# Heavy rains control the floating macroplastic inputs into the sea from coastal Mediterranean rivers: A case study on the Têt River (NW Mediterranean Sea)

M. Laverre, Philippe Kerhervé, M. Constant, L. Weiss, B. Charrière, M.

Stetzler, D. González-Fernández, Wolfgang Ludwig

► To cite this version:

M. Laverre, Philippe Kerhervé, M. Constant, L. Weiss, B. Charrière, et al.. Heavy rains control the floating macroplastic inputs into the sea from coastal Mediterranean rivers: A case study on the Têt River (NW Mediterranean Sea). Science of the Total Environment, 2023, 877, pp.162733. 10.1016/j.scitotenv.2023.162733. hal-04743302

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Distributed under a Creative Commons Attribution 4.0 International License
HEAVY RAINS CONTROL THE FLOATING MACROPLASTIC INPUTS
INTO THE SEA FROM COASTAL MEDITERRANEAN RIVERS: A CASE
STUDY ON THE TET RIVER (NW MEDITERRANEAN SEA).

M. LAVERRE¹,², P. KERHERVÉ¹,²*, M. CONSTANT¹,²,³, L. WEISS¹,²,⁴, B. CHARRIÈRE¹,², M.

STETZLER¹,²,⁵, D. GONZÁLEZ-FERNÁNDEZ⁶, W. LUDWIG¹,²

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¹ Université de Perpignan Via Domitia, Centre de Formation et de Recherche sur les Environnements Méditerranéens, UMR 5110, 52 Avenue Paul Alduy, F-66860 Perpignan cedex, France

²CNRS, Centre de Formation et de Recherche sur les Environnements Méditerranéens, UMR 5110, F-66860, Perpignan, France

³ present address: Université Lille, Institut Mines-Télécom, Université Artois, Junia, ULR 4515LGCgE, Laboratoire de Génie Civil et géo-Environnement, F-59000 Lille, France ; orcid.org/0000-0003-4808-4373

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⁴ present address: IRD, LEGOS, UMR 5566, 31400 Toulouse, LOPS, UMR 6523, 29280 Plouzané, France

⁵ present address: Center for Arctic Gas Hydrate, Environment and Climate (CAGE), Department of Geosciences, UiT-The Arctic University of Norway, Tromsø, Norway

⁶ Department of Biology, University Marine Research Institute INMAR, University of Cádiz and European University of the Seas, Puerto Real, Spain

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<footnote>* Corresponding author: Philippe Kerhervé (kerherve@univ-perp.fr)</footnote>
<figure><img src="image_2.png" /></figure>

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Abstract

This study focuses on the relevance of small watersheds in the macroplastic pollution of coastal environments. It aims to identify and quantify in terms of composition, number and mass, current riverine flows of floating macroplastics (>2.5 cm). Estimates are based on 66 visual monitoring of total litter over a 4-year-period (2016-2019) in a small coastal Mediterranean river, the Têt River (NW Mediterranean Sea). The plastic fraction represented 97 % of the observed litter, mainly cigarette butts (20.5 %), polystyrene fragments (18.8 %) and light packaging (16.3 %). The Têt River is characterized by frequent flash-flood events caused by heavy rain, that can induce a sudden rise of the water discharge. Such hydroclimatic forcing greatly influence macroplastic flows, both in terms of their average compositions and loads. We have estimated that 354,000 macroplastic items, corresponding to 0.65 tons, are discharged annually from the Têt River into the sea, and that 73 % of them are released during rain events (~6 % of the year). The short observation distance from the water surface allowed to exhibit the great abundance of small litter (80% of them were <10 cm) and to evaluate to 1.8g the average mass of floating plastics. Our results suggest that remediation actions must be taken on rainy days and target small litter in order to significantly limit macroplastic inputs from rivers to the sea. Moreover, the large share of cigarette butts in macrolitter inputs demonstrates

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that reducing ocean pollution cannot be achieved solely by improving waste management, but that
changes in social behavior are also needed to stem waste production at the source.

