Natural Factors of Microplastics Distribution and Migration in Water: A Review

Abstract

Microplastics are widely present worldwide and are of great concern to scientists and governments due to their toxicity and ability to serve as carriers of other environmental pollutants. The abundance of microplastics in different water bodies varied significantly, mainly attributed to the initial emission concentration of pollutants and the migration ability of pollutants. The migration process of microplastics determines the abundance, fate, and bioavailability of microplastics in water. Previous studies have proved that the physicochemical properties of water bodies and the properties of microplastics themselves are important factors affecting their migration, but the change in external environmental conditions is also one of the main factors controlling the migration of microplastics. In this paper, we focus on the effects of meteorological factors (rainfall, light, and wind) on the distribution and migration of microplastics and conclude that the influence of meteorological factors on microplastics mainly affects the inflow abundance of microplastics, the physical and chemical properties of water, and the dynamics of water. At the same time, we briefly summarized the effects of aquatic organisms, water substrates, and water topography on microplastics. It is believed that aquatic organisms can affect the physical and chemical properties of microplastics through the physical adsorption and in vivo transmission of aquatic plants, through the feeding behavior, swimming, and metabolism of animals, and through the extracellular polymers formed by microorganisms, and can change their original environmental processes in water bodies. A full understanding of the influence and mechanism of external environmental factors on the migration of microplastics is of great theoretical significance for understanding the migration law of microplastics in water and comprehensively assessing the pollution load and safety risk of microplastics in water.

1. Introduction

Microplastics (MPs) refer to plastic particles with a size between 1 μm and 5 mm and are also a class of macromolecular polymers with high heterogeneity. These microplastics are widely distributed in aquatic and terrestrial ecosystems and pose potential risks to ecosystems and humans through inhalation, ingestion, and skin contact, as well as through the food chain [1]. It has become a hot topic of concern in global political and academic circles. However, it elaborated the concept of MPs for the first time, which also triggered the rapid growth of microplastic research in environmental media [2]. Plastic debris in the environment is a growing pollution problem, and a large number of studies have shown that MPs are ubiquitous on the planet and in polar regions, seawater, rivers, lakes, urban water bodies, underground water bodies, soils, the Qinghai-Tibet Plateau [3,4,5,6], food [7], and organisms [8]. “Plastics—the Fast Facts 2023” reported that global rate production rose from 1.7 million tons in 1950 to 367 million tons in 2020, in addition to 403 million tons in 2022 due to the COVID-19 pandemic in 2019 [9]. As of 2015, humans produced at least 6.9 billion tons of plastic waste [10], and 11% of plastic waste enters aquatic ecosystems every year [11]. According to the previous study, by 2030, about 53 million metric tons of plastic waste will still be entering the water environment each year, even with global efforts to reduce and manage plastic waste [12]. Therefore, the migration of microplastics in the aquatic environment affects the security of the entire ecosystem.

Similar to plankton in size, aquatic organisms quickly ingest MPs; the bisphenol A, phthalates, flame retardants, and coloring metals carried by them are toxic and can accumulate in organisms, disrupt the stability of biological cells, cause cytotoxicity [13,14], affect the normal physiological function of cells [15], and destroy biological tissues and organs [16], which endanger the health of organisms. Recent studies have confirmed that microplastics have entered the human placenta, brain, and other tissues and organs, and, amazingly, nanoplastics can also break through the blood-brain barrier [17] and may also be an accomplice in causing Parkinson’s disease [18]. At the same time, drinking water is also an important medium of microplastic pollution; every 1 L of plastic bottled water contains up to 240,000 microplastic particles, of which microplastics account for 10% and 90% may be nanoplastics [19]. Shockingly, there is direct evidence that microplastics and nanoplastics have penetrated human arteries and have helped increase the risk of serious diseases such as heart disease, stroke, and death [20]. It is worth noting that MPs are not only toxic in their constituents but also have a significant enrichment effect on environmental pollutants due to their small particle size and large specific surface area, and they are also good carriers of heavy metals [21], organic pollutants [22], and biotoxins [23] in the environment. There are a wide range of sources of MPs in natural water systems, including domestic sewage [24], atmospheric deposition [25], agricultural irrigation [26], seawater recharge [27], tire particles [28], etc.

Water bodies are important places for the convergence of primary and secondary microplastics in the environment, and, at the same time, water bodies are important functional elements in the environment, which play an extremely important role in global ecosystems and human health [17,20,25]. Primary microplastics are microplastics products mainly used for personal care products, while secondary microplastics are the products of mechanical stress, ultraviolet radiation, and biodegradation of large-sized plastics. They enter natural water bodies through atmospheric settlement, ground runoff, and domestic wastewater. The migration of microplastics in water is the key to the fate of microplastics. The migration behavior of microplastics in water mainly includes vertical sedimentation, suspension and floating, horizontal migration, aggregation, sedimentation, and undercurrent exchange [29,30,31,32]. Studies have pointed out that the properties of microplastics affect their migration process. The physical and chemical properties of water components can change the migration pathways of microplastics [31,33,34]. However, there is still a lack of review literature on the influence of external natural factors such as aquatic plants and rainfall on the migration of microplastics in water bodies [35,36,37]. Based on this, this paper briefly reviews the natural influencing factors of microplastic migration in water bodies, including meteorological factors, aquatic organisms, sediment, and topography of water bodies. Understanding the migration of microplastics in water is of great theoretical significance for comprehensively assessing the pollution load and safety risk of microplastics in water.

2. Meteorological Factors

Microplastics will be affected by various meteorological factors in the open environment of water bodies. Here, we review the effects of precipitation, sunlight radiation, and wind on microplastics’ distribution, migration, and fate in water bodies (Figure 1).

