Recent developments in the science of microplastics transport highlight their long-range movement as well as the nuances of transport modeling.
The recent growth in scientific and regulatory interest in microplastics has paralleled their increasing detection in diverse settings, and prompted the realization that microplastics can potentially be transported long-distances in the environment. Microplastics have been found not only in close proximity to plastics manufacturing facilities, but also in locations far from known discharges, such as rivers, oceans, polar waters, sea ice, groundwater, and even in glaciers. However, the majority of the investigations of microplastics have focused on their presence in the world’s oceans, where buoyant microplastics accumulate after being transported by currents from their point of discharge over long durations.
Microplastics have been found not only in close proximity to plastics manufacturing facilities, but also in locations far from known discharges,
such as rivers, oceans, polar waters, sea ice, groundwater, and even in glaciers.”
Ocean surveys have detected microplastics worldwide, but have also identified high spatial variation among the sampled regions. One recent global study found the highest microplastics concentrations in the Mediterranean Sea and the lowest concentrations in the Indian Ocean. This study investigated the hypothesis that microplastics concentrations are lower in the southern hemisphere than in the northern hemisphere due to the lower plastics inputs in the southern hemisphere, finding support for it for sampling in the Pacific Ocean, but not for the Atlantic Ocean where sampling indicated nearly equal microplastic concentrations for the northern and southern portions of the Atlantic Ocean (Eriksen et al., 2014).
The discovery of microplastics in far-flung settings, coupled with the occasional counterintuitive and contradictory findings, has raised questions such as:
To work towards answering these questions, the study of microplastics transport in the environment is rapidly developing.
Characteristics such as size, composition, density, and propensity to aggregate with other materials control the transport of microplastics in water (see Figure). As such, microplastics are in some ways similar to other commonly discharged substances, such as sediments or dissolved chemicals, but also differ in significant ways that affect their transport potential. For example, microplastics can be similar in size to suspended sediments and colloids, but are often much lighter – the density of microplastics can be half that of sediments and is often similar to the density of water. However, their buoyancy also depends on the density of the transport medium, leading to different behavior in freshwater and marine environments, e.g., polystyrene microplastics may sink in freshwater but remain buoyant in saltwater. Additionally, processes such as biofouling, aggregation, or fragmentation can affect the size and density of microplastics during transport.
Despite these challenges, researchers are making progress in adapting existing tools for use in modeling microplastics transport in the environment. Numerical models simulating the transport of microplastics can provide important insights into their behavior, but care must be taken when adapting particle-tracking modules and dissolved chemical transport modules for modeling microplastics. Multiple researchers have modified existing particle-tracking modules by using laboratory measurements of microplastic rising or settling velocities, which are affected by particle composition, size, and shape (SFEI, 2019; Ballent et al., 2013). Others have modified the same models with empirical attachment efficiencies of microplastic particles to suspended sediments – a process by which microplastics can settle more rapidly as they attach to and aggregate with suspended sediments (Besseling et al., 2017). An additional process that recent studies have attempted to modify for use with microplastics is the amount of stress required to resuspend particles that have settled to the bottom of a waterway (Ballent et al., 2013). Despite these additional calibrations of existing models, validating their accuracy in the field remains a significant challenge.
Moreover, models that are calibrated for microplastics behavior in one aquatic environment may become less accurate and errors may compound as microplastics are transported further away to environments with different water chemistry and potentially different rates of aggregation between plastics and sediments. For example, near points of discharge, large and small-sized microplastics may settle rapidly because they have higher density than the surrounding water and due to aggregation with sediments, respectively (Besseling et al., 2017). Therefore, intermediate-sized microplastics will predominantly be transported beyond the immediate discharge area into a saltwater environment, resulting in a very different particle size distribution in that environment, for which the original model may not have been calibrated.
Ultimately, existing models (used for sediments or dissolved chemicals) can also be used to simulate the transport of microplastics, but one should carefully consider the model assumptions used. The science on microplastics transport is rapidly developing both in terms of modifications to existing models as well as in terms of how environmental observations are changing our understanding of microplastics behavior. Even small differences in assumptions about particle behavior, or which processes are considered, can lead to potentially diverging results.
Ballent, A; Pando, S; Purser, A; Juliano, MF; Thomsen, L. 2013. “Modelled transport of benthic marine microplastic pollution in the Nazare Canyon.” Biogeosciences 10(12):7957-7970. doi: 10.5194/bg-10-7957-2013.
Besseling, E; Quik, JTK; Sun, M; Koelmans, AA. 2017. “Fate of nanoand microplastic in freshwater systems: A modeling study.” Environ. Pollut. 220(Pt. A):540-548. doi: 10.1016/j.envpol.2016.10.001.
Eriksen, M; Lebreton, LC; Carson, HS; Thiel, M; Moore, CJ; Borerro, JC; Galgani, F; Ryan, PG; Reisser, J. 2014. “Plastic pollution in the world’s oceans: More than 5 trillion plastic pieces weighing over 250,000 tons afloat at sea.” PLoS ONE 9(12):e111913. doi: 10.1371/ journal.pone.0111913.
San Francisco Estuary Institute (SFEI). 2019. “Understanding Microplastic Levels, Pathways, and Transport in the San Francisco Bay Region.” SFEI-ASC Publication #950. 402p., October. Accessed at https://www.sfei.org/sites/default/files/biblio_files/Microplastic Levels in SF Bay – Final Report.pdf.