Rivers as potential macroplastic shredders

In an October issue of Water Matters, we featured a newly published article by Polish and Dutch scientists on macroplastic fragmentation in rivers in a literature review. The authors of the article agreed to bring our readers closer to this important issue. Their work is likely to contribute to the development of a method for assessing the plastic waste footprint, or the amount of microplastic generated from plastic waste in the environment.

Plastic is everywhere, but in rivers it turns to microplastic faster

The widespread use of plastic over the past 70 years has contributed to its massive delivery to the environment. It is estimated that a total of as much as 4.9 billion tons of plastic waste has been shipped since the middle of the last century[1], and the presence of this new synthetic material has been confirmed in most aquatic and terrestrial environments around the world[2]. The risks posed by plastic waste emissions vary depending on the type of plastic itself and where it goes.

A review of existing experimental work shows, for example, that microplastics (particles less than 5 mm) that are harmful to organisms are produced most quickly by destroying common waste materials constructed of polystyrene foam or various types of film fragments[3]. Particularly if they end up in an environment where there is a high intensity of external mechanical (e.g., wind, water movement) and biochemical factors (e.g., oxygen availability and UV radiation) that promote fragmentation (Song et al., 2017). A well-known example of such an environment, often cited in the literature, is beaches, where wave action and the availability of sunlight create countless opportunities for its degradation and fragmentation, both mechanically and biochemically[4].

Observing the way rivers function and their dramatic macro-plastic pollution, one can conclude that the riverbed, like the seacoast, can be considered a microplastic factory[5]. This is because it is characteristic of this environment that there is a continuous or periodic flow of water and transport of debris, which (as in the case of wave action on the beach), promotes mechanical interactions of macroplastics with the river bed, debris and water. In addition, riverbeds are generally not covered with vegetation, which increases the exposure of these zones to sunlight, which significantly accelerates plastic degradation and photochemical fragmentation (Liro et al., 2023b).

Conceptual model of the determinants of macroplastic fragmentation in rivers

In order to be able to determine in the future how much microplastic is formed during the fragmentation of different types of macroplastic in rivers, a theoretical and conceptual basis is needed to select empirically testable (e.g., in experiments) questions and hypotheses. The formation of microplastics during macroplastic fragmentation is of increasing interest to scientists and researchers, but the existing literature lacked a basis for analyses of this phenomenon in rivers. Therefore, the team, which included Dr. Maciej Liro of the Polish Academy of Sciences, Anna Zielonka, M.Sc. of Jagiellonian University, and Dr. Eng. Tim van Emmerik of Wageningen University in the Netherlands, undertook to develop a conceptual model of the factors determining the intensity of macroplastic fragmentation in riverbeds[6].

Based on the results of laboratory experiments to date, two types of factors controlling the rate of macroplastic fragmentation in the river have been distinguished. The first are internal factors resulting from the type of polymers and shape of the macroplastic (Figure 1), and the second are external factors related to the climate and hydromorphology of the trough (Figure 2).

Microplastic formation in rivers – internal factors

As for intrinsic factors, a model built from information gleaned from existing laboratory experiments shows that the fragmentation rate of different polymers can vary by as much as 1,000 to 10,000 times, and is highest for polystyrene (such as white disposable cups) or foamed polystyrene (polystyrene). The shape of the macroplastic is also an important internal factor affecting the rate of fragmentation. There are, for example, studies showing that film fragments, due to their high surface area-to-weight ratio, will fragment 260-1100 times faster than more compact macroplastics[7].

It can therefore be assumed that waste generated from single-use plastics (e.g., white polystyrene cups and cutlery, polystyrene trays and various types of plastic packaging) will be particularly susceptible to conversion to microplastics in the river (Figure 1). Put another way, their plastic footprint, or the amount of microplastic created from them in the environment at any given time, will be the largest[8].

Microplastic formation in rivers – external factors

The team of researchers also demonstrated how climate and river hydromorphology can control the course of macroplastic fragmentation. Climate has a decisive influence on whether macroplastic is transported in the riverbed continuously or only periodically, which determines the possibility of mechanical and biochemical fragmentation. For example, macroplastic can be deposited on the surface of sediments or riverine vegetation, where biochemical fragmentation will be more important; it can also be transported in the riverbed in flotation (transport on the water surface), in suspension (transport between the bottom and the water surface) or as bottom debris (transport with frequent contact with the river bottom), where mechanical fragmentation will be more important (Figure 2).

The developed model therefore suggests that the riverbed may be the site of microplastic formation from various types of macroplastics transported in the lower, middle and upper parts of the water column in the riverbed.

As Dr. Maciej Liro points out, the publication is primarily intended to support the planning of future experimental and modeling work aimed at directly measuring the plastic footprint of specific wastes deposited in rivers. Such information is currently unavailable, and is crucial to understanding the secondary production of micro- and nanoplastics in the environment and determining the associated risks to the river ecosystem and human health.

Authors: Anna Zielonka, MA, is a doctoral student at the Institute of Geography and Spatial Management at the Jagiellonian University. His doctoral thesis examines organic carbon stocks in the Arctic. In his research, he combines various methods – especially remote sensing and big data – to better understand natural processes. Author of 9 peer-reviewed scientific publications.

Dr. Maciej Liro is an assistant professor at the Institute of Nature Conservation of the Polish Academy of Sciences, where he leads a team studying the functioning of mountain rivers in the Anthropocene. Starting in 2021. directs the grant “Macroplastics in mountain and foothill rivers” funded by the SONATA program, National Science Center. Author of more than 30 peer-reviewed scientific publications.

The article benefited from, among other things. From the works:

[1] Geyer R., Jambeck J., Lavender Law K., 2017 Production, use, and fate of all plastics ever made. Sci. Adv., 3, e1700782.

[2] Jambeck J.R., Geyer R., Siegler T., Perryman M., Anrady A., Narayan R., Lavender Law K., 2015. Plastic waste inputs from land into the ocean. Science, 347, 768-771.

[3] Liro M., van Emmerik T.H.M., Zielonka A., Gallitelli L., Mihai F.C., 2023a. The unknown fate of macroplastic in mountain rivers. Sci. Total Envi., 865, 161224.

[4] Corcoran P.L., Biesinger M.C., Grifi M., 2009. Plastic and beaches: A degrading relationship. Marine Pollution Bulletin, 58, 80-84.

[5] Liro M., van Emmerik T.H.M., Zielonka A., Gallitelli L., Mihai F.C., 2023a. The unknown fate of macroplastic in mountain rivers. Sci. Total Envi., 865, 161224.

Liro M., Zielonka A., van Emmerik T.H.M., 2023b. Macroplastic fragmentation in rivers. Environment International, 180, 108186.

[6] Liro M., Zielonka A., van Emmerik T.H.M., 2023b. Macroplastic fragmentation in rivers. Environment International, 180, 108186.

[7] Chamas A., Moon H., Zheng J., Qiu Y., Tabassum T., Jang J.H., Abu-Omar M., Scott S.L., Suh S., 2020. Degradation Rates of Plastics in the Environment. ACS Sustainable Chem. Eng. 8, 9, 3494-3511

[8] Liro M., Zielonka A., van Emmerik T.H.M., 2023b. Macroplastic fragmentation in rivers. Environment International, 180, 108186.