2.1. Rainfall

Previous studies have shown that rainfall-induced surface runoff may be one of the important pathways for microplastics to enter water in the terrestrial environment. Rainfall can also flush suspended microplastics from the atmosphere into the water environment. At the same time, rainfall will also change the hydrodynamic conditions, which will affect the occurrence and migration of the original microplastics in the water body. We systematically summarized the existing literature, searched 126 relevant articles with TS = (microplastics AND (rainfall OR rain) AND water) using the WOS database, and finally sorted out 60 articles on the effects of rainfall on microplastics in water systems using the PRISMA [38] literature screening process (Table 1). Through systematic analysis, the paper concluded that rainfall affects the distribution and migration of microplastics in water in four aspects (Figure 2).

The intensity and process of rainfall significantly affect the abundance of microplastics in water, and heavy rainfall affects the dynamics of water, which in turn affects the migration of microplastics. Hydrodynamic conditions affect the sedimentation and resuspension of microplastics [39]. The effect of rainfall on the abundance of microplastics in water bodies mainly showed positive effects, but there were also different results (Table 1). However, it is pointed out that there is a negative correlation between the abundance of microplastics and the flow velocity in sediments, and strong hydrodynamic conditions increase the horizontal migration of microplastics, which is not conducive to the deposition of microplastics [40]. Studies have shown that the content of microplastics in the Vuachere and Venoge rivers in Switzerland increases significantly after rain [41]. Another study pointed out that rainfall intensity and drought periods are important factors affecting microplastics in freshwater environments [42]. It has been suggested that rainstorm erosion can bring terrestrial microplastics into water bodies and increase their abundance in water bodies [43,44]. The particulate matter brought into the water body during the heavy rainfall process interacts with the plastic particles, which converts the large-sized plastics into small-sized plastic particles, thereby changing the migration and occurrence state of microplastics. The microplastics deposited in the water body will also be resuspended with the disturbance of heavy rainfall, thereby increasing the horizontal movement of microplastics in the water body [45]. It was shown that sustained rainfall increases the abundance and diversity of microplastics in surface waters, while the opposite trend was observed for microplastics in sediments [46]. Rainfall intensity positively correlates with the abundance of microplastics in urban water bodies [47,48]. However, some studies have pointed out a negative or no correlation between the abundance of microplastics in water bodies and rainfall, as rainfall and flooding may dilute the abundance of microplastics in groundwater and affect the migration of microplastics [49,50]. However, that also suggested that rainfall can dilute microplastic pollution in flowing water bodies such as rivers, mainly because the abundance of microplastics in rainfall is lower than that in rivers. The microplastics in rivers are washed out to the downstream areas. Still, compared with closed and regional water bodies, rainfall will inevitably reduce the concentration of microplastics in the regional atmosphere and increase the abundance of microplastics in water [51]. Previous research has shown that flood events lead to the release of large amounts of microplastics in rivers [52]. The process of rainfall also has a certain effect on the distribution and migration of microplastics in water, and it concluded that the abundance of microplastics in rainwater is higher in the early stage of rainfall, and the concentration in water decreases as rainfall continues [53]. Drought, as opposed to rainfall, is also one of the factors affecting microplastics in water bodies, and studies have shown that the concentration of microplastics in rainwater is positively correlated with the number of dry days before rainfall [42,54]. During three consecutive days of rainfall, the concentration of microplastics in river surface water was twice that of coastal seawater, and the number of microplastics decreased by 90% at 2 h after rainfall [55], which also indicates that different stages of rainfall also change the distribution of microplastics in water bodies.

The type of rainfall may also change the migration capacity of microplastics in the water by affecting the physical and chemical properties of the water body; the difference in ionic composition in the rainwater will affect the ion content of the water body, which in turn will affect the migration of microplastics, and previou study has shown that there is a significant positive correlation between the abundance of microplastics in the sediment and the concentration of Ca²⁺/Mg²⁺ in the overlying water [56]. The increase of ionic strength will enhance the agglomeration of microplastics, and at the same time, the electric double layer between the microplastics and the medium will be compressed, reducing the electrostatic repulsion, thus reducing the migration ability of microplastics [57]. The migration capacity of microplastics decreases with the increase in the concentration of high-valent cations. In the presence of high-valent cations, microplastic particles are more likely to agglomerate, so they tend to settle in water and have a weakened horizontal migration ability [58]. Prolonged acidic rainfall may lower the pH of water bodies. It has been shown that the migration capacity of microplastics in water usually increases with a pH increase. The increase in pH will reduce microplastic agglomeration, affect functional group deprotonation, increase the negative charge between microplastics and the medium, and enhance the electrostatic repulsion [59,60]. Small-scale water bodies are generally more susceptible to rainfall types than large-scale ones. Interestingly, it has been reported that coral reefs can trap microplastics, and acid rain can lead to bleaching and increased release of stored microplastics on coral reefs, allowing microplastics to redistribute [61]. As far as we know, the research on the direct impact of rainfall types on microplastics in water has not been reported, and it is necessary to systematically explain the effects of different rainfall types on the distribution and migration of microplastics in water from a combination of indoor quantitative experiments and field measurements.

It is easy to overlook that rainfall tends to change the content of dissolved organic matter in water bodies, thus affecting the migration of microplastics. When the dissolved organic matter in the water body increases, it will reduce the agglomeration capacity of microplastics and adsorb on the surface of microplastics, increase the negative charge, enhance electrostatic repulsion, and enhance the steric hindrance effect between microplastics and the medium, covering the deposition sites of microplastics on the surface of the medium, thereby increasing the migration ability of microplastics [62,63].

Table 1. Literature summary of microplastics in sewer systems during rainfall events.

