# Microplastics in Water Resources: Threats and Challenges
## 2 Microplastics in the Aquatic Environment
### MPs in Drinking Water Sources  
As discussed above, MPs enter surface waters through various pathways, including wet and dry atmospheric deposition, runoff from urban [47] and agricultural areas [48], as well as through wastewater [49]. Surface water contamination presents an obstacle to using these waters as sources for domestic purposes [50; 51]. The presence of MPs in water has increased interest in more detailed issues, such as the presence of these contaminants in drinking water treatment processes and the effectiveness of water treatment plants in removing them [52]. The presence of MPs in drinking water sources poses a significant challenge to water treatment processes, increasing costs and technical requirements, which can hinder the provision of clean water to communities. It has been observed that the removal efficiency of MPs in a simple treatment plant (including coagulation, flocculation, and sand filtration) is lower (about 70% MP removal efficiency) than in more advanced drinking water treatment stations (about 81%), which include coagulation and flocculation, sand filtration, and granular-activated carbon filtration [53]. Studies on the efficiency of MP removal in a pilot water treatment plant in Switzerland indicated that the coagulant does not play a significant role in the elimination of microplastic particles (the efficiency of MP removal after sand filtration was 95% without the coagulant and 92% with the coagulant). The coagulant played a more significant role in the removal of MPs in the form of synthetic fibers. The removal efficiency was 69% and 96%, respectively, without and with the coagulant [52]. In light of the threat posed by the presence of MPs in the water supplied by networks using surface waters, an alternative could be the use of groundwater. However, even groundwater is not free of the risk of MP contamination [54]. Small micro and nanoplastics have a particular ability to migrate vertically in soil. Due to their size, microplastics easily penetrate microorganisms and move with them within the soil profile, eventually leading to their entry into groundwater [55]. In shallow soil layers, agricultural practices, soil erosion, and soil fauna activities, such as those of earthworms, play important roles [56]. The presence of MPs in groundwater has been repeatedly confirmed. For example, [PERSON] et al. [57] verified the presence of MPs in groundwater on Dawanshan Island. The collected water samples contained microplastics in amounts ranging from 34 to 64 particles per liter, with polyester fibers accounting for a significant 80% of the total. According to the authors, the presence of MPs in the groundwater of the island is due to their infiltration with surface water and wells contaminated with microplastics. [PERSON] et al. [58] highlighted the still insufficient knowledge on MP contamination in groundwater. They pointed out potential factors that contribute to groundwater contamination with MPs, such as groundwater recharge from standing water bodies used for agricultural purposes and watercourses near waste disposal sites. Scientists emphasize the need for comprehensive solutions to protect groundwater, as it is a crucial and strategic resource for the development of civilization [59]. Reports from researchers in the Czech Republic indicate the presence of MPs in quantities ranging from 2 to 20 particles per liter in water from deep wells and inactive mines. This finding is particularly surprising given that the water samples were collected from depths of about 700 m [60]. Mechanisms of transport or water contamination pathways at such great depths remain unknown. Given the surprising presence of MPs so far from potential sources, there are concerns among scientists about errors in groundwater and soil studies. It was found that in most studies, blank samples were not used, MPs were incorrectly classified by mixing shapes and forms, and filters with too large pore sizes or materials that could be potential sources of MPs were used. According to [PERSON] et al.