Microplastics: An introduction to environmental transport processes

Microplastic pollution is widespread across the globe, pervading land, water, and air. These environments are commonly considered independently, however, in reality these are closely linked. This review gives an overview of the background knowledge surrounding sources, fate and transport of microplastics within the environment. We introduce a new “Plastic Cycle” concept in order to better understand the processes influencing flux and retention of microplastics between and across the wide range of environmental matrices. As microplastics are a pervasive, persistent and potentially harmful pollutant, an understanding of these processes will allow for assessment of exposure to better determine the likely long‐term ecological and human health implications of microplastic pollution. WIREs Water 2018, 5:e1268. doi: 10.1002/wat2.1268


| PRESENCE AND SOURCES OF MICROPLASTICS WITHIN THE ENVIRONMENT
There are many ways in which plastics can be released to the environment, either as primary microplastics or as larger plastic items ("macroplastics") which will break down to form secondary microplastics ( Figure 2). Primary microplastics from domestic products, such as microbeads, can be present in waste water and subsequently discharged to rivers, while nurdles can be lost to freshwaters during production processes. Examples of secondary microplastic sources include intentional release (illegal dumping), mismanaged waste (litter) or unintentional losses (e.g., fishing gear and loss of shipping cargo) (Boucher & Friot, 2017), with the magnitude of different sources and pathways for microplastic release varying between the terrestrial, freshwater, and marine environments.

| Microplastics on land
All plastic is manufactured on land and, other than maritime or fishing uses, it is also where the majority of plastic is used in consumer products. The pathways for release of waste consumer products to land include direct littering and inefficient waste management, for example, loss during the waste disposal chain, industrial spillages, or release from landfill sites (Figure 2a; Lechner & Ramler, 2015;. Modern agricultural practices make use of plastic in a variety of ways including as mulches, which can degrade in situ, in addition to bale twine and wrapping which can be improperly disposed of (Nizzetto, Futter, & Langaas, 2016). These items can degrade to form secondary microplastics within the environment.
Microplastics may also be released directly to land along with sewage sludge applied to agricultural land as a fertilizer. Wastewater treatment plants are quite effective at removing microplastic particles from the wastewater stream, often with~99% removal (Carr, Liu, & Tesoro, 2016;Murphy, Ewins, Carbonnier, & Quinn, 2016;Talvitie, Mikola, Setala, Heinonen, & Koistinen, 2017), and many of these particles will settle to the sludge. It is estimated that throughout Europe, between 125 and 850 t of microplastics per million inhabitants are added annually to agricultural soils as a result of sewage sludge application . Horton, Walton, Spurgeon, Lahive, and Svendsen (2017) calculated that 473,000 to 910,000 metric tons of plastic waste is retained within European continental environments (terrestrial and freshwater) annually, which includes microplastics derived from sewage sludge, in addition to predicted inputs of litter and inadequately managed waste. Where plastics are not transported from land to rivers or the sea, this could lead to massive accumulation. However, few studies have investigated abundance of microplastics within terrestrial environments, or linked abundance to input pathways, therefore, it is not currently possible to directly link accumulation with specific environmental characteristics or anthropogenic activities.

