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Hydrology as a Driver of Floating River Plastic Transport

Hydrology as a Driver of Floating River Plastic Transport IntroductionPlastic debris and other anthropogenic litter has negative impacts on ecosystem health and human livelihood (van Emmerik & Schwarz, 2020). Despite several global initiatives to tackle this emerging environmental challenge, plastic production and leakage into the environment is expected to further grow in the coming decades (Borrelle et al., 2020). Rivers have been assumed to be the main conveyors of land‐based plastic waste into the ocean (Meijer et al., 2021; Schmidt et al., 2017). However, recent work has suggested that plastic pollution can be retained within river systems for years to decades, and potentially even longer (van Emmerik et al., 2022). Plastics accumulate on riverbanks, in vegetation, around hydraulic structures, and within estuaries, where they are exposed to environmental weathering leading to degradation and fragmentation (Delorme et al., 2021). The secondary micro‐ and nanoplastics that arise from this may lead to additional environmental risks, and may eventually be exported into the ocean (Koelmans et al., 2022). Understanding transport and retention dynamics is therefore crucial for optimizing monitoring strategies, risk assessments, and interventions to reduce plastic pollution.Reliable observational data are imperative for improving fundamental understanding of plastic transport processes in rivers. However, plastic and anthropogenic litter monitoring efforts have been limited to date, as the scientific field is still emerging. Several measurement techniques have been developed in recent years, including visual counting from bridges, the use of drones, and net sampling (van Emmerik & Schwarz, 2020). Yet, direct comparison of available data remains complicated due to the lack of harmonized measurement methods and protocols (González‐Fernández & Hanke, 2017; Wendt‐Potthoff et al., 2020). As a consequence, thorough comparative analyses of driving processes of river plastic transport are limited to date. Several case studies have revealed that plastic transport can vary both seasonally, and spatially along the course of a river (Castro‐Jiménez et al., 2019). For individual rivers, the observed variation was explained by, for example, the response to river flow, the abundance of plastic accumulating floating vegetation, or wind and rainfall (C. T. Roebroek, Hut, et al., 2021; Schirinzi et al., 2020; Schreyers et al., 2021). Due to the limited spatial and temporal extent of these studies, the challenge of arriving at a general understanding of the role of hydrology, wind dynamics, human factors, and other factors on variability in floating plastic transport remains largely unresolved. Many of the world's assumed most polluted rivers flow into the ocean through complex delta systems (Best, 2019). For such rivers, the transport and retention dynamics are further complicated by the tidal dynamics and river network architecture (Duncan et al., 2020; Haberstroh et al., 2021).Our paper focuses on the Rhine‐Meuse delta, which is one of the major European river networks (van Emmerik et al., 2020). Here, we present the results of an extensive year‐long monitoring effort of floating plastic in the Dutch Rhine, IJssel, and Meuse rivers. The main goal of this paper is to explore the role of hydrology on the spatial and temporal variation of floating plastic transport. Field data on floating plastic were collected at a total of 26 locations along the studied rivers from January to December 2021. Seven locations were measured each month, and two additional measurements were done during peak discharge events. The data at these locations were used to assess the seasonal dynamics, quantify the difference between upstream and downstream, and explore correlations with measured river discharge. The 19 remaining locations were measured three times between June and December 2021, and were used to investigate the spatial variation of floating plastic along the rivers. We combine observations of floating plastic with an openly available data set on mass statistics of over 16,000 items sampled on riverbanks in the same period (van Emmerik & de Lange, 2022) to estimate the mass transport at the seven key locations. Our paper presents three key findings. First, we demonstrate the strong response of floating plastic transport to peak discharge events (Sections 3.1 and 3.2). Second, we show that floating plastic transport is higher around urban areas, and in the most downstream sections of all three rivers (Section 3.3). Finally, our results emphasize that estimates of floating plastic mass transport and export into the ocean are still highly uncertain due to limited data, and insufficient understanding of the driving processes (Sections 3.4, 3.5, and 3.6).With this paper reveal the non‐trivial variation of floating plastic in time and space in the Rhine‐Meuse delta using novel field monitoring data. Both have societal and scientific implications, for example, for designing long‐term monitoring programs, or planning prevention and reduction strategies (Wendt‐Potthoff et al., 2020). Most importantly, we identified several urgent knowledge gaps related to the role of hydrology, tidal dynamics, and factors determining spatial variations. Future work should address these open challenges to advance the fundamental understanding of plastic transport dynamics. The results from this paper are of direct relevance for other river deltas around the world, as they emphasize the urgent need for investing in data collection to unraveling the complicated transport and retention dynamics in such rivers. Finally, our paper shows that river plastic pollution is a transboundary challenge, which calls for further harmonization of methods for data collection and planning of interventions.MethodsStudy AreaWe measured floating plastic and other anthropogenic litter at 26 measurement locations distributed across the Dutch reaches of the Rhine (IJssel, Waal, Nederrijn) and Meuse rivers (see Figure 1) between 28 January and 7 December 2021. The Rhine enters the Netherlands from Germany at Spijk, and splits into the main Waal, IJssel and Nederrijn. The Waal is the main branch, and joins the Nederrijn‐Lek branch at Rotterdam before flowing into the North Sea. The IJssel flows into Lake IJssel at Kampen. The Meuse enters the Netherlands from Belgium at Eijsden, and discharging into the tidal Hollands Diep estuary. Here, the Meuse is joined by a Rhine distributary before reaching the North Sea.1FigureMeasurement locations along the Rhine, IJssel, and Meuse rivers. The large symbols represent the locations where measurements were done monthly and during the peak discharge events. The small symbols represent the locations where three measurements were done between June and December 2021. The thickness of the rivers represent the share of annual discharge in the Rhine‐Meuse delta based on data from the Netherlands Directorate‐General for Public Works and Water Management (Ministerie van Infrastructuur en Waterstaat, 2019; Reeze et al., 2017).Floating Plastic MeasurementsFloating macroplastic and macrolitter (>0.5 cm) were measured using the visual counting method developed by González‐Fernández and Hanke (2017) and van Emmerik et al. (2018), for which all items floating at the surface are counted from bridges. This quantitative method was developed as part of the RIverine and Marine floating macro litter Monitoring and Modeling of Environmental Loading (RIMMEL) project to quantify plastic litter flow from rivers into the ocean across Europe (González‐Fernández et al., 2021). For large‐scale and long‐term monitoring, visual counting is often preferred as it is cost‐efficient and no other equipment or infrastructure is required (such as nets, boats, cranes on bridges) (Aisyah et al., 2022; Schöneich‐Argent et al., 2020). Only bridges that are safe and legally accessible, for example, presence of pedestrian or bicycle paths, were selected. At each location, three to 12 observation points were selected, depending on the river width. The majority of the locations had five or six points (23 out of 26), two locations had three points, and only the downstream Meuse location had 12 points. For a measurement, all visible floating items were counted within a predefined observation track. The minimum observable item size depends on the bridge height (8–20 m), but was estimated to be at least 2.5 cm for all locations. Note that the width of the observation tracks depends on the field of view and the height above the water, and there varied between bridges and between points on the same bridge (12–34 m). The observation track width was quantified by selecting a reference object (e.g., bridge column, buoy, orange peels) and measuring the distance to the observation point. The sum of the observation track widths per bridge covered between 25% and 85% of the total river width. On each measurement day each point was measured four times for a five‐minute period. The total floating plastic flux F [items h−1] was calculated using:1F=∑i=1Sfi‾wi1S⋅W⋅T $F=\sum\limits _{i=1}^{S}\frac{\overline{{f}_{i}}}{{w}_{i}}\frac{1}{S}\cdot W\cdot T$With mean or median plastic flux observation f‾ $\overline{f}$ [items h−1] for observation point i, total number of observation points S, observation track width wi [m], total river width W [m], and extrapolation period T (e.g., hour, day, year). Since observations were done across the river width, the cross‐sectional distribution may also be explored in future studies. This aspect is however outside the scope of this work.Plastic flux can be both positive (toward downstream) and negative (toward upstream) in areas influenced by tidal dynamics. We aimed to only measure plastic flux during low tide, with discharge and plastic flux in downstream direction. Only at Rotterdam (Rhine) and Moerdijk (Meuse) negative plastic fluxes were occasionally observed. In this study we only focus on transport in downstream direction, and therefore use the absolute values of the measured plastic fluxes for the downstream locations to calculate the mean and median. More in‐depth analysis of the effect of the tide is outside the scope of this work.We measured floating plastic to quantify seasonality and the spatial variation along the river. All 26 locations (for details, see Appendix A) were measured three times (Table A1). For the measurement locations, we selected (a) the most upstream and downstream bridge for all rivers, and (b) each safely accessible bridge for the main Rhine and Meuse branches. The locations along the Rhine were measured in July, October and December, and the locations along the Meuse in June, September, and December. The seasonality was assessed using monthly measurements at the seven core locations from January to December 2021; the Rhine at Nijmegen and Rotterdam, the IJssel at Arnhem and Kampen, and the Meuse at Maastricht (starting late February), Ravenstein, and Moerdijk. Each month, all locations were measured within a 3‐day period. Additional measurements were done during the peak discharge in early February for all core locations except for Maastricht. A second set of additional measurements were done for the Meuse locations in July during the floods. At Maastricht, measurements were done on 3 days, and at Ravenstein and Moerdijk on 1 day. At Ravenstein and Moerdijk, three to four observations were done for each point. For Maastricht each observation point was measured once per day, and therefore we used all observations during the 3 days to calculate the mean and median values for transport during the flood peak. All measurements were done by trained students and staff from Wageningen University, Open University, University of Applied Science Zuyd and Rijkswaterstaat.1TableEstimated Yearly Floating Plastic Flux Transport in Items/Hour and Tonnes/YearLocationFloating transportItem transport F [million items/year]Mass transport M [tonnes/year]Mean mass/itemMedian mass/itemSpecific categoriesAggregatedSpecific categoriesAggregatedMeanMedianMean FMedian FMean FMedian FMean FMedian FMean FMedian FRhineNijmegenLitter1.91.632.424.925.519.65.74.41.10.8Plastic1.71.428.124.48.97.85.04.30.70.6RotterdamLitter4.03.165.550.252.740.48.26.32.21.7Plastic3.52.756.849.218.516.07.16.21.51.3IJsselArnhemLitter0.90.810.58.111.68.91.71.30.50.4Plastic0.80.79.17.94.13.51.51.30.30.3KampenLitter2.42.628.622.032.024.55.84.41.31.0Plastic2.12.324.821.511.29.75.04.30.90.8MeuseMaastrichtLitter2.71.838.929.835.227.05.94.51.51.1Plastic2.31.533.729.212.310.75.14.51.00.8RavensteinLitter1.30.820.115.417.513.43.82.90.70.6Plastic1.20.717.415.16.15.33.32.80.50.4MoerdijkLitter3.82.452.540.350.138.47.35.62.11.6Plastic3.32.145.539.517.615.36.45.51.41.2Note. The mass calculations were calculated using three combinations of input. First, we estimated the yearly floating item transport based on the mean and median observed item flux. Second, the calculations were done using both mean and median mass per item. Third, we used the aggregated item statistics, and the category specific item statistics. Note that the range of values refer to the estimates based on the mean (first value) and median (second value) item flux. The mass statistics were taken from (van Emmerik & de Lange, 2022).The floating plastic data sets were tested for normality using the Anderson–Darling test. To test whether the mean and median plastic flux was significantly different between locations we used the Kruskal–Wallis (mean) and Wilcoxon rank sum (median) tests for non‐normally distributed populations. We also used these tests to investigate whether the spring/summer (March–September) observations were higher or lower than the fall/winter (October–February) observations.Plastic and Other Litter CompositionWe adapted the visual counting method to determine the composition of the floating plastic. Plastic items were classified into 16 categories, based on material and use (see full list in Appendix B). As most litter items found in aquatic environments are plastic (González‐Fernández et al., 2021; Morales‐Caselles et al., 2021), we included seven more detailed plastic categories. The classification is a combination of the plastic categories, and the material and usage categories from the River‐OSPAR protocol (van Emmerik et al., 2020). For the 110 most common plastic items in the Dutch rivers, we assigned one of the 16 categories used for the visual counting. The specific item list including categories can be found Appendix B (Table B1). When the floating plastic flux is relatively low (approximately 50 items per 5 min, per segment), the categorization can be done by a single surveyor. For increased plastic flux it is recommended to work in pairs (observer and