2. Microplastics as Pollutants of Aquatic Ecosystems: State of Research for Freshwater Bodies in the Russian Federation
2.1. Overview of the Sources, Sampling, and Analysis of Microplastics in Aquatic Ecosystems
The term “microplastics” was first used in 2004 by Thompson et al.
[12] to describe microscopic plastic particles that accumulated in the water column and bottom sediments of the UK coastal aquatic ecosystems. Later, it was proposed to classify all plastic particles smaller than 5 mm as MPs
[37]. The Group of Experts on the Scientific Aspects of Marine Environmental Protection
[38] expanded the concept by defining it as “plastic particles less than 5 mm in diameter, including nano-sized particles (down to 1 nm)”
[39][40].
Some authors have proposed to consider particles smaller than 1 mm along the longest axis as MPs and to classify particles larger than 1 mm as mesoplastics (up to 25–100 mm)
[41]. One of the suggested definitions of MPs is as follows: “Microplastics are any synthetic solid particle or polymeric matrix, with regular or irregular shape and with size ranging from 1 μm to 5 mm, of either primary or secondary manufacturing origin, which are insoluble in water”
[12]. The U.S. National Oceanic and Atmospheric Administration (NOAA website), which is actively working towards developing a methodology for studying the distribution of MPs in aquatic environments, and other authors
[8][42] also support the current definition of MPs as polymer particles less than 5 mm along the longest axis. There are special terms for fine fractions of MPs, such as “nanoplastics” for particles from 1 nm to 1 μm
[43] and “mini-microplastic” for particles less than 330 μm
[44]. Small MP particles (up to 0.45 mm, including nanoplastics) are also referred to by the general term “submicroplastic”
[45].
Small particles are formed in the aquatic environment in the course of the sequential decomposition of larger plastic materials, mainly as a result of the action of physical and chemical factors
[10]; these are the so-called “secondary MPs”. Plastics can also enter water systems directly in the micro-sized (<5 mm) form
[11]. This group of MPs is referred to as “primary MPs”.
MPs enter freshwater ecosystems from various point and diffuse sources
[46]. Diffuse sources (for example, plastic waste coming with watercourses from a catchment area, with groundwater) are distributed over large areas, while point sources combine direct entry with wastewater, including sewage, agricultural wastewater, storm water, industrial wastewater and others. Primary MPs enter aquatic ecosystems in the form of granules used in many industrial processes (raw materials for the production of plastic products, industrial abrasives, components of paint coatings, drilling fluids, etc.) and in personal care products
[42]. The amount of primary MPs entering the oceans annually is estimated at 0.8–2.5 million tons
[11]. Secondary MPs are also widespread in aquatic environments. They are formed from the fragmentation of larger plastic products, including plastic waste, synthetic textiles, etc. Quantitative estimates suggest that between 4.8 and 12.7 million tons of plastics enter water bodies annually due to poor waste management
[47]. The rates of secondary MP formation are not clearly determined, since this process is complex and depends both on the properties of the material itself and on environmental factors. The leading factor in the degradation of plastics is photochemical oxidation under the action of ultraviolet radiation from sunlight
[4][48]. Photooxidation occurs most rapidly on beaches and in open ground conditions; in water, this process is greatly slowed down due to lower ambient temperatures. The formation of biofilms on plastic surfaces also significantly (up to 99%) reduces the effect of ultraviolet radiation
[49]. Other significant environmental factors affecting the decay of large fragments to MPs include wind action, waves, aggressive chemical environments, and microbial degradation processes
[50]. The cumulative effect of these factors on the formation of secondary MPs has not been studied enough. However, it is known that the smaller the fragments, the faster their further decomposition is, with the formation of MPs under the influence of UV radiation and mechanical abrasion for all types of polymers
[51]. High temperatures accelerate the processes of plastic degradation, in particular its photodegradation
[52].
Washing of synthetic textiles has recently been recognized as one of the largest sources of microfibers
[53]. It is estimated that millions of fibers enter the wastewater during a typical home clothing wash
[54]. Ross et al.
