# Lagrangian Modeling of Marine Microplastics Fate and Transport: The State of the Science
## 1 Introduction  
Aquatic environments, including oceans, coastal regions, estuaries, seas, and rivers, as major planetary water resources, have been affected by marine plastic debris accumulation and distribution, which has triggered global environmental concern and irreversible pollution consequences [1, 2, 3]. Macro (\(>\) 1 cm) [4], meso (5 mm-2 cm) [4], micro (\(<\) 5 mm) [5], and nano-size (1-100 nm) [6] plastics pollution has become a critical environmental problem due to their persistence, toxicological properties, and destructive effects on not only Earth's hydrosphere [7] but also aquatic organisms [8, 9, 10, 11], wildlife, and human health [12, 13, 14, 15]. The tendency of microplastics to adsorb and react with wastewater agents is relatively high due to their physical properties, such as hydrophobicity [16], as well as the large ratio of their surface area to volume, allowing them to adsorb toxic pollutants (e.g., heavy metals) and invasive species [17]. Microplastics are classified into two categories: primary and secondary. Primary microplastics are mainly detected with an original size of less than 5 mm in textiles and some personal care products [2, 18]. They can be transported through discharges of water treatment plants, rivers, surface run-off, and wind into aquatic environments [19]. Large plastic debris under fragmentation may produce secondary microplastics through some processes including physical, chemical, and biological interactions andphoto-degradation [20; 21]. The secondary microplastics originate mainly from industrial resin pellets, fishing nets, and other thrown-away plastic litters [22], and they can also be formed from the fragmentation of any larger plastic particle in the ocean, regardless of its source. Microplastics in the environment majorly consist of secondary microplastics [23]. The possibility of microplastics breaking down into nanoplastics may increase during their exposure to the environment, leading to higher environmental risks given the nano-size nature of these materials [24]. In addition, plastic pollution impacts have been observed at economic and social scales [25]. The tourism industry accounts for a considerable percentage of the gross domestic product (GDP) of many countries, which indicates the market value of all services provided within a country's territory during a specific temporal period; thus, governments schedule various plans to attract more tourists and introduce different sightseeing places in order to boost tourism income. Nevertheless, beaches confront some challenges with respect to their sustainability in terms of preservation and operation from the touristic and economic perspectives. The uncontrolled release of microplastics causes the pollution of the seawater and the shore. Plastic debris enters the ocean directly or through rivers by uncontrolled dumping from land-based or maritime sources, such as fishing or shipping sectors, and recreational activities [26; 27; 28]. Plastics will be degraded and broken down into microplastics (i.e., particles less than 5 mm in size) after entering the ocean if not treated at early stages. Microplastics are found throughout the water column in oceans and also on the seabed [29; 30]. Numerous studies have been conducted to quantify and characterize microplastics in aquatic environments [4; 21; 22; 23] including lakes [23; 31], rivers [32; 33; 34; 35; 36], and shorelines [37] by employing different field surveys and using a set of lab experiments. Recent advances in computer science and technology have also promoted the application of Machine Learning (ML) approaches in the automated quantification and characterization of microplastics [38; 39; 40; 41; 42; 43]. Microplastics in water undergo some transformational processes such as biofouling [44], degradation and fragmentation [16; 18; 45], and hetero- and homo-aggregation [45; 46], known also as particle-particle interactions [47], which influence their fate in their receiving environment.