| Microplastics in freshwater environments
Freshwaters represent the most complex system regarding microplastic transport and retention, as they receive microplastics from the terrestrial environment, function as conduits for microplastics to the marine environment (Figure 2b), act as a means of microplastic production through breakup of larger items and act as sinks retaining microplastics in sediments. Additionally, "freshwater" represents rivers, streams, ditches, lakes and ponds, all with very different characteristics.
Larger plastic items can enter the freshwater environment through inadequate waste disposal, either through littering or loss from landfill and transported from land via wind or surface runoff. In addition to macroplastics, there are significant direct inputs of microplastics to freshwater systems. Agricultural drainage and runoff from farmland can result in input of agricultural plastics or sewage-sludge derived fibers and microbeads. Storm drainage and urban runoff is often unfiltered and untreated, and can contain microplastics from degraded road paint and wear from vehicles (Boucher & Friot, 2017;. Despite the efficiency of wastewater treatment plants in removing microplastics, direct effluent input can also contain microplastics (Murphy et al., 2016). Additionally, during very high flow conditions, combined sewage overflows (CSOs) are designed to release untreated sewage into surrounding rivers to reduce the pressure on drainage systems, releasing both micro-and macroplastic waste. Studies suggest that although hotspots of microplastics may occur in close proximity to urban areas, the majority of microplastics are likely to enter waterbodies as a result of drainage systems and thus attention must also be paid to inputs including CSOs, storm drains, and effluent outfalls, which may be set apart from the most densely populated areas (Browne et al., 2011;. Although the majority of freshwater microplastic studies tend to focus on rivers, it is understood that microplastics are also prevalent within ponds and lakes (Free et al., 2014;Imhof, Ivleva, Schmid, Niessner, & Laforsch, 2013;Vaughan, Turner, & Rose, 2017). In the same way as rivers, these will receive inputs from land runoff and wind-blown debris, however, due to the enclosed nature of lakes it is likely that inputs of microplastics to standing waterbodies will lead to accumulation over time (Vaughan et al., 2017).

| Microplastics in the marine environment
The presence and abundance of microplastics within the oceans have been widely studied. Sources of microplastics to marine environments are widespread, as oceans are generally considered to be the ultimate sink for all plastic within the environment (Browne et al., 2011;Law & Thompson, 2014). In addition to the inputs from rivers, plastics will also enter oceans directly via mismanaged maritime or fishing waste, including abandoned fishing gear, accidental cargo loss, and illegal dumping. This will most likely be in the form of macroplastic waste that will degrade to form microplastics within the marine environment ( Figure 2c). Microplastics have been found to be widespread throughout various locations and within marine organisms worldwide, with ocean currents leading to specific areas of accumulation such as the well-known "Great Pacific Garbage Patch" (Zhang, Zhang, Feng, & Yang, 2010). Models have been developed to investigate transport processes and fate of microplastics within the oceans (Ballent, Purser, de Jesus Mendes, Pando, & Thomsen, 2012;Kowalski, Reichardt, & Waniek, 2016;Sherman & van Sebille, 2016) which may also add to our understanding of the processes that influence microplastic transport within freshwater environments.

| Microplastics in the atmosphere
It has been recently recognized that due to their lightweight nature, many microplastic particles will become suspended and transported within the air as "urban dust" (Dehghani, Moore, & Akhbarizadeh, 2017;Dris, Gasperi, Saad, Mirande, & Tassin, 2016). These commonly originate from road dust (e.g., tyre and paint particles) and fibers from synthetic textiles, especially from soft furnishings (Dris et al., 2017; and can lead to deposition of microplastics to land or aquatic environments. Although urban dust will originate especially in cities and highly populated areas, air currents and wind can lead particles to be transported far from the source (Zylstra, 2013). Weather events such as heavy rainfall will facilitate the deposition of particles to land (Dris et al., 2016). Given the diverse range of sources, the varying characteristics of particles affecting their behavior and the range of environmental factors influencing particle transport, airborne microplastic contamination is extremely difficult to trace and predict. It is not currently known to what extent atmospheric fallout contributes to aquatic and terrestrial contamination, therefore, more research is needed in this area.

