soil was reduced, it does not produce an order of magnitude transition over an order of magnitude for the soil of the same soil. Recommendations and perspectives The oil industry is of great significance to the development of the national economy, but the problem of soil and water pollution caused by it cannot be neglected. It is very important to study the influence of oil pollution on soil water characteristics in northern Shaanxi. It should be further studied to further clarify the influence of oil pollution on soil hydrodynamic characteristics, so as to provide theoretical basis for oil pollution remediation.
Keywords: Oil pollution; Soil water; Soil wettability; Saturated hydraulic conductivity