**Key words:** *floating macroplastics, Mediterranean river, riverine pollution, visual monitoring, temporal variability.*

**1. Introduction**

The exceptional growth of the production and consumption of plastics has been such that the current
era has already been called the “Age of Plastics” (Bergmann 2015). Our consumption of this cheap
and light material has led to a massive pollution of the natural environment, which has become an
international concern. In 2015, the accumulated mass of plastic waste generated worldwide was
estimated at 6.3 giga tons (Gt) (Geyer et al. 2017). Among them, 79 % are accumulated in landfills
and natural environments. According to the latest projections, these authors estimated that the
amount of plastic waste would reach about 12 Gt by 2050.

None of the commonly used plastics are biodegradable. This means that most of the plastic that has
been produced so far and ended up in natural environments is still present in one form or another. In
literature, macroplastics are defined as large items of plastics >2.5 cm in size (Lee et al. 2015;
Gonzalez-Fernandez et al. 2021; Multisanti et al. 2022). Those macroplastics are able to fragment,
due to photo- and thermo-oxidative degradation notably, into much smaller pieces commonly called
microplastics. So far, macroplastic pollution of aquatic ecosystems has been poorly investigated in
comparison with the widely studied microplastic pollution (Gallitelli & Scalici 2022). Furthermore,
few studies have focused on rivers to measure plastic flows (Blettler et al. 2018), nor their transfer
dynamics (Al-Zawaidah et al. 2021). This lack of data is likely linked to a delay in the
standardization of plastic monitoring strategies on rivers, while the European Marine Strategy
Framework Directive (MSFD, 2008) has governed monitoring at sea since its adoption in 2008.
However, it has been estimated that around 80 % of marine litter originates from land-based sources
(Morales-Caselles et al. 2021), mainly through rivers (Lebreton & Andrady 2019) and during flood
events (Roebroek et al. 2021). Transfer dynamics of macroplastics as well as their amount are still largely unknown, which explains the great uncertainties on the estimates of global flows (Schmidt et al. 2017; Roebroek et al. 2022).

Lately, a few studies have tried to estimate the amount of continental plastic waste discharged into the ocean from land via modeling and via the assessment of the mismanaged plastic waste (MPW) (Jambeck et al. 2015; Lebreton et al. 2017). Meijer et al. (2021) provided a new model approach that take into account local specific features, considering the probability for macrolitter to reach each river and thus the ocean. They evaluated that 0.8 to 2.7 million tons (Mt) of MPW could have reached the sea from more than 1000 coastal rivers. Their model was computed based on the field data of 67 different rivers plus data on MPW. However, those MPW data are not specific to each watershed but result from a proportional calculation from the country to the watershed scale. This evidence the need for more accurate estimates at the regional scale. Direct visual field observations downstream on rivers may improve the assessments of plastic flows at the watershed scale and, therefore, should allow to establish locally-adapted countermeasures against plastic pollution.

Castro-Jiménez et al. (2019) and González-Fernández et al. (2021) provided the first estimates of floating macrolitter in surface waters from European rivers, based on visual observations during a 1-year period (2016–2017) and within the framework of the RIMMEL project (Riverine and Marine floating macrolitter Monitoring and Modeling of Environmental Loading) from the European Commission-Joint Research Centre (EC-JRC). In our study, we focused on floating riverine macroplastics from the Têt River (Gulf of Lion, NW Mediterranean Sea). The objective of this work is to provide annual number and mass flows of plastics assumed to be discharged into the Gulf of Lion, completed by an exhaustive composition description. This work is partly based on the RIMMEL protocol that was extended over a 4-year period (data collection in 2016, 2018 and 2019) and through different meteorological and hydrological conditions to gain understanding on short-term variability (e.g. after rain events). This is particularly relevant in such highly reactive regimes as Mediterranean rivers. Until we know more about the factors that control the plastic flows, we cannot
give accurate estimates of the global input of plastics to the sea. Thus, we provide a complete
category description according to two meteorological conditions: the dry and rainy periods; and three
water discharge categories: low, moderate and high. As the Têt River is representative of numerous
coastal Mediterranean rivers, such a quantitative and qualitative monitoring of floating litter would
deliver relevant outcomes for oceanographers, watershed syndicates or even municipalities in the
context of heavy plastic consumption and growing demography.