scribe). In some cases the plastic flux becomes too high to categorize the individual items (van Lieshout et al., 2020). The latter was the case during the additional July measurements in Maastricht. In some cases the plastic flux becomes too high to categorize the individual items. The latter was the case during the additional July flood measurements in Maastricht. Here, only plastic items were counted and no further categorization was done. Also note that the categorization was added to the protocol after January. For all measurements, the categorization was done by a single surveyor.Mass Transport EstimatesWe estimated the floating plastic mass transport M at each location by combining the observed floating plastic flux F, and the average mass per item m‾ $\overline{m}$ (Vriend et al., 2020). We estimated the mass transport using the following two equations:2M=F⋅m‾ $M=F\cdot \overline{m}$3M=∑j=116Fj⋅mj‾ $M=\sum\limits _{j=1}^{16}{F}_{j}\cdot \overline{{m}_{j}}$Equation 2 can be used when only general statistics on the average mass per item were available. Equation 3 can be used in case more detailed mass statistics for the different litter categories j were available. We applied both equations to investigate the effect of increased data availability. We calculated the mass transport using both the mean and median values for the litter flux and mass statistics. In total, this yielded eight values of total yearly mass transport for each location. For the mass statistics we used a detailed data set of over 16,000 sampled and analyzed macrolitter items, collected from riverbanks at the same time as the visual counting measurements (van Emmerik & de Lange, 2022). We use this data set to calculate the mean and median mass per item for (a) all items, (b) all plastic and non‐plastic items, and (c) all 16 item categories.Correlation With HydrologyWe explore the correlation with hydrology by comparing the observed floating plastic flux with discharge time series at some of the measurement locations. Discharge data was only available for locations outside the tidal influence: Nijmegen (Rhine), Arnhem and Kampen (IJssel), and Maastricht and Ravenstein (Meuse). Note that for Kampen, we used the nearest station of Olst, located 35 km upstream. All data are publicly available from the Directorate‐General for Public Works and Water Management (Rijkswaterstaat, https://waterinfo.rws.nl/). For the five locations we calculated the Spearman and Pearson correlations between the observed daily mean plastic flux, and the mean discharge during the observation period of the matching floating plastic observation.Results and DiscussionSeasonality of Floating Plastic TransportFloating plastic flux showed several clear peaks during the year, especially for the locations along the Meuse and the downstream location on the Rhine (Figure 2). The strongest increase was observed for the Meuse river. In July, the plastic flux increased with a factor 4 for Maastricht (Upstream; 1,374 vs. 306 items/hour) and Moerdijk (downstream; 1,571 vs. 436 items/hour), and 6 for Ravenstein (midstream; 857 vs. 153 items/hour), compared to the yearly mean transport. In February, the plastic flux increased with a factor 1.5 in Ravenstein and Moerdijk. Both increases are associated to the discharge peak in February and the flood event in the upstream regions of the Meuse in July. Between 13 and 20 July, severe floods occurred in the Meuse basin, leading to broken discharge records in the Dutch part of the river (Strijker et al., 2021). The return period of the measured discharge at Maastricht and Ravenstein were 200 and 50 years, respectively.2FigureObserved mean daily floating plastic flux for (a) the Rhine at the upstream (Nijmegen) and downstream (Rotterdam) locations, (b) the IJssel at the upstream (Arnhem) and downstream (Kampen) locations, and (c) the Meuse at the upstream (Maastricht), midstream (Ravenstein), and downstream location (Moerdijk). In February, the annual peak discharge occurred in the Rhine, IJssel, and Meuse, and in July an extreme flood event occurred in the upstream regions of the Meuse.At Rotterdam, close to the river mouth, two peaks were observed in February and June. The February peak (1,284 items/hour) was 2.8 times higher than the yearly mean (459 items/hour) and the June peak (1,625 items/hour) 3.5 times higher than average. The February peak was a response to the annual discharge peak, which will be further discussed in Section 3.2. The June peak did not correspond to any hydrometeorological events, but may be explained by increased outdoor activity after suspension of several COVID‐19 pandemic related measures. Note that the measurement location is in the middle of Rotterdam, the second largest city in the Netherlands, and home to Europe's largest port. Floating plastic may be introduced along the riverbanks of the city, but can also flow toward the city from the port areas (downstream of the measurement location) during flood tide. No evident peak or seasonal variation was observed at the upstream location at Nijmegen.Floating plastic transport at the IJssel showed an increase of 60% during the February peak discharge (414–666 items/hour). During the remainder of the measurement period the plastic flux at both the upstream and downstream locations remained relatively constant. After July the plastic flux downstream decreased (33–113 items/hour), compared to the period before July (120–666 items/hour). The decrease may be explained by the flushing effect of the discharge peak in July (Hurley et al., 2018).The floating litter transport showed a significant seasonal variation, with higher values during the spring/summer than during the fall/winter at Kampen (p < 0.01), Rotterdam (p < 0.01), Ravenstein (p = 0.03), and Moerdijk (p = 0.02). The upstream locations did not show a significant difference. As we omitted the observations done during the February and July peaks for this specific analysis, these results suggest that other factors may influence the seasonal variation in litter flux. The role of river discharge will be further explored in the next section. Future work should focus on investigating the influence of other seasonal effects, such as human activities, shipping, tidal dynamics, and other hydrometeorological variables (Schirinzi et al., 2020).Correlation Between Floating Plastic Transport and HydrologyAt four of the five tested locations (Meuse: Maastricht, Ravenstein; Rhine: Nijmegen; IJssel: Arnhem and Kampen) the floating plastic flux is strongly positively correlated to discharge (Spearman ρ = 0.59–0.66, p = 0.02–0.05; Pearson ρ = 0.74–0.90, p = 0.01). The observed discharge peaks in February and July therefore explain the increased floating plastic flux at those locations (Figure 3). The found correlations in the Meuse and IJssel confirm the hypotheses posed by previous work on the link between discharge and plastic flux (Castro‐Jiménez et al., 2019; C. T. Roebroek, Harrigan, et al., 2021; Schirinzi et al., 2020). Only at Nijmegen a negative, non‐significant correlation was found. There is no clear explanation for the deviating results here, and it is most likely a combination of the timing of the measurements (peaks were missed), and actual absence of a strong relation between discharge and plastic flux at Nijmegen. The absence of a correlation here emphasizes that although plastic flux and discharge may be correlated at some locations, an actual more generalized relation is most likely more complicated and non‐trivial (C. T. Roebroek, Hut, et al., 2021). As can be seen in Figures 3f and 3g, the slope of any linear approximation of the relation between discharge and plastic flux would yield varying degrees of steepness. For IJssel, Kampen and Maastricht, Meuse, the slope seems steeper than for IJssel, Arnhem and Meuse, Ravenstein. A simple linear model may be a suitable approach to reconstruct a higher resolution time series for a limited historical period at a specific location. Due to the variation in (cor)relation between discharge and plastic transport, transferability to other locations within and across river systems remains rather limited.3FigureThe observed mean daily floating plastic flux and discharge for the measurement locations without tidal influence. (a) IJssel at the upstream location Arnhem (Spearman ρ = 0.59, p = 0.05; Pearson ρ = 0.81, p < 0.01). (b) IJssel at the downstream location Kampen (Spearman ρ = 0.66, p = 0.02; Pearson ρ = 0.74, p < 0.01). (c) Meuse at the upstream location Maastricht (Spearman ρ = 0.60, p = 0.03; Pearson ρ = 0.90, p < 0.01). (d) Meuse at the midstream location Ravenstein (Spearman ρ = 0.60, p = 0.02; Pearson ρ = 0.76, p < 0.01). Note that the discharge time series is interrupted as a result of the July flood, probably due to failure of the gauge. (e) Rhine at the upstream location Nijmegen (Spearman ρ = −0.16, p = 0.61; Pearson ρ = −0.19, p = 0.55). (f) Discharge versus floating plastic for the Rhine and IJssel location. (g) Discharge floating plastic versus floating plastic for the Meuse locations.Spatial Variation Along the Rhine and MeuseFor both the Rhine and Meuse the highest floating plastic flux was observed at the most upstream locations (200–400 items/hour), and closest to the river mouth (100–250 items/hour). These observations suggest that a substantial amount of plastic is already transported in the river from across the border, and floating plastic may in fact accumulate in the tidal zone.Emmerich am Rhein (upstream, Figure 4a) is located before the rivers splits, and the drop from 330 items/hour to 150 items/hour (Nijmegen) may be explained by the distribution of plastic over the different branches. Downstream of Nijmegen there is again an increase, especially in July (at Ewijk, 400 items/hour). Around the measurement locations there are various recreational areas, and river ports along the river, which may be considered as a source of plastic. During October and December, the plastic flux remains low until it reaches Rotterdam. In July a peak was observed around Gorinchem (70 km from the river mouth), which may be related to the urban, recreational and industrial areas, and shipping activities. The variation along the Meuse is lower than for the Rhine. Except for a peak in Roermond (230 km from the river mouth) in December (206 items/hour), the floating plastic flux is relatively stable between Maaseik and Peerenboom (20–50 items/hour). At Moerdijk another peak was observed (50–240 items/hour). Between Peerenboom and Moerdijk, the Meuse is joined by a side branch of the Rhine, which may transport some plastic from the Rhine system into the Meuse estuary.4FigureLongitudinal profiles of floating plastic flux for (a) the Rhine in July, October, and December 2021, and (b) the Meuse in June, September, and December 2021.All three rivers have significantly higher mean and median floating plastic fluxes in the most downstream location compared to the upstream location (see Figure 5). The multiplication factors between the upstream and downstream locations are 1.4 (Meuse), 2.8 (IJssel), and 2.1 (Rhine). The difference in the upstream and downstream mean and medians is not significant for all rivers. For the IJssel, both the mean (p = 0.0196) and median (p = 0.021) downstream flux is significantly higher than the upstream flux. In the Meuse, both the median and mean of the upstream (mean p = 0.0141, median p = 0.0088) and downstream locations (mean p = 0.0117, median p = 0.0059) are larger than the midway values. The difference between Maastricht and Moerdijk is less significant (mean p = 0.2801, median p = 0.2917). For the Rhine, the difference in the mean is not very significant (p = 0.1740), and the median is not different at all (hypothesis not rejected, p = 0.1823). Note that during specific months, such as during the flood peak in July, plastic transport can be much larger upstream than downstream.5FigureThe difference between the upstream, downstream and midstream plastic flux observations at the (a) Rhine, (b) IJssel, and (c) Meuse rivers.A logical reason for the increase is the additional plastic that may be introduced in the rivers. However, the results from the Meuse show that this may not always be the case, as the intermediate locations almost all show lower values compared to the upstream and river mouth. A second explanation could be related to the urban and industrial areas around the downstream locations. The Rhine and IJssel transverse Rotterdam and Kampen, respectively, and the downstream Meuse location is neighbored by heavy industry and shipping infrastructure.Another likely reason for the increased downstream values is the (temporary) accumulation in the river mouth. Due to the tidal dynamics, the river flow alternates direction diurnally (Blondel & Buschman, 2022; López et al., 2021; Okuku et al., 2022). The floating plastic within the tidal zone therefore also flows back and forth, increasing the likelihood of accumulation on riverbanks, or deposition on the riverbed (Acha et al., 2003; Tramoy et al., 2020). Note that for both the Rhine and Meuse, the most downstream location was still 30–50 km upstream from the river mouth. The lack of suitable measurement locations (i.e., safe bridges), and the complex tidal dynamics make it challenging to accurately estimate the actual emission of floating plastic into the sea.Plastic and Litter CompositionThe majority of the 3,293 categorized items (44% of the total counted items) were plastic (86.7%). Only wood (3.5%) and paper (3.8%) items contributed more than 1%. In total 4,244 items were not categorized, which was mainly due to the high transport fluxes during the July flood. Counting per individual categories was not possible. Note that with our categorization, cigarette butts were counted as paper, in contrast to some other studies which label them as plastic. Most plastic items were soft (56.6%), with POsoft (39.5%) and Multilayer (17.1%) as the most abundant categories. These categories include items such as food packaging, soft fragments, bags, and foils. Hard plastic items made up 30.3% (15.6% POhard, 7.7% EPS, 6.0% PS, 1.1% PET), and 13.1% were non‐identified items. On average, the floating plastic composition is similar to the plastic found on the Dutch riverbanks (85.1% plastic, 33.4% POsoft, 16.1% POhard) (van Emmerik et al., 2020). The plastic composition in the Dutch rivers is similar to the European average (82%), which was based on one year of measurements in 42 rivers across the continent (González‐Fernández et al., 2021). A clear difference was found for the plastic bottles, which was much lower in the Dutch rivers (1.1%) than the European mean (almost 10%). The composition is also in line with global statistics, with an average of 50%–55% soft items, and relatively low abundance of PET (<5%) (van Calcar & van