| TRANSPORT PROCESSES
It is widely considered that the ocean represents a sink for a large proportion of microplastics, with the terrestrial and freshwater environments acting as important sources and pathways for microplastics to the sea (Jambeck et al., 2015;Lechner et al., 2014). Due to their lightweight nature and potential for widespread dispersal it is also likely that air currents act as a means of particulate transport, contributing to microplastic contamination on land and within aquatic systems (Cai et al., 2017;Dris et al., 2016). A number of studies have provided evidence for macro-and microplastic litter reaching oceans from rivers (Lebreton et al., 2017;Morritt, Stefanoudis, Pearce, Crimmen, & Clark, 2014; with particles often originating on land . However, it is increasingly becoming recognized that far from being merely conveyor belts for waste plastic, freshwaters, and soils can act as sinks themselves, retaining much of the microplastic pollution that they receive (Castañeda, Avlijas, Simard, Ricciardi, & Smith, 2014;. In some cases, due to the proximity and scale of plastic inputs, certain terrestrial and freshwater areas could actually accumulate microplastics at higher concentrations than in the ocean (Castañeda et al., 2014;. For future understanding of microplastic pollution within the environment it will therefore be important to link sources, particle behaviors and transport mechanisms, to understand how and where microplastics will accumulate. Agricultural soils may be an important source for microplastics to rivers through the application of sewage sludge as fertilizer, although it is likely that a high proportion will also be retained. A study on microplastic retention within soils found synthetic fibers derived from sewage sludge retained within treated agricultural soil up to 15 years after the last sludge application (Zubris & Richards, 2005). This study also suggested that accumulation hotspots can occur even at depth, with fibers found at greater than 25-cm depth in areas where downward drainage flow through the soil was high (Zubris & Richards, 2005). Retention within soils will be further facilitated by processes such as bioturbation which will draw particles away from the surface and into the deeper layers of the soil (Lwanga et al., 2017). Agricultural and forest soils are more likely to retain particles than urban land due to permeable soils and lower rates of overland flow (Nizzetto, Bussi, Futter, Butterfield, & Whitehead, 2016).
Where particles do enter rivers, they will be subject to the same transport processes which mobilize other sediments, such as sand and silt, in channels. In simple terms, the faster the river flow the more energy it has, and thus it can entrain and transport a greater volume of particles (Knighton, 2014). However, in the case of microplastics, most rivers are likely to be supply-limited with respect to transport, meaning rivers will be capable of transporting all plastics that are delivered to them. Despite the buoyancy of many plastics, where river energy drops, for example, in slow-moving sections of water, it is likely that microplastics will settle out along with sinking sediment particles. Additionally, this sediment deposition may aid in the burial of microplastic particles, whether microplastics are simultaneously deposited or are already present within the sediment (Corcoran, Moore, & Jazvac, 2014). It is therefore likely that on their journey throughout the freshwater environment, many particles will also be retained within sediments (Nizzetto, Buzzi, et al., 2016;. Within lakes where sediment accumulation rates are high, it has been suggested that retention and incorporation of microplastics into sediments could lead to burial and long-term preservation within the sediment (Corcoran et al., 2014(Corcoran et al., , 2015. The density and shape of microplastic particles will have important effects on their transport and retention in sediments. Although many polymer particles have low densities, so are buoyant and will float, there are also many types of polymer that are denser than water and so will naturally sink. Dense plastics include commonly used polymers such as polyvinyl chloride (PVC), polyethylene terephthalate (PET), and nylon (Table 1), in addition to polymer composites such as those found in paints . The density of plastic polymers is also not constant, with the growth of microalgae on particles (biofouling) increasing their density, leading to them sinking and being deposited in sediments (Lagarde et al., 2016). Additionally, size and shape play a role in retention of microplastics within sediments, with irregularly shaped particles having highly complex settling mechanics compared to spherical particles (Bridge & Bennett, 1992). For buoyant particles, those which are irregularly shaped are most likely to be drawn down from the surface of the water and be retained underwater, rather than return to the surface, compared to spherical particles (Ballent et al., 2012). In river bed sediments, larger microplastic particles have been found to be more likely to be retained (Nizzetto, Bussi, et al., 2016). However, previous work on comparable sediment particles has shown that shape may have a greater influence than size, with larger platelike particles more likely to be mobilized in preference to finer, spherical particles (Prager, Southard, & Vivoni-Gallart, 1996). This difference in particle behavior, dependent on size, shape, and density, illustrates the complexity in predicting and modeling microplastic fate and transport in river environments.
Sediment transport and deposition in rivers also has a great degree of temporal and spatial variability. At a local scale, instantaneous, small-scale changes in turbulence can apply energy to an area of river bed and act to entrain previously deposited particles (Nelson, Shreve, McLean, & Drake, 1995). At a wider scale, higher energy flows from floods are likely to lead to resuspension of dense microplastics along with other sediment particles (Hoellein et al., 2017;Knighton, 2014). At longer timescales, progressive change in the morphology of river channels could lead to erosion of river bars or banks, remobilizing previously deposited microplastics from floodplain sediment as has been shown for heavy metals (Lecce & Pavlowsky, 1997;Walling et al., 2003).
Due to currents, winds and the large area covered, once they reach the oceans (micro)plastics can be rapidly and widely dispersed, travelling significant distances from the source (Van Sebille, England, & Froyland, 2012). Additionally, microplastics are subject to vertical transport within the oceans due to biofouling, egestion in fecal pellets and incorporation into marine snows (sinking detritus) (Cole et al., 2016;Kowalski et al., 2016;Rummel, Jahnke, Gorokhova, Kühnel, & Schmitt-Jansen, 2017). This wide-ranging vertical and horizontal transport is highlighted by the fact that microplastics have been discovered in all locations that have been investigated, including in the deep sea, Southern Ocean, and Arctic ice cores (Cincinelli et al., 2017;Obbard et al., 2014;Woodall et al., 2014).
Little is known about the processes governing transport of microplastics within the air, although it is understood that this is likely to be a significant transport pathway of microplastics (Dris et al., 2016(Dris et al., , 2017. Importantly, this mode of transport is likely to lead to the widest dispersal as it is the least limited by environmental boundaries, influenced mainly by the directions of air movement rather than the unidirectional flows that are generally the case on land and within waterbodies. Due to the limited data currently available, further research will be needed to better understand the processes involved in atmospheric microplastic transport and how this links with aquatic and terrestrial contamination (Dris et al., 2016).

