Study On Controlling Factors And Monitoring Technology Of NAPLs Underground Migration

At present the remediation of Non-aqueous Phase Liquids (NAPLs) contaminated soil is difficulty, NAPLs pollution period is long, the NAPLs migration characteristic in soil and its controlling factors are not very clear, and the pollution scope and extent is difficult to determine. To study the laws and monitoring technique of NAPLs migration in subsurface soil will provide valuable basic data and experimental support for subsequent repair work and make repair measures more economic efficient. In this paper the Light Non-aqueous Phase Liquids (LNAPLs) and Dense Non-aqueous Phase Liquids (DNAPLs) migration in different lithology combination are simulated, and the rules and controlling factors are analyzed. At the same time LNAPLs is taken as an example to study the electrical response characteristics and mechanism of different kinds of pollution soils, the resistivity formula is set up, and the corresponding electric models are developed. In addition, the dynamic monitoring of LNAPLs vertical transport process underground is carried out by resistivity method. Finally, the LNAPLs three-dimensional migration process in soil is explored using resistivity monitoring method. The aims are to understand the NAPLs transport processes and mechanism under different controlling factors and impacting mechanism systematically, and discuss the electrical rules of LNAPLs polluting underground aquifer, monitoring method and electrical reflection to different transport processes to study NAPLs pollution and residual mechanism further.The concrete works and main results are as follows:(1) NAPLs transport simulation tests are carried out using diesel and tetrachloroethylene through three kinds of different lithology combination indoor, the controlling factors of free NAPLs migration path and speed are analyzed thoroughly, and capillary force equilibrium models of NAPLs interface and laws of migration in different soil zone are developed. Results show that the effect laws of NAPLs type, soil initial water content, the local low permeability lens, leakage position and lithologic mutation interface on NAPLs migration path and velocity are unlike. The NAPLs movement rules should be expressed in accordance with partitions as related to the initial water content namely dry soil area, capillary zone and water-saturated zone. The vertical movement and lateral extension of NAPLs are interdependent, and the vertical migration is given priority to in leakage initial stage. The vertical migration is hindered by lateral growth when meeting local low permeability lens, lithology mutation interface or capillary fringe. Vertical migration reduction is mainly caused by the pore resistance, NAPLs residual and lateral spreading and the causes of corresponding increase of horizontal expansion speed are the decrease of vertical velocity and the increase of buoyancy.(2) The water level increase and reduce and rainfall process are simulated indoor, the influences of the water level fluctuation and rainfall leaching on NAPLs redistribution are studied, and the final spatial distribution characteristics of NAPLs are represented by multispectral image analysis. Results show that the water level fluctuation lead to NAPLs capture for two phase flow saturation area, and make residual NAPLs to release for two phase flow unsaturated zone. Rainfall mainly affects NAPLs redistribution through two mechanisms, namely the infiltration displacement and lifting water level. NAPLs pollution concentration decreases from the center to the side of the migration body, and the pollution degree in the upper zone of the local low permeability layer and the top/bottom of lithologic mutation interface is heavy.(3) The influences of water saturation, oil saturation and porosity on resistivity of LNAPLs contaminated soil is studied by the Miller Soil Box using sand, silt and silty loam as test soil with motor oil, diesel oil and gasoline as LNAPLs. Then the primary and secondary of influence factors is developed through the grey relational grade analysis of the data. And then the pore fluid mixed saturation is introduced to Archie formula. At last the electrical conductivity models of LNAPLs contaminated soil are put forward. The results show that the resistivity of LNAPLs polluted soil decreases as power function with the increase of water saturation in consistent with Archie formula and the determination coefficient is99%. Under different initial moisture content the resistivity change principles of LNAPLs polluted soil are discrepant along with contamination extent variation. The resistivity change rule of polluted soil is complex which appears low resistivity anomaly when the water content is5%, and the resistivity reduces first and then rises with the oil saturation increase when the water content is15%. Both formation factor F and water saturation exponent n of LNAPLs contaminated soil reduced and the impact degree on resistivity is water saturation>oil saturation>porosity. The calculated value and the measured value are more close to each other after introducing the mixed pore fluid saturation to the Archie formula.(4) The LNAPLs vertical diffusion dynamic process is monitored by self-made Resistivity Automatic Monitoring System through indoor simulation test, and the migration front of different time is determined by wavelet analysis. The results show that the resistivity change trends caused by diesel infiltration are diverse in different water-saturated sand, and floating oil thickness can be estimated through resistivity change curve. Wavelet analysis can be used to determine LNAPLs migration front, the calculated value agrees with the observed value well and the relative error is between0.8%and1.6%.(5) LNAPLs underground three-dimensional migration processes is simulated and monitored using self-made three dimensional seepage water tank and high density mini-electrode array, the reflection ability of high density resistivity method for LNAPLs migration process is discussed. Then the detection effect of the self-made Resistivity Automatic Monitoring System for multipoint measurement simultaneously is explored for LNAPLs space transport. The results show that the NAPLs pollution process is reflected comprehensively by the Pole-Pole device. The apparent resistivity section performs high resistance anomaly regard to LNAPLs pollution, however it can’t distinguish the pollution range and it is necessary to reconstruct the resistivity profiles through inversion. The apparent resistivity is bigger after log inversion, but it can well reflect LNAPLs migration scope of different time and oil-water displacement process by rainfall leaching. Three dimensional resistivity images can roughly reflect LNAPLs pollution scope in space which agrees with chemical analysis results well after the hydrogeological conditions changes. Multiple resistivity rods can be used to monitor dynamically at the same time, and the data can be transmitted synchronously and doesn’t interfere with each other.