In the case of fresh water, the main sources of microplastics can be industrial and municipal wastewater, but agricultural activities also generate a large number of microplastic particles. The water purification technology of wastewater treatment plants has a great influence on the content of microparticles in the surrounding surface waters. In municipal wastewater, the microparticle content of toothpastes, beauty products and detergents used in households can be considered as a primary source of microplastics, while microfibers from textile washing can be considered as a secondary source of microplastics. Pellets and microbeads that enter natural waters via industrial effluents are primary sources of microplastics. Particles resulting from the decomposition of larger plastic wastes are considered secondary microplastics [52], [53].

Microplastic pollution in surface waters not only has a harmful effect on the aquatic ecosystem, but also on humans through drinking water produced from fresh surface waters and through the food chain (e.g. by eating fish). The amount of microplastic particles transported by rivers to the oceans adds to the already significant microplastic content of seas and oceans. According to some authors, for example, the Danube River can carry up to 4.2 tons of plastic per day into the Black Sea [54].

Studies researching fresh and drinking water show that PE and PP are the most common types of polymers in these waters. PS, PVC, PET, PA, acrylates and PEST can also be detected in large quantities. They also contain PUR, nylon and polyester. The most frequently detected particle form is fragment, in addition to this there are a large number of fibers, films, foams and pellets. Spherical, elongated, flaky particles and films can also be found in the tested waters [18].

Microplastic concentration measured in surface waters show great variety. These differences may arise from different geographical and industrial conditions, but mostly from the fact that different research groups used different sampling and analytical methods. In general, the number of microplastic particles smaller than 20 µm is the highest due to fragmentation in the environment. The mesh size of most commercial sampling devices ranges from 20 to 3000 µm. Although a lower limit of 1 µm for microplastic particles has already been agreed, meshes with much larger pore sizes are used, thus the smaller MP fractions, which account for 35-90% of all MP present in the marine environment, are omitted from the sampling [55]. Understandably, research groups that use meshes and filters with smaller pore sizes during sampling and sample preparation obtain higher concentrations of microplastic particles. The comparability of the data is also complicated due the use of different measurement units (particle/kg, particle/L, particle/m3, mg/km2, particle/km2 or g/m3). Microplastic concentration measured in surface waters is also influenced by the natural forces, the weather, and the changing of seasons. Higher microplastic concentrations are usually measured in rainy, windy weather [56]. According to a study on the effects of storms, the abundance of microplastic particles in the river increased during two days of heavy rain from 400 particles/m3 before storm event to 17,383 particles/m3 after the event; the author emphasized that rain and storms are key events in microplastic pollution of surface waters [57].