**2. Materials and methods**

**2.1. Study area and environmental parameters**

The Têt River is a typical small Mediterranean coastal river, discharging into the Gulf of Lion (NW
Mediterranean Sea) (Fig.1a). The mean long-term annual water discharge is about 10 m³.s⁻¹ in the
Têt River (Ludwig et al. 2004), but ongoing climate change significantly decreased this value during
recent decades (Lespinas et al. 2009; Labrousse et al. 2020). Water discharge is highly variable,
oscillating between 1 and 240 m³.s⁻¹ over the 4 years of the survey (with an average of about 7 m³.s⁻¹)
as this river is characterized by extreme low-water stages and occasional flash-flood events (Serrat et
al. 2001). Flash-floods, which are a common phenomenon in coastal Mediterranean river basins, are
short and extreme events occurring during a few hours or days provoked by marine storms reaching
coastal mountains (Bourrin et al. 2008). The result is massive rainfall and a sudden river discharge of
fresh water, sediments and litter to the coastal zone, significantly contributing to the total riverine
matter flowing into the Mediterranean Sea (Sadaoui et al. 2018).

The Têt River watershed can be separated in two major areas (Fig.1b). The upstream zone is characterized by mountains, where the river has its source at the foot of the Carlit peak (2,405 m above sea level). The upstream zone remains natural and poorly urbanized. The downstream zone is the coastal plain of the Têt River, which is characterized by agricultural activities, and also includes the most densely populated city in the Pyrénées-Orientales (Perpignan, about 120,000 inhabitants). Population density of the Têt River watershed (150 pers.km⁻²) is 1.7 times greater than the Rhône
River's one (Constant et al. 2020) for a total area of 1,400 km². There are two major dams crossing the Têt River (Bouillouses and Vinça) which lie upstream the densely populated coastal plains (Fig.1b). The Têt River watershed is regularly exposed to a strong continental wind called "Tramontane" associated with dry periods. Such intense winds can mobilize plastic litter from landfills and other MPW sources to the nearest drainage ditch or waterway.

<figure><img src="image_3.png" /><figcaption>Figure 1: Map showing a) the location of the Têt River watershed in the South of France (Gulf of Lion, NW Mediterranean Sea), b) close-up of the Têt River watershed's topography, the two dams crossing the river (Bouillouses and Vinça), the city of Perpignan and the location of the fording (modified from SMTBV). c) Photo of the fording in Villelongue-de-la-Salanque where 66 collection sessions were performed from 2016 to 2019.</figcaption></figure>

## 2.2. Data collection

Data collection was initiated within the framework of the project RIMMEL held by the EC-JRC (González-Fernández & Hanke, 2017). Macrolitter monitoring was carried out by two persons against current by visual observations of floating litter at the Villelongue-de-la-Salanque's fording (2 m above the water surface) (Fig.1c). This location offered the opportunity to observe small floating items such as fragments down to 1 cm and to collect them quickly with a telescopic swimming pool net (up to 4m long) before they flow under the fording. Since remote detection does not allow to
determine accurately the mass of macrolitter, we collected all the floating items (few exceptions
during high water discharges). During the sampling with the net, the second watcher recorded what
we saw on a field notebook to compare visual interpretation with what we collected during the same
sampling, thus avoiding misidentification. The fording, located on the Têt River at around 5 km from
the river mouth, is the most downstream and convenient place for scientific sampling. While some
litters could be trapped in the lower reach between the fording and the coast, some other inputs may
exist. Furthermore, the Mediterranean Sea is microtidal and we do not observe effective "back and
forth" movements in the mouth of the Têt River. Thus, we choose to assume that all the observed and
collected floating macrolitter would have reached the sea from this location.