[55] revealed the intense pollution of the Arctic waters with synthetic fibers, mainly polyester particles, coming with ocean currents and with atmospheric flows from the south. The study
[56] showed that it is microfibers that represent the major part of MP particles in the surface layers of the Arctic waters, as a result of the breakdown of larger synthetic materials. These are shipping waste (primarily discharges of liquid household waste), fishing waste (pieces of plastic nets), as well as sewage brought by currents and waste from offshore platforms. This conclusion was confirmed by a recent study in the Arctic seas adjacent to the Russian Federation
[34][57], which showed that most of the particles found in the surface water layer are microfibers of polymers, such as polyethylene terephthalate (PET), polypropylene (PP), and polyethylene (PE).
In general, street runoff, wastewater treatment plants, and atmospheric transfer from land are cited as the largest channels for MPs’ entry into aquatic ecosystems
[11]. Many authors have confirmed that MP concentrations are often elevated near point sources, such as large population centers, wastewater treatment plants, landfills, and plastic manufactories, and they decrease with distance from the sources
[58][59][60]. Considerable attention is paid to assessing the contribution of treatment facilities to the pollution of water environment with MPs. Although most wastewater treatment plants generally have relatively high MP removal rates (over 95%)
[61][62], many wastewater treatment plants are not effective at capturing certain types of particles characterized by small size and high buoyancy, such as microspheres and microfibers
[63][64]. Industrial sources can also cause the entry of primary MPs into surface waters. In addition to municipal wastewater treatment plants, high concentrations of MPs in the aquatic environment are observed when sampling in the immediate vicinity of plastic factories and other industrial enterprises. MPs are used in various industrial processes as raw materials (“primary granules”) and as a part of abrasive products, and they enter watercourses with regular or accidental releases
[65]. Point sources of MPs are characterized by specific “profiles” that reflect the nature of pollution. Microspheres used in personal care and cosmetic products, along with synthetic textile microfibers, are most abundant near wastewater outlets
[63][64]; high concentrations of polyester fibers have also been observed near textile factories
[66]; microgranules are typical for sites located near the production of plastic products
[67]; and fragments of composite thermoplastics containing reflective glass spheres can be associated with the entry of road marking components into surface waters with storm runoff
[68]. Most MPs entering the seas from rivers are represented by particles of synthetic polymers left after incomplete wastewater treatment (42%) and microfibers of synthetic fabrics (29%), followed by fragments and fibers from the breakdown of plastic waste (19%) and microspheres from personal care products and industrial sources (10%)
[69].
Both special and improvised means can be used to take samples of surface waters and bottom sediments for the quantitative analysis of MPs. Trawl nets of various modifications, such as Manta trawl, neuston and plankton nets, and pump filtration, are used to sample MPs from water, while bottom sediments are collected using bottom grabs or hand tools
[32][70][71][72]. Then, laboratory processing of the samples is performed to selectively extract MPs and get rid of other particles, such as minerals and organics. The identification of MP particles becomes more difficult in the presence of organic residues, which also have a low density and are not removed during the separation of particles in saturated salt solutions. Therefore, the elimination of impurities of biological origin, which is achieved by using acidic, alkaline, and enzymatic hydrolysis or strong oxidizing agents, is of great importance for adequate quantitative and qualitative analyses of plastic particles
[73].
It is now accepted that visual analysis using stereomicroscopy can be used to obtain preliminary quantitative data on the presence of MPs in aquatic ecosystems. For the qualitative identification of plastic particles, spectroscopic methods are used, such as Raman and IR-Fourier spectroscopy, and pyrolysis gas chromatography/mass spectrometry (pyrolysis GC-MS). The chemical composition of particles is determined to understand the ratio of polymer types in the samples and obtain more accurate estimates of the origin and sources of MPs
[70]. For particle analysis, some researchers also use scanning electron microscopy combined with energy-dispersive spectroscopic analysis (SEM-EDS) (for example,
[74][75]. According to some estimates, the imperfection of the sampling methodology leads to an underestimation of the real concentrations of MPs in water and bottom sediments of freshwater ecosystems
[76]. The use of different methods by different research groups for sampling natural waters and sediments and their subsequent laboratory analysis for MP content in many cases make it difficult to compare quantitative data.