Country	Event	Sample	Abundance *	Effect **	Ref.
China	Rain	Rainwater	146~8629 items/m2	Positive	[50]
China	Rain	Pearl River	219.8 ± 160.5 n/L (before); 474 ± 259.7 n/L (after)	Positive	[46]
Nigeria	Rain	Oxbow Lake	3.70 items/L (dry season); 3.08 items/L (rainy season)	Negative	[64]
China	Rain	Rainwater	1.1 × 1013 particle/day (wet); 7.4 × 1012 particles/day (dry)	Positive	[65]
Canada	Rain	Catchments	33.5 pieces/L (rain); 19.1 pieces/L (baseflow)	Positive	[66]
Japan	Rain	Surface water	35 items/L (light rainfalls); 929 items/L (moderate); 331 items/L (heavy)	Positive	[67]
Australia	Rain, storm	Cooks River Estuary	0.4 particles/L (before storm and heavy rain); 17.38 particles/L (after)	Positive	[68]
China	Rain	Qing River	1.16 n/L (before); 1.04 n/L (after)	Negative	[51]
Brazil	Rain	Goiana Estuary	0.56 n/100 m3 (rainy); 0.62 n/100 m3 (dry)	Positive	[69]
China	Rain	Urban river	29.98 n/L (dry season); 90.99 n/L (wet season)	Positive	[70]
Sri Lanka	Rain	Beira Lake and Canal	lake: 0.011 (dry), 0.007 (wet); canal: 0.003 (wet), 0.002 (dry)	No	[71]
China	Rain	Runoff	6.0 items/L (beginning); 1.0~4.0 items/L (during); 0.7 items/L (end)	Positive	[72]
China	Rain, flood	Dafangying River	18.62 ± 7.12 items/m3	Positive	[73]
China	Rain	Chaohu Lake	2133 ± 1534 n/m3 (dry season); 1679 ± 1577 n/m3 (wet season)	Negative	[74]
Brazil	Rain	Jurujuba Cove	14.4~202.8 n/L (rainy season); 91.2~137.4 n/L (dry season)	Negative	[75]
Brazil	Rain	Fish farms	81.12 items/L (dry); 236.96 items/L (rainy)	Positive	[76]
South Africa	Rain	Crocodile River	1058 (cool-dry season); 625 (hot-dry season); 625 particles/m3 (hot-wet season)	Positive	[77]
Singapore	Rain	Sea	164.5 particles/mL	Positive	[78]
China	Rain	Lake	0.59 items/L	Positive	[79]
China	Rain	Rainwater	141 (spring); 140 (winter); 102 (summer); 78 particles/(m2·d) (autumn)	Positive	[80]
China	Rain	Maowei Sea	2.8 particles/L (rainy season); 4.29 particles/L (dry season)	Negative	[81]
Mexico	Rain	Runoff	177.13 particles/L	Positive	[47]
USA	Rain	Estuarine rivers	90,007 pieces/km2 (summer); 95425 pieces/km2 (fall)	Positive	[82]
Indian	Rain, Storm	Manipal Lake	0.423 particles/L (monsoon); 0.117 particles/L (post-monsoon)	Positive	[83]
Malaysia	Rain, wind	Sepanggar Bay water	106.6 ± 23.0 (SWM); 63.0 ± 8.0 (NEM); 31.2 ± 6.7 particles/m3 (INTER)	Positive	[84]
China	Rain	Rainwater	229 n/(m2·d) (wet deposition); 125 n/(m2·d) (dry deposition)	Positive	[85]
Colombian	Rain	Estuaries	0.33 items/m3 (high rain); 0.085 items/L (low rain)	Positive	[86]
Canada	Rain	Urban runoff	186 particles/L	Positive	[87]
France	Rain	Liane River	35.5 (heavy rain); 5.1 (light rain); 12.4 particles/m3 (no rain)	Positive	[88]
China	Rain	Karst groundwater	4.50 items/L	Positive	[89]
Brazil	Rain	Paraíba do Sul River	1~12 particles/m3 (low water season); 1~18.3 particles/m3 (high water season)	Positive	[90]
China	Rain	Xincun Lagoon Bay	60.9 ± 21.5 items/L (rainy season); 72.6 ± 23.7 items/L (dry season)	Negative	[91]
Turkey	Flood	Mediterranean Region river	539,189 MPs/km2 (before flood); 7,699,716 MPs/km2 (afterwards)	Positive	[92]
France	Stormwater	Catchment outlet	29 items/L	Positive	[93]
USA	Rain	Tampa Bay surface water	2.2 particles/L (rain in OTB site); 1.0 particles/L(average)	Positive	[94]
Indian	Monsoonal rainfall	Udyavara River	530.14 ± 352 particles/m3	Positive	[95]
Vietnam	Rain	Saigon River	53 items/L (rainy season); 75 items/L (dry season)	No	[96]
India	Rain	Netravathi River	36.86 ± 23.12 (2020 monsoon); 70.5 ± 61.22 MP/m3 (after)	Positive	[97]
Belgium	Rain	Flanders surface water	0.48 MPs/L	No	[98]
Argentina	Rain	Lake	100 (spring)~180 MPs/m3 (summer)	Positive	[99]
China	Rain, flood	Yangtze Estuary	300 n/kg (1954 flood at ECS1); 1000 n/kg (1998 flood at CCYY1)	Positive	[100]
China	Rain, typhoons	Seawater	63.6 ± 37.4 items/L (before typhoon); 89.5 ± 20.6 items/L (after typhoon)	Positive	[101]
Indonesia	Rain	Jakarta River	9.80 ± 4.79 (rainy season); 8.01 ± 4.82 particles/m3 (dry season)	Positive	[102]
Brazil	Rain	Acaraí Lagoon	1.4~3.4 n/L	Negative	[103]
Australia	Rain	Perth metropolitan waters	47,164 pieces/km2 (heavy rain in May); 2461 pieces/km2 (March)	Positive	[104]
India	Rain	Mandovi-Zuari estuarine	107 particles/L (wet season); 99 particles/L (dry season)	Positive	[105]
Australia	Rain	Storm drains	139.43 items/effort (before); 132.6 items/effort (during); 294.5 items/effort (after)	Positive	[106]
Finland	Rain	Surface flow wetland	104 MPs/m3 (inflow); 200 (outflow addition deposition)	Positive	[107]
Thailand	Rain	Runoff	1.3 ± 1.3 particles/L (wet season); 2.8 ± 0.9 particles/L (dry season)	Positive	[108]
India	Rain	Sharavathi River sediment	2.5~57.5 pieces/kg (pre-monsoon); 0~15 pieces/kg (post-monsoon)	Positive	[109]
China	Rain	Donghu Lake	5.84 ± 2.95 items/L (equilibrium state); 8.27 ± 5.65 items/L (during rain); 7.60 ± 4.04 items/L (after rainfall)	Positive	[110]
China	Rain	WWTP	36.2~126.2 particles/L(rain); 38.9~75.3 particles/L (no rain)	Positive	[111]
China	River	Hanjiang rRiver	30.9 (base flow); 80.2~114.5 (flood)	Positive	[112]
China	Rain, typhoon	Surface seawater in Hong Kong	0.02 items/L (dry season); 0.10 items/L (wet season)	Positive	[113]
China	Rain	Harbor and coastal sediments	36.5 ± 52.5 items/kg (dry season); 22.6 ± 23.2 items/kg (wet season)	Positive	[114]
Italy	Rain, wind	Lake	0.82~1.24 particles/m3 (before); 2.42~4.41 particles/m3 (after)	Positive	[115]
Portugal	Rain	Estuary	263 items/kg (no rain); 205 items/kg (rain)	No	[116]
China	Rain	Jiaozhou Bay sea water	0.174 pieces/m3 (heavy rain in May); 0.05 piece/m3 (no rain in November)	Positive	[117]
USA	Rain	Outfalls	0.30 ± 0.10~0.80 ± 0.33 MP/L (rain)	Negative	[118]
Italy	Rain	Mugnone Creek	3.5 × 108 items/day (wet season in 2019); 5.2 × 106 (dry season 2020)	Positive	[119]
Lithuania	Rain	WWTP	2982 ± 54 MP/L (wet season); 1964 ± 50 MP/L (dry season)	Positive	[120]
Germany	Rain	Weser River	219.05 items/m3 (no rain day); 14,536.1 items/m3 (rain day)	Positive	[121]

