Module I: Sources and entry paths

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Litter in the environment is anno 2020 a versatile and widely discussed topic. Plastic litter has come into focus, with particular attention being paid to the accumulation points, the so-called hotspots, in the environment. However, it is important to consider the entire life cycle with the route within the environment in order to understand the problem and to derive measures for action. Therefore, the topic is presented here in 5 different modules.

This module focuses not only on the sources of litter, but also on its entry paths to explain how and where it becomes part of the environment. To take actions towards litter free rivers and streams (LIVES) it is an important part to understand the problem at its beginning. This module is structured into three parts:

    1. Introduction – what is litter?
    2. What is a source – what is an entry path?
    3. The example of the Meuse River – getting to know some numbers!

In addition, the transport paths and sinks (module 2) as well as litter traps (module 3), policy responses (module 4) and monitoring (module 5) are considered to be able to get a holistic view of the problem and identify possible solutions.

In this module, we have prepared for you some explanatory videos, figures, tables, questionnaires, cloze tests and informative texts which will help you to answer the following questions:

  • What exactly is litter?
  • What is the significance of plastic litter and what are the sources of it for river pollution?
  • Where and how does plastic litter enter the environment?
  • Why is the Meuse River a suitable example and what is the current situation there with plastic litter?

Introduction – What is litter?

I. What is litter?

Litter defines everything you (including you, as a single person, but also you, as owner of a factory etc.) want or need to dispose, which includes manufacturing and industrial, construction and demolition, mining and municipal waste (more information in the European Waste Framework Directive 2008/98/EG). Normally, many of these dispose waste in a dustbin or waste container, but if it is thrown into the environment, in which it is not properly disposed of, it accumulates there.

Waste is therefore as diverse as the materials from which the products are made or which are needed for the production. Such materials are for example paper and cardboard, glass, aluminium, wood, rocks, textiles and plastics. In recent years there has been a growing debate about the use of plastic as a material and, consequently, about plastic litter. This was initially based on numerous images of plastic accumulation in the environment or animals entangled in plastic, that resulted in and opened up a new field of research into the effects of plastics on the environment and human health.

Plastic litter poses a particularly serious problem due to its persistence, which is the main reason why all plastic items that end up in the environment continuously accumulate there. In this module, the focus is set on sources and entry paths of plastic litter to provide the basis for future retention and reduction of it into the environment.

Within research there are many terms used synonymously for plastic litter which makes uniform research, definitions and comparability considerably difficult:

  • macro litter (Andrady, 2011)
  • anthropogenic litter (Chin and Fung, 2018)
  • plastic litter (Bond et al., 2018)
  • marine litter (Hengstmann et al., 2017)
  • marine plastic (Barnes et al., 2018)
  • plastic debris (Derraik, 2002).

Since research has mainly focused on very small plastic particles (diameter < 5 mm) in the environment, so-called microplastics, the differentiation from larger plastics with the term ‘macroplastic’ (diameter ≥ 5 mm) is useful (Lechthaler et al., 2020).

II. From macro- to microplastic

With the entry of macroplastic into the environment, microplastic can arise. The origin of microplastic from macroplastic is caused by different degradation processes that can be divided into physical, chemical and biological categories. The physical category includes mechanical and thermal processes, the chemical one photochemical, oxidative and hydrolytic processes and the biological category a bacterial process. These processes cause degradation and fragmentation and lead thus subsequently to the development of microplastics.

III. LIVES: Project focus on (macro-)plastics as riverine litter

The LIVES project focuses on riverine litter and especially macroplastic in fluvial systems with the main focus on the Meuse River in the Euregion Meuse-Rhine. The project aim is to find not only technical, but also social solutions to face the current problem of the polluted aquatic environment. The flow chart (Figure 1) differentiates between (plastic) litter, macro- and microplastic.

Flow chart:


Figure 1: Flow chart showing the differences between litter, plastic litter, macro- and microplastic (own graphic).

2. What is a source – what is an entry path?

I. The difference between a source and an entry path for plastic litter (definition)

To understand the problem and to develop possible reduction approaches it is important to distinguish between sources and entry paths, as sources are the origin of the material, but it only enters the environment via entry paths (Waldschläger et al., 2020).

II. The source of plastic litter

Production must be considered as the source of plastic litter. The manufacturing of products makes it possible for such litter to be released into the environment. In 2018, 359 million tonnes (Mt) of plastic were produced worldwide with more than half (51%) coming from Asia. In Europe, 64.4 Mt, 18.5% of the global production, were manufactured, where the packaging industry is the main user group, accounting for 39.9% of the plastic produced, while the construction industry (19.8%), the automotive industry (9.9%), the electronics industry (6.2%), household and leisure use (4.1%) and agriculture (3.4%) require only comparatively small shares (PlasticsEurope, 2019).

Figure 2 shows the production shares of plastics in Europe in 2016/17 including the different densities of the polymers. Furthermore, the usage and disposal of the produced plastic items is displayed (Lechthaler et al., 2020).


Figure 2: From the production of plastics to the usage and disposal in Europe 2016/17 (Lechthaler et al., 2020).

III. Examples of entry paths

Humans are entirely responsible for the fact that loads of plastic litter are released into the environment. Such litter can enter the environment anywhere at any time. Starting from the entry, you can distinguish between land- and ocean-based entry paths. The main connection between land- and ocean-based entry are freshwater systems which can transport plastic litter into the oceans (cf. Figure 3).

