Study On Migration Mechanism And Simulation Of Light Nonaqueous-phase Liquids In Vadose Zone

Light non-aqueous phase pollutant (LNAPLs) is the typical organic compounds, can cause serious soil and groundwater pollution after spilled into vadose zone,the issue of organic pollution has become the core topic of groundwater environment protection.Therefore, under the guidance of the porous medium hydrodynamics solute migration dynamics and the hydrogeology theory, the paper make the diesel oil represents LNAPLs, simulated the pollution behavior of two-dimensional region through the laboratory physical model test, with emphasis on the disciplinarian in vadose zone, and also established the migration multi-phase flow mathematical model of LNAPLs in vadose zone from the perspective of three-phase flow, revealed the the migration mechanism and the influencing factor qualitatively and quantitatively. the main research results are as follows:(1)The displacement experiments between oil-water has carried out in different media by self-physical simulation test device,and revealed migration laws and distribution characteristics of LNAPLs in the vadose zone. The results indicated that diesel oil transfer dominated by the vertical pattern at the initial stage of diesel leakage beause of the gravity controlling by the medium porosity and permeability, the frontal area maintains the rule, the round arc migration before the front on the edge of the capillary. After the front reached the edge of the capillary zone, the degree of front development grows slowly, diesel oil transport with the horizontal expansion and always along the edge of the capillary zone, vertically enters in the capillarity area and the ground water part is very limited. The oil frontal have different distribution characteristics in different media, the oil frontal can be divided into residual oil zone and capillary band together oil in homogeneous medium,and in thin coarse-type medium shows combination of gathered-residual-gathered oil zone,in containing sandwich-type medium shows residual-gathered oil zone alternating type.The characteristic of oil stream frontal concerned with the lithological character, initial aqueous phase pressure, flow field changes. The maximum growth width of frontal area confined by the porosity and the medium penetrating capability, The frontal area crosswise growth enlarges when has weak infiltration medium, it reflected the weak permeable medium has the obvious retardarce to the pollutant retardation.The laws lay the foundation to reveal the mechanism.(2) The relationship between s-p in two-phase system has obtained by sandy funnel test devices, and extended it into three-phase system using conversion theory of F. F. Fagerlund, studied the relationship of k-s-p in three-phase systemly.The results shows the k-s curve shape of oil and water is on concave no matter with which kind of lithological, the aqueous phase relative permeability increases slowly but the oil phase and gas phase relative permeability reduces quickly before the isotonia spot. After being bigger than an isotonia spot degree of saturation, the aqueous phase relative permeability increases but the oil phase reduces the to zero slowly.The results lay the foundation for analysis of multiphase flow and the mathematical simulation.(3) The multiphase flow dynamics model describing LNAPLs in vadose zone migtration was established from the perspective of multi-phase flow, and solve the model using TOUGH2 software, the inversion parameters are basically consistent with the measured parameters,it can provide quantitative support for forecast and analysis mechanism of LNAPLs pollution.(4) Based on physics-simulation and mathematical models, has studied migration mechanism of the LNAPLs in the vadose zone qualitatively and quantitatively.The results shows that the controlling factors of LNAPLs migration in the vadose zone are lithology, structure, and initial soil moisture content.Under a certain amount of leakage in homogeneous media,LNAPLs can reach the capillary zone. easily concerned with the media thickness level. The coarse granular media mains horizontal expansion after the front reached the capillary zone and transverse area in coarse-grained phase is larger than in the fine-grained phase, but the LNAPLs saturation of gathering zone in fine-grained phase is larger than in coarse-grained phase.Lateral migration of oil phase occurred when the water phas saturation threshold of 0.45 regardless of what kind of medium.For the hydrophilic medium, when the water phase occupies more than 45% pore space, LNAPLs migration blocked by water phase, the migration of LNAPLs show horizontal development.In non-homogeneous structure of strata.of sand-based medium,the weak penetration medium shows a certain blocking effect,but for long-term leakage source, it is not sufficient to completely block the downward movement of LNAPLs.Therefore, we should attach importance to pipeline leaks and other issues.The research can provides the theory basis and the experimental support for the quantitative research of migration, pollution forecast and prevention of organic pollutant in underground environment system, also have some theoretical value and practical significance for ensuring the sustainable use of groundwater resources and pollution control effectively.