Module II: Transport paths and sinks

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Figure 1: Average plastic input per year determined by modelling for the 20 most polluted rivers (Figure from Lechthaler et al. (2020), data by Lebreton et al. (2017b))

For the year 2010, a global input of 4.6m to 12.7m tonnes of plastic waste from coastal regions into the oceans was estimated (Jambeck et al., 2015). Lebreton et al. (2017a) assume an annual marine input between 1.15m and 2.41m tonnes of plastic waste through rivers, based on waste management, population density and hydrological information, with 74% of the input occurring between May and October. According to them, the top 20 polluting rivers, mostly located in Asia and shown in Figure 1, account for 67% of the global emissions of plastic waste through rivers. 

Click here to see a video about: 12.7 million tonnes of plastic waste

Why do you think more plastic waste is found in Asian rivers? One reason for this is the inadequate waste management infrastructure in most Asian countries, which means that plastic is released into the environment more often. In addition, small packaging sizes, so-called sachets, are often sold in Asia because a large part of the population is poor and can only afford small quantities of a product at a time (Zerowaste Europe, 2016). In addition, we Europeans export some of our plastic waste to Asia because it can be disposed of there at low cost. Thus we contribute directly to the increased pollution on site. 

Plastic waste in rivers is not transported evenly distributed over the water column, but a large proportion floats on the water surface. Depending on the density of the plastic item, on the entrapped air and the condition the item is in (biofouling, dirt…), a plastic item is either buoyant or it settles to the river bed. Strong currents, for example during floods, can remobilise the plastic item from the bottom of the river and transport it further. The pathway of a plastic item in a river is therefore dependent on the flow lines at the water surface and the force exerted by wind on floating debris. It can also be trapped by vegetation or man-made structures in the river, such as dams or bridges. 

In the LIVES project we aim to trap litter from the upper part of the water column (see Module 3), as a high amount of the plastic is suspected to be transported in the upper 0.5 m (Van Emmerik et al., 2019).    

An example: the Meuse

Van der Wal et al. (2013) focused on the riverine input of plastic litter in the North Sea, especially through the Meuse. According to them, Rhine, Meuse, Scheldt and Ems discharge between 785 and 7850 m³ of plastic waste per year into the North Sea. The Meuse is herein responsible for 110-1100 m³ of plastic waste per year. Van Emmerik et al. (2020) looked more closely on riverbank litter, and identified median plastic loads of 2,430 items/km in the Meuse. The highest densities of plastic items were found at upstream and at downstream locations, interestingly not in between. Different concentrations of plastic litter were also identified between spring (2,830 items/km) and fall (845 items/km). They found a lot of plastic bottles, food wrappings and packaging, caps, lids and cotton swabs. 

In comparison to other rivers, the Meuse is therefore less contaminated than most Asian rivers. However, a lot of European litter can be found on beaches of the Philippines (https://www.breakfreefromplastic.org/globalbrandauditreport2018/), indicating a transport of plastic debris from Europe to Asia. 

Lakes

For indicating litter concentrations in lakes, mostly the unit items/km² is used. Six lakes in Switzerland were investigated by Faure et al. (2015) and an average of 1,800 plastic items/km² were detected when using a floating manta trawl on the water surface. A manta trawl is a net system for sampling the surface of water bodies. The net is made of a thin mesh (often 330 μm mesh size), and the whole trawl is towed behind a scientific research vessel or lowered from a bridge, as illustrated in Figure 2. In addition to the samples, an input of 55 tons of plastics/year was modelled for Lake Geneva (Boucher et al., 2019). 

Figure 2: (a) Manta trawl being lowered into the water; (b) Manta trawl in the water during microplastic sampling; (c) Attachment of the manta trawl at the research vessel

Ever wondered how much 1,800 plastic items/km² is? If you were swimming in this lake, how much floating plastic would you find on average? We tried to illustrate it in Figure 3. 