Emmerik, 2019).Plastic composition can change considerably over time. We do find that when more items were observed, the plastic composition is more distributed, and closer to the mean statistics. Strongly deviating composition is often related to the low number of observed items. During periods with high observed plastic, the percentage of non‐identified items is often higher. These results emphasize one of the major limitations of the visual counting method. For high plastic fluxes, especially during discharge peaks, not all items can be categorized by a single surveyor. The uncertainty may be reduced by working in teams of two surveyors, one observer and one scribe. However, previous studies have emphasized that for extremely high plastic fluxes the categorization cannot be done by visual observations anymore (van Lieshout et al., 2020). Cameras may provide a solution, as recorded videos allow for counting by multiple people and at slower speeds. Future developments may even include further automation of plastic observations. Preliminary results from rivers in Jakarta show that during floating plastic flux peaks, the camera‐based estimates were structurally higher than the visual counting‐based estimates (van Lieshout et al., 2020). Plastic composition is important to identify sources, understand transport processes, and improve risk assessments. Most plastic is mobilized during peak discharge, which underscores the importance of composition analysis during those events.Floating plastic composition is relatively constant between measurement locations. For almost all locations, at least 79% of the items were plastic. Only in Maastricht, the most upstream Meuse location, the plastic content was lower (21%). During the July flood event, the plastic flux was however too large (1,374 items/hour on average) to categorize individual items. When these items are excluded, also here the plastic content increases to 92%. When comparing the seven locations where monthly measurements were done, the composition statistics remains similar. In Nijmegen, the upstream location Rhine, POsoft was higher (48%) than at the other locations (28%–35%). Previous studies have suggested that soft plastics may be found less in downstream regions of rivers, as they are more likely to entangle in riparian vegetation or accumulate on riverbanks (van Emmerik et al., 2022). For the Rhine the percentage of soft plastics decreased from 68% to 46% from upstream to downstream locations, but for the IJssel (54%–50%) and Meuse (50%–45%) it remained within limited range.Floating Plastic Mass TransportThe estimated annual item transport of the Rhine, IJssel and Meuse were consistently larger at the most downstream locations, and varied between 2.4 and 4.0 million items/y (2.1–3.5 million plastic items/year), see Table 1. The Rhine transported the most items (2.7–3.5 million items/year), followed by the IJssel (2.4–2.6 million items/year) and the Meuse (2.3–3.8 million items/year). All three rivers are among the European top polluted rivers measured to date, with similar values to the Danube (∼1.8–3.0 million items/year), Tiber (∼2 million items/year), and Drini (∼1.2 million items/year) (González‐Fernández et al., 2021).The plastic mass transport closest to the river mouth was largest for the Rhine (mean: 16.0–58.8 t/y; median: 1.3–6.3 t/y), followed by the Meuse (mean: 15.3–45.5 t/y; median: 1.2–6.4 t/y), and the IJssel (mean: 9.7–24.8 t/y; median: 0.8–5.0 t/y), see Table 1. The downstream mass transport was higher for all three rivers. Similar to the item transport, the Meuse had the lowest mass transport midway at Ravenstein. The mass transport estimates vary almost by an order of magnitude, depending on whether the mean or median item statistics are used. A similar range was found during an assessment of mass transport of three German rivers (Schöneich‐Argent et al., 2020). Plastic has the highest share when the median item transport F is used, and the lowest when the aggregated item mass statistics are used. Our calculations show that because of the large discrepancies in the mean and median for both item transport and item‐mass statistics, the estimates of total yearly mass transport come with substantial uncertainty.The distribution of the mass transport in Rhine, Meuse, and IJssel branches do not follow the distribution of total annual discharge. The Rhine at Rotterdam accounts for 54% of the yearly discharge into the ocean from the Rhine‐Meuse delta, but only conveys 25% of the annual item transport and 41% of the mass transport. At Moerdijk 40% of the item transport and 36% of the mass transport was estimated, against 32% of the river discharge. The IJssel at Kampen accounts for 14% of the discharge, but 35% of item transport and 24% of the mass transport. The contribution of the item and mass transport at Moerdijk seems to be most in line with the river discharge, the Rhine distributes relatively low, and the IJssel relatively high amounts of plastic. These results again emphasize the non‐trivial relation between discharge and plastic transport, especially when comparing river branches or different river systems.The mean mass transport values are close to recent model estimates by Meijer et al. (2021). The model estimates for the Rhine (56.2 t/y) and IJssel (23.7 t/y) are well within our calculated range. The highest agreement between the model estimates and our observation based values was found when using the mean item statistics of the specific item categories. For the Meuse, most of our transport estimates are higher than the modeled values (22.7 t/y). The observation based approach included measurements during two peak discharge events, with substantially higher floating plastic fluxes. The model based estimates only use average yearly input data, and therefore does not capture the seasonal dynamics or extreme values. Our findings emphasize the further development of modeling approaches that better represent the temporal dynamics of driving forces and retention dynamics (C. Roebroek et al., 2022).Previous assessments estimated the mass transport downstream of the Rhine between 0.5 and 3.5 t/y (Vriend et al., 2020) and 5.8–58.4 t/y (van der Wal et al., 2015). Vriend et al. (2020) based their estimates on observations during low discharge, and are closer to our lowest estimates based on the mean. The values presented by van der Wal et al. (2015) are closer to our higher estimates. When plastic flux is low, it is more likely that the few observed items statistics are close to the median item statistics. During periods of high plastic flux, especially during extreme hydrological conditions, the likelihood of larger and heavier items being transported increases (Liro et al., 2020). There is no consensus yet on whether using mean or median statistics results in more realistic estimates of mass transport. However, our results suggest that a hybrid approach may be the way forward. During periods of low plastic flux, median items statistics can be used, whereas during periods of high plastic flux the mean statistics may be more realistic.The estimates that used the aggregation item‐mass statistics are lower, and plastics make up a smaller share of the total mass transport. Other studies that analyzed the mass of sampled litter generally find that plastics constitute a share larger than 80% (Schöneich‐Argent et al., 2020; Treilles et al., 2022; van Calcar & van Emmerik, 2019). We therefore recommend using the item‐mass statistics of the specific categories for future estimates. Openly available databases (van Emmerik & de Lange, 2022) can be used for more accurate estimates in case limited resources are available for detailed data collection.Synthesis and OutlookHydrology plays an important but complex role in floating plastic transport in rivers. For five out of six locations we found significant correlations between discharge and plastic transport. However, the response to changing discharge varies substantially between rivers. Most global river plastic transport models assume a general relation between discharge (or surface runoff) and river plastic transport (Lebreton et al., 2017; Meijer et al., 2021). A recent study already revealed that the correlations between floating plastic flux, discharge and wind varies greatly between different rivers (C. Roebroek et al., 2022). With our work we highlight that such (cor)relations also clearly vary within river systems. Increased discharge is often associated with increased preceding rainfall, higher water levels, and higher flow velocity. Rainfall, especially with high intensity and in urban areas, can be a driver of plastic transport from land into rivers. Plastic can be transported over land, although the main mechanisms are assumed to be through direct littering, combined sewer overflow, or discharge of urban drainage on surface water systems (Treilles et al., 2021, 2022). When water levels and flow velocity increases, parts of the riverbanks and floodplains may become inundated. If the mobilizing forces are large enough this may (re)mobilize accumulated plastic (Liro et al., 2020). All the factors above vary greatly per location, and depend on mismanaged plastic waste rates, urban water system characteristics, and river characteristics. Future work should focus on identifying the governing transport and retention principles, that can be used to better explain and forecast plastic flux dynamics and link it to their sources. One way forward is to include plastic concentration‐discharge analyses, as the hysteresis patterns reveal whether increased discharge leads to dilution or enrichment of plastic pollution at specific locations (Hashemi et al., 2020). In turn, describing the concentration‐discharge dynamics helps to identify the sources of the observed additional river plastic transport.Discharge peaks, and floods in particular, are one of the main drivers of floating plastic transport. During the Meuse floods of July 2021, the transport increased with a factor 4–6 compared to the yearly means. Compared to the lowest observed values, the transport during extreme discharge was ∼30–50 times higher. The large spread of plastic transport emphasizes the skewed distribution over time. Similar to sediment and woody debris transport, it seems that also most plastic transport occurs in a relatively short amount of time (Hooke, 2019; Ruiz‐Villanueva et al., 2019). Our findings are in line with previous studies on the role of floods on mobilizing and transporting plastics during flood events regionally and globally (Hurley et al., 2018; C. T. Roebroek, Harrigan, et al., 2021). The strong response to high discharge values may have important implications for the transport and fate dynamics, and for development of monitoring and intervention strategies. For reliable estimates of floating plastic transport, it may not be necessary to increase the measurement frequency. During regular discharge conditions, the plastic transport shows relatively low variation. It is imperative however to monitor during peak events, as most transport may occur during those times. The fate of plastic during peaks events remains unclear. Previous work found increased plastic concentrations on riverbanks in the most downstream reaches of the Rhine‐Meuse delta after floods (van Emmerik et al., 2020), suggesting that the high values for floating plastic do not necessary result in export into the ocean. A growing amount of evidence suggests that the majority of mobile plastics may be entrapped on floodplains, on riverbanks or in riparian vegetation (Cesarini & Scalici, 2022).This study excluded any plastics below the surface, either suspended in the water column or sunk to the river bed. To date it remains unclear what share of floating plastics is to the total plastic transport. In some cases, the highest plastic concentrations were measured both at the surface and close to the river bed (Blondel & Buschman, 2022). Other studies reported a rather uniform distributed of plastics over the water column Broere et al., 2021, Haberstroh et al., 2021, or a clear peak concentration at the surface (Haberstroh et al., 2021). The few available studies demonstrate that the vertical distribution is far from trivial, and may depend on flow conditions, and plastic item characteristics (e.g., size, shape, effective buoyancy) (Kuizenga et al., 2022). The main challenge remains data collection below the surface, as it involves heavy equipment such as nets, boats, and cranes (Blondel & Buschman, 2022; Liedermann et al., 2018), or relies on novel technology that is still under development, including sonar (Broere et al., 2021). Future work should focus on improving estimating plastic transport below the surface by combining new measurement methods, a better understanding of settling velocities, and empirical models to relate surface observations to the total transport.Our paper demonstrates the importance of basin scale quantitative assessments, especially in complex river deltas. To date, most river plastic assessments, also in large rivers, have focused on single locations within river basins (González‐Fernández et al., 2021; Vriend et al., 2020). Although this has resulted in new insights regarding the local driving mechanisms that determine the temporal variation, many challenges regarding the transport and retention dynamics across large river deltas remain unresolved. One of the main challenges in plastic research focuses on closing the mass balance of plastics in the open ocean (Weiss et al., 2021). As it is assumed that a considerable share comes from land‐based sources, and is conveyed to the ocean through river systems, it is imperative that the transport dynamics between rivers and the sea are better quantified and understood. Several works have investigated the travel paths of macroplastics along river systems, demonstrating that the majority of items are removed, or retained on riverbanks, in vegetation, at infrastructure, or otherwise (Duncan et al., 2020; Schreyers et al., 2021; Tramoy et al., 2020; van Emmerik et al., 2022). Also our results show that these dynamics are not trivial, and we emphasize the need for additional monitoring efforts in other large river deltas that are expected to emit large amounts of plastics into the ocean.Our study emphasized the importance of understanding plastic transport in tidal areas. Despite the largest values found in the downstream regions, it is not at all certain to say how much of these are emitted into the ocean. In rivers around the world, high concentrations of plastics are found around the estuary (Acha et al., 2003; Núñez et al., 2021; Ryan & Perold, 2021; Tramoy et al., 2020). At the same time, observational evidence of floating plastics actually flowing into the ocean remain limited. Partly this is caused by the lack of