* The average abundance of microplastics is mainly represented in the literature. Meanwhile, Get Data software v 2.2 (http://getdata-graph-digitizer.findmysoft.com/ (accessed on 29 April 2024)) was used to manually extract the measured data when the data information in the literature was presented graphically. ** represented the effect between the abundance of microplastics in water and the degree of rainfall.

Table 1. Literature summary of microplastics in sewer systems during rainfall events.

Country	Event	Sample	Abundance *	Effect **	Ref.
China	Rain	Rainwater	146~8629 items/m2	Positive	[50]
China	Rain	Pearl River	219.8 ± 160.5 n/L (before); 474 ± 259.7 n/L (after)	Positive	[46]
Nigeria	Rain	Oxbow Lake	3.70 items/L (dry season); 3.08 items/L (rainy season)	Negative	[64]
China	Rain	Rainwater	1.1 × 1013 particle/day (wet); 7.4 × 1012 particles/day (dry)	Positive	[65]
Canada	Rain	Catchments	33.5 pieces/L (rain); 19.1 pieces/L (baseflow)	Positive	[66]
Japan	Rain	Surface water	35 items/L (light rainfalls); 929 items/L (moderate); 331 items/L (heavy)	Positive	[67]
Australia	Rain, storm	Cooks River Estuary	0.4 particles/L (before storm and heavy rain); 17.38 particles/L (after)	Positive	[68]
China	Rain	Qing River	1.16 n/L (before); 1.04 n/L (after)	Negative	[51]
Brazil	Rain	Goiana Estuary	0.56 n/100 m3 (rainy); 0.62 n/100 m3 (dry)	Positive	[69]
China	Rain	Urban river	29.98 n/L (dry season); 90.99 n/L (wet season)	Positive	[70]
Sri Lanka	Rain	Beira Lake and Canal	lake: 0.011 (dry), 0.007 (wet); canal: 0.003 (wet), 0.002 (dry)	No	[71]
China	Rain	Runoff	6.0 items/L (beginning); 1.0~4.0 items/L (during); 0.7 items/L (end)	Positive	[72]
China	Rain, flood	Dafangying River	18.62 ± 7.12 items/m3	Positive	[73]
China	Rain	Chaohu Lake	2133 ± 1534 n/m3 (dry season); 1679 ± 1577 n/m3 (wet season)	Negative	[74]
Brazil	Rain	Jurujuba Cove	14.4~202.8 n/L (rainy season); 91.2~137.4 n/L (dry season)	Negative	[75]
Brazil	Rain	Fish farms	81.12 items/L (dry); 236.96 items/L (rainy)	Positive	[76]
South Africa	Rain	Crocodile River	1058 (cool-dry season); 625 (hot-dry season); 625 particles/m3 (hot-wet season)	Positive	[77]
Singapore	Rain	Sea	164.5 particles/mL	Positive	[78]
China	Rain	Lake	0.59 items/L	Positive	[79]
China	Rain	Rainwater	141 (spring); 140 (winter); 102 (summer); 78 particles/(m2·d) (autumn)	Positive	[80]
China	Rain	Maowei Sea	2.8 particles/L (rainy season); 4.29 particles/L (dry season)	Negative	[81]
Mexico	Rain	Runoff	177.13 particles/L	Positive	[47]
USA	Rain	Estuarine rivers	90,007 pieces/km2 (summer); 95425 pieces/km2 (fall)	Positive	[82]
Indian	Rain, Storm	Manipal Lake	0.423 particles/L (monsoon); 0.117 particles/L (post-monsoon)	Positive	[83]
Malaysia	Rain, wind	Sepanggar Bay water	106.6 ± 23.0 (SWM); 63.0 ± 8.0 (NEM); 31.2 ± 6.7 particles/m3 (INTER)	Positive	[84]
China	Rain	Rainwater	229 n/(m2·d) (wet deposition); 125 n/(m2·d) (dry deposition)	Positive	[85]
Colombian	Rain	Estuaries	0.33 items/m3 (high rain); 0.085 items/L (low rain)	Positive	[86]
Canada	Rain	Urban runoff	186 particles/L	Positive	[87]
France	Rain	Liane River	35.5 (heavy rain); 5.1 (light rain); 12.4 particles/m3 (no rain)	Positive	[88]
China	Rain	Karst groundwater	4.50 items/L	Positive	[89]
Brazil	Rain	Paraíba do Sul River	1~12 particles/m3 (low water season); 1~18.3 particles/m3 (high water season)	Positive	[90]
China	Rain	Xincun Lagoon Bay	60.9 ± 21.5 items/L (rainy season); 72.6 ± 23.7 items/L (dry season)	Negative	[91]
Turkey	Flood	Mediterranean Region river	539,189 MPs/km2 (before flood); 7,699,716 MPs/km2 (afterwards)	Positive	[92]
France	Stormwater	Catchment outlet	29 items/L	Positive	[93]
USA	Rain	Tampa Bay surface water	2.2 particles/L (rain in OTB site); 1.0 particles/L(average)	Positive	[94]
Indian	Monsoonal rainfall	Udyavara River	530.14 ± 352 particles/m3	Positive	[95]
Vietnam	Rain	Saigon River	53 items/L (rainy season); 75 items/L (dry season)	No	[96]
India	Rain	Netravathi River	36.86 ± 23.12 (2020 monsoon); 70.5 ± 61.22 MP/m3 (after)	Positive	[97]
Belgium	Rain	Flanders surface water	0.48 MPs/L	No	[98]
Argentina	Rain	Lake	100 (spring)~180 MPs/m3 (summer)	Positive	[99]
China	Rain, flood	Yangtze Estuary	300 n/kg (1954 flood at ECS1); 1000 n/kg (1998 flood at CCYY1)	Positive	[100]
