# Effect of Water Content on Light Nonaqueous Phase Fluid Migration in Sandy Soil
## 1 Introduction  
Light nonaqueous phase fluids (LNAPLs) can contaminate soil and groundwater and even endanger human life in the event of leaks. In oil extraction and petrochemical production areas, oil spills damage soil structure, reduce soil permeability, and affect microbial growth. Approximately 8 million tons of oil enter the environment every year globally [1], with approximately 600,000 t in China, of which approximately 70,000 t flows into the soil, causing soil pollution [2]. Following soil contamination, oil can easily seep into groundwater through precipitation, surface water, and irrigation channel water [3], thereby polluting the groundwater. In addition, LNAPLs can volatilize into the atmosphere and be taken into the human body via breathing, skin contact, and ingestion, for example, affecting the normal functions of the liver, kidneys, and other organs, and even have considerable carcinogenic, teratogenic, and mutagenic effects on the human body [4]. In 2004, a pipelineleaked thousands of tons of crude oil at Yan'an in Shaanxi Province, resulting in the contamination of hundreds of acres of farmland [5]. In 2015, an oil leak from the Changqing oilfield contaminated dozens of kilometers of rivers, causing serious contamination of the soil environment in Wuqi County, Yan'an. Chemically contaminated sites account for 44.1% of the Beijing-Tianjin-Hebei Construction Land Soil Pollution Risk Control and Remediation List, with petroleum hydrocarbons accounting for up to 42.10% [6]. In 2009, a diesel spill occurred in the Weinan branch of the China National Petroleum Corporation's Lanzhou-Zhengzhou-Changsha oil pipeline during the commissioning of the sub-transmission, which resulted in around 100 kiloiters of diesel flowing into the Chishui River, a tributary of the Wei River. According to Russian scientists, 7% of global crude oil extracted each year enters the environment and contaminates soil and groundwater [7]. Thus, there is an urgent need to investigate the migration mechanism of LNAPLs as well as their occurrence state. The process of petroleum pollutant migration includes convection, diffusion, dispersion, dissolution, volatilization, adsorption, and desorption (Figure 1). Along with groundwater level fluctuations, different pollutant components are continuously exchanged, migrated, adsorbed, desorbed, and diffused in the unsaturated zone, groundwater fluctuation zone, and aquifer [6; 8]. [PERSON] et al. [9] combined HYDRUS-1D software and the TOUGH program to establish numerical simulations and study the effects of migration and retention of LNAPLs during dynamic fluctuations of groundwater level caused by the freeze-thaw process. [PERSON] et al. [10] presented a model to study the influence of groundwater table fluctuations on oil thickness in a well related to an LNAPL-contaminated aquifer, which was compared with field data obtained by continuous measurement of the vertical position of LNAPL in wells. [PERSON] et al. [11] developed an LNAPL simulator to simulate the migration process of LNAPL under water table fluctuations, which is capable of portraying the migration of pollutants relatively accurately. LNAPL moves downward vertically in the unsaturated zone, gradually replacing capillary water. Following an oil leak, it migrates downward into the unsaturated zone through gravity. When it reaches the groundwater level, it floats on the water surface and diffuses laterally, but the overall trend is downward migration [12; 13]. The distribution of water content in the unsaturated zone is the main factor controlling the distribution pattern of LNAPL. The occurrence of LNAPLs in the unsaturated zone can be divided into four categories: free-flowing liquid phase, adsorbed phase (adsorbed on the surface of soil particles), dissolved phase (dissolved in water), and volatile phase (volatilized in the atmosphere) [4; 14]. In the saturated zone, the oil phase and water phase exist. The migration and transformation of LNAPL in the subsurface environment is extremely complicated.