observations, as river mouths are often difficult to monitor. The available data do suggest that the majority of plastics do not leave the estuary (López et al., 2021). Future work may focus on collecting more observations within the complex tidal areas with bidirectional flow dynamics. High temporal resolution measurements during full tidal cycles may shed additional light on the factors that determine net emission or accumulation across temporal time scales.Estimates of mass transport and emission into the ocean have become important figures for policymakers, stakeholders, and initiatives focused on environmental plastic reduction. Studies such as Jambeck et al. (2015) and Schmidt et al. (2017) presented straightforward numbers on global plastic input into the ocean, and the contribution of rivers. Our work shows that mass transport estimates of specific rivers remain highly uncertain, even when relatively large and detailed data sets are available. For the floating plastic item transport estimates, using the mean and median yielded very similar results (38% difference at most). The mass transport estimates however varied more than an order of magnitude for all locations. A potential source of uncertainty is the use of mass statistics of riverbank plastics, rather than floating plastics. Future work should further investigate to what extent plastic characteristics vary between river compartments. As established by C. Roebroek et al. (2022), the largest uncertainty in mass transport estimates lies within the highly variable mass statistics of (plastic) litter items. The variation in our mass transport estimates for each of the three rivers confirm this uncertainty. Future efforts may therefore explore the use of more probabilistic descriptions of item characteristics (Kooi & Koelmans, 2019) and transport modeling approaches (C. Roebroek et al., 2022). Rather than selecting a fixed value for assessments, a probabilistic description can result in an ensemble of possible outcomes with various degrees of certainty.Finally, we would like to emphasize the importance of international and transboundary harmonization of monitoring strategies. The current data collection only focused on the Dutch reaches of the Rhine and Meuse rivers. We demonstrated that the longitudinal profiles are non‐trivial, and similar measurements along the full course of the river may give additional insights in points of entry and retention. Also for policy and management practices it is key that data are collected and reported consistently (Wendt‐Potthoff et al., 2020). For example, to establish material flow analyses (Lobelle et al., 2022), or to assess the efficacy of interventions (Helinski et al., 2021). Riverbank monitoring in the Netherlands (van Emmerik et al., 2020) and Germany (Kiessling et al., 2019) is both done through citizen science approaches, but the used protocols are quite different in terms of spatiotemporal coverage and level of detail (Wendt‐Potthoff et al., 2020). The recent RIMMEL project (González‐Fernández et al., 2021) showcased how the straightforward visual counting method can be applied in a pan‐European effort to harmonize floating plastic monitoring. The missing link that can connect the point scale to the European or global scale is the river basin scale, the natural system boundary of plastic mobilization, transport, and retention dynamics. We therefore stress the necessity for further development of basin‐wide approaches and monitoring strategies.ConclusionsHydrology is an important driver of floating plastic mobilization, transport and retention dynamics. Especially during peak discharge events, a strong response in plastic flux was observed. The highest plastic flux was observed during the Meuse floods of July 2021. The exact relations between hydrology and plastic transport are however non‐trivial, and vary strongly between and along rivers. Fundamental work is necessary to arrive at a more general understanding of plastic transport mechanisms.Plastic mass transport estimates remain highly uncertain, in most cases larger than an order of magnitude. The uncertainty is largely due to the skewed distribution in item‐mass statistics, with large differences in the means and medians. The high estimates of mass transport were in good agreement with previous model results. The remaining discrepancy was related to the inclusion of peak discharge events in our approach. Future work should explore the development of probabilistic approaches to describe item‐mass statistics, and model river plastic transport.The largest uncertainty is found in the transport estimates in the areas under tidal influence. Current data do not allow for estimating the net emission or accumulation of plastic. It remains therefore unknown whether the observed floating plastic at the most downstream locations flow into the ocean, or remain within the river systems. Estuaries are assumed to be a major sink for plastic pollution. Additional measurements are required to further explore the transport dynamics in the Dutch Rhine‐Meuse estuaries and beyond.Plastic pollution is a global challenge that requires international and transboundary harmonization of monitoring approaches. We demonstrated how relatively simple measurements can be done across a complex river delta at the national scale, yet revealing crucial new insights on the seasonality and spatial variation. As hydrology is an important driver of river plastic transport, river basin wide approaches for monitoring and intervening are required to address this environmental stressor within its natural system boundaries.With this paper we highlight the importance of consistent field data to understand the role of hydrology on the transport dynamics, temporal variation, and spatial distribution of floating plastics. The presented insights are crucial for planning further fundamental research, optimize long‐term monitoring strategy, and develop international collaboration for river plastic monitoring.AAppendixOverview of Measurement LocationsTable A1 presents the overview of the measurement locations along the Rhine, IJssel and Meuse Rivers.A1TableOverview of the Measurement Locations Along the Rhine, IJssel, and Meuse RiversLocationDist. to mouth [km]Coordinates [lon, lat]River width [m]Obs pointsObsTotal itemsTotal hoursMeasurements 2021 x* = additional measurements during discharge peakJFMAMJJASONDRhine ‐ WaalEmmerich am Rhein (DE)17151.828926, 6.2263014205601005xxxNijmegen14151.852691, 5.857029380623923620xx*xxxxxxxxxxEwijk13151.885791, 5.737637500555515xxxBeneden‐Leeuwen11551.889436, 5.497387200560345xxxZaltbommel9351.818882, 5.260073200559425xxxGorinchem7051.827146, 4.942190500540273xxPapendrecht5351.823282, 4.705814300560425xxxAlblasserdam4651.856393, 4.654418400558325xxxRotterdam East3651.904052, 4.654418500561315xxxRotterdam Center3151.909284, 4.486466500629841225xx*xxxxxxxxxxRhine ‐ NederrijnArnhem14151.958200, 5.937085112524272xxRhine ‐ IJsselArnhem11351.969409, 5.95912971314123812xxxxxxxxxxxxKampen652.559602, 5.918914213631555026xxxxxxxxxxxxMeuseMaastricht29150.846234, 5.6972501106294444126xxxxx*xxxxxMaaseik (BE)25451.092855, 5.79835280332173xxxRoermond22751.198261, 5.980660150555525xxxVenlo20251.368746, 6.161304150555185xxxWell17951.548057, 6.099343150554165xxxGennep15851.693214, 5.959068120555125xxxHeumen14551.758523, 5.838436150560105xxxNederasselt13751.794507, 5.66346414055294xxxRavenstein13151.769005, 5.735756120526654122xx*xxxxx*xxxxxHedel9551.739671, 5.268502140560225xxxHeesbeen8451.736041, 5.11817515056085xxxPeerenboom6751.719815, 4.890445300560135xxxMoerdijk4951.718369, 4.63606810001261755652xx*xxxxx*xxxxxTotal31907537268BAppendixItem Category ListTable B1 presents the used item category list, with the Item ID, the original Dutch description, the translation in English, and the material category.B1TableItem Categories With Their Original Item ID, the Original Description in Dutch, the Description in English, and the Material Category (POSoft: Soft Polyolyfins; POHard: Hard Polyolefins; PET: Polyethylene Terephthalate; PS: Polystyrene; EPS: Expanded Polystyrene)Item IDDescription (Dutch)Description (English)Material category1plastic_6_packringenSix pack ringPO soft2plastic_tassenBagPO soft3plastic_kleine_plastic_tasjesSmall bagPO soft4.1plastic_drankflessen_groterdan_halveliterBottle (>= 0.5 L)PET4.2plastic_drankflessen_kleinerdan_halveliterBottle (<0.5 L)PET4.3plastic_wikkels_van_drankflessenBottle labelPO soft5plastic_verpakking_van_schoonmaakmiddelenCleaning product packagingPO hard6plastic_voedselverpakkingen_frietbakjes_etcFood packagingPS7plastic_cosmeticaverpakkingenCosmetics packagingPO hard9plastic_motorolieverpakking_groterdan50cmMotor oil packaging (>= 50 cm)PO hard10plastic_jerrycansJerrycanPO hard13plastic_krattenCratePO hard14plastic_auto_onderdelenCar partsPO hard15plastic_doppen_en_dekselsCaps and lidsPS16plastic_aanstekersLighterPO hard20plastic_speelgoedToyPS21plastic_plastic_bekers_of_delen_daarvanCupPS24plastic_netzakkenNet bagPO soft25plastic_handschoenen_huishoudelijkCleaning glovePO soft113plastic_handschoenen_professioneelGlovePO soft31plastic_touw_diameter_groterdan_1cmRopePO soft32plastic_touw_diameter_kleinerdan_1cmRopePO soft35plastic_sportvisspullenFish gearPO soft36plastic_breekstaafjesGlowstickPO hard38plastic_emmersBucketPO hard40plastic_industrieel_verpakkingsmateriaalIndustrial packagingPO soft42plastic_helmenHelmetPO hard43plastic_geweerpatronenGun roundsPO hard57plastic_schoenenShoePO hard117.1plastic_plastic_stukjes_0_2_5cm_hard_plasticHard fragment (<5 cm)PO hard46.1plastic_plastic_stukjes_2_5_50cm_hard_plasticHard fragment (>= 5 cm)PO hard117.2plastic_plastic_stukjes_0_2_5cm_zacht_plasticSoft fragment (<5 cm)PO soft46.2plastic_plastic_stukjes_2_5_50cm_zacht_plasticSoft fragment (>= 5 cm)PO soft48plastic_overig_plasticOther plasticOther plastic1172plastic_piepschuim_0_2_5cmFoam fragment (<5 cm)EPS462plastic_piepschuim_2_5_50cmFoam fragment (>= 5 cm)EPS6.1plastic_piepschuim_voedselverpakkingenFoam food packagingEPS47.1plastic_plastic_folies_groterdan_50cmFoil (>= 50 cm)PO soft47.2plastic_hard_plastic_groterdan_50cmHard other (>= 50 cm)PO hard22.1plastic_rietjesStrawPS19plastic_snoep_snack_chipsverpakkingFood wrappingMultilayer472plastic_piepschuim_groterdan_50cmFoam (>50 cm)EPS212plastic_piepschuim_bekersFoam cupEPS22plastic_bestekCutleryPS481plastic_biofilm_waterfiltertjesWater filterPO hard11plastic_kitspuitenCaulking gunPO hard39plastic_kunststof_band_tiewrapsCable tiePO hard19.1plastic_lolliestokjesStickPO hard8plastic_motorolieverpakking_kleinerdan50cmMotor oil packaging (<50 cm)PO hard2.1plastic_vuilniszakkenGarbage bagPO soft17plastic_schrijfwarenPenPO hard35.1plastic_visdraadFishing wirePO soft43.1plastic_vuurwerkFireworkPO hard22.1plastic_borden_newPlatePS22.2plastic_roerstaafjes_newMixing stickPS38.1plastic_bloempotten_newPlant potPO hard39.1plastic_plakband_newTapePO soft49rubber_ballonnenBalloonRubber52rubber_bandenTireRubber53rubber_overig_rubberOther rubberRubber54textiel_kledingClothingTextile55textiel_vloerbedekkingCarpetTextile44textiel_schoeiselShoewareTextile59textiel_overig_textielOther textileTextile60papier_tassenPaper bagPaper61papier_kartonCartonPaper63papier_sigarettenverpakkingCigarette packPaper64papier_sigarettenfiltersCigarette filterPaper65papier_kartonnen_bekersCarton cupPaper66papier_krantenNewspaperPaper67papier_papier_overigOther paperPaper62.1papier_drankkartonDrink cartonPaper67.1papier_ondefinieerbaarOther paperPaper68hout_kurkCorkWood69hout_pelletsPelletWood72hout_ijsstokjesStickWood73hout_kwastenPaintbrushWood74hout_overig_hout_keinderdan_50cmOther wood (<50 cm)Wood75hout_overig_hout_groterdan_50cmOther wood (>= 50 cm)Wood81metaal_aluminiumfolieAluminium foilMetal81.1metaal_capsulesMetal capsuleMetal78metaal_drankblikjesDrink canMetal79metaal_elektriciteitsdraadElectrical wireMetal83metaal_oud_ijzerIron partMetal77metaal_kroonkurkenMetal bottle capMetal84metaal_oliedrumOil drumMetal88metaal_omheinigsdraad_prikkeldraadBarbed wireMetal76metaal_spuitbussenSpray canMetal86metaal_verfblikPaint canMetal80metaal_visloodFish leadMetal82metaal_voedselblikkenFood canMetal120metaal_wegwerpbarbecuesSingle use grillMetal89metaal_overig_metaal_kleinerdan_50cmOther metal (<50 cm)Metal90metaal_overig_metaal_groterdan_50cmOther metal (>= 50 cm)Metal91glas_flessen_potttenPotGlass92glas_lampen_tl_lampenTube lampGlass93glas_overig_glasOther glassGlass7sanitair_cosmeticaCosmeticsSanitary98sanitair_plastic_wattenstaafjesCotton swabPO hard982sanitair_kartonnen_wattenstaafjesCarton cotton swabSanitary102.2sanitair_vochtige_doekjesWet tissueSanitary97sanitair_condoomsCondomSanitary99sanitair_maandverband_en_verpakkingen_ervanSanitary towelSanitary18sanitair_plastic_kam_borstelHair brushPO hard100sanitair_tampons_en_tamponapplicatorsTampon (applicator)Sanitary102.3sanitair_tissues_wc_papierToilet paperSanitary101sanitair_toiletverfrissersToilet refresherPO hard102sanitair_overig_sanitairOther sanitarySanitary103medisch_verpakkingenMedical packagingMultilayer104medisch_spuitenSyringeMedical105medisch_overig_medischOther medicalMedicalAcknowledgmentsThe authors are very thankful to all students and volunteers who participated in the fieldwork and lab analysis: Tom Barendse, Boaz Kuizenga, Jiaheng Zheng, Titus Kruijssen, Belle Holthuis, Aline Looijen, Siebolt Folkertsma, Lianita Suryawinata, Kryss Waldschläger, Anna Schwarz, Rosalie Mussert, Lisanne Middelbeek, Roos Kolkman, Joël Kampen, Gijs Roosen, Evelien Castrop, Maartje Wadman, Olga Dondoli, Khoa Thi, Wessel van der Meer, Tijme Rijkers, Laura Wilson, Berte Mekonen, Willen de Rooij, Pepijn van Aubel, Lauren Quiros, Ida Meyenberg. This research was partly funded by the Netherlands Ministry of Infrastructure and Water Management, Directorate‐General for Public Works and Water Management (Rijkswaterstaat). This paper is partly based on the technical report Pilot monitoring drijvend zwerfafval en macroplastics in rivieren: Jaarmeting 2021 (https://doi.org/10.18174/566475). The work of TvE is supported by the Veni research program The River Plastic Monitoring Project with project number 18211, which is (partly) funded by the Dutch Research Council (NWO). The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.Data Availability StatementAll data are openly available through http://doi.org/10.4121/19447199.ReferencesAcha, E. M., Mianzan, H. W., Iribarne, O., Gagliardini, D. A., Lasta, C., & Daleo, P. (2003). The role of the rıo de la plata bottom salinity front in accumulating debris. 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Hydrology as a Driver of Floating River Plastic Transport

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Wiley
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© 2022 The Authors. Earth's Future published by Wiley Periodicals LLC on behalf of American Geophysical Union.