Thanks to the narrow width of the Têt River (~35 m), floating macrolitter were collected from the
water surface layer along the entire width of the fording (except during flood events; see details
below). Back to the laboratory, collected macro litter were dried in an oven (40°C overnight),
identified, classified, and weighed individually. Litter were sorted using the EC-JRC classification
compatible with the MSFD MasterList (MSFD, 2008; González-Fernández & Hanke, 2017),
allowing the harmonized monitoring of macroplastics. This MasterList is composed of more than
two hundred item names pooled in 8 main categories describing the material types: synthetic
polymers/plastic, rubber, metal, manufactured wood, glass/ceramics, clothes/textile, paper/cardboard
and others/diverse.

A total of 66 collection sessions (30 min each) were performed from June 2016 to April 2019 (no
data collection in 2017) (see sampling representativeness in Table S1). Among them, 16 sessions
were done during rainy days (RD, days with rainfall >10 mm or the day following a rain event >10
mm) and 50 sessions during dry days (DD, all other days). The sessions were further classified
according to water discharge categories: low (<5 m³.s⁻¹), moderate (5-25 m³.s⁻¹) and high (>25 m³.s⁻¹).
These limits were chosen since they best reflect the hydroclimatic conditions in the field. It should
also be noted that all three categories correspond to significant parts of the river discharge in general
(see Table S2). Percentage of the river width considered depends on the water discharge at the time
of the monitoring (21/66 sessions were performed on the entire river width). When a flood was
occurring, macrolitter were too numerous to be counted across the entire river width (Sup. File 1).
For these sessions, macrolitter number and mass flows were extrapolated over 100 % of the river
section afterwards, considering a homogeneous distribution of litter across the river section. Since we
focused only on plastic items, we defined NB<sub>std</sub> (in items·30min<sup>-1</sup>) as the number of macroplastics
collected in 30 minutes over 100 % of the river section and M<sub>std</sub> (in g·30min<sup>-1</sup>) as the dry mass of the
macroplastics collected in 30 minutes over 100 % of the river section.

In order to assess of the plastic flows and their temporal variability, we selected the “Têt partielle pont Joffre” discharge gauging station, 7 km upstream from our sampling site, with less than 0.001% missing values during the investigated period from 2016 to 2019. Instantaneous water discharges for the 66 collection sessions (in m³.s⁻¹, averaged every 30 minutes) were obtained from the tool Hydroportail (v1.3.2) available at http://www.hydro.eaufrance.fr and were multiplied by 1800 (60 s × 30 min) to give a close approximation of the total water volume that has flowed during 30 minutes (V₃₀, in m³·30min⁻¹). Rainfall and wind data in Perpignan were provided by the internet portal of Infoclimat (https://www.infoclimat.fr). These stations also corresponded to the meteorological stations closest to the sampling site (~10 km).

**2.3. Statistical analysis**

As normality of distributions was never observed, *Shapiro-Wilk tests* were used. We used the *non-parametric Spearman correlation tests* to assess the relations that may exist between NB<sub>std</sub> and the wind speed, M<sub>std</sub> and the wind speed, and also NB<sub>inst</sub> and M<sub>inst</sub>. We used the *non-parametric Mann-Whitney U-tests* to detect significant differences in NB<sub>inst</sub> and M<sub>inst</sub> between RD and DD. To determine this difference, the rank sums of the two groups were used rather than the means. For all tests, p-values<0.05 were considered as statistically significant.

Afterwards, we calculated macroplastic number and mass concentrations (NB<sub>std</sub>/V<sub>30</sub> and M<sub>std</sub>/V<sub>30</sub>, providing items·m<sup>-3</sup> and g·m<sup>-3</sup>) to accurately estimate the annual flows of plastic waste, based on V<sub>30</sub>
values associated with each collection session. The 66 sessions were classified into subgroups to provide a single mean concentration for each of the six hydroclimatic conditions defined (i.e., 2 rainfall conditions × 3 water discharge conditions). The mean was selected against the median because we took partially into account the influence of environmental variability (meteorological and hydrological specificities), limiting large discrepancies on our estimates, as it was the case for previous studies. Then, we estimated macroplastics loading every 30 min (items.30min⁻¹ or g.30min⁻¹) by extrapolating those concentrations to all V₃₀ values calculated from the 2016 to 2019 period, according to their hydroclimatic belonging. The annual number flow in items.y⁻¹ and mass flow in g.y⁻¹ (converted in t.y⁻¹) were estimated as the sum of all the 30 minutes loading.