| THE PLASTIC CYCLE
Currently, environmental microplastic research commonly focuses on independent environmental "compartments," as highlighted above: terrestrial, freshwater and marine, and more recently, atmosphere (Dris et al., 2016). However, with regard to movement, transport and fate of particulate (and chemical) matter, in reality these environmental compartments are very closely interlinked, with indistinct and permeable boundaries. Interactions between compartments can vary depending on weather and environmental conditions. This means the abundance and fate of microplastics in any given environment will be dependent on the degree of connectivity with adjacent environments, which can be highly variable in space and time. Further, processes that affect microplastics within one compartment can influence the way that a particle behaves within another. For example, degradation, association with chemicals or acquisition of an organic coating on particles derived from a terrestrial environment are factors that can have a significant bearing on particle behavior and ecological interactions once within the freshwater environment. Therefore, it is not appropriate to consider these environments as separate, discrete regions governed by different processes (Horton, Walton, et al., 2017).
Microplastics are now so ubiquitous throughout the globe that a paradigm shift is needed, considering them as integrated into earth surface processes. A novel way of conceptualizing microplastic pollution within the environment is through a "Plastic Cycle" (Figure 3). There are many pathways by which microplastics may travel between environmental compartments, from land via rivers to the sea. However, although the dominant transport direction will be from land to the marine environment, it is not necessarily the case that microplastics that reach the oceans will remain there, as they can return to land with high tides and storm events. This is highlighted not only in the abundance of plastic washed up on beaches following storm events (Figure 2d; Esiukova, 2017), but also in the fact that microplastic particles can be found even on the shores of remote and uninhabited islands (Imhof et al., 2017;Lavers & Bond, 2017). Similarly, other transport pathways are not unidirectional, for example, particles within rivers may return to land during flooding events (Horton, Walton, et al., 2017). There are also regions where the compartmental boundaries blur, for example, estuaries can contain predominantly fresh or marine water depending on the state of the tides, while ephemeral rivers only flow at specific times of year, for example, drying out completely during the summer. In the case of dryland rivers, these may even cease to flow for multi-year periods (Tooth, 2000). During these dry periods terrestrial organisms may be exposed to riverine microplastic deposits in these environments. Furthermore, dryland rivers readily mobilize previously deposited sediments in flow events (Reid & Laronne, 1995;Tooth, 2000), meaning these environments could experience large-scale pulses of microplastic transport. In fact, most rivers are characterized by seasonal flows, meaning the transfer of microplastics from land to rivers and the mobilization of microplastics from river sediments will be highly variable throughout the year. Conceptual model representing the "Plastic Cycle" concept (wastewater treatment, WWT). Orange boxes represent sinks, blue boxes represent transport mechanisms and arrows represent transport pathways. Atmospheric microplastics are not included within the model as they cannot be attributed to a specific compartment or route of transport Microplastic research should therefore seek to consider these environmental associations and interactions to enhance understanding of how marginal environments may inhibit, alter or facilitate the movement, or sequestration of microplastics.