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10.1029/2022ef002811
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Abstract

IntroductionPlastic debris and other anthropogenic litter has negative impacts on ecosystem health and human livelihood (van Emmerik & Schwarz, 2020). Despite several global initiatives to tackle this emerging environmental challenge, plastic production and leakage into the environment is expected to further grow in the coming decades (Borrelle et al., 2020). Rivers have been assumed to be the main conveyors of land‐based plastic waste into the ocean (Meijer et al., 2021; Schmidt et al., 2017). However, recent work has suggested that plastic pollution can be retained within river systems for years to decades, and potentially even longer (van Emmerik et al., 2022). Plastics accumulate on riverbanks, in vegetation, around hydraulic structures, and within estuaries, where they are exposed to environmental weathering leading to degradation and fragmentation (Delorme et al., 2021). The secondary micro‐ and nanoplastics that arise from this may lead to additional environmental risks, and may eventually be exported into the ocean (Koelmans et al., 2022). Understanding transport and retention dynamics is therefore crucial for optimizing monitoring strategies, risk assessments, and interventions to reduce plastic pollution.Reliable observational data are imperative for improving fundamental understanding of plastic transport processes in rivers. However, plastic and anthropogenic litter monitoring efforts have been limited to date, as the scientific field is still emerging. Several measurement techniques have been developed in recent years, including visual counting from bridges, the use of drones, and net sampling (van Emmerik & Schwarz, 2020). Yet, direct comparison of available data remains complicated due to the lack of harmonized measurement methods and protocols (González‐Fernández & Hanke, 2017; Wendt‐Potthoff et al., 2020). As a consequence, thorough comparative analyses of driving processes of river plastic transport are limited to date. Several case studies have revealed that plastic transport can vary both seasonally, and spatially along the course of a river (Castro‐Jiménez et al., 2019). For individual rivers, the observed variation was explained by, for example, the response to river flow, the abundance of plastic accumulating floating vegetation, or wind and rainfall (C. T. Roebroek, Hut, et al., 2021; Schirinzi et al., 2020; Schreyers et al., 2021). Due to the limited spatial and temporal extent of these studies, the challenge of arriving at a general understanding of the role of hydrology, wind dynamics, human factors, and other factors on variability in floating plastic transport remains largely unresolved. Many of the world's assumed most polluted rivers flow into the ocean through complex delta systems (Best, 2019). For such rivers, the transport and retention dynamics are further complicated by the tidal dynamics and river network architecture (Duncan et al., 2020; Haberstroh et al., 2021).Our paper focuses on the Rhine‐Meuse delta, which is one of the major European river networks (van Emmerik et al., 2020). Here, we present the results of an extensive year‐long monitoring effort of floating plastic in the Dutch Rhine, IJssel, and Meuse rivers. The main goal of this paper is to explore the role of hydrology on the spatial and temporal variation of floating plastic transport. Field data on floating plastic were collected at a total of 26 locations along the studied rivers from January to December 2021. Seven locations were measured each month, and two additional measurements were done during peak discharge events. The data at these locations were used to assess the seasonal dynamics, quantify the difference between upstream and downstream, and explore correlations with measured river discharge. The 19 remaining locations were measured three times between June and December 2021, and were used to investigate the spatial variation of floating plastic along the rivers. We combine observations of floating plastic with an openly available data set on mass statistics of over 16,000 items sampled on riverbanks in the same period (van Emmerik & de Lange, 2022) to estimate the mass transport at the seven key locations. Our paper presents three key findings. First, we demonstrate the strong response of floating plastic transport to peak discharge events (Sections 3.1 and 3.2). Second, we show that floating plastic transport is higher around urban areas, and in the most downstream sections of all three rivers (Section 3.3). Finally, our results emphasize that estimates of floating plastic mass transport and export into the ocean are still highly uncertain due to limited data, and insufficient understanding of the driving processes (Sections 3.4, 3.5, and 3.6).With this paper reveal the non‐trivial variation of floating plastic in time and space in the Rhine‐Meuse delta using novel field monitoring data. Both have societal and scientific implications, for example, for designing long‐term monitoring programs, or planning prevention and reduction strategies (Wendt‐Potthoff et al., 2020). Most importantly, we identified several urgent knowledge gaps related to the role of hydrology, tidal dynamics, and factors determining spatial variations. Future work should address these open challenges to advance the fundamental understanding of plastic transport dynamics. The results from this paper are of direct relevance for other river deltas around the world, as they emphasize the urgent need for investing in data collection to unraveling the complicated transport and retention dynamics in such rivers. Finally, our paper shows that river plastic pollution is a transboundary challenge, which calls for further harmonization of methods for data collection and planning of interventions.MethodsStudy AreaWe measured floating plastic and other anthropogenic litter at 26 measurement locations distributed across the Dutch reaches of the Rhine (IJssel, Waal, Nederrijn) and Meuse rivers (see Figure 1) between 28 January and 7 December 2021. The Rhine enters the Netherlands from Germany at Spijk, and splits into the main Waal, IJssel and Nederrijn. The Waal is the main branch, and joins the Nederrijn‐Lek branch at Rotterdam before flowing into the North Sea. The IJssel flows into Lake IJssel at Kampen. The Meuse enters the Netherlands from Belgium at Eijsden, and discharging into the tidal Hollands Diep estuary. Here, the Meuse is joined by a Rhine distributary before reaching the North Sea.1FigureMeasurement locations along the Rhine, IJssel, and Meuse rivers. The large symbols represent the locations where measurements were done monthly and during the peak discharge events. The small symbols represent the locations where three measurements were done between June and December 2021. The thickness of the rivers represent the share of annual discharge in the Rhine‐Meuse delta based on data from the Netherlands Directorate‐General for Public Works and Water Management (Ministerie van Infrastructuur en Waterstaat, 2019; Reeze et al., 2017).Floating Plastic MeasurementsFloating macroplastic and macrolitter (>0.5 cm) were measured using the visual counting method developed by González‐Fernández and Hanke (2017) and van Emmerik et al. (2018), for which all items floating at the surface are counted from bridges. This quantitative method was developed as part of the RIverine and Marine floating macro litter Monitoring and Modeling of Environmental Loading (RIMMEL) project to quantify plastic litter flow from rivers into the ocean across Europe (González‐Fernández et al., 2021). For large‐scale and long‐term monitoring, visual counting is often preferred as it is cost‐efficient and no other equipment or infrastructure is required (such as nets, boats, cranes on bridges) (Aisyah et al., 2022; Schöneich‐Argent et al., 2020). Only bridges that are safe and legally accessible, for example, presence of pedestrian or bicycle paths, were selected. At each location, three to 12 observation points were selected, depending on the river width. The majority of the locations had five or six points (23 out of 26), two locations had three points, and only the downstream Meuse location had 12 points. For a measurement, all visible floating items were counted within a predefined observation track. The minimum observable item size depends on the bridge height (8–20 m), but was estimated to be at least 2.5 cm for all locations. Note that the width of the observation tracks depends on the field of view and the height above the water, and there varied between bridges and between points on the same bridge (12–34 m). The observation track width was quantified by selecting a reference object (e.g., bridge column, buoy, orange peels) and measuring the distance to the observation point. The sum of the observation track widths per bridge covered between 25% and 85% of the total river width. On each measurement day each point was measured four times for a five‐minute period. The total floating plastic flux F [items h−1] was calculated using:1F=∑i=1Sfi‾wi1S⋅W⋅T $F=\sum\limits _{i=1}^{S}\frac{\overline{{f}_{i}}}{{w}_{i}}\frac{1}{S}\cdot W\cdot T$With mean or median plastic flux observation f‾ $\overline{f}$ [items h−1] for observation point i, total number of observation points S, observation track width wi [m], total river width W [m], and extrapolation period T (e.g., hour, day, year). Since observations were done across the river width, the cross‐sectional distribution may also be explored in future studies. This aspect is however outside the scope of this work.Plastic flux can be both positive (toward downstream) and negative (toward upstream) in areas influenced by tidal dynamics. We aimed to only measure plastic flux during low tide, with discharge and plastic flux in downstream direction. Only at Rotterdam (Rhine) and Moerdijk (Meuse) negative plastic fluxes were occasionally observed. In this study we only focus on transport in downstream direction, and therefore use the absolute values of the measured plastic fluxes for the downstream locations to calculate the mean and median. More in‐depth analysis of the effect of the tide is outside the scope of this work.We measured floating plastic to quantify seasonality and the spatial variation along the river. All 26 locations (for details, see Appendix A) were measured three times (Table A1). For the measurement locations, we selected (a) the most upstream and downstream bridge for all rivers, and (b) each safely accessible bridge for the main Rhine and Meuse branches. The locations along the Rhine were measured in July, October and December, and the locations along the Meuse in June, September, and December. The seasonality was assessed using monthly measurements at the seven core locations from January to December 2021; the Rhine at Nijmegen and Rotterdam, the IJssel at Arnhem and Kampen, and the Meuse at Maastricht (starting late February), Ravenstein, and Moerdijk. Each month, all locations were measured within a 3‐day period. Additional measurements were done during the peak discharge in early February for all core locations except for Maastricht. A second set of additional measurements were done for the Meuse locations in July during the floods. At Maastricht, measurements were done on 3 days, and at Ravenstein and Moerdijk on 1 day. At Ravenstein and Moerdijk, three to four observations were done for each point. For Maastricht each observation point was measured once per day, and therefore we used all observations during the 3 days to calculate the mean and median values for transport during the flood peak. All measurements were done by trained students and staff from Wageningen University, Open University, University of Applied Science Zuyd and Rijkswaterstaat.1TableEstimated Yearly Floating Plastic Flux Transport in Items/Hour and Tonnes/YearLocationFloating transportItem transport F [million items/year]Mass transport M [tonnes/year]Mean mass/itemMedian mass/itemSpecific categoriesAggregatedSpecific categoriesAggregatedMeanMedianMean FMedian FMean FMedian FMean FMedian FMean FMedian FRhineNijmegenLitter1.91.632.424.925.519.65.74.41.10.8Plastic1.71.428.124.48.97.85.04.30.70.6RotterdamLitter4.03.165.550.252.740.48.26.32.21.7Plastic3.52.756.849.218.516.07.16.21.51.3IJsselArnhemLitter0.90.810.58.111.68.91.71.30.50.4Plastic0.80.79.17.94.13.51.51.30.30.3KampenLitter2.42.628.622.032.024.55.84.41.31.0Plastic2.12.324.821.511.29.75.04.30.90.8MeuseMaastrichtLitter2.71.838.929.835.227.05.94.51.51.1Plastic2.31.533.729.212.310.75.14.51.00.8RavensteinLitter1.30.820.115.417.513.43.82.90.70.6Plastic1.20.717.415.16.15.33.32.80.50.4MoerdijkLitter3.82.452.540.350.138.47.35.62.11.6Plastic3.32.145.539.517.615.36.45.51.41.2Note. The mass calculations were calculated using three combinations of input. First, we estimated the yearly floating item transport based on the mean and median observed item flux. Second, the calculations were done using both mean and median mass per item. Third, we used the aggregated item statistics, and the category specific item statistics. Note that the range of values refer to the estimates based on the mean (first value) and median (second value) item flux. The mass statistics were taken from (van Emmerik & de Lange, 2022).The floating plastic data sets were tested for normality using the Anderson–Darling test. To test whether the mean and median plastic flux was significantly different between locations we used the Kruskal–Wallis (mean) and Wilcoxon rank sum (median) tests for non‐normally distributed populations. We also used these tests to investigate whether the spring/summer (March–September) observations were higher or lower than the fall/winter (October–February) observations.Plastic and Other Litter CompositionWe adapted the visual counting method to determine the composition of the floating plastic. Plastic items were classified into 16 categories, based on material and use (see full list in Appendix B). As most litter items found in aquatic environments are plastic (González‐Fernández et al., 2021; Morales‐Caselles et al., 2021), we included seven more detailed plastic categories. The classification is a combination of the plastic categories, and the material and usage categories from the River‐OSPAR protocol (van Emmerik et al., 2020). For the 110 most common plastic items in the Dutch rivers, we assigned one of the 16 categories used for the visual counting. The specific item list including categories can be found Appendix B (Table B1). When the floating plastic flux is relatively low (approximately 50 items per 5 min, per segment), the categorization can be done by a single surveyor. For increased plastic flux it is recommended to work in pairs (observer and