2.2. Abundance and Distribution of Microplastics in Freshwater Ecosystems
The accumulation and transport of plastic debris and microparticles in continental freshwater ecosystems has become the subject of systematic research only since 2010
[58][77][78][79]. Terrestrial fresh waters need intensive research on the extent and peculiarities of pollution with MPs, as they represent the most important source of plastic pollution to the oceans
[76][80]. The knowledge of the distribution of MPs in freshwater systems remains limited, but it is clear that the amount of pollutants carried by rivers is enormous. The global model of plastic transport to the World Ocean from river flows estimated the total flux at 1.15 to 2.41 million tons
[81], with the 20 most polluted world rivers carrying about 67% of the total volume of transported artificial polymers. Quantitative estimates had been performed for large European rivers. The modeling of MP transport in the Danube basin and the calculated data based on the actual concentrations in the surface waters indicate an annual transport of 500 to 1534 tons of MPs into the Black Sea
[69][82][83]. For the Po and Rhine Rivers, the calculated values of MP transport to the seas are 120–399 tons and 20–105 tons per year, respectively
[69][83]. The Neva River is presumably one of the main sources of MPs entering the waters of the Gulf of Finland
[33], while the Northern Dvina is a source of pollution for the White and Barents Seas
[34][36][84].
The second reason for the intensive research on freshwater ecosystems, especially rivers, is that they are valuable water resources subjected to pollution
[68]. Despite the common belief that most land-based plastic is transported directly to oceans by rivers, available evidence suggests that the bulk of the pollutants accumulates in rivers and their floodplains
[85][86][87][88]. It is believed that lakes act as filters, retaining MPs in land surface waters as they flow into the continental seas and the World Ocean, and become the primary MP reservoir
[89]. As a river flows into a lake, MPs are carried by the surface currents of the lake and can be concentrated in small temporary whirlpools
[90]. Wind driven surface currents, especially during storms, also transport and deposit significant amounts of plastics on lake shorelines
[91]. Rivers and lakes act not only as transit systems on their way to the ocean, but also as reservoirs of plastic pollution. River estuaries function in the same way: for example, the estuary of the Neva River, at its confluence with the Gulf of Finland of the Baltic Sea, serves as an accumulator of marine debris and MPs in the Neva Bay, where the maximum concentrations of MPs in the water and on the coasts are observed
[33].
Regression analysis of the data from several dozen freshwater bodies around the world showed that the part of the world with the highest MP content is Asia, followed (in descending order) by North America, Africa, Oceania, South America, and Europe
[92]. The average MP concentrations in the surface waters of freshwater bodies around the world vary from tenths to hundreds and thousands of items per cubic meter. However, quantitative estimates are highly dependent on the methods used for sampling and MP detection. For example, when sampling with a neuston network with a mesh diameter of 0.33 mm in the tributaries of the Great Lakes in the United States and visualizing particles under a binocular microscope, the researchers registered the presence of 4.2 MP items per cubic meter on average
[93]. Similar results of 5.60 and 5.57 items/m
3 were obtained for the surface waters of the Rhine River (from Basel to Rotterdam)
[94] and Elba River
[95], respectively, using a 0.30 mm and a 0.15 mm mesh and visual analysis with selective particle identification by (Fourier-transform infrared spectroscopy) FT-IR spectroscopy. The average concentration of MP particles in the water of the Danube River discovered during multiple sampling using a 0.50 mm mesh and subsequent visual analysis was an order of magnitude lower, 0.32 items/m
3 [82]. On the contrary, much higher average values (more than 100 items/m
3) were detected using a light microscopy in the surface waters of a Seine tributary, the Marne River, when sampling with a Manta trawl with a mesh of 0.08 mm
[96]. One of the highest average MP concentrations in river waters, registered by a visual method in samples taken from the Los Angeles River (USA) using a Manta trawl and various types of nets, reached 3473.42 items/m
3 in some seasons
[77]. However, it cannot be concluded that a finer mesh diameter when filtering water for MP collection is unequivocally associated with an increase in the number of plastic particles. Thus, in the waters of lakes in China, after direct filtration of water samples using a 45 μm sieve and subsequent visual analysis with verification using a Raman spectroscopy, an average of 617 to 7050 particles per cubic meter was recorded
[97][98]. Hundreds and thousands of MP particles per cubic meter of water were detected in the Nakdong River in South Korea using 20 μm filters
[99], and the volume of MPs carried by the river per year was estimated by the authors at 5.4–11 trillion particles or 53.3–118 tons.
MPs are also ubiquitous in freshwater bottom sediments where their concentrations vary from single and tens of particles per kg dry weight
[100][101] to thousands; for example, in the sediments of the Amsterdam canals, the MP content was found to reach 10,500 particles per kg dry weight
[102]. Bottom sediments are considered to be a MP depot, the accumulation of particles in which is the result of sedimentation on a long-term scale.
