places, e.g. blockage in sewage drains and the combined costs of removing these plastics as well as the loss in tourists will have a negative effect on the tourism sector (Drinkwater & Moy 2017).
2.5 Freshwater Plastic Pollution
Freshwater plastic pollution is an emerging hazard. To overcome this global challenge, reliable data on river plastic transport and accumulation is key. Approximately 4 million tons of plastic waste enters the oceans via rivers every year. River health is intrinsically linked to marine health; there is no separating the two. Why are people more concerned about plastic in the ocean than they are about plastic in rivers? Most ocean plastic ‘litter’ originates on land, with major rivers providing important source‐to‐sink pathways into the ocean. How effectively the problem of plastic pollution in the ocean can be resolved is dependent on establishing effective methods for surveying, quantifying, and modeling pollution pathways on land and examining how these can vary through time. Because it will be a herculean task to clean up the plastic debris that have already filled up the oceans, we must take necessary steps and precautions to reduce the amount of plastic entering the marine environment. To prevent the plastic debris from entering the sea, the path taken by the plastic has to be elucidated so that immediate and efficient steps can be taken.
To date, there is very little available data on the quantification and characterization of riverine MPs. As the impact of plastic pollution increases with decreasing particle size, the investigation of MPs (particles <5 mm) is particularly relevant. MPs particles, ranging from microns to millimeters in size, pose a significant risk to natural ecosystems and habitats. However, despite the potential ecological impacts from MPs pollution, the ability to accurately predict MP transport by environmental flows (e.g. in rivers, estuaries, and coastal currents) is limited. It is important to understand how they are transported to predict their dispersion and behavior, and ultimately understand their impact on ecological and human health. The extensive quantification of major rivers as sources of MPs pollution is yet to be established, and extensive research is needed to focus on transport pathways, fluxes, and fate of this emerging pollutant to understand the threat it poses to human health and ecosystems across the world (Alimi et al. 2018; Browne 2015; de Souza Machado et al. 2018; Horton et al. 2017; Horton and Dixon 2018; Nizzetto et al. 2016; Wagner et al. 2014).
2.5.1 Sources of Freshwater Microplastics
According to literature, MPs sources can be categorized as primary sources or secondary sources (Andrady 2011; Cole et al. 2011). We can refer to primary sources as those MP particles released into the environment which were manufactured in the range of micrometers, whereas secondary sources are those sources that produce the MPs by the process of disintegration and/or breakdown of larger plastic particles due to various physical and chemical mechanisms (e.g. exposure to sun rays, weathering, mechanical wear and tear, etc.). Faure et al. (2012) and Eriksen et al. (2013) propose three major sources: (i) effluent from wastewater treatment plants (WWTP); (ii) sewage treatment overflow during high‐volume rain events; and (iii) runoff from agricultural or public lands. In recent literature, sources of MPs in personal care and cosmetic products and clothing have been reported (Gouin et al. 2015). Sundt et al. (2014) tried to attempt a detailed study of source apportionment for Norway; primary as well as secondary sources that release MPs and in the study, and made the conclusion that the MPs from tire dust are the sources for the largest contribution in the Baltic Sea with a small contribution from other consumer products. Data for other terrestrial sources of MPs are almost nonexistent, for example, agricultural and urban soils (Lwanga et al. 2017). Literature suggests that potential sources of MPs from agricultural equipment or use of WWTP products in agriculture along with landfill waste disposal sites soil pose a threat to the environment, which have not been assessed in detail (Wagner et al. 2014). As per literature, polythene (PE) and polyethylene terephthalate (PET) are found to be the major polymer type found in freshwater samples (Figure 2.3, Li et al. 2020).
Figure 2.3 Composition of microplastics found in freshwater samples
Source: Modified from Li et al. 2020.
2.5.2 Studies on Freshwater Plastic Pollution from around the World
As we are becoming increasingly aware of the problems caused by plastic to marine life, the terrestrial environments and freshwaters are now being recognized as the source and transport pathways of plastics to the oceans (Horton et al. 2017), yet how MPs move from terrestrial to marine environments is poorly understood due to scarcity of data (Eerkes‐Medrano et al. 2015). Thus, there is an ardent need to focus on the freshwater sources of plastic pollution and discover ways to minimize it before it reaches the oceans. Studies in freshwater environments have received some focus in recent years, for example, recent studies include lakes (Horton et al. 2017; Imhof et al. 2016; Fischer et al. 2016); tributaries of the Great Lakes (Baldwin et al. 2016), the Seine River (Dris et al. 2015, 2018), various rivers in Switzerland (Faure et al. 2015), the Rhine River (Mani et al. 2015), various river sites near Chicago (McCormick et al. 2014), and the Danube River (Klein et al. 2015; Lechner et al. 2014). Some researchers working on river plastic pollution (Blettler & Wantzen 2019; Islam & Tanaka 2004; Jambeck et al. 2015; Lebreton et al. 2017; Lechner et al. 2014; Nollkaemper 1994; Schmidt et al. 2017) have identified riverine source to be an important contributor to the ocean environment. It is now a widely accepted fact that rivers are major carriers of plastic debris to the marine environment (Carr et al. 2016; Jambeck et al. 2015; Lebreton et al. 2017) for primary as well as secondary MPs (Andrady 2011). Castro‐Jiménez et al. (2019) describe the transport of macro‐plastics from the Rhone to the Mediterranean, and research from Indonesia (van Emmerik et al. 2019a) studied the movement of plastic wastes through rivers to the oceans. The ongoing research on freshwater plastic pollution includes topics such as monitoring and analyzing concentrations of MP river debris (Dris et al. 2015; Mani et al. 2015; Verster et al. 2017; Yonkos et al. 2014), outflow flux estimations to the marine environment (Lebreton et al. 2017; Mani et al. 2015; Nizzetto et al. 2016; Schmidt et al. 2017; Siegfried et al. 2017), MP source apportionment and identification (Carr et al. 2016; Leslie et al. 2013; McCormick et al. 2016; Murphy et al. 2016; Yonkos et al. 2014; Ziajahromi et al. 2016), and the effect of MPs on freshwater life forms (Besseling et al. 2017; Dris et al. 2015; Hoellein et al. 2017; Wagner et al. 2014). Researchers suggest that population, land use, basin characteristics, and hydrology (Lebreton et al. 2017) have a significant relationship with riverine plastic concentrations, and have found that MPs are concentrated in rivers near areas with high population density (Mani et al. 2015; Yonkos et al. 2014). Some researchers have used various mathematical models to theoretically estimate the transport of MPs in watersheds (Nizzetto et al. 2016).
Two of the areas in riverine plastic pollution studies where there is almost no data is how MPs transport changes along the river, and that temporal variation remains unknown. More studies can help in increasing our understanding of the origins, sinks, and accumulation zones in catchments. Most studies that we have come across focused on the movement of MPs plastic in specific river cross‐sections (Crosti et al. 2018; van Emmerik et al. 2018) or the output from complete river systems (Tramoy et al. 2019). A study conducted in the Los Angeles rivers indicated significant temporal variations in plastic transport within one year (Moore et al. 2011). A study in rivers of Indonesia even suggested monthly variations on plastic transport (van Emmerik et al. 2019a,b).
The plastic emission from Asian rivers is estimated to be significantly high, which may be due to various factors such as high population density, a large quantity of primary MPs production, and hydrological regimes with heavy rainfalls. This results in huge MP waste transport from Asian continent to the oceans; 86% of the total global input, with an estimated annual input of 1.21 million tons (Lebreton