An overview of possible entry paths is presented in Table 1 regarding an entry from land or ocean, based on different literature.

Table 1: Land- and ocean-based entry of plastic litter (Mehlhart and Blepp, 2012; Macfadyen et al., 2009).

In addition, the following Figure 3 shows an estimated, worldwide mass balance of plastic litter entering the environment and being transported there. With regard to fluvial systems, the annual volume inputs are assumed to be as follows (Lechthaler et al., 2020):

  • 6,300 Mt/year from littering
  • 4,977 Mt/year from agricultural areas
  • 2.0 Mt/year inland-based litter
  • 0.8 Mt/year in wastewater, whereof stormwater can discharge litter into rivers
  • 0.75 Mt/year cigarette butts.



Figure 3: Life-cycle of plastics within the environment with information on flow rates, if available (Lechthaler et al., 2020).

Make here the first questionnaire of module 1!

To illustrate the path of a product until it accumulates in the environment, this path is shown below using the example of a cigarette butt. Cigarette butts, consisting of a synthetic polymer, represent a very large proportion of plastic waste in the environment.

The Way of a cigarette butt through the environment

Since it is assumed that 4.5 trillion cigarette filters are released into the environment every year (Slaughter et al., 2011) resulting in 750.000 t/a (Novotny and Slaughter, 2014) it is particularly useful to illustrate the path of a cigarette butt through the environment.

Cigarette butts, mostly consisting of the polymer cellulose acetate (CA), are declared as the most common waste. Toxic chemicals that are retained there can then be released into the environment. Furthermore, the material of the filter is durable.

So, what happens if someone throws away his/her cigarette butt, maybe within the city, and it ends up on the street?

Click here to see the video: The way of a cigarette butt through the Environment

As the project specifically deals with the catchment area of the Meuse River and it also functions as an entry path for every kind of litter, the third part informs on the Meuse River system.

3. The example of the Meuse River – getting to know some numbers!


I. Why are we looking at the Meuse River Catchment?

The Meuse, as part of the Rhine River system, provides a very good example of the possible input of plastic litter via rivers as well as the resulting transport and subsequent accumulation in the oceans, as the river flows into the North Sea.

II. What is the added value of studying a cross-border river system?

Since the Meuse flows through several countries and also has tributaries from these different countries, the current situation regarding pollution with plastic litter can be reconstructed, and suitable solutions for different problems can be found. In addition, knowledge from different areas and from stakeholders with different backgrounds can be shared.

III. The Meuse River as an example for every river system in the world (international application)

With the work on technical solutions for the retention of plastic litter in fluvial systems (rivers) and the additional work on environmental education, participation and citizen science, the Meuse River is a showcase example and knowledge can be transferred to other rivers. This knowledge can be used everywhere in a similar context.

The following questionnaire gives a brief insight into the Meuse and its catchment area. After entering the answer, you will receive further information on the question.

Make here the second questionnaire of module 1!

Related information material / additional and further reading

The issue of litter and especially plastic litter in the environment and specifically also in the aquatic environment is now relatively present due to media attention. As a result, numerous publications have been published on the subject, as well as online books, webinars, apps and homepages. In the following, a selection of these materials related to sources and entry paths is presented.


I. Publications

  • Araújo, M.C.B., & Costa, M.F. (2019). A critical review of the issue of cigarette butt pollution in coastal environments. Environmental research, 172, 137-149.
  • Geyer, R., Jambeck, J., & R. and Law, K.L. (2017). Production, use, and fate of all plastics ever made. Science advances, 3(7), e1700782.
  • Kedzierski, M., Frère, D., Le Maguer, G., & Bruzaud, S.(2020). Why is there plastic packaging in the natural environment? Understanding the roots of our individual plastic waste management behaviours. Science of the Total Environment, 740, 139985.
  • Kim, L.-H., Kayhanian, M., & Stenstrom, M.K. (2004). Event mean concentration and loading of litter from highways during storms. Science of the Total Environment, 330(1-3), 101-113.
  • Lechthaler, S., Waldschläger, K., Stauch, G., & Schüttrumpf, H. (2020). The Way of Macroplastic through the Environment. Environments, 7(10), 73.
  • Schwarz, A.E., Ligthart, T.N., Boukris, E., & van Harmelen, T. (2019). Sources, transport, and accumulation of different types of plastic litter in aquatic environments: A review study. Marine Pollution Bulletin, 143,  92-100.
  • Van Emmerik, T., Roebroek, C., de Winter, W., Vriend, P., Boonstra, M., & Hougee, M. (2020). Riverbank macrolitter in the Dutch Rhine-Meuse delta. Environmental research letters, 15(10), 104087.
    • Waldschläger, K., Lechthaler, S., Stauch, G., & Schüttrumpf, H. (2020). The way of microplastic through the environment – Application of the source-pathway-receptor model (review). Science of the Total Environment, 713, 136584

II. Books



Click here if you are interested in the references of this module.


The author of module 1:

Simone Lechthaler

Simone Lechthaler

Research Associate at RWTH Aachen University

Simone Lechthaler takes her doctorate on microplastic detection in fluvial sediments and thus is doing research in the field of (micro-)plastic in the environment. More information regarding the Institute of Hydraulic Engineering and Water Resource Management at RWTH Aachen University are available here!

Click here to visited her ORCID profile with current publications!