Macroplastic Storage in the Mountain and Foothill Rivers (SONATA)

Mountain rivers are typically seen as relatively pristine ecosystems, supporting numerous goods (e.g., water resources) for human populations living not only in the mountain regions but also downstream from them. However recent evidence suggests that mountain river valleys in populated areas can be substantially polluted by macroplastic (plastic item >25 mm). It is unknown how distinct characteristics of mountain rivers modulate macroplastic routes through them, which makes planning effective mitigation strategies difficult. To stimulate future works on this gap, we present a conceptual model of macroplastic transport pathways through mountain river. Based on this model, we formulate four hypotheses on macroplastic input, transport and mechanical degradation in mountain rivers. Then, we propose designs of field experiments that allow each hypothesis to be tested. We hypothesize that some natural characteristics of mountain river catchments can accelerate the input of improperly disposed macroplastic waste from the slope to the river. Further, we hypothesize that specific hydromorphological characteristics of mountain rivers (e.g., high flow velocity) accelerate the downstream transport rate of macroplastic and together with the presence of shallow water and coarse bed sediments it can accelerate mechanical degradation of macroplastic in river channels, accelerating secondary microplastic production. The above suggests that mountain rivers in populated areas can act as microplastic factories, which are able to produce more microplastic from the same amount of macroplastic waste inputted into them (in comparison to lowland rivers that have a different hydromorphology). The produced risks can not only affect mountain rivers but can also be transported downstream. The challenge for the future is how to manage the hypothesized risks, especially in mountain areas particularly exposed to plastic pollution due to waste management deficiencies, high tourism pressure, poor ecological awareness of the population and lack of uniform regional and global regulations for the problem.

Mountain rivers are typically seen as relatively pristine ecosystems, supporting numerous goods (e.g., water resources) for human populations living not only in the mountain regions but also downstream from them. Recent evidence suggests, however, that mountain river valleys in populated areas can be substantially polluted by macroplastic (plastic item > 5 mm). It is, however, unknown how distinct characteristics of mountain rivers modulate macroplastic routes through them, which makes planning effective mitigation strategies difficult. To stimulate future works on this gap, here, we present a conceptual model of macroplastic transport pathways through mountain river. Based on this model, we formulate four hypotheses on macroplastic input, transport and degradation in mountain rivers. Then, we propose designs of field experiments that allow each hypothesis to be tested. We hypothesize that some natural characteristics of mountain river catchments (e.g., steep valley slopes, mass movements occurence) can accelerate the input of improperly disposed macroplastic waste from the slope to the river. Further, we hypothesize that specific hydromorphological characteristics of mountain rivers (e.g., high flow velocity) accelerate the downstream transport rate of macroplastic and, together with the presence of shallow water and coarse bed sediments, can accelerate mechanical degradation of macroplastic in river channels, accelerating secondary microplastic production. The above suggests that mountain rivers in populated areas can act as microplastic factories, which are able to produce more microplastic from the same amount of macroplastic waste inputted into them (in comparison to lowland rivers that have a different hydromorphology). The produced risks can not only affect mountain rivers but can also be transported downstream. The challenge for the future is how to manage the hypothesized risks, especially in mountain areas particularly exposed to plastic pollution due to waste management deficiencies, high tourism pressure, poor ecological awareness of the population and lack of uniform regional and global regulations for the problem.

Macroplastic storage in mountain rivers remains unexplored and it is unknown how river morphology and different surface types of river areas modulate this process. Therefore, we sampled macroplastic debris stored on the surface of emergent river areas with different vegetation cover and on wood jams in a channelized, single-thread reach and an unmanaged, multi-thread reach of the Dunajec River in the Polish Carpathians. Total amounts of macroplastic debris retained in these reaches were then estimated on the basis of mean mass of macroplastic deposited on unit area of each surface type and the area of this surface type in a given reach. Exposed river sediments and areas covered with herbaceous vegetation stored significantly lower amounts of macroplastic debris (0.6 and 0.9 g per 1 m2 on average) than wooded islands and wood jams (respectively 6 g and 113 g per 1 m2). The amounts of macroplastic debris stored on wood jams exceeded 19, 129 and 180 times those found on wooded islands, areas covered with herbaceous vegetation and exposed river sediments. Wooded islands and wood jams covering 16.7% and 1.5% of the multi-thread reach stored 43.8% and 41.1%, respectively, of the total amount of macroplastic stored in that reach, whereas these surface types were practically absent in the channelized reach. Consequently, the unmanaged, multi-thread reach, 2.4 times wider than the neighbouring channelized reach, stored 36 times greater amount of macroplastic per 1 km of river length. Our study demonstrated that the storage of macroplastic debris in a mountain river is controlled by channel management style and resultant river morphology, which modulate river hydrodynamics and a longitudinal pattern of the zones of transport and retention of macroplastic conveyed by river flow.

