# A Critical Review of the Modelling Tools for the Reactive Transport of Organic Contaminants
## 2 Processes
### Transport  
Despite numerous studies and years of research into hydrocarbon migration, the process remains in the discovery phase. When hydrocarbons enter an aquifer, they move downward under gravity through a vadose zone and partition into subsequent phases [99; 100; 101; 102]:  
* mobile fluid--the mobile free product, mobile non-aqueous liquid phase (NAPL);
* residual liquid--an immobile, trapped portion of the hydrocarbons that occurs above and/or below the water table and is an additional, persistent source of contamination;
* aqueous--dissolved in groundwater and soil moisture;
* sorbed--adsorbed to soil particles;
* volatile--hydrocarbons in the gaseous state, which have evaporated (depending on Henry's constant) from the free product layer and/or dissolved phase and occur mainly in the unsaturated zone. Once NAPLs reach the water table, they can form a visible free-product layer that floats on the surface of the groundwater--LNAPLs (Figure 3). The soluble compounds are dissolved while they are denser than water. DNAPLs migrate vertically downwards into the saturated zone. Both phases, the dissolved and the free, migrate or spread laterally in the direction of the decreasing hydraulic gradient [103]. Long-term studies of the free-product plume have shown that it is not a continuous phase of hydrocarbons, its content rarely reaches 50%, and the remaining pore space is filled with water and air; this phenomenon is called partitioning [101]. In addition, recent studies have suggested replacing the image of an elongated contaminant plume with a characteristic oval smooth redox zone with the concept of a plume fringe, where the shape of the contaminated zone is sharp and pointed [104]. The mobility and transport of NAPL are controlled by several parameters, including water abstraction near the leak, groundwater fluctuations, properties of the liquid phase (density, viscosity, solubility, vapor pressure, volatility, and interfacial tension), properties of the soil in which it spreads (pore size distribution, initial moisture content), structural and geological conditions, and saturation functions (residual saturation and relative permeability) [105; 106]. The fate of a contaminant plume is also determined by the nature of the medium. The transport of dissolved substances in karstified fractured environments usually occurs through favourable tectonic structures and fracture networks [107]. Figure 3: The concept of LNAPL migration with the decisive processes, source: [99]. The forces that drive DNAPL movement are in turn dependent on the density and viscosity of the DNAPLs (Figure 4), the permeability of the aquifer, i.e., the heterogeneity, the groundwater velocity, and the DNAPL saturation and distribution [108]. Both low flow velocities and the likelihood of DNAPLs becoming trapped in pore spaces lead to the formation of residual bubbles and ganglia [109; 110]. When the degree of DNAPL saturation in the soil is greater than the residual DNAPL saturation, the DNAPL plume is referred to as a pool [111]. The distribution of DNAPL sources among pools and ganglia leads to the use of the simple metric of the ratio of ganglia to-pool to quantify mass distribution (GTP) [112]. This parameter has been used in many environmental studies to determine the relationship between source zone geometry, dissolution rate, release time, and mass recovery [113; 114]. In a homogeneous porous medium, mass transfer from a pool is expected to be proportional to the flow rate through the DNAPL and to the interface between the DNAPL and the aqueous phase. The low saturation of the DNAPL in the pool caused the groundwater to circulate through the pool, allowing the contaminants to escape at an early stage of contamination. The transfer of contaminants from a pool zone is again mainly due to the dispersive component. Since the advective flow through the pool is suppressed by the high DNAPL saturation, it is responsible for the accumulation of contaminants in the mature contamination phase [113; 115; 116].