China	Rain, typhoons	Seawater	63.6 ± 37.4 items/L (before typhoon); 89.5 ± 20.6 items/L (after typhoon)	Positive	[101]
Indonesia	Rain	Jakarta River	9.80 ± 4.79 (rainy season); 8.01 ± 4.82 particles/m3 (dry season)	Positive	[102]
Brazil	Rain	Acaraí Lagoon	1.4~3.4 n/L	Negative	[103]
Australia	Rain	Perth metropolitan waters	47,164 pieces/km2 (heavy rain in May); 2461 pieces/km2 (March)	Positive	[104]
India	Rain	Mandovi-Zuari estuarine	107 particles/L (wet season); 99 particles/L (dry season)	Positive	[105]
Australia	Rain	Storm drains	139.43 items/effort (before); 132.6 items/effort (during); 294.5 items/effort (after)	Positive	[106]
Finland	Rain	Surface flow wetland	104 MPs/m3 (inflow); 200 (outflow addition deposition)	Positive	[107]
Thailand	Rain	Runoff	1.3 ± 1.3 particles/L (wet season); 2.8 ± 0.9 particles/L (dry season)	Positive	[108]
India	Rain	Sharavathi River sediment	2.5~57.5 pieces/kg (pre-monsoon); 0~15 pieces/kg (post-monsoon)	Positive	[109]
China	Rain	Donghu Lake	5.84 ± 2.95 items/L (equilibrium state); 8.27 ± 5.65 items/L (during rain); 7.60 ± 4.04 items/L (after rainfall)	Positive	[110]
China	Rain	WWTP	36.2~126.2 particles/L(rain); 38.9~75.3 particles/L (no rain)	Positive	[111]
China	River	Hanjiang rRiver	30.9 (base flow); 80.2~114.5 (flood)	Positive	[112]
China	Rain, typhoon	Surface seawater in Hong Kong	0.02 items/L (dry season); 0.10 items/L (wet season)	Positive	[113]
China	Rain	Harbor and coastal sediments	36.5 ± 52.5 items/kg (dry season); 22.6 ± 23.2 items/kg (wet season)	Positive	[114]
Italy	Rain, wind	Lake	0.82~1.24 particles/m3 (before); 2.42~4.41 particles/m3 (after)	Positive	[115]
Portugal	Rain	Estuary	263 items/kg (no rain); 205 items/kg (rain)	No	[116]
China	Rain	Jiaozhou Bay sea water	0.174 pieces/m3 (heavy rain in May); 0.05 piece/m3 (no rain in November)	Positive	[117]
USA	Rain	Outfalls	0.30 ± 0.10~0.80 ± 0.33 MP/L (rain)	Negative	[118]
Italy	Rain	Mugnone Creek	3.5 × 108 items/day (wet season in 2019); 5.2 × 106 (dry season 2020)	Positive	[119]
Lithuania	Rain	WWTP	2982 ± 54 MP/L (wet season); 1964 ± 50 MP/L (dry season)	Positive	[120]
Germany	Rain	Weser River	219.05 items/m3 (no rain day); 14,536.1 items/m3 (rain day)	Positive	[121]

* The average abundance of microplastics is mainly represented in the literature. Meanwhile, Get Data software v 2.2 (http://getdata-graph-digitizer.findmysoft.com/ (accessed on 29 April 2024)) was used to manually extract the measured data when the data information in the literature was presented graphically. ** represented the effect between the abundance of microplastics in water and the degree of rainfall.

2.2. Ultraviolet Radiation

The effect of sunlight radiation on the distribution and transport of microplastics in water bodies is twofold (Figure 3). On the one hand, sunlight radiation will change microplastics’ physicochemical properties and surface morphology, making them more susceptible to aging and decomposition. In the natural environment, the microplastics in the water body easily decompose slowly into microplastics and nano-sized microplastics with smaller particle sizes under ultraviolet irradiation, especially the microplastics floating on the surface of the water body. The action of ultraviolet rays and oxygen will gradually age and decompose under long-term exposure to sunlight, and the surface morphology and functional groups will change accordingly; therefore, the light affects the distribution and migration law of microplastics through the changes in the particle size and surface characteristics of microplastics [122,123]. Photoaging will roughen the surface of microplastics, cracks will appear, and the density of polyethylene microplastics will increase, changing the water body’s migration process. Previous research pointed out that the color of plastics may also be an influencing factor affecting the aging and degradation of microplastics in water, and sunlight exposure is the main reason for the aging of plastics, which can easily trigger the chain reaction and chain breakage of plastic polymers, resulting in the cracking of plastics into microplastics [124]. Photoaging often changes the color of the plastic polymer, and the color of the plastic itself can also affect the absorption of sunlight.