scribe). In some cases the plastic flux becomes too high to categorize the individual items (van Lieshout et al., 2020). The latter was the case during the additional July measurements in Maastricht. In some cases the plastic flux becomes too high to categorize the individual items. The latter was the case during the additional July flood measurements in Maastricht. Here, only plastic items were counted and no further categorization was done. Also note that the categorization was added to the protocol after January. For all measurements, the categorization was done by a single surveyor.Mass Transport EstimatesWe estimated the floating plastic mass transport M at each location by combining the observed floating plastic flux F, and the average mass per item m‾ $\overline{m}$ (Vriend et al., 2020). We estimated the mass transport using the following two equations:2M=F⋅m‾ $M=F\cdot \overline{m}$3M=∑j=116Fj⋅mj‾ $M=\sum\limits _{j=1}^{16}{F}_{j}\cdot \overline{{m}_{j}}$Equation 2 can be used when only general statistics on the average mass per item were available. Equation 3 can be used in case more detailed mass statistics for the different litter categories j were available. We applied both equations to investigate the effect of increased data availability. We calculated the mass transport using both the mean and median values for the litter flux and mass statistics. In total, this yielded eight values of total yearly mass transport for each location. For the mass statistics we used a detailed data set of over 16,000 sampled and analyzed macrolitter items, collected from riverbanks at the same time as the visual counting measurements (van Emmerik & de Lange, 2022). We use this data set to calculate the mean and median mass per item for (a) all items, (b) all plastic and non‐plastic items, and (c) all 16 item categories.Correlation With HydrologyWe explore the correlation with hydrology by comparing the observed floating plastic flux with discharge time series at some of the measurement locations. Discharge data was only available for locations outside the tidal influence: Nijmegen (Rhine), Arnhem and Kampen (IJssel), and Maastricht and Ravenstein (Meuse). Note that for Kampen, we used the nearest station of Olst, located 35 km upstream. All data are publicly available from the Directorate‐General for Public Works and Water Management (Rijkswaterstaat, https://waterinfo.rws.nl/). For the five locations we calculated the Spearman and Pearson correlations between the observed daily mean plastic flux, and the mean discharge during the observation period of the matching floating plastic observation.Results and DiscussionSeasonality of Floating Plastic TransportFloating plastic flux showed several clear peaks during the year, especially for the locations along the Meuse and the downstream location on the Rhine (Figure 2). The strongest increase was observed for the Meuse river. In July, the plastic flux increased with a factor 4 for Maastricht (Upstream; 1,374 vs. 306 items/hour) and Moerdijk (downstream; 1,571 vs. 436 items/hour), and 6 for Ravenstein (midstream; 857 vs. 153 items/hour), compared to the yearly mean transport. In February, the plastic flux increased with a factor 1.5 in Ravenstein and Moerdijk. Both increases are associated to the discharge peak in February and the flood event in the upstream regions of the Meuse in July. Between 13 and 20 July, severe floods occurred in the Meuse basin, leading to broken discharge records in the Dutch part of the river (Strijker et al., 2021). The return period of the measured discharge at Maastricht and Ravenstein were 200 and 50 years, respectively.2FigureObserved mean daily floating plastic flux for (a) the Rhine at the upstream (Nijmegen) and downstream (Rotterdam) locations, (b) the IJssel at the upstream (Arnhem) and downstream (Kampen) locations, and (c) the Meuse at the upstream (Maastricht), midstream (Ravenstein), and downstream location (Moerdijk). In February, the annual peak discharge occurred in the Rhine, IJssel, and Meuse, and in July an extreme flood event occurred in the upstream regions of the Meuse.At Rotterdam, close to the river mouth, two peaks were observed in February and June. The February peak (1,284 items/hour) was 2.8 times higher than the yearly mean (459 items/hour) and the June peak (1,625 items/hour) 3.5 times higher than average. The February peak was a response to the annual discharge peak, which will be further discussed in Section 3.2. The June peak did not correspond to any hydrometeorological events, but may be explained by increased outdoor activity after suspension of several COVID‐19 pandemic related measures. Note that the measurement location is in the middle of Rotterdam, the second largest city in the Netherlands, and home to Europe's largest port. Floating plastic may be introduced along the riverbanks of the city, but can also flow toward the city from the port areas (downstream of the measurement location) during flood tide. No evident peak or seasonal variation was observed at the upstream location at Nijmegen.Floating plastic transport at the IJssel showed an increase of 60% during the February peak discharge (414–666 items/hour). During the remainder of the measurement period the plastic flux at both the upstream and downstream locations remained relatively constant. After July the plastic flux downstream decreased (33–113 items/hour), compared to the period before July (120–666 items/hour). The decrease may be explained by the flushing effect of the discharge peak in July (Hurley et al., 2018).The floating litter transport showed a significant seasonal variation, with higher values during the spring/summer than during the fall/winter at Kampen (p < 0.01), Rotterdam (p < 0.01), Ravenstein (p = 0.03), and Moerdijk (p = 0.02). The upstream locations did not show a significant difference. As we omitted the observations done during the February and July peaks for this specific analysis, these results suggest that other factors may influence the seasonal variation in litter flux. The role of river discharge will be further explored in the next section. Future work should focus on investigating the influence of other seasonal effects, such as human activities, shipping, tidal dynamics, and other hydrometeorological variables (Schirinzi et al., 2020).Correlation Between Floating Plastic Transport and HydrologyAt four of the five tested locations (Meuse: Maastricht, Ravenstein; Rhine: Nijmegen; IJssel: Arnhem and Kampen) the floating plastic flux is strongly positively correlated to discharge (Spearman ρ = 0.59–0.66, p = 0.02–0.05; Pearson ρ = 0.74–0.90, p = 0.01). The observed discharge peaks in February and July therefore explain the increased floating plastic flux at those locations (Figure 3). The found correlations in the Meuse and IJssel confirm the hypotheses posed by previous work on the link between discharge and plastic flux (Castro‐Jiménez et al., 2019; C. T. Roebroek, Harrigan, et al., 2021; Schirinzi et al., 2020). Only at Nijmegen a negative, non‐significant correlation was found. There is no clear explanation for the deviating results here, and it is most likely a combination of the timing of the measurements (peaks were missed), and actual absence of a strong relation between discharge and plastic flux at Nijmegen. The absence of a correlation here emphasizes that although plastic flux and discharge may be correlated at some locations, an actual more generalized relation is most likely more complicated and non‐trivial (C. T. Roebroek, Hut, et al., 2021). As can be seen in Figures 3f and 3g, the slope of any linear approximation of the relation between discharge and plastic flux would yield varying degrees of steepness. For IJssel, Kampen and Maastricht, Meuse, the slope seems steeper than for IJssel, Arnhem and Meuse, Ravenstein. A simple linear model may be a suitable approach to reconstruct a higher resolution time series for a limited historical period at a specific location. Due to the variation in (cor)relation between discharge and plastic transport, transferability to other locations within and across river systems remains rather limited.3FigureThe observed mean daily floating plastic flux and discharge for the measurement locations without tidal influence. (a) IJssel at the upstream location Arnhem (Spearman ρ = 0.59, p = 0.05; Pearson ρ = 0.81, p < 0.01). (b) IJssel at the downstream location Kampen (Spearman ρ = 0.66, p = 0.02; Pearson ρ = 0.74, p < 0.01). (c) Meuse at the upstream location Maastricht (Spearman ρ = 0.60, p = 0.03; Pearson ρ = 0.90, p < 0.01). (d) Meuse at the midstream location Ravenstein (Spearman ρ = 0.60, p = 0.02; Pearson ρ = 0.76, p < 0.01). Note that the discharge time series is interrupted as a result of the July flood, probably due to failure of the gauge. (e) Rhine at the upstream location Nijmegen (Spearman ρ = −0.16, p = 0.61; Pearson ρ = −0.19, p = 0.55). (f) Discharge versus floating plastic for the Rhine and IJssel location. (g) Discharge floating plastic versus floating plastic for the Meuse locations.Spatial Variation Along the Rhine and MeuseFor both the Rhine and Meuse the highest floating plastic flux was observed at the most upstream locations (200–400 items/hour), and closest to the river mouth (100–250 items/hour). These observations suggest that a substantial amount of plastic is already transported in the river from across the border, and floating plastic may in fact accumulate in the tidal zone.Emmerich am Rhein (upstream, Figure 4a) is located before the rivers splits, and the drop from 330 items/hour to 150 items/hour (Nijmegen) may be explained by the distribution of plastic over the different branches. Downstream of Nijmegen there is again an increase, especially in July (at Ewijk, 400 items/hour). Around the measurement locations there are various recreational areas, and river ports along the river, which may be considered as a source of plastic. During October and December, the plastic flux remains low until it reaches Rotterdam. In July a peak was observed around Gorinchem (70 km from the river mouth), which may be related to the urban, recreational and industrial areas, and shipping activities. The variation along the Meuse is lower than for the Rhine. Except for a peak in Roermond (230 km from the river mouth) in December (206 items/hour), the floating plastic flux is relatively stable between Maaseik and Peerenboom (20–50 items/hour). At Moerdijk another peak was observed (50–240 items/hour). Between Peerenboom and Moerdijk, the Meuse is joined by a side branch of the Rhine, which may transport some plastic from the Rhine system into the Meuse estuary.4FigureLongitudinal profiles of floating plastic flux for (a) the Rhine in July, October, and December 2021, and (b) the Meuse in June, September, and December 2021.All three rivers have significantly higher mean and median floating plastic fluxes in the most downstream location compared to the upstream location (see Figure 5). The multiplication factors between the upstream and downstream locations are 1.4 (Meuse), 2.8 (IJssel), and 2.1 (Rhine). The difference in the upstream and downstream mean and medians is not significant for all rivers. For the IJssel, both the mean (p = 0.0196) and median (p = 0.021) downstream flux is significantly higher than the upstream flux. In the Meuse, both the median and mean of the upstream (mean p = 0.0141, median p = 0.0088) and downstream locations (mean p = 0.0117, median p = 0.0059) are larger than the midway values. The difference between Maastricht and Moerdijk is less significant (mean p = 0.2801, median p = 0.2917). For the Rhine, the difference in the mean is not very significant (p = 0.1740), and the median is not different at all (hypothesis not rejected, p = 0.1823). Note that during specific months, such as during the flood peak in July, plastic transport can be much larger upstream than downstream.5FigureThe difference between the upstream, downstream and midstream plastic flux observations at the (a) Rhine, (b) IJssel, and (c) Meuse rivers.A logical reason for the increase is the additional plastic that may be introduced in the rivers. However, the results from the Meuse show that this may not always be the case, as the intermediate locations almost all show lower values compared to the upstream and river mouth. A second explanation could be related to the urban and industrial areas around the downstream locations. The Rhine and IJssel transverse Rotterdam and Kampen, respectively, and the downstream Meuse location is neighbored by heavy industry and shipping infrastructure.Another likely reason for the increased downstream values is the (temporary) accumulation in the river mouth. Due to the tidal dynamics, the river flow alternates direction diurnally (Blondel & Buschman, 2022; López et al., 2021; Okuku et al., 2022). The floating plastic within the tidal zone therefore also flows back and forth, increasing the likelihood of accumulation on riverbanks, or deposition on the riverbed (Acha et al., 2003; Tramoy et al., 2020). Note that for both the Rhine and Meuse, the most downstream location was still 30–50 km upstream from the river mouth. The lack of suitable measurement locations (i.e., safe bridges), and the complex tidal dynamics make it challenging to accurately estimate the actual emission of floating plastic into the sea.Plastic and Litter CompositionThe majority of the 3,293 categorized items (44% of the total counted items) were plastic (86.7%). Only wood (3.5%) and paper (3.8%) items contributed more than 1%. In total 4,244 items were not categorized, which was mainly due to the high transport fluxes during the July flood. Counting per individual categories was not possible. Note that with our categorization, cigarette butts were counted as paper, in contrast to some other studies which label them as plastic. Most plastic items were soft (56.6%), with POsoft (39.5%) and Multilayer (17.1%) as the most abundant categories. These categories include items such as food packaging, soft fragments, bags, and foils. Hard plastic items made up 30.3% (15.6% POhard, 7.7% EPS, 6.0% PS, 1.1% PET), and 13.1% were non‐identified items. On average, the floating plastic composition is similar to the plastic found on the Dutch riverbanks (85.1% plastic, 33.4% POsoft, 16.1% POhard) (van Emmerik et al., 2020). The plastic composition in the Dutch rivers is similar to the European average (82%), which was based on one year of measurements in 42 rivers across the continent (González‐Fernández et al., 2021). A clear difference was found for the plastic bottles, which was much lower in the Dutch rivers (1.1%) than the European mean (almost 10%). The composition is also in line with global statistics, with an average of 50%–55% soft items, and relatively low abundance of