# A Critical Review of the Modelling Tools for the Reactive Transport of Organic Contaminants
## 3 Modelling
### Screening Models  
A screening process can be broadly defined as a decision-making step to either proceed with a comprehensive environmental assessment or to conclude with no further action [366]. During this initial screening phase, information regarding the contaminated site, sensitive environmental receptors, presence and concentrations of hazardous pollutants, and their potential migration pathways is gathered. When a detailed quantitative risk assessment (DQRA) is warranted, modelling tools are often utilized to replicate the fate of contaminants in media such as groundwater, as well as exposure parameters [367; 368; 369]. Hence, screening models should furnish a framework for a detailed description (including quantity and size) of chemical sources in the environment and the probable migration pathways. Screening models can take into account different processes that take place either in the vadose or in the saturated zone. However, the potential user should be aware that these tools are often based on a simplification of the local flow conditions and do not take into account, for example, the heterogeneity of the soil. Most of the simple models use the advection-dispersion equation (ADE1) and assume steady-state transport conditions [367]. Simplification of some processes and properties may be intentional and desirable, as models used in the initial environmental assessment and regulatory process should not be overly complicated [370; 371]. Most screening models are analytical, so they do not require much computational power and the calculations are usually fast. The small and well-structured input data, embedded functions, and a very intuitive user interface ensure that these models are easy to use and check. The use of mathematical modelling provides the scientific basis for regulation and policy and can support both early decisions on risk reduction and further remedial action [119; 370; 372; 373]. The utility and validation of some screening models are reviewed by the Council for Regulatory of Environmental Modelling (CREM). In addition, the National Research Council (NRC) established the Committee on Models in the Decision Process in 2005 to address all scientific and technical issues related to the selection and use of computational and statistical models in EPA's decision-making processes. Additional efforts to disseminate knowledge about environmental assessment models and the behaviour of chemicals in the subsurface include initiatives such as the US EPA Chemistry Dashboard. This platform offers data from numerous external databases and predictive models. Another resource is SMaRT Search (EPA Science Models and Research Tools), a searchable database of environmental modelling tools. Furthermore, the advancement and utilization of environmental fate models have been promoted in recent decades through collaborative efforts between model developers and users in working groups, as well as through the organization of workshops and knowledge-exchange platforms. In summary, the simple screening tools are used for a number of listed reasons:  
* Limited data requirements that do not compromise the accuracy of the mathematical representation. Screening models require little data per se, and the representation of even large systems is accurate in simple models, as the statistical relationships required to assess uncertainty can often be expressed more realistically at the aggregate level [374]. * Lower implementation costs. Simpler models are more cost-effective in terms of time and resources [375]. * Computational simplicity. Simplicity is favoured by some practitioners and decision makers who need an approximate time frame for remediation and the mass to be relocated. Screening models are parsimonious, i.e., these models achieve the expected level of explanation or prediction with as few parameters as possible [376].