# A Critical Review of the Modelling Tools for the Reactive Transport of Organic Contaminants
## 2 Processes
### Dissolution  
When a hydrocarbon is mixed with water, it theoretically dissolves up to its solubility limit, and the excess is present as a free product. Solubility is also related to a parameter known as the soil saturation limit. This is the pollutant concentration at which the pore air and water in the soil are saturated with the chemical and the adsorption limits of the soil  
Figure 4: The concept of DNAPL migration through vadose and saturated zones according to [109].  
particles are reached. Above this limit, the pollutant can be present in the free phase. If the hydrocarbon concentration is below the saturation limit, all pollutants present in the subsoil are distributed in three phases: dissolved, in vapor, and sorbed. The saturation limit in the soil depends on the water solubility, the dry bulk density of the soil, the soil-water partition coefficient, the porosity, and the Henry constant [120]. Understanding the dissolution processes in the source zone is crucial for a valid assessment of the mass balance for solvent transport models and for what follows after the lifetime of the pollutant source.  
Ideally, equilibrium dissolution is expressed by Raoult's law of mass transfer in porous media [121], where the aqueous equilibrium concentration of a component in a mixture of NAPL is the result of phase mole fraction and solubility. In a non-ideal multicomponent mixture, which is usually the NAPL source at a heterogeneous contaminated site, Raoult's law may not be valid because the transfer to the aqueous phase depends on the abundance and intermolecular interactions between the components in the non-aqueous and aqueous phases [122; 123]. These interactions are computed using a thermodynamic approach and parameterized by an activity coefficient, which factors in the non-ideal distribution [124; 125].  
The activity coefficient and molar fraction undergo variation as the dissolution process unfolds, influenced by changes in the composition of the remaining NAPL and the solubility of individual components within the mixture [126; 127]. Approximations for the activity coefficients can be made using the universal quasi-chemical functional group activity coefficient (UNIFAC) method, which has demonstrated strong agreement with experimentally derived ternary phase diagrams [128; 129].  
The rate-limited, or non-equilibrium, mass transfer of NAPL becomes more realistic for multicomponent mixtures, particularly when the compounds are thermodynamically distinct. This process unfolds in three phases: initially, there are constant concentrations at or near equilibrium; then, this is followed by a rapid decline from the equilibrium concentration to levels three orders of magnitude below; and finally, tailing occurs when only about 0.5% of the original charge remains [130; 131; 132]. The non-ideal dissolution of NAPL residues and the dilution of effluent have been the subjects of numerous fundamental studies [133; 134; 135; 136; 137; 138; 139; 140; 141; 142; 143].  
The solubility of hydrocarbons decreases with higher molecular weight, attributed to factors such as longer chain length (>C20) or an increase in aromatic rings (more than three aromatic rings) [144; 145]. Generally, hydrocarbons with higher molecular weights are less prone to volatilization or dissolution. Instead, they tend to disperse into NAPL and soils, where they can be adsorbed by organic material or minerals [78; 146]. This process results in the depletion of more soluble compounds and the enrichment of residues with less soluble ones, potentially leading to their precipitation as solids--a phenomenon known as solidification [147]. Additionally, the solubility of hydrocarbons is influenced by factors such as the temperature of the medium and the salinity of the groundwater [78; 148; 149]. The size of a plume (dissolved phase) is typically determined by the transfer of hydrocarbon-soluble compounds from the free product and/or residual phase, along with the hydrogeological properties of the aquifer [150].