On the other hand, sunlight exposure will affect the change of atmospheric gradient and change the evaporation of water, so that sunlight radiation will bring about the linkage of wind and rain, and at the same time, sunlight radiation will affect the temperature of the water body and affect the change of dissolved oxygen and other water physical and chemical properties in the water. The change in the water’s physical and chemical properties and the resulting change in biological activities will change the distribution and migration of microplastics. The study pointed out that the increase in solar radiation, the increase in temperature, and the melting of glaciers, ice, and snow will increase the total abundance of microplastics in the water body. The microplastic pollution in a remote lake on the Qinghai-Tibet Plateau is caused by the microplastics released by the melting of glaciers. Both light exposure and the resulting physical weathering affect the decomposition of plastics, change the size of microplastics, and then affect the migration and distribution of microplastics in water bodies. An increase in the temperature of a water body can lead to changes in the aquatic community. The study reported that predation by benthic organisms increases when water temperatures rise, so abundant biological activity may lead to the resuspension of deposited microplastics [125]. Sunlight exposure drives the circulation of global ocean currents, which is also an important factor affecting the migration of microplastics in water bodies. However, mathematical models were used to predict the transport pathways of plastic pollution in the wake of ocean circulation [126].

2.3. Wind

Wind is an important path for microplastics to travel from land sources to water sinks, and also pulls microplastics to remote and alpine areas that are inaccessible to people, such as the Arctic [127] and Qinghai-Tibet Plateau [128]. Typhoons in China increased the concentration of microplastics in the aquatic environment of its immediate vicinity [101]. A previous study found that hurricane-induced turbulence redistributed microplastics in coastal waters [129]. It has been noted that during storm events, microplastics can be transported by rapid flow and deposited in deeper locations when turbulence slows down [130]. Microplastics in water of different depths before and after the storm in Santa Nika Bay were investigated, and it was found that the abundance of microplastics on the surface and mid-water increased after the storm, while the abundance of microplastics decreased on the seafloor [131]. Wind can significantly impact microplastics floating on the surface of the water, especially polystyrene foam. Floating microplastics increase their horizontal migration capacity under the influence of wind [132]. The sedimentation flux of microplastic polymers in water increases with the increase in wind speed, and high-density microplastic polymers are more affected by wind force than low-density microplastics [133]. So, it was found that in the northwestern Mediterranean, strong winds were followed by five times as many plastic particles floating on the ocean’s surface, which facilitated the mixing of plastic particles in the surface water column and their vertical redistribution [134]. However, the abundance of microplastics in the waters of Lake Zurich and Lake Constance decreased after vertical mixing caused by strong winds, mainly because the winds before sampling in Lake Zurich and Lake Constance may have reduced the number of particles measured due to vertical mixing [41]. It is worth noting that the increased winds due to global warming strengthen the circulation of microplastics in the atmosphere, hydrosphere, and pedosphere. In the future, the impact of wind on microplastics should be viewed from a global perspective; however, large-scale field detection is very difficult, so it is necessary to develop corresponding models and locally measured data to explore the impact of wind on microplastics.

3. Aquatic Life

Aquatic organisms in the aquatic environment are crucial to the distribution and migration of microplastics, including the adsorption, interception, and internal absorption of aquatic plants, the carrying and ingestion of aquatic animals, and the interaction between microorganisms and microplastics in water (Figure 4).

3.1. Aquatic Plants

The interaction between microplastics and aquatic plants affects the normal growth of aquatic plants and changes the occurrence state and migration ability of microplastics. Aquatic plants can affect the migration of microplastics in water through surface adsorption, absorption, transport, and accumulation, thereby changing the exposure concentration of free microplastics in water and reducing the bioavailability of other animals, plants, and humans [135,136,137] (Figure 3). Therefore, aquatic plants are considered to be an important way to slow down the migration of microplastics in water bodies. Another study reported that seaweed has the ability to trap microplastics [138]. The results showed that the three aquatic plants had the effect of capturing and removing polystyrene, and the roots of aquatic plants had the strongest ability to capture micro/nanoplastics in the water body, which could reach 6250 μg/g, and different root structures may affect the absorption and transport of micro/nanoplastics [135]. Polystyrene (PS) microplastics will be significantly aggregated on the root surface of plants, especially at the root tips; microplastics tightly adhere to the roots and can remain adhered to them after washing [139]. Smaller nanoscale microplastics move from adherent root hairs to columnar vascular bundles inside roots [140]. However, hydroponically grown rice seedlings were exposed to PS microplastics, and it was found that the microplastics were distributed in the rice root system and the intercellular space [141]. Studies have shown that small-sized microplastics can enter plants through the stomata on the leaf surface, binding microplastics to aquatic plants and changing their migration process [142].

3.2. Aquatic Animals

Aquatic animals are direct victims of exposure to microplastics in water bodies, and these animals alter the abundance and migration of microplastics in water bodies through ingestion and epidermal contact. Microplastics ingested by aquatic organisms accumulate in different organs in the body through the digestive system, and the microplastics accumulated in aquatic organisms of different nutritional levels can be transmitted within the food chain in the preying relationship between organisms, ultimately endangering human health. Currently, the research on microplastics in organisms mainly focuses on fish (Figure 5), followed by crustaceans, mollusks, and annelids [143]. Fish are also the main body, and the focus of attention is on the impact of microplastics in water bodies. Microplastics have been detected in more than 728 fish species [144,145]. Fish change the migration of microplastics in water through ingestion, transport, digestion, and excretion. Their ingestion behavior is the premise of controlling the transport, digestion, and excretion of microplastics in fish and the intake of microplastics by aquatic animals, including active ingestion (foraging or accidental ingestion) and passive ingestion (food chain transmission). In a survey on microplastic exposure in food fish off the east coast of Brazil, it was found that 62.5% of Atlantic mackerel were exposed to microplastics, while 33% of them were oblique megalodons. Similarly, aquatic organisms can also ingest microplastics in the terrestrial water environment, and the concentration of microplastics in carp, crucian carp, etc. is 1~6 [146]. Previous studies suggested that copepod aquatic organisms would ingest smaller microplastics, which would accumulate in the anterior midgut and eventually be excreted in dense feces, thus changing the migration path of microplastics in the water body [147]. Vroom et al. pointed out that PS in the intestinal tract of Calanus finmarchicus forms aggregates that can account for 30 to 90 percent of the intestinal volume, and these aggregates are excreted in the form of feces, which, combined with the diurnal vertical migration of aquatic animals, transports microplastics to deeper waters [148]. Different feeding patterns correlate with microplastic loading in aquatic animals [149]. The study showed that a certain concentration of microplastics was detected in the skin tissue, proving that the adhesion of the aquatic animal epidermis or the skin tissue is the carrier of microplastic migration in the water body [150]. Interestingly, recent studies have shown that microplastics have been detected in rotifers collected from all marine and freshwater sites and have shown that the grinding effect of rotifers chewing is considered to be an important producer of nanoplastics in the aquatic environment and that rotifers alter the migration of microplastics in the aquatic environment by affecting the size of the plastics [151].