In addition to the difficulties associated with comparing data obtained by different methods, surface water bodies have other features making it difficult to quantify the MP content. The processes of transport and redistribution of MPs in surface water bodies, especially in river systems, are complex. The mobilization, transport, dispersion, and accumulation of plastics in rivers depend on hydrological and meteorological conditions, including wind speed and direction
[103][104], flow velocity, and water level and discharge
[105][106]. The hydromorphological and dynamic features of surface water bodies determine the zones of accumulation and the surface transport of MPs
[80].
The ways of MP transport in aquatic ecosystems are complex. They are currently best understood for marine and brackish waters and include surface drift, distribution in the water column due to vertical mixing, aggregation near shores and natural obstacles, and sedimentation
[107]. In addition to the dynamic conditions of aquatic environments, MP distribution is affected by their physical characteristics (shape, particle size, and polymer density), and, as a result, particles in aquatic environments demonstrate a high variability of dynamic properties, including settling/rising velocity, critical shear stress, and re-suspension threshold
[108][109].
To a large extent, the fate of MPs in marine and brackish water ecosystems depends on the type of polymer. Typically, PP and PE are low-density plastics that are relatively buoyant and are carried by currents, while PVC, PS, polyester, and polyamides are considered high-density plastics that tend to sink
[110][111]. However, even PP and PE can acquire a higher density as a result of the addition of mineral additives
[112]. The denser varieties of plastics tend to sink and reach the bottom sediments. A significant number of MPs are eventually buried in deep ocean sites
[113] and accumulate in food chains
[114].
The hydrodynamics influencing MP behavior in freshwaters is relatively poorly studied in comparison to that in the waters of the seas. However, it is known that, since the specific gravity of plastics primarily affects their distribution in the surface waters, the water column, and the bottom sediments, MPs are distributed in freshwater systems along a gradient from top downward, with a pronounced increase in concentrations from the surface to the bottom
[79][107]). The difference between the MP content in fresh surface waters and bottom sediments can be enormous. Thus, MP concentrations in the sediments of the Elbe River in Germany are, on average, 600,000 times higher than those in the water
[95].
Due to the peculiarities of the transport and the accumulation of MPs in rivers, it is difficult to interpret the data when studying MP distribution in cases where it is impossible to take samples along the transect. The selection and analysis of point samples presents a mosaic picture and does not always reflect the real MP concentrations. Fluctuations in the distribution of MPs in the water along the river are due to the fact that complete mixing and redistribution of pollutants can occur at a considerable distance from the source or confluence site with another watercourse; currents, turbulence, and wind action can contribute to the accumulation of floating particles in bends, and the slowing down of currents may be associated with biofouling and the sinking of denser fragments to the bottom
[79]. In addition, a set of point measurements is only a “snapshot”, which makes it difficult to estimate the total particle flux, and therefore, for surface water bodies, it is preferable to conduct spatiotemporal studies to determine the average MP concentrations over a certain representative period of time
[83].
Despite the fact that the distribution and the abundance of MPs in freshwater ecosystems are influenced by many factors, in general, MP concentrations in freshwater ecosystems, especially in rivers, are directly dependent on population size and population density, but are also influenced by such factors as the efficiency of wastewater treatment, volumes of discharged wastewater, and remoteness from urbanized, industrial, and agricultural centers
[76][115]. The majority of MP particles entering freshwater ecosystems are secondary in origin. They form as a result of the destruction of larger plastic items, such as single-use packaging, tires, and fibers, and they are also represented by particles of paving and car paints
[68]. These types of MPs enter water bodies and streams along with surface and agricultural runoff or directly from plastic waste as a result of inefficient waste management
[116]. Storm runoff is another major source of MPs entering freshwater bodies. These types of wastewater introduce to the aquatic environment the particles from car tire abrasion and road markings
[68][117]. However, high MP concentrations in river waters are often associated with primary MPs. The plastics in the Austrian Danube were mainly in the form of industrial raw materials, such as pellets and flakes
[82]. Two studies showed that most of the plastics found in surface waters came from cosmetics or textiles
[118][119]. In a tributary of the Ob River, the Tom River, synthetic microspheres were identified, the share of which reached 56.8% of the total amount of particles found downstream of a large industrial center of Western Siberia, the city of Kemerovo
[120].