In 2012, a range of 4.81×104 particles/km2 [58] microplastic pollution was found in Lake Geneva with a 300 µm size net and with visual identification. In 2013, 0.027 particles/m3 were detected in the Great Lakes area as microplastic pollution using the SEM-EDS method (the size of the particles found was > 355 µm) [59]. In 2014, also with visual observation and this time with a 500 µm size net, 0.314 microplastic particles/m3 were detected [60] in the Austrian part of the Danube. In the same year, water sample was collected at the Goiana River estuary in Brazil using a 300 µm plankton net and passed through a 45 µm pore size filter, there the microplastic concentration was 3.1×10-4–2.6×10-3 particles/m3 and the microplastic abundance was 4.14×103 particles/m3 in a sample collected at the Yangtze River estuary where a pump which included a 32 µm filter and a 1.2 µm membrane filter was used [61]. Also in 2014, microplastic concentrations of 4-108 particles/m3 (the size of the particles found was > 80 µm) were measured in the Seine and the Marne rivers [62]. In Hövsgöl Lake, Mongolia, 0.12 particles/m3 (the size of the particles found was > 355 µm) and at the Tamar estuary in the United Kingdom, 0.028 particles/m3 of microplastic pollution (the size of the particles found was > 270 µm) were detected using FTIR analysis. In 2015, German researchers found 387 particles/m3 of microplastics in the Rhine River also using FTIR analysis (the size of the particles found was > 300 µm) [59]. In 2016, a research team found 4,30×104 particles/ km2 of microplastics in the Great Lakes area, they used a Neuston net with a mesh size of 333 µm and a 125 µm filter. Another team measured a concentration of 2.58×104 particles/m3 and a concentration of 6.8×106 particles/km2 in Taihu Lake in China using a plankton net 333 µm mesh size and membrane filters with pore sizes of 5 µm and 100 µm. The latter research team was able to support the results obtained by visual observation by FTIR and SEM-EDS tests. In the same year, microplastic pollution of 0.82 to 4.41 particles/m3 (the size of the particles found was > 300 µm) in Lake Bolsena and Lake Chiusi, Italy and 24 to 7.7 particles/m3 of microplastics (the size of the particle found was > 153 µm) in the Raritan River in New Jersey were found [49]. In 2017, Anderson and his colleagues [63] observed microplastic pollution of 1.93×105 particles/km2 in Lake Winnipeg, Canada using SEM-EDS. And in the Netherlands, 2.7×104 particles/m3 on the coast of the North Sea and 105 particles/m3 of microplastics (> 10 µm) in the canals of Amsterdam were detected using FTIR analysis. In the same year, 1660–8925 particles/m3 (the size of the particles found was > 50 µm) in the Han and the Wuhan rivers, China were measured using FTIR for confirming the microscopic identification, and research teams measured microplastic concentrations of 105–9×105 particles/m3 (the size of the particles found was > 300 µm) depending on the sampling site in the Elbe River, Germany. 96.5% of the polymers identified in the Elbe were also PE, as previously mentioned, but PS, PP and PA particles were also among the particles found. Microplastic concentrations of 104–105 particles/m3 were found in the area of the Three Gorges Dam, China [59]. In 2018, concentrations of 1.72×105–5.19×105 particles/m3 were found in the Saigon River, Vietnam, using FTIR [64] and 900–4650 particles/m3 were found in Dongting Lake and Lake Hong, China using a stereomicroscope, the polymers were examined with Raman spectroscopy [65]. In the same year, concentrations of 0.01–95.8 particles/L were measured in the Teltow Canal in Berlin using SWIR spectroscopy [66] and 5×103–7.58×105 particles/km2 were detected in Qinghai Lake [67] by stereomicroscopic identification followed by Raman spectroscopic examination. In 2020, Deng and his colleagues [68] analyzed the microplastic content of surface waters and their sediments near a Chinese textile factory. Using µFTIR, microplastic concentrations in the waters ranged from 2.1 to 71.0 particles/L, and the microplastic content of the sediment was higher in all cases. The most common type of polymer was polyester, reflecting the impact of industrial activities [68].

In Hungary, surveys investigating the microplastic pollution of surface waters have been carried out since 2017, Bordós and his colleagues investigated the presence of microplastics in fish ponds and natural water bodies (Lake Balaton, the Zala River, Lake Tisza) [69]. The dominant polymer was polypropylene and polyethylene in the waters, polypropylene and polystyrene in the sediment. Microplastic particles were detected in 92% of the tested water samples, at a concentration of 3.52–32.05 particles/m3, while 69% of the sediment samples contained plastic particles in the amount of 0.46–1.62 particles/kg. Furthermore, the rivers of Hungary were investigated in the framework of the Parányi Plasztiktalány Projekt (Tiny Plastic Puzzle, a Hungarian project on the topic of microplastics) between 2017–2018, the results of the tests – according to which the pollution was the highest in the Danube, 50 particles/m3 – can be found on the mikromuanyag.hu site [70]. In the Tisza River, there are 4.9 particles larger than 300 µm, but smaller than 2 mm per cubic meter and 62.5 particles ranged from 15 to 300 µm. Based on the results of the sediment samples, 1 kg of Tisza sediment contains an average of 1.76 microplastics. Microplastics were found in a concentration of 12.1 particles in the Rába River and 1.7 particles in the Ipoly River.

In the field of environmental microplastics research, a significant proportion of the currently published results are uncertain due to inadequate sampling, detection, and quantification methods. For example, the experience of comprehensive analyses shows that the concentration of microplastics in drinking water and (mainly surface) water bases that serve as a basis for drinking water can change by ten orders of magnitude (1×10-2–108 particles/m3) [18], implying such a large difference within a water type that methodological questions arise and indicate that the standardization of sampling methods plays a key role in the comparability of the results. The Hungarian authors mentioned above also achieved significant progress in the development and standardization of sampling methods [71]; the method they developed and validated showed that the filtering efficiency of the methods is different for plastic of different shapes. Furthermore, filtration results are also affected by the water depth of sampling: surface sampling is more effective than sampling in the deeper layers of the water column. The results indicate that it is more important to determine the environmental efficiency of the entire sampling process than to measure only the recovery of the filtration device. This suggests that particle counts reported and detected in environmental monitoring studies may be lower than actual environmental concentrations.