# A Critical Review of the Modelling Tools for the Reactive Transport of Organic Contaminants
## 1 Introduction
the development of standards for soil and water remediation based on the models and their uncertainty and sensitivity analyses [22; 23; 24; 25], timeframes for remediation [26; 27; 28], and health risk assessments related to indoor and outdoor exposure [29; 30; 31]. These parameters are of critical importance to practitioners and environmental engineers whose goal is to revitalise polluted sites or return them to their natural state. The main turning points in hydrocarbon research have been the introduction of new remediation methods such as the use of nanoparticles [32; 33], new devices/methods to measure pollutant concentrations [34; 35; 36], detailed monitoring [37; 38; 39; 40; 41], or the coupling of other techniques such as isotope measurements [42; 43; 44]. Remediation of hydrocarbon contaminated sites is an emerging field of research [45]. It has been reported that the number of papers published annually on this topic is increasing at an average annual growth rate of 9.22% [46]. Despite this large number of publications, only a few of them mention the need for further progress, which still needs to be improved, as hydrocarbons are among the most hazardous pollutants, posing a high risk to health and the aquatic environment. So what does the future of hydrocarbon research look like? Looking at the articles from the last decades, one might conclude that some studies will continue to be site-specific, focusing on high-risk sites [47; 48], large-scale sites [49; 50; 51; 52], or areas with extreme conditions [53; 54; 55; 56; 57]. Certainly, some articles will refer to laboratory experiments and the dynamics of reactions affecting the attenuation of hydrocarbons in soils and water [58; 59; 60]. When it comes to studying the fate of hydrocarbons in the subsurface, there is still room for mesoscale laboratory studies [61; 62; 63] or field experiments [64; 65; 66; 67], as these types of studies better reflect the ongoing processes in heterogeneous soils and the growth of bacterial communities in the quasi-real environment. It is likely that new studies will be required to address the remaining unresolved challenges in remediating hydrocarbon-contaminated soils, such as heterogeneity [68; 69], realistic representation of volatile compound transport [70; 71], accurate injection of oxidants or reductants [72; 73], and modelling of these processes with new modelling tools [74; 75] or with models that are well known but applied in novel ways [76; 77]. Overviews of the fate and transport of organic pollutants and their bioremediation with updated knowledge of the mechanisms, the dynamics of the processes, and new modelling tools will also play an important role for future researchers to keep up with the trends [45; 78; 79; 80]. Stochastic models are particularly interesting and can be used to construct probable scenarios, policy rules, guidelines, and risk assessments. They seem to be good solutions for complex systems where heterogeneity and randomness play a crucial role in the distribution of pollutants. These tools are described in the following sections together with examples of their application. A brief description of the application of machine learning methods in hydrogeological modelling is also included. This method, which offers high accuracy, is dynamically integrated into hydrological and hydrogeological modelling [56; 81; 82]. The aim of this article is to provide a comprehensive overview of existing solutions for modelling the transport and removal of hydrocarbons from the subsurface. This is facilitated by several objectives: the description of the processes controlling hydrocarbon migration and remediation required to build the conceptual model, the presentation of the most common mathematical models with their characteristics and limitations, and the presentation of the theoretical background of the different modelling approaches and their weaknesses and strengths.