3.3. Water Microorganisms

Biofilms are integral structures formed by the growth and development of protozoa, bacteria, algae, and fungi [152]. Bacterial colonies within biofilms secrete extracellular polymeric substances (EPSs). EPS is considered a viscous colloidal substance that plays a key role in microbial colonization and contaminant migration [153]. In the aquatic environment, microorganisms can attach to microplastics, forming biofilms, which affect the sedimentation and transport of microplastics in water [154,155,156]. The biofilm formed by microorganisms on the surface of microplastics will affect the surface morphology and physicochemical properties of microplastics and then affect the migration behavior of microplastics. Due to the presence of biofilms, the hydrophobicity, density, functional groups, size, surface, and roughness of microplastics change. As previously reported, the sedimentation rate of PET and PS attached to the biofilm was 1% and 4% faster than the original state [157]. Biofilm increases the roughness of the surface of microplastics. After biofilm formation, the surface roughness, number of pores, and surface area of microplastics increase [158]. Recent studies have shown that the biofilm on the surface of microplastics can be used as a kind of surface roughness, and the sedimentation rate can be changed by changing the resistance coefficient of microplastics during the sedimentation process. It has also been pointed out that the multilayer structure and anisotropy of microbial biofilms are not exactly equivalent to the roughness of traditional particles [159]. The spectral absorption summit of the functional groups of PE plastics was exposed to seawater changes, producing an additional peak at 1700, which is believed to be mainly due to the C==O produced by the decomposition of microplastics [156]. There are some differences in the study of biofilm alteration of the surface hydrophobicity of microplastics. Attachment to biofilm has been reported to reduce the hydrophobicity of PE [160], but it has also been reported that biocontaminated plastic surfaces will increase hydrophobicity [161]. At the same time, microorganisms are also considered an ideal way for microplastics to be degraded. The aerobic environment on the water body’s surface will promote microorganisms to degrade microplastics into carbon dioxide and water. In the anoxic environment of deep-water bodies, microplastics will be decomposed into carbon dioxide, water, and methane under the action of anaerobic microplastics. In the process of degradation, the original structure of microplastics will change significantly between polymers. Between polymers and plastic additives, the characteristics of plastics will change and the specific surface area of microplastics exposed to the environment will increase, which will promote the attachment and reproduction of microorganisms and the occurrence of physicochemical reactions. After the interaction between microplastics and microorganisms, their particle size continues to decrease, and smaller microplastics are easily suspended on the surface of the water body again, changing the vertical migration path. These smaller microplastics are also more easily ingested by aquatic animals, changing their migration process.

4. Water Matrix: Suspended Particulate, Sediment, and Topography

Geographical location and land use were excluded from this study, considering that these two factors are closely related to human activities [162], so the effects of natural factors on microplastics in water bodies did not take into account the effects of geographical location and land use. The migration capacity of microplastics is related to the roughness of the medium.

4.1. Suspended Particulate

The rougher the surface of the sand grains, the easier it is for microplastics to be deposited and the lower their migration capacity. The suspended sediment in the water body can disrupt the stability of microplastics [163,164] and significantly affect the migration behaviors of microplastics, such as accumulation, suspension, and infiltration. Studies have pointed out that the aggregation behavior of microplastics and suspended sediment increases the density of microplastic aggregates, affecting the vertical distribution and long-term migration of microplastics [165,166]. The interaction between MPs and suspended sediment is influenced by water environmental conditions (e.g., hydrodynamic characteristics, concentration and type of ions, dissolved or granular organic/inorganic colloids, microorganisms, and phytoplankton) and physicochemical properties of MPs/sediment itself (e.g., particle Zeta potential size, charge distribution, and surface polar functional groups) [163,167]. Studies have reported that about 2–9% of all microplastic mass leaving Switzerland is transported by sediment [3]. In the water environment, microplastics combine with other suspended particles to change the density of microplastics so that they migrate from the surface water body to the deeper water body; once the microplastics change the suspension state, under the action of complex meteorological factors, the migration path will be more complex [168]. Studies have shown that microplastics form aggregates with natural mineral particles, such as kaolin and other clay particles, and negatively charged alginate and iron oxide in water form electrostatic interaction with microplastic particles, which makes the two form larger aggregates and affects the migration process of microplastics in water [167,169].