PET (<5%) (van Calcar & van Emmerik, 2019).Plastic composition can change considerably over time. We do find that when more items were observed, the plastic composition is more distributed, and closer to the mean statistics. Strongly deviating composition is often related to the low number of observed items. During periods with high observed plastic, the percentage of non‐identified items is often higher. These results emphasize one of the major limitations of the visual counting method. For high plastic fluxes, especially during discharge peaks, not all items can be categorized by a single surveyor. The uncertainty may be reduced by working in teams of two surveyors, one observer and one scribe. However, previous studies have emphasized that for extremely high plastic fluxes the categorization cannot be done by visual observations anymore (van Lieshout et al., 2020). Cameras may provide a solution, as recorded videos allow for counting by multiple people and at slower speeds. Future developments may even include further automation of plastic observations. Preliminary results from rivers in Jakarta show that during floating plastic flux peaks, the camera‐based estimates were structurally higher than the visual counting‐based estimates (van Lieshout et al., 2020). Plastic composition is important to identify sources, understand transport processes, and improve risk assessments. Most plastic is mobilized during peak discharge, which underscores the importance of composition analysis during those events.Floating plastic composition is relatively constant between measurement locations. For almost all locations, at least 79% of the items were plastic. Only in Maastricht, the most upstream Meuse location, the plastic content was lower (21%). During the July flood event, the plastic flux was however too large (1,374 items/hour on average) to categorize individual items. When these items are excluded, also here the plastic content increases to 92%. When comparing the seven locations where monthly measurements were done, the composition statistics remains similar. In Nijmegen, the upstream location Rhine, POsoft was higher (48%) than at the other locations (28%–35%). Previous studies have suggested that soft plastics may be found less in downstream regions of rivers, as they are more likely to entangle in riparian vegetation or accumulate on riverbanks (van Emmerik et al., 2022). For the Rhine the percentage of soft plastics decreased from 68% to 46% from upstream to downstream locations, but for the IJssel (54%–50%) and Meuse (50%–45%) it remained within limited range.Floating Plastic Mass TransportThe estimated annual item transport of the Rhine, IJssel and Meuse were consistently larger at the most downstream locations, and varied between 2.4 and 4.0 million items/y (2.1–3.5 million plastic items/year), see Table 1. The Rhine transported the most items (2.7–3.5 million items/year), followed by the IJssel (2.4–2.6 million items/year) and the Meuse (2.3–3.8 million items/year). All three rivers are among the European top polluted rivers measured to date, with similar values to the Danube (∼1.8–3.0 million items/year), Tiber (∼2 million items/year), and Drini (∼1.2 million items/year) (González‐Fernández et al., 2021).The plastic mass transport closest to the river mouth was largest for the Rhine (mean: 16.0–58.8 t/y; median: 1.3–6.3 t/y), followed by the Meuse (mean: 15.3–45.5 t/y; median: 1.2–6.4 t/y), and the IJssel (mean: 9.7–24.8 t/y; median: 0.8–5.0 t/y), see Table 1. The downstream mass transport was higher for all three rivers. Similar to the item transport, the Meuse had the lowest mass transport midway at Ravenstein. The mass transport estimates vary almost by an order of magnitude, depending on whether the mean or median item statistics are used. A similar range was found during an assessment of mass transport of three German rivers (Schöneich‐Argent et al., 2020). Plastic has the highest share when the median item transport F is used, and the lowest when the aggregated item mass statistics are used. Our calculations show that because of the large discrepancies in the mean and median for both item transport and item‐mass statistics, the estimates of total yearly mass transport come with substantial uncertainty.The distribution of the mass transport in Rhine, Meuse, and IJssel branches do not follow the distribution of total annual discharge. The Rhine at Rotterdam accounts for 54% of the yearly discharge into the ocean from the Rhine‐Meuse delta, but only conveys 25% of the annual item transport and 41% of the mass transport. At Moerdijk 40% of the item transport and 36% of the mass transport was estimated, against 32% of the river discharge. The IJssel at Kampen accounts for 14% of the discharge, but 35% of item transport and 24% of the mass transport. The contribution of the item and mass transport at Moerdijk seems to be most in line with the river discharge, the Rhine distributes relatively low, and the IJssel relatively high amounts of plastic. These results again emphasize the non‐trivial relation between discharge and plastic transport, especially when comparing river branches or different river systems.The mean mass transport values are close to recent model estimates by Meijer et al. (2021). The model estimates for the Rhine (56.2 t/y) and IJssel (23.7 t/y) are well within our calculated range. The highest agreement between the model estimates and our observation based values was found when using the mean item statistics of the specific item categories. For the Meuse, most of our transport estimates are higher than the modeled values (22.7 t/y). The observation based approach included measurements during two peak discharge events, with substantially higher floating plastic fluxes. The model based estimates only use average yearly input data, and therefore does not capture the seasonal dynamics or extreme values. Our findings emphasize the further development of modeling approaches that better represent the temporal dynamics of driving forces and retention dynamics (C. Roebroek et al., 2022).Previous assessments estimated the mass transport downstream of the Rhine between 0.5 and 3.5 t/y (Vriend et al., 2020) and 5.8–58.4 t/y (van der Wal et al., 2015). Vriend et al. (2020) based their estimates on observations during low discharge, and are closer to our lowest estimates based on the mean. The values presented by van der Wal et al. (2015) are closer to our higher estimates. When plastic flux is low, it is more likely that the few observed items statistics are close to the median item statistics. During periods of high plastic flux, especially during extreme hydrological conditions, the likelihood of larger and heavier items being transported increases (Liro et al., 2020). There is no consensus yet on whether using mean or median statistics results in more realistic estimates of mass transport. However, our results suggest that a hybrid approach may be the way forward. During periods of low plastic flux, median items statistics can be used, whereas during periods of high plastic flux the mean statistics may be more realistic.The estimates that used the aggregation item‐mass statistics are lower, and plastics make up a smaller share of the total mass transport. Other studies that analyzed the mass of sampled litter generally find that plastics constitute a share larger than 80% (Schöneich‐Argent et al., 2020; Treilles et al., 2022; van Calcar & van Emmerik, 2019). We therefore recommend using the item‐mass statistics of the specific categories for future estimates. Openly available databases (van Emmerik & de Lange, 2022) can be used for more accurate estimates in case limited resources are available for detailed data collection.Synthesis and OutlookHydrology plays an important but complex role in floating plastic transport in rivers. For five out of six locations we found significant correlations between discharge and plastic transport. However, the response to changing discharge varies substantially between rivers. Most global river plastic transport models assume a general relation between discharge (or surface runoff) and river plastic transport (Lebreton et al., 2017; Meijer et al., 2021). A recent study already revealed that the correlations between floating plastic flux, discharge and wind varies greatly between different rivers (C. Roebroek et al., 2022). With our work we highlight that such (cor)relations also clearly vary within river systems. Increased discharge is often associated with increased preceding rainfall, higher water levels, and higher flow velocity. Rainfall, especially with high intensity and in urban areas, can be a driver of plastic transport from land into rivers. Plastic can be transported over land, although the main mechanisms are assumed to be through direct littering, combined sewer overflow, or discharge of urban drainage on surface water systems (Treilles et al., 2021, 2022). When water levels and flow velocity increases, parts of the riverbanks and floodplains may become inundated. If the mobilizing forces are large enough this may (re)mobilize accumulated plastic (Liro et al., 2020). All the factors above vary greatly per location, and depend on mismanaged plastic waste rates, urban water system characteristics, and river characteristics. Future work should focus on identifying the governing transport and retention principles, that can be used to better explain and forecast plastic flux dynamics and link it to their sources. One way forward is to include plastic concentration‐discharge analyses, as the hysteresis patterns reveal whether increased discharge leads to dilution or enrichment of plastic pollution at specific locations (Hashemi et al., 2020). In turn, describing the concentration‐discharge dynamics helps to identify the sources of the observed additional river plastic transport.Discharge peaks, and floods in particular, are one of the main drivers of floating plastic transport. During the Meuse floods of July 2021, the transport increased with a factor 4–6 compared to the yearly means. Compared to the lowest observed values, the transport during extreme discharge was ∼30–50 times higher. The large spread of plastic transport emphasizes the skewed distribution over time. Similar to sediment and woody debris transport, it seems that also most plastic transport occurs in a relatively short amount of time (Hooke, 2019; Ruiz‐Villanueva et al., 2019). Our findings are in line with previous studies on the role of floods on mobilizing and transporting plastics during flood events regionally and globally (Hurley et al., 2018; C. T. Roebroek, Harrigan, et al., 2021). The strong response to high discharge values may have important implications for the transport and fate dynamics, and for development of monitoring and intervention strategies. For reliable estimates of floating plastic transport, it may not be necessary to increase the measurement frequency. During regular discharge conditions, the plastic transport shows relatively low variation. It is imperative however to monitor during peak events, as most transport may occur during those times. The fate of plastic during peaks events remains unclear. Previous work found increased plastic concentrations on riverbanks in the most downstream reaches of the Rhine‐Meuse delta after floods (van Emmerik et al., 2020), suggesting that the high values for floating plastic do not necessary result in export into the ocean. A growing amount of evidence suggests that the majority of mobile plastics may be entrapped on floodplains, on riverbanks or in riparian vegetation (Cesarini & Scalici, 2022).This study excluded any plastics below the surface, either suspended in the water column or sunk to the river bed. To date it remains unclear what share of floating plastics is to the total plastic transport. In some cases, the highest plastic concentrations were measured both at the surface and close to the river bed (Blondel & Buschman, 2022). Other studies reported a rather uniform distributed of plastics over the water column Broere et al., 2021, Haberstroh et al., 2021, or a clear peak concentration at the surface (Haberstroh et al., 2021). The few available studies demonstrate that the vertical distribution is far from trivial, and may depend on flow conditions, and plastic item characteristics (e.g., size, shape, effective buoyancy) (Kuizenga et al., 2022). The main challenge remains data collection below the surface, as it involves heavy equipment such as nets, boats, and cranes (Blondel & Buschman, 2022; Liedermann et al., 2018), or relies on novel technology that is still under development, including sonar (Broere et al., 2021). Future work should focus on improving estimating plastic transport below the surface by combining new measurement methods, a better understanding of settling velocities, and empirical models to relate surface observations to the total transport.Our paper demonstrates the importance of basin scale quantitative assessments, especially in complex river deltas. To date, most river plastic assessments, also in large rivers, have focused on single locations within river basins (González‐Fernández et al., 2021; Vriend et al., 2020). Although this has resulted in new insights regarding the local driving mechanisms that determine the temporal variation, many challenges regarding the transport and retention dynamics across large river deltas remain unresolved. One of the main challenges in plastic research focuses on closing the mass balance of plastics in the open ocean (Weiss et al., 2021). As it is assumed that a considerable share comes from land‐based sources, and is conveyed to the ocean through river systems, it is imperative that the transport dynamics between rivers and the sea are better quantified and understood. Several works have investigated the travel paths of macroplastics along river systems, demonstrating that the majority of items are removed, or retained on riverbanks, in vegetation, at infrastructure, or otherwise (Duncan et al., 2020; Schreyers et al., 2021; Tramoy et al., 2020; van Emmerik et al., 2022). Also our results show that these dynamics are not trivial, and we emphasize the need for additional monitoring efforts in other large river deltas that are expected to emit large amounts of plastics into the ocean.Our study emphasized the importance of understanding plastic transport in tidal areas. Despite the largest values found in the downstream regions, it is not at all certain to say how much of these are emitted into the ocean. In rivers around the world, high concentrations of plastics are found around the estuary (Acha et al., 2003; Núñez et al., 2021; Ryan & Perold, 2021; Tramoy et al., 2020). At the same time, observational evidence of floating plastics actually flowing into the