Study On Migration Law Of LNAPLs At Layered Heterogeneous Interface In Vadose Zone

LNAPLs（light non-aqueous liquids）are often represented by petroleum pollutants in underground storage tanks,oil pipelines and oil fields.After leaking,LNAPLs often contaminate soil and groundwater.As an important channel connecting surface water and groundwater,aeration zone affects the transport mode of LNAPLs and decides the time when LNAPLs reach groundwater.Once LNAPLs enter the soil environment,they will not only harm the whole soil environment,but also risk groundwater.Owing to the different sedimentary conditions of the medium,the vadose zone in nature tends to be heterogeneous.The complex and changeable vadose zone environment will have a significant impact on the migration behavior of LNAPLs.In this paper,diesel oil is used to represent LNAPLs,and the ratio of permeability coefficient of upper and lower media(R=K_up/K_below)is used to represent the difference of particle size between upper and lower media.The migration law of diesel oil at heterogeneous interface in vadose zone was studied by simulated tank experiment.The effects of R value,water content of medium and inclination angle of interface on migration behavior were mainly discussed,which could provide theoretical basis for fine characterization of site contamination plume and risk assessment of groundwater.The main conclusions are as follows:1.A capillary isolation zone will be formed at the horizontal heterogeneous interface of R<1 type.When the oil flow front reaches the interface,it will preferentially develop along the interface level and the vertical migration will be hindered.When a certain amount of diesel oil accumulates,it will break through the capillary isolation zone and show obvious downward migration behavior.The thickness of the diesel lens formed at this time can be estimated by the difference between the dehumidification pressure of the upper fine medium and the moisture absorption turning pressure of the lower coarse medium.2.The thickness of lens formed by diesel oil at R<1 horizontal heterogeneous interface decreases with the increase of water content,and diesel oil is easier to break through capillary isolation zone.When water content is 7%,the change is obvious,mainly because displaceable pore water appears at this time.When water content reaches 10%,the interface basically loses its interception ability.Within a certain range（0%¹0%）,the time of diesel arriving at the interface of dielectric first decreases and then increases with the increase of water content.The residence time of diesel wetting front at the interface of dielectric decreases with the increase of moisture content,and the retention of diesel oil decreases.3.The wetting front curvature of the lower fine medium is smaller than that of the upper coarse medium when diesel reaches the R>1 interface.The difference of wetting front curvature between upper and lower media decreases first and then increases with the increase of water content.When the water content is 0%,there is a turning point of the rate from high to low at the interface.The turning point of the rate first decreases and then increases with the increase of the water content of the medium.At the same moisture content,the rate transition of wetting front at the interface increases with the increase of R.4.R<1 inclined layered heterogeneous interface has certain conductivity to diesel oil.Increasing the inclined angle of the interface can enhance its conductivity,but this conductivity will be greatly weakened by increasing the moisture content of the medium.According to the theoretical formula proposed by Steenhuis,the reasons are analyzed.It is concluded that increasing the water content of the medium reduces the permeability coefficient of diesel oil at the interface and the difference of capillary pressure between the upper and lower layers.