Figure 3: Illustration on how much 1.800 plastic items/km² are. Please be aware: the particles can also be smaller that plastic bottles

Oceans

Plastics represent the largest proportion of floating marine waste (Galgani et al. 2015) and account for about 75% of the total volume (Pieper et al. 2019). In the first scientifically published studies on plastic pollution of the marine environment, Carpenter and Smith Jr. (1972) examined the water surface of the Sargasso Sea with a neuston net, which is similar to a manta trawl, and detected concentrations of 3,500 items/km² or 290 g/km². Colton et al. (1974) investigated the water surface (1 m deep) of the north-western Atlantic with a neuston net and detected an average load of 77.7 grams of plastic/km².

Debris floating on the surface of the oceans worldwide are estimated to be 389.9 billion pieces with a weight of 233,460 tons (Eriksen et al., 2014). The greatest contamination is found in the North Pacific with a share of 34.6% in terms of the number of plastic items and 36.1% in terms of weight of the total global contamination. High densities of plastic items were found in coastal water of the Southeast Pacific, that decreased with higher distance to the coast (Thiel et al., 2018). However, when reaching the South Pacific Subtropical Gyre (SPSG), the densities of plastic debris are high again, consisting mainly of large fragments (95.4%), lines (17.7%) and buoys (7.6%) (Thiel et al., 2018). 

Figure 4: Comparison on how much plastic is floating on the surface of the oceans, based on data from Eriksen et al. (2014)

Figure 5 gives an overview on the estimated amount of floating debris in the oceans and the location of the garbage patches.

Figure 5: Location of the garbage patches and illustration of the amounts of plastic items in the different oceans (data from Eriksen et al. (2014))

Figure 6: Data based on a citizen science study in the Mediterranean Sea by Consoli et al. (2020)

After so many numbers: Which environmental compartment is most polluted? This question is difficult to answer, because we still know too little. Basically, it can be said that both rivers and oceans are heavily polluted, with hotspots in the oceans particularly affected, such as the garbage patches or certain coastal areas.

Make here the first questionnaire of module 2!

2. (Micro-)plastic from sink to source

Why is it important to know about the plastic sources, its transport and its sinks in the aquatic environment? The sources must first be defined to be able to reduce the input of plastic into the environment. If this is not possible, the plastic waste must be removed from the watercourse, for example with extraction technologies. For this purpose we need extensive knowledge about the transport routes of plastic waste in the water body in order to identify reasonable location for measures. This is especially important for the use of plastic traps. If it is not possible to remove the plastic from the water body during transport, the only remaining option is to remove the plastic in its sinks in the water body. There, the waste accumulates and can be removed. But where are these environmental sinks of plastic waste?

So how does plastic move from the economy to the environment? Watch the following clip to get a brief history of plastic.

Click here to see an interesting video!

An example: The Meuse

Focusing on the Meuse, we will now identify possible sources, transport paths and sinks of plastic debris. The Meuse is 874 km long and flows through France, Belgium and the Netherlands. Along the Meuse are on the one hand numerous towns and villages (among others, Maastricht, Roermond and Venlo), which offer a home to over 9 million inhabitants, and on the other hand several industrial areas. Additionally, 9 harbours are at the Meuse, namely Verdun, Sedan, Charleville-Mézières, Namur, Liège, Maastricht, Maasbracht, Roermond and Venlo and the Meuse is mostly navigable. The Meuse also serves as a recreational area, so that tourists occasionally leave plastic waste behind. Several tributaries flow into the Meuse, the larger ones being the Semois, Sambre, Ourthe, Rur, Chiers and Dommel. All of the above-mentioned characteristics can be considered as sources of plastic waste. 