ocean remain limited. Partly this is caused by the lack of observations, as river mouths are often difficult to monitor. The available data do suggest that the majority of plastics do not leave the estuary (López et al., 2021). Future work may focus on collecting more observations within the complex tidal areas with bidirectional flow dynamics. High temporal resolution measurements during full tidal cycles may shed additional light on the factors that determine net emission or accumulation across temporal time scales.Estimates of mass transport and emission into the ocean have become important figures for policymakers, stakeholders, and initiatives focused on environmental plastic reduction. Studies such as Jambeck et al. (2015) and Schmidt et al. (2017) presented straightforward numbers on global plastic input into the ocean, and the contribution of rivers. Our work shows that mass transport estimates of specific rivers remain highly uncertain, even when relatively large and detailed data sets are available. For the floating plastic item transport estimates, using the mean and median yielded very similar results (38% difference at most). The mass transport estimates however varied more than an order of magnitude for all locations. A potential source of uncertainty is the use of mass statistics of riverbank plastics, rather than floating plastics. Future work should further investigate to what extent plastic characteristics vary between river compartments. As established by C. Roebroek et al. (2022), the largest uncertainty in mass transport estimates lies within the highly variable mass statistics of (plastic) litter items. The variation in our mass transport estimates for each of the three rivers confirm this uncertainty. Future efforts may therefore explore the use of more probabilistic descriptions of item characteristics (Kooi & Koelmans, 2019) and transport modeling approaches (C. Roebroek et al., 2022). Rather than selecting a fixed value for assessments, a probabilistic description can result in an ensemble of possible outcomes with various degrees of certainty.Finally, we would like to emphasize the importance of international and transboundary harmonization of monitoring strategies. The current data collection only focused on the Dutch reaches of the Rhine and Meuse rivers. We demonstrated that the longitudinal profiles are non‐trivial, and similar measurements along the full course of the river may give additional insights in points of entry and retention. Also for policy and management practices it is key that data are collected and reported consistently (Wendt‐Potthoff et al., 2020). For example, to establish material flow analyses (Lobelle et al., 2022), or to assess the efficacy of interventions (Helinski et al., 2021). Riverbank monitoring in the Netherlands (van Emmerik et al., 2020) and Germany (Kiessling et al., 2019) is both done through citizen science approaches, but the used protocols are quite different in terms of spatiotemporal coverage and level of detail (Wendt‐Potthoff et al., 2020). The recent RIMMEL project (González‐Fernández et al., 2021) showcased how the straightforward visual counting method can be applied in a pan‐European effort to harmonize floating plastic monitoring. The missing link that can connect the point scale to the European or global scale is the river basin scale, the natural system boundary of plastic mobilization, transport, and retention dynamics. We therefore stress the necessity for further development of basin‐wide approaches and monitoring strategies.ConclusionsHydrology is an important driver of floating plastic mobilization, transport and retention dynamics. Especially during peak discharge events, a strong response in plastic flux was observed. The highest plastic flux was observed during the Meuse floods of July 2021. The exact relations between hydrology and plastic transport are however non‐trivial, and vary strongly between and along rivers. Fundamental work is necessary to arrive at a more general understanding of plastic transport mechanisms.Plastic mass transport estimates remain highly uncertain, in most cases larger than an order of magnitude. The uncertainty is largely due to the skewed distribution in item‐mass statistics, with large differences in the means and medians. The high estimates of mass transport were in good agreement with previous model results. The remaining discrepancy was related to the inclusion of peak discharge events in our approach. Future work should explore the development of probabilistic approaches to describe item‐mass statistics, and model river plastic transport.The largest uncertainty is found in the transport estimates in the areas under tidal influence. Current data do not allow for estimating the net emission or accumulation of plastic. It remains therefore unknown whether the observed floating plastic at the most downstream locations flow into the ocean, or remain within the river systems. Estuaries are assumed to be a major sink for plastic pollution. Additional measurements are required to further explore the transport dynamics in the Dutch Rhine‐Meuse estuaries and beyond.Plastic pollution is a global challenge that requires international and transboundary harmonization of monitoring approaches. We demonstrated how relatively simple measurements can be done across a complex river delta at the national scale, yet revealing crucial new insights on the seasonality and spatial variation. As hydrology is an important driver of river plastic transport, river basin wide approaches for monitoring and intervening are required to address this environmental stressor within its natural system boundaries.With this paper we highlight the importance of consistent field data to understand the role of hydrology on the transport dynamics, temporal variation, and spatial distribution of floating plastics. The presented insights are crucial for planning further fundamental research, optimize long‐term monitoring strategy, and develop international collaboration for river plastic monitoring.AAppendixOverview of Measurement LocationsTable A1 presents the overview of the measurement locations along the Rhine, IJssel and Meuse Rivers.A1TableOverview of the Measurement Locations Along the Rhine, IJssel, and Meuse RiversLocationDist. to mouth [km]Coordinates [lon, lat]River width [m]Obs pointsObsTotal itemsTotal hoursMeasurements 2021 x* = additional measurements during discharge peakJFMAMJJASONDRhine ‐ WaalEmmerich am Rhein (DE)17151.828926, 6.2263014205601005xxxNijmegen14151.852691, 5.857029380623923620xx*xxxxxxxxxxEwijk13151.885791, 5.737637500555515xxxBeneden‐Leeuwen11551.889436, 5.497387200560345xxxZaltbommel9351.818882, 5.260073200559425xxxGorinchem7051.827146, 4.942190500540273xxPapendrecht5351.823282, 4.705814300560425xxxAlblasserdam4651.856393, 4.654418400558325xxxRotterdam East3651.904052, 4.654418500561315xxxRotterdam Center3151.909284, 4.486466500629841225xx*xxxxxxxxxxRhine ‐ NederrijnArnhem14151.958200, 5.937085112524272xxRhine ‐ IJsselArnhem11351.969409, 5.95912971314123812xxxxxxxxxxxxKampen652.559602, 5.918914213631555026xxxxxxxxxxxxMeuseMaastricht29150.846234, 5.6972501106294444126xxxxx*xxxxxMaaseik (BE)25451.092855, 5.79835280332173xxxRoermond22751.198261, 5.980660150555525xxxVenlo20251.368746, 6.161304150555185xxxWell17951.548057, 6.099343150554165xxxGennep15851.693214, 5.959068120555125xxxHeumen14551.758523, 5.838436150560105xxxNederasselt13751.794507, 5.66346414055294xxxRavenstein13151.769005, 5.735756120526654122xx*xxxxx*xxxxxHedel9551.739671, 5.268502140560225xxxHeesbeen8451.736041, 5.11817515056085xxxPeerenboom6751.719815, 4.890445300560135xxxMoerdijk4951.718369, 4.63606810001261755652xx*xxxxx*xxxxxTotal31907537268BAppendixItem Category ListTable B1 presents the used item category list, with the Item ID, the original Dutch description, the translation in English, and the material category.B1TableItem Categories With Their Original Item ID, the Original Description in Dutch, the Description in English, and the Material Category (POSoft: Soft Polyolyfins; POHard: Hard Polyolefins; PET: Polyethylene Terephthalate; PS: Polystyrene; EPS: Expanded Polystyrene)Item IDDescription (Dutch)Description (English)Material category1plastic_6_packringenSix pack ringPO soft2plastic_tassenBagPO soft3plastic_kleine_plastic_tasjesSmall bagPO soft4.1plastic_drankflessen_groterdan_halveliterBottle (>= 0.5 L)PET4.2plastic_drankflessen_kleinerdan_halveliterBottle (<0.5 L)PET4.3plastic_wikkels_van_drankflessenBottle labelPO soft5plastic_verpakking_van_schoonmaakmiddelenCleaning product packagingPO hard6plastic_voedselverpakkingen_frietbakjes_etcFood packagingPS7plastic_cosmeticaverpakkingenCosmetics packagingPO hard9plastic_motorolieverpakking_groterdan50cmMotor oil packaging (>= 50 cm)PO hard10plastic_jerrycansJerrycanPO hard13plastic_krattenCratePO hard14plastic_auto_onderdelenCar partsPO hard15plastic_doppen_en_dekselsCaps and lidsPS16plastic_aanstekersLighterPO hard20plastic_speelgoedToyPS21plastic_plastic_bekers_of_delen_daarvanCupPS24plastic_netzakkenNet bagPO soft25plastic_handschoenen_huishoudelijkCleaning glovePO soft113plastic_handschoenen_professioneelGlovePO soft31plastic_touw_diameter_groterdan_1cmRopePO soft32plastic_touw_diameter_kleinerdan_1cmRopePO soft35plastic_sportvisspullenFish gearPO soft36plastic_breekstaafjesGlowstickPO hard38plastic_emmersBucketPO hard40plastic_industrieel_verpakkingsmateriaalIndustrial packagingPO soft42plastic_helmenHelmetPO hard43plastic_geweerpatronenGun roundsPO hard57plastic_schoenenShoePO hard117.1plastic_plastic_stukjes_0_2_5cm_hard_plasticHard fragment (<5 cm)PO hard46.1plastic_plastic_stukjes_2_5_50cm_hard_plasticHard fragment (>= 5 cm)PO hard117.2plastic_plastic_stukjes_0_2_5cm_zacht_plasticSoft fragment (<5 cm)PO soft46.2plastic_plastic_stukjes_2_5_50cm_zacht_plasticSoft fragment (>= 5 cm)PO soft48plastic_overig_plasticOther plasticOther plastic1172plastic_piepschuim_0_2_5cmFoam fragment (<5 cm)EPS462plastic_piepschuim_2_5_50cmFoam fragment (>= 5 cm)EPS6.1plastic_piepschuim_voedselverpakkingenFoam food packagingEPS47.1plastic_plastic_folies_groterdan_50cmFoil (>= 50 cm)PO soft47.2plastic_hard_plastic_groterdan_50cmHard other (>= 50 cm)PO hard22.1plastic_rietjesStrawPS19plastic_snoep_snack_chipsverpakkingFood wrappingMultilayer472plastic_piepschuim_groterdan_50cmFoam (>50 cm)EPS212plastic_piepschuim_bekersFoam cupEPS22plastic_bestekCutleryPS481plastic_biofilm_waterfiltertjesWater filterPO hard11plastic_kitspuitenCaulking gunPO hard39plastic_kunststof_band_tiewrapsCable tiePO hard19.1plastic_lolliestokjesStickPO hard8plastic_motorolieverpakking_kleinerdan50cmMotor oil packaging (<50 cm)PO hard2.1plastic_vuilniszakkenGarbage bagPO soft17plastic_schrijfwarenPenPO hard35.1plastic_visdraadFishing wirePO soft43.1plastic_vuurwerkFireworkPO hard22.1plastic_borden_newPlatePS22.2plastic_roerstaafjes_newMixing stickPS38.1plastic_bloempotten_newPlant potPO hard39.1plastic_plakband_newTapePO soft49rubber_ballonnenBalloonRubber52rubber_bandenTireRubber53rubber_overig_rubberOther rubberRubber54textiel_kledingClothingTextile55textiel_vloerbedekkingCarpetTextile44textiel_schoeiselShoewareTextile59textiel_overig_textielOther textileTextile60papier_tassenPaper bagPaper61papier_kartonCartonPaper63papier_sigarettenverpakkingCigarette packPaper64papier_sigarettenfiltersCigarette filterPaper65papier_kartonnen_bekersCarton cupPaper66papier_krantenNewspaperPaper67papier_papier_overigOther paperPaper62.1papier_drankkartonDrink cartonPaper67.1papier_ondefinieerbaarOther paperPaper68hout_kurkCorkWood69hout_pelletsPelletWood72hout_ijsstokjesStickWood73hout_kwastenPaintbrushWood74hout_overig_hout_keinderdan_50cmOther wood (<50 cm)Wood75hout_overig_hout_groterdan_50cmOther wood (>= 50 cm)Wood81metaal_aluminiumfolieAluminium foilMetal81.1metaal_capsulesMetal capsuleMetal78metaal_drankblikjesDrink canMetal79metaal_elektriciteitsdraadElectrical wireMetal83metaal_oud_ijzerIron partMetal77metaal_kroonkurkenMetal bottle capMetal84metaal_oliedrumOil drumMetal88metaal_omheinigsdraad_prikkeldraadBarbed wireMetal76metaal_spuitbussenSpray canMetal86metaal_verfblikPaint canMetal80metaal_visloodFish leadMetal82metaal_voedselblikkenFood canMetal120metaal_wegwerpbarbecuesSingle use grillMetal89metaal_overig_metaal_kleinerdan_50cmOther metal (<50 cm)Metal90metaal_overig_metaal_groterdan_50cmOther metal (>= 50 cm)Metal91glas_flessen_potttenPotGlass92glas_lampen_tl_lampenTube lampGlass93glas_overig_glasOther glassGlass7sanitair_cosmeticaCosmeticsSanitary98sanitair_plastic_wattenstaafjesCotton swabPO hard982sanitair_kartonnen_wattenstaafjesCarton cotton swabSanitary102.2sanitair_vochtige_doekjesWet tissueSanitary97sanitair_condoomsCondomSanitary99sanitair_maandverband_en_verpakkingen_ervanSanitary towelSanitary18sanitair_plastic_kam_borstelHair brushPO hard100sanitair_tampons_en_tamponapplicatorsTampon (applicator)Sanitary102.3sanitair_tissues_wc_papierToilet paperSanitary101sanitair_toiletverfrissersToilet refresherPO hard102sanitair_overig_sanitairOther sanitarySanitary103medisch_verpakkingenMedical packagingMultilayer104medisch_spuitenSyringeMedical105medisch_overig_medischOther medicalMedicalAcknowledgmentsThe authors are very thankful to all students and volunteers who participated in the fieldwork and lab analysis: Tom Barendse, Boaz Kuizenga, Jiaheng Zheng, Titus Kruijssen, Belle Holthuis, Aline Looijen, Siebolt Folkertsma, Lianita Suryawinata, Kryss Waldschläger, Anna Schwarz, Rosalie Mussert, Lisanne Middelbeek, Roos Kolkman, Joël Kampen, Gijs Roosen, Evelien Castrop, Maartje Wadman, Olga Dondoli, Khoa Thi, Wessel van der Meer, Tijme Rijkers, Laura Wilson, Berte Mekonen, Willen de Rooij, Pepijn van Aubel, Lauren Quiros, Ida Meyenberg. This research was partly funded by the Netherlands Ministry of Infrastructure and Water Management, Directorate‐General for Public Works and Water Management (Rijkswaterstaat). This paper is partly based on the technical report Pilot monitoring drijvend zwerfafval en macroplastics in rivieren: Jaarmeting 2021 (https://doi.org/10.18174/566475). The work of TvE is supported by the Veni research program The River Plastic Monitoring Project with project number 18211, which is (partly) funded by the Dutch Research Council (NWO). The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.Data Availability StatementAll data are openly available through http://doi.org/10.4121/19447199.ReferencesAcha, E. M., Mianzan, H. W., Iribarne, O., Gagliardini, D. A., Lasta, C., & Daleo, P. (2003). The role of the rıo de la plata bottom salinity front in accumulating debris. 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Journal

Earth's Future – Wiley

Published: Aug 1, 2022

Keywords: macroplastic; anthropocene; floods; delta; pollution; water quality

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