Amounts of macroplastic debris stored on different elements of mountain rivers are unknown, but such data are crucial to plan future mitigation activities in these fragile ecosystems. We determined the amounts of macroplastic stored on different surface types (geomorphic units and wood jams) in two reaches of the Dunajec River in the Polish Carpathians. A wide, multi-thread reach stored 36 times more macroplastic per 1 km of river length than the upstream-located narrow, channelized reach (1495.4 kg vs. 41.8 kg). In the multi-thread reach, 43.8% and 41.1% of macroplastic was stored on wooded islands and wood jams that covered, respectively, 16.7% and 1.5% of the area of active river zone. The median of macroplastic mass stored on wood jams equalled 113.2 g/m 2 and was 180 times higher than on exposed river sediments, 129 times higher than in the areas overgrown with herbaceous vegetation and 19 times higher than on wooded islands. The results indicated that multi-thread reaches of mountain rivers supporting extensive wooded islands and numerous wood jams are hot-spots of macroplastic storage, whereas channelized reaches lacking these surface types act as transport reaches for macroplastic debris. Thus, multi-thread reaches of mountain rivers in populated areas can be used as target zones for river cleaning actions and downstream ends of channelized reaches as the location for installation of macroplastic trapping infrastructure.

Processes of macroplastic (plastic particles > 5 mm) storage and remobilization in rivers have been overlooked so far, but are of crucial importance for the estimation of plastic accumulation and transport and associated risks. We present a conceptual model that defines phases of the macroplastic route through a fluvial system and systematizes their main controls. We divided macroplastic route into (1) input, (2) transport, (3) storage, (4) remobilization and (5) output phases. Phase 1 is mainly controlled by humans, phases 2–4 by fluvial processes, and phase 5 byboth types of controls. We hypothesize that natural characteristics of fluvial systems and their modification by dam reservoirs and flood embankments construction are key controls onmacroplastic storage and remobilization in rivers. The zone of macroplastic storage can be definedas a river floodplain inundated since the beginning of widespread disposal of plastic waste to the environment in the 1960s and the remobilization zone as a part of the storage zone currently influenced by floodwaters and bank erosion. The amount of macroplastic in both zones can beestimated using data on the abundance of surface- and subsurface-stored macroplastic, and the lateral and vertical extent of the zones. A demonstrated diversity of factors controlling the route of macroplastic through a fluvial system requires a broader, transdisciplinary perspective including humans who not only dispose plastic, but are also affected by it both physically and aesthetically, and who may remove it from rivers.

Woreczki foliowe przyczepione do drzew i plastikowe butelki unoszące się na wodzie stały się w ostatnich latach częścią krajobrazu rzecznego. Masowa produkcja plastiku i związana z nią dostawa plastikowych odpadów do środowiska rozpoczęła się w latach 60. ubiegłego stulecia. Od tamtego czasu obecność cząstek plastiku stwierdzono w wodach morskich i śródlądowych całego globu. W związku z dającym się precyzyjnie określić momentem zapoczątkowania emisji plastikowych odpadów do środowiska oraz wspomnianą powszechnością tego zjawiska obecność plastiku w osadach uznano za jeden ze wskaźników nowej epoki geologicznej – antropocenu. Oprócz rozpoznania znaczenia cząstek plastiku dla rekonstrukcji sedymentologicznych potwierdzono, że jego obecność w środowisku wpływa negatywnie na zdrowie ludzi i zwierząt (dotyczy szczególnie mikroplastiku) oraz pogarsza estetykę krajobrazu (zwłaszcza makroplastik). Problem ten jest istotny, ponieważ do roku 2050 przewidywany jest sześciokrotny wzrost produkcji plastiku. W tym artykule pokazujemy, jak można wykorzystać istniejącą wiedzę o funkcjonowaniu środowiska rzecznego w prognozowaniu sposobu transportu i miejsc depozycji dużych cząstek plastiku (tzw. makroplastiku) w rzekach, co może pomóc w planowaniu infrastruktury i działań mających na celu usunięcie tego typu odpadów z rzek. W ten sposób człowiek może się przyczynić do poprawy zdegradowanego stanu ekologicznego rzek i krajobrazu rzecznego.

The paper presents a conceptual model of the route of macroplastic debris (>5 mm) through a fluvial system, which can support future works on the overlooked processes of macroplastic storage and remobilization in rivers. We divided the macroplastic route into (1) input, (2) transport, (3) storage, (4) remobilization and (5) output phases. Phase 1 is mainly controlled by humans, phases 2-4 by fluvial processes, and phase 5 by both types of controls. We hypothesize that the natural characteristics of fluvial systems and their modification by dam reservoirs and flood embankments construction are key controls on macroplastic storage and remobilization in rivers. The zone of macroplastic storage can be defined as a river floodplain inundated since the beginning of widespread disposal of plastic waste to the environment in the 1960s and the remobilization zone as a part of the storage zone influenced by floodwaters and bank erosion. The amount of macroplastic in both zones can be estimated using data on the abundance of surface-and subsurface-stored macroplastic and the lateral and vertical extent of the zones. Our model creates the framework for estimation of how much plastic has accumulated in rivers and will be present in future riverscapes.