For transport in the water, buildings that cross the river are particularly interesting because the plastic waste can be retained there. There are numerous bridges along the Meuse and weirs that interrupt the river. In the Dutch section alone, seven weirs are installed to ensure a minimum water depth of three meters. These buildings can be temporal sinks of plastic waste. In addition, the many tributaries with high gradient, especially in the Ardennes and the Meuse up to Roermond, lead to an increased risk of flooding, which can lead to an accumulation of plastic waste in alluvial areas or the higher banks.

Figure 8: Meuse (Source: Wikipedia, CC-BY-SA 3.0)

However, if the plastic waste is not held back in the river or at it’s banks, it will end up in the North Sea and can be distributed from there in the world’s oceans. How this happens is illustrated in the following video: 

Click here to see an interesting video!

The final sink for most plastic items in the aquatic environment is supposed to be the ocean floor, with an estimation of up to 99% of all environmental plastic ending up there (Koelmans et al., 2017). Additionally, the garbage patches are temporal sinks where the floating plastic accumulates for some time before settling to the ocean floor or being released back into the ocean. Floating plastic items are often washed ashore at beaches, where they can also accumulate. 

Make here the second questionnaire of module 2!

3. The Search for the Missing Plastic

The previous chapters almost seem as if we already know enough about the input, transport and the sinks of plastic waste in the environment. This is deceptive. All comparisons between the amount of plastic that is discharged into the environment and the concentration levels that are detected in the environment lead to large differences. So far, we have only been able to detect a fraction of the plastic in the environment that is supposed to be discharged. This is how the term “missing plastic” came up – and the question of where this plastic accumulates or what has happened to it.

But let us start with some numbers: In 2015, Jambeck et al. calculated the amount of plastic waste that enters the marine environment from coastal regions (< 50 km from the coast). Based on plastic waste generation, coastal population and waste management practices (data from 2010), they came to the conclusion that about 8 million tons of plastic are being introduced annually into the oceans from the coastal regions alone, as illustrated in Figure 9. 

Figure 9: Some data on plastic production and input into the oceans (Source: OurWorldInData)

Additionally, as you have already learned in subchapter 1, between 1.15 and 2.41 million tonnes of plastic waste are supposed to enter the oceans every year from rivers (Lebreton et al., 2017b). Accordingly, the amount of plastic waste in the oceans should increase by about 9 to 10 million tonnes each year. And how much plastic waste did we find so far in the oceans? 

Here it becomes a little tricky. Obviously not the whole ocean surface was examined regarding its plastic load. The figures given below are projections of punctual sampling and results of hydronumeric simulations, so the data should be viewed with caution. Eriksen et al. (2014) estimated that 270,000 tonnes are floating in the oceans, based on 24 expeditions and oceanographic models. 

Thus, the annual input (9 million tonnes) and the actually detected plastic in the oceans (270,000 tonnes) differ by a factor of 33. Additionally, the annual input should be multiplied by 70 years, because this is how long we have been producing plastic and how long it has been released into the environment (although it might have been less than 9 million tonnes in 1950). Where is the rest? How could we lose it? 

Part of the answer is obvious: Up to now, samples have been taken mainly from the surface of the oceans, neither from the marine water column (which is on average 3.7 km deep) nor from the seafloor. Initial studies focusing on this have already been able to detect large amounts of plastic in underwater canyons and Koelmans et al. (2017) estimated that 99.8% of plastic that has entered the oceans since 1950 had settled from the ocean surface layer, with an additional 9.4 million tonnes settling per year. This would indeed explain the small amount found on the surface and agrees with the assumption that the ocean floor is a final sink of plastic waste.

We also do not know how much plastic is on the beaches, ingested by animals or has been degraded by bacteria. 

Click here to see a video about: Microplastic

Of course the calculations could also be wrong. Theoretically, much less plastic could be introduced into the oceans, for example because it is retained in the rivers and thus does not reach the oceans. Therefore, we have to learn more about the behaviour of plastic waste in the rivers and about the retention in the rivers. But the ultimate goal of all efforts should be to prevent the plastic from entering, spreading and accumulating in the environment. 

References

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

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