4.2. Sediments

Water sediments contain a large number of microplastics, which migrate due to changes in natural conditions such as wind, waves, and tides and can also move through human disturbances [170]. The roughness, particle size structure, and organic matter composition of the sediment all affect the migration and distribution of microplastics. It has been shown that microplastics deposited in coarse-grained sediments are more likely to be resuspended, while sediments with higher viscosity and rich organic matter are less likely to be resuspended [171]. The retention and accumulation of MPs and aggregates in sediments can gradually clog the pores between sediment particles and inhibit or even completely hinder the osmotic migration of MPs in sediments [172]. So, it has been pointed out that constructed wetlands can effectively remove microplastics in water bodies, with a removal efficiency of more than 90%, which significantly changes the migration process of microplastics in this water body, and the matrix particle size of wetlands and the design of wetlands are the key factors affecting the removal and migration of microplastics in water bodies [173]. In the treatment of rural domestic sewage, it was found that the smaller the particle size of the matrix, the higher the removal efficiency of microplastics, which may be caused by the greater friction of the microplastics per unit volume of sediment. Researchers reported that microplastics’ removal and retention efficiency in the sand-based reactor was close to 100%, which was significantly higher than that of the gravel-based reactor [174]. The biofilm formed on the sedimentary matrix of the water body affects the migration of microplastics, and it was observed that the biofilm growing on the water matrix reduces the pore space and increases the viscosity of the matrix, thereby enhancing the substrate’s ability to retain microplastics [175]. The nature of sediments is also a factor affecting the migration of microplastics in water, and it has been suggested that positively charged sediments are more likely to adsorb negatively charged microplastics [176].

4.3. Water Topography and Landform

The topography and appendages of the water body can affect the migration of microplastics in the water body. It has been shown that artificial structures such as dams, sand bars, and diversion walls can reduce a water body’s flow and thus increase the microplastic sedimentation rate [177,178,179]. River morphology can affect the velocity of water bodies or the migration of microplastics through bend interception. Compared with curved channels, straight channels are more likely to cause the horizontal migration of microplastics [171]. The study showed that the abundance of microplastics in sediment samples from the straight channel of the Thames River in Canada was lower than that collected at the inner and outer bends [171]. River morphology can also affect the undercurrent exchange behavior of particles through advection pumping, scouring, and silting exchange. When the shape of the riverbed is uneven, the surface pressure of the riverbed changes, which drives the movement of pore water so that colloids and plastic particles enter the riverbed and migrate. In the riverbed scouring zone, water flow affects the release and accumulation of pore water, resulting in the exchange of riverbed and colloidal particles [180,181,182].

5. Conclusions and Outlook

The migration of microplastics in water determines the distribution, fate, and ecological risk of microplastics, and the current research mainly focuses on the influence of physical and chemical properties of water bodies on the migration of microplastics and the research on microplastics under natural conditions is not sufficient. This paper focuses on the distribution and migration of microplastics in water and focuses on the influence of meteorological factors on microplastics, concluding that the influence of meteorological factors on microplastics mainly affects the distribution and migration of microplastics by affecting the abundance of microplastics inflow, affecting the physical and chemical properties of water bodies and water dynamics. At the same time, we also briefly summarized the effects of aquatic organisms’ water substrates and water topography on microplastics. It is believed that aquatic organisms can affect the physical and chemical properties of microplastics through the physical adsorption and in vivo transmission of aquatic plants, through the feeding behavior, moving, and metabolism of animals and the EPSs formed by microorganisms, changing their original environmental processes in the water body. Future research on microplastic migration should pay more attention to the following.

• Carry out indoor quantitative simulation experiments and outdoor long-term observations on the migration of microplastics under changes in meteorological conditions and understand the migration mechanism and controlling factors of microplastics in the field at a large scale. Attention should be paid to the changes in water environmental conditions in different seasons to better evaluate microplastics’ interannual variation characteristics. Global climate change has led to more frequent extreme weather events, and the transport of microplastics in water bodies and their sediments may be significantly affected by climate change, so it is necessary to strengthen the research on the global distribution and circulation of microplastics under climate change scenarios in the future.
• In different types of water bodies, topography and landforms, as well as the types of particulate matter in water bodies, are very different, and more attention needs to be paid to the movement mechanism of microplastics in different types of lakes and rivers. It is important to clarify the agglomeration behavior and co-migration mechanism between particulate matter and microplastics in clear water to predict microplastics’ environmental behavior and risks accurately. The migration process of microplastics in seawater and surface water has been partially studied, but more attention needs to be paid to the migration of microplastics in groundwater and special ecological regions such as karst water [89].
• The impact of water temperature change on the transport of microplastics and the impact of ocean current processes on the global migration of microplastics were investigated. Currently, most of the research on the migration mechanism of microplastics in water bodies is carried out in the laboratory, which is quite different from the actual environment in which microplastics are located, and the model microplastics used are also significantly different from those in the real environment.
• The interactive migration model of external environmental factors, internal water characteristics, and the physical and chemical properties of microplastics can be constructed. Systematic research can be carried out on multiple factors and scales to have a more comprehensive understanding of the migration process of microplastics in the real environment and better supervise their ecological risks [126,169]. At the same time, large-scale data acquisition needs to be standardized before validating the migration model to increase the applicability of measurements. Strengthening the study of the movement mechanics of microplastics and the spatial mode of pollution, determining the risk location of pollutant accumulation, and better delineating the area with the greatest risk of potential negative effects of microplastic pollution will help to focus on water protection.
• Microplastics in water bodies are important carriers of other pollutants, and the synergistic migration behavior and mechanism of bacteria and viruses of other pollutants, especially newly polluted antibiotics, and microplastics in the actual water environment are still unknown and need to be further studied and improved [183].
• Microplastics are constantly changing in the actual environment and may be affected by various factors in the migration, decomposition, and transformation process. Their original physical and chemical properties will also change.
• Combined with the characteristics of microplastic pollution and the law of migration, an efficient governance system with scientific classification, source treatment, and migration control as the main contents and a sub-regional, sub-type, and sub-time supervision mechanism should be established.

In general, external environmental factors affect the distribution and migration of microplastics, Therefore, understanding and mastering the change of external environmental conditions can effectively control the pollution distribution and migration behavior of microplastics in water bodies. For example, after strong weather events, the exposure abundance of microplastics in water bodies may increase significantly, so the intake of microplastics can be reduced by reducing the use of water bodies. By mastering the settlement law of microplastics in different rivers, we can selectively use water resources in different locations, and then control the exposure of microplastics in water bodies. However, for aquatic organisms, effective source control is the main way to reduce their environmental risks.