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 et al. 2017). Lebreton et al. (2017) estimated that the Chinese Yangtze River catchment is the largest contributor, followed by the Ganges River catchment. With the growing awareness in recent years, some studies are focusing on Asian rivers (Blettler et al. 2018; van Emmerik et al. 2019a; Jambeck et al. 2015; OCMCBE 2015). Surface samplings at the Chinese Yangtze River mouth showed considerably higher plastic concentrations than any other sampled river worldwide (Zhao et al. 2014), with a reported 4137 particles per cubic meter. The significant differences between sampled estuarine concentrations and nearshore monitoring in the area confirmed that the Yangtze River is a major regional source of plastic input into the marine environment. Kataoka et al. (2019) reported of the MP concentrations on 29 Japanese river surfaces, which may be a source of MPs for the MP hotspot in the East Asian seas. They found MPs in 31 of the 36 sites and demonstrated that concentration of MPs in the river basins were dependent on population density, urbanization, and biological oxygen demand (BOD), which suggested that river water quality and plastic pollution in rivers are related.
2.5.3 The Problem of Freshwater Microplastics in Developing Countries
The problem of freshwater plastic pollution occurs more in developing countries due to the lack of proper waste management systems in these countries. As a result, the waste is directly disposed of into open areas or water bodies like rivers, from which they reach the oceans (OCMCBE 2015; Kaza et al. 2018). As observed by van Emmerik et al. (2019a), rivers in South East Asia are known to be major contributors of plastic waste into the marine ecosystem, but with no proper observation and monitoring of these riverine plastic wastes, the sources and fate of these MPs remain unknown. The current research on freshwater plastic pollution is mainly focused on the developed countries with almost no proper monitoring in the developing countries (Blettler et al. 2018; Blettler & Wantzen 2019). This disparity is mainly because the countries doing quality research in sciences (especially experimental) are countries with a high‐income economy (OECD 2017). From an environmental perspective this cannot be sustainable, as the developing countries are generating the largest amount of solid wastes due to high population growth and high consumption rate, with no proper solid waste management (UNHSP 2016). Furthermore, many developed countries are dumping their solid wastes by exporting to these low‐income countries, adding to the environmental issues in these countries. Lastly, the largest river drainage systems are located in these countries (Latrubesse 2008) and they are now recognized as the largest potential contributors of plastic to the marine ecosystem. Thus, due to these reasons, extensive quantification and monitoring of sources and transport pathways, as well as fluxes of riverine MPs in these countries is urgently required.
2.5.4 Status of India's Freshwater Plastic Problem
The plastic consumption rate among the Indian population is among the highest in the world, and results in the generation of approximately 5.6 million tons of plastic waste per year (Toxics Link 2014). Not surprisingly, India is described as the twelfth largest contributor of marine plastic pollution in the world (Jambeck et al. 2015), but very few studies to date have investigated the presence of