| IMPLICATIONS
It is clear from the research published to date that microplastics are abundant and widespread across the globe, and that their rate of input is increasing. The main concern with this is the potential damage that microplastics may cause to ecosystems. Large-scale macroplastic waste has been prominent within the global media in contributing to the deaths of numerous marine animals including whales, turtles, and seabirds (Jacobsen, Massey, & Gulland, 2010;Pierce, Harris, Larned, & Pokras, 2004;Santos, Andrades, Boldrini, & Martins, 2015). A variety of studies have also shown harm by microplastics to a wide variety of smaller aquatic organisms including zooplankton and large invertebrates including mussels, crabs, and fish larvae (Browne, Dissanayake, Galloway, Lowe, & Thompson, 2008;Lu et al., 2016;Rehse, Kloas, & Zarfl, 2016). Harm may occur as a result of physical damage due to clogging of the gut or gills, or internal lacerations following ingestion due to sharp edges (Wright, Thompson, & Galloway, 2013). Damage to organisms and populations at lower trophic levels has the potential for knock-on effects in food webs, either due to reduced populations of smaller organisms leading to a reduced food source, or due to predators ingesting large numbers of contaminated prey and concentrating microplastics in their own bodies (Mattsson et al., 2017;Watts et al., 2014). Additionally, toxicity or bioaccumulation of chemicals associated with the plastics may occur, for example, organic pollutants sorbed to plastics may become available to organisms following ingestion, while plasticizer chemicals can leach out within the environment (Besseling, Wegner, Foekema, van den Heuvel-Greve, & Koelmans, 2013;Lithner, Damberg, Dave, & Larsson, 2009). Microplastics may have implications for soil ecosystem function, for example, experimental studies have shown effects of microplastics on reproduction of earthworms-a key organism for nutrient cycling and aeration within soils (Lwanga et al., 2016;Rillig, 2012). This will be especially pertinent for agricultural areas given the likely prevalence of microplastics on agricultural land . The resultant chemical or particulate toxic effects to organisms could have detrimental impacts on agricultural productivity (Steinmetz et al., 2016).
Recently, concerns have been raised about the possible consequences of widespread microplastic pollution on human health, with microplastics highly likely to be ingested or inhaled on a regular basis (Van Cauwenberghe & Janssen, 2014; Wright & Kelly, 2017). The potential for health implications has been highlighted by workers in textile industries suffering respiratory disorders following inhalation of synthetic particulate matter (Wright & Kelly, 2017), although this has not yet been directly compared to the effects of nonpolymeric dust such as cotton fibers, which may be similarly inhaled (Pauly et al., 1998). As little clinical data is available on short-or long-term health effects of this microplastic exposure, this remains a priority research question to be addressed.

| CONCLUSIONS
Microplastics are widespread throughout terrestrial, freshwater, marine, and atmospheric systems. They are easily dispersed away from their sources, can be generated in the environment from larger plastic items, and may ultimately end up being retained within a specific location due to incorporation into soils and sediments. Alternatively, they may continuously cycle throughout different environments influenced by weather and currents. Although particle properties will influence behavior and fate, this is not the only determining factor, as biological, chemical, and physical interactions will also affect particle transport. In order to develop a holistic understanding of the drivers, magnitude and effects of microplastic pollution at a large system scale, it will be necessary for future research to consider interactions between microplastics and the environment across the range of environmental matrices, and how the fate of microplastics may affect their ecological impact. ACKNOWLEDGMENTS A.A.H. would like to thank Elma Lahive for her input during the preparation of this manuscript. S.J.D. would like to thank Iseult Lynch and John Lewin for their stimulating discussions which helped form some of the ideas presented in this manuscript, and to Nick Kettridge for supporting the writing process. This work was funded by the UK Natural Environment Research Council through National Capability funding of the Centre for Ecology and Hydrology Pollution and Environmental Risk Program.