# A Critical Review of the Modelling Tools for the Reactive Transport of Organic Contaminants
## 1 Introduction  
Contamination of soil and groundwater by organic pollutants is a global problem that needs to be addressed, especially at historically contaminated industrial areas, such as brownfields or abandoned sites. Despite their tainted past, these areas, often located in urban industrial regions and low-income communities, hold potential for revitalization through remediation efforts. In 2016, 1.38 million potentially contaminated sites were registered, 98% of them in 11 countries [1]. According to projections, this number is likely to increase, as potentially polluting activities took place at a total of 2.8 million sites [2, 3, 4]. Mineral oils and heavy metals are the main contaminants, accounting for about 60% of soil pollution. In the EEA countries, around 235,000 contaminated sites have been remediated [4], which corresponds to 17% of the potentially contaminated sites currently registered. The cost of managing contaminated soils in Europe amounts to around 6.5 billion euros per year [5]. The cost of a site investigation is usually between EUR 5000 and EUR 50,000 (60% of reported cases), while the cost of remediation projects is usually between EUR 50,000 and EUR 500,000 (40% of reported cases) [3]. The cost of remediation and restoration can be even higher, ranging from EUR 150,000 to EUR 2 billion, as it is difficult to separatethese measures [6]. As hydrocarbon pollution is a widespread problem, many spatial planners and decision-makers are faced with the problem of rapid and cost-effective remediation. Decisions on remediation strategies require tools and guidelines that provide a reliable estimate of the time and labour required to reduce the plume and source of contamination [7; 8]. Numerical modelling can be an important method to study the fate of hydrocarbons in the aquatic environment [9; 10; 11]. It can precede any on-site action, estimate the extent of contamination, and help select the most effective and economical remediation methods. They can therefore help scientists, engineers, and decision makers to significantly improve their knowledge and optimize site management. Numerical models can also help to reduce the costs associated with long-term monitoring programs and sampling campaigns by identifying biogeochemical processes and quantifying their long-term effects. One of the first applications of computer modelling to estimate groundwater flow dates back to the 1960s when Renson, together with Appel and Webster, proposed the use of finite differences and digital computers to track water flow [12], but very few involved hydrocarbon seeps. Since the 1970s, however, the number of journal articles describing models for the transport of hydrocarbons in water has increased (Figure 1), with an abrupt increase in recent years. There have been several breakthroughs in knowledge during these decades, e.g., the involvement of microorganisms in oil spills was described in detail as early as the 1970s [13], and in the same year a fundamental theoretical concept of the chemical and biological processes associated with the presence of hydrocarbons in soil and groundwater was formulated [14]. The theoretical and numerical definition and realisation of multiphase transport of hydrocarbons can also be regarded as a fundamental advance in knowledge in the 1980s [15; 16]. The first numerical models were based on a single organic pollutant whose degradation was simulated either by a single electron acceptor [17; 18] or by several electron acceptors [19]. In the 1990s, the groundwater flow model--MODFLOW--was coupled with the reaction modules, and since then the term multispecies reactive transport has become established [20; 21]. An important part of the hydrocarbon studies has also been  
Figure 1: Number of articles in which the following keywords are mentioned: hydrocarbon, transport, model, and water in scientific journals; data from Google Scholar.