# Microplastics in Water Resources: Threats and Challenges
## 2 Microplastics in the Aquatic Environment  
### Sources and Transport Pathways of Microplastics to Aquatic Ecosystems  
Aquatic ecosystems, such as rivers, lakes, seas, and oceans, have become sites of intense accumulation of MPs [15]. Until recently, the vast majority of global research focused on the issue of MPs pollution in marine environments, as it was believed that rivers merely served as transport routes for MPs from terrestrial areas to seas and oceans. However, MPs have been shown to also be retained in riverine and lacustrine sediments, with concentrations as high as those in marine environments. There are many factors that make freshwater bodies particularly susceptible to this type of pollution. Inland water areas and adjacent land remain under strong anthropogenic influence, including residential, industrial, agricultural, transportation, and tourism activities. As a result, MPs can be introduced into aquatic environments from various sources, such as industrial emissions, discharge of municipal and industrial wastewater, and runoff from urban and agricultural areas by rainwater [16,17]. Figure 1 schematically presents the sources and transport pathways of MPs to aquatic environments. MPs migrate in the environment, and their movement results from various transport mechanisms. One of these is atmospheric transport. MPs, as components of suspended dust, can be carried by wind over long distances. Airborne MPs settle on the surfaces of soils and waters, contaminating them even in locations far from direct emission sources [18]. It has been shown that the efficiency of transporting MPs through the air is influenced by their size and shape. An interesting and unexpected finding of the analysis conducted by [PERSON] et al. [19] is the increased proportion of blue-colored MPs at significant distances from areas under anthropogenic influence compared to other MPs. According to [PERSON] et al. [20], up to 84% of plastics suspended in the air are MPs lifted from roads due to vehicle movement. Tire abrasion generates MPs in the form of both fragments and fibers, predominantly in the size range of 2.5-350 \(\upmu\)m [21]. The amount of tire rubber particles in suspended dust near roads is reported to range from 5% to 28% for particles smaller than 10 \(\upmu\)m. Analyzing these estimates, the emission of tire particles and road dust as a source of microplastic particles in the air is significant [22]. Another important mechanism for understanding the distribution of MPs in the environment is river transport. Plastic particles, both large and microscopic, can be carried by river currents for long distances. Consequently, this leads to the contamination of their outlets: lakes, reservoirs, and even seas and oceans [12; 23; 24]. For example, [PERSON] et al. [25] demonstrated using the Yangtze River, which flows into the East China Sea, that the load of MPs carried by rivers can be high and significantly contribute to marine pollution. The annual amount of MPs transported by this river to the sea was estimated at 7000 tons. Oceanic currents also play a key role in the spread of MPs. Plastic from various land areas can reach the oceans, where it becomes part of the global marine transport pathways for MPs [26]. In 2014, the distribution of floating plastic debris around the Kuroshio Current was studied, which transports plastics from coastal waters of Asian countries to the subtropical gyre of the North Pacific. The average abundance and mass of plastic debris on the sea surface were 100,376 particles/km\({}^{2}\) and 446.16 g/km\({}^{2}\), respectively. The debris was predominantly composed of small (\(\leq\)1 mm) fragmented white particles (1 mm) of polyethylene and polypropylene. This evidence points to the global reach of MPs pollution. Ocean currents are extensive, allowing polymer pollution to be transported over intercontinental distances [27]. The discussion of the accumulation of MPs in the oceans cannot overlook the Great Pacific Garbage Patch.