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Chapter 1 Introduction NAPL (Non-aqueous Phase Liquid) is a typical subsurface organic pollutant�� such as tetrachloroethylene�� trichloroethylene�� and methylbenzene. Although the solubility of NAPLs in water is quite low (for example�� the solubility of tetrachloroe- thylene in water is only 0.015 g/100 mL)�� their harmfulness to the groundwater environment is surprising. Many NAPLs pose a high risk of cancer. The World Health Organization��s drinking water standard requires that benzene and trichloroethylene contain no more than 0.005 mg per liter of water�� and vinyl chloride�� which poses a higher cancer risk�� must contain no more than 0.002 mg per liter of water. Although the solubility of many NAPLs in groundwater is quite low�� it is still above the safety standard for drinking water. NAPLs are divided by the density of pure water. One is heavier than water and is referred to as DNAPL (dense NAPL)�� while another is lighter than water and is referred to as LNAPL (light NAPL). Compared to LNAPL�� DNAPL has a higher density than water and sinks continuously after entering the groundwater. DNAPL has the characteristics of concealment and persistence�� and the problems of migration and mass transfer in fractured groundwater are more complex. DNAPL is a very special organic pollutant. Its migration and mass transfer process has the characteristics of soluble and insoluble pollutants. This is mainly because most liquid DNAPL cannot be completely dissolved in a short time and discharged from the subsurface with the runoff�� but remain in the frac medium for a long time in the form of a ��DNAPL pollution pool�� that slowly dissolves and expands its plume with the runoff from the subsurface. The process of DNAPL migrating with the water flow as a liquid in a non-aqueous phase and DNAPL migrating with the water flow after being dissolved by the water is referred to here as mass transfer. The mass transfer process of DNAPL in fractured groundwater consists of the migration of an immiscible two-phase flow and the migration of solutes after the DNAPL is dissolved. The mass transfer of contaminant in fractured media plays an important role in the surface environment problems (e.g.�� contaminant degradation (Sale et al.�� 2008)�� nuclear waste disposal (Bagalkot and Kumar�� 2016)�� and bioremediation (Song et al.�� 2014). In recent decades�� the investigation of dilution and mixing processes in fractured media has been focused by the scholars in various fields (Anna et al.�� 2014; Soltanian et al.�� 2015; Dou et al.�� 2018a; Dou et al.�� 2018b). The characterization of the spreading and mixing processes of the conservative solute is instrumental in understanding and assessing reactive solute transport�� which is necessary for studying complex chemical biological reaction transport in groundwater. Although many studies (Shapiro and Brenner�� 1988; Pini et al.�� 2016) have provided new insights into the mechanisms and properties of the mixing behavior at the Darcy scale or the larger field scale�� little attentions have been focused on porous media at the pore scale. There is always an influence of the smaller-scale processes on the larger scale behaviors (Bear�� 2013). Therefore�� to understand and evaluate mixing processes at pore-scale with suitable methods is very significant (Rolle et al.�� 2013). The migration process of DNAPL in fractured groundwater can be divided into three stages: The first stage is the overall intrusion stage of DNAPL; the second stage is the comigration stage of DNAPL and water flow; the third stage is the residual dissolution stage of DNAPL�� as shown in Figure 1.1.1. The residual DNAPL of fractured groundwater refers to the DNAPL that cannot be displaced by water flow alone under the conditions of natural water flow and pressure. Since the dissolution rate of DNAPL is affected by its interface area�� hydrodynamic conditions�� and geometric characteristics of fractures�� its dissolution content in water can be ignored compared to the penetration of DNAPL in a short time. Generally�� the migration process of DNAPL in the first and second stages is considered only as two-phase immiscible flow migration process�� while the solubility of DNAPL in water is ignored. In the third stage�� the remaining DNAPL is relatively stable and no longer migrates as a whole�� but continuously dissolves in the water body and increases its dissolved pollution plume by the effect of groundwater discharge. In this stage�� the problem of DNAPL dissolution process and solute transport with DNAPL as solute must be considered. The first and second stages directly affect the distribution characteristics of DNAPL residues in fractured media in the third stage�� and the third stage is an important reason for the persistence and concealment of DNAPL pollutants. The spatial heterogeneities of the pore space in aquifers lead to the complex fluid flow and anomalous solute transport. As the occurrence of preferential flow paths and/o