# A Critical Review of the Modelling Tools for the Reactive Transport of Organic Contaminants
## 2 Processes  
As early as the 1970s, the fate of oil spills was described in terms of various processes, including advection, spreading and dispersion, dissolution, biodegradation, evaporation, and emulsification. However, due to the limited or lack of knowledge of analytical expressions for many of these processes and associated factors, the mathematical models were often simplified, resulting in several important processes being omitted [83]. In the early 1980s, the understanding of the fate and behaviour of spilled crude oil was extended to nine processes: advection, dispersion, evaporation, dissolution, emulsification, auto-oxidation, biodegradation, and sinking/sedimentation [84]. In the mathematical models, however, the focus remained on advection, dispersion, and evaporation.  
However, stochastic reactive 3D flow transport was developed as early as 1985, taking into account first-order kinetics for degradation, adsorption, dispersion, spreading and retardation through a doubly porous medium [85]. This method proved to be sophisticated, cost-effective, and robust and enabled accurate calculations of pollutant concentrations in the plume of chlorinated hydrocarbons. Almost simultaneously, the BIOPLUME model [17; 86] was developed, an early attempt to link 2D transport of dissolved hydrocarbons and biodegradation under low-oxygen conditions. This model was applied in aerobic aquifers to predict the release of organic pollutants by various processes such as convection, dispersion, mixing, and biodegradation.  
In the 1980s, when computing power was still relatively modest, two parallel branches of modelling (deterministic and stochastic) were successfully formulated and implemented (Figure 2). At that time, however, modelling was primarily the preserve of engineers, scientists, and professionals who had the necessary knowledge and tools. After almost a decade, in response to the needs of practitioners, screening tools were developed to facilitate the use of these models in a user-friendly spreadsheet environment [87; 88; 89]. Another significant advance was the introduction of the modular open-source 3D groundwater flow model MODFLOW, together with MT3D and RT3D, which enabled the simulation of the transport of multiple species in water.  
In the following sections, the processes controlling the migration of hydrocarbons are described in detail to improve the understanding of the underlying theory behind the algorithms incorporated into the mathematical models. While most models had to account for transport in the saturated zone, a few also can consider infiltration through the vadose zone. In addition, key processes such as dispersion, sorption, and biodegradation, which play a central role in controlling the concentrations of solutes in the subsurface, are explained in detail. Furthermore, lesser-known processes pertaining to the removal of contaminants, such as biodegradation or chemical oxidation, which are modelled by the included tools, are also discussed. Some additional processes that are not explicitly considered in the models but are important for the natural attenuation, leakage, and transport of hydrocarbons can be found in the cited literature [46; 90; 91; 92; 93].  
Figure 2: Timeline of key knowledge breakthroughs and models for the transport of hydrocarbon, references: [12,83,84,94,95,96,97,98].