When possible, measures of uncertainty were added to means and were calculated as % SD = (mean / standard deviation) × 100.

## 3. Results

### 3.1. Characterization of floating macrolitter

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>
<caption>Figure 2: Top 10 in terms of (a.) percentage of total abundance (n=1168), (b.) percentage of total mass (m=3.9 kg) and (c.) frequency (percentage of observations) based on raw data (non standardized) collected during 66 collection sessions.</caption>

During 66 collection sessions, a total of 1,168 items (raw, non standardized) were recorded and identified within 65 item names and 8 categories of material (Fig. S3). They were quasi exclusively made of synthetic polymers (97.0 %). Rubber accounted for 1.2 % of the items and all the other categories accounted for less than 0.5 % each. The most abundant items were cigarette butts that accounted for 20.5 % of all items recorded (Fig.2a), followed by polystyrene (PS) fragments (18.8 %), light packaging (16.3 %), plastic fragments (12.8 %, other than PS), cotton buds (6.7 %) and bottle tops and rings (3.5 %). The 10 most recorded items in abundance accounted for 84.7 % of total items recorded (Fig.2a).

In terms of mass, we collected a total of 3.9 kg of floating macrolitter. The average mass of a floating item (all types of litter included) was 3.4 g. Considering plastic only, a mean mass per sampled plastic piece of 1.8 g was found and 80 % of sampled plastic items were <1 g, whatever their size (Fig. S5 and S6). The heaviest items found (glass bottles and balls) were not plastic items (Fig.2b). Despite being lightweight materials, the pool of non-identified PS and plastic fragments accounted for 14.6 % of the total mass, as they were abundant (31.6% of the total items). In terms of mass, the top 10 items represented 81.4 % of the total mass.

Light packaging items were recorded in nearly 70 % of the collection sessions (46/66 sessions) (Fig.2c). Plastic and PS fragments were respectively present 47.0 % and 42.4 % of the sessions. Cigarette butts were mostly mobilized in large quantities during RD (12/16 RD sessions) and were not so present during DD.

**3.2. Floating macroplastics dynamic and composition**

Because of the predominance of plastic items in our dataset, we focus in the following only on this category of litter. Macroplastic flows and environmental parameters, such as rainfall and daily water
discharge for the 4-year-period are represented in Fig. 3 (additional information for each monitoring
session are available in Sup. File 1). Sampling in the Têt River was performed for a wide range of
daily water discharges from 1.1 to 94.9 m³.s⁻¹. This hydrological range (< 100 m³.s⁻¹) represents
99.5 % of the daily water discharges recorded over 2016-2019.

<figure><img src="image_7.png" /><figcaption>Figure 3: Evolution of hydroclimatic conditions : daily water discharge (black sticks) and rainy events (red dots) over the entire study period (2016-2019). Temporal location of the 66 sampling days (red dots) and the number of plastics observed on these days (yellow dots).</figcaption></figure>

NB<sub>std</sub> values ranged from 0 (9/66 sessions) to 1,752 items.30min<sup>-1</sup> (Sup. File 1). The highest NB<sub>std</sub> (1,752 items.30min<sup>-1</sup>), 25 times higher than the mean NB<sub>std</sub> (70 items.30min<sup>-1</sup>; Fig.4a), happened in October 2018 during the fourth RD after a 3-month-long dry period, with a moderate daily discharge rate (21.3 m<sup>3</sup>.s<sup>-1</sup>) (Fig. 3). We found a strong correlation between NB<sub>std</sub> and M<sub>std</sub> (Spearman's correlation, rho=0.83, p<2.0 10<sup>-16</sup>). Macroplastic mass flows varied from 0 to 2,785 g.30min<sup>-1</sup> (Sup. File 1). The maximum M<sub>std</sub> of 2,785 g.30min<sup>-1</sup> was obtained the same day as the highest NB<sub>std</sub>, and was 20 times superior to the mean M<sub>std</sub> (140 g.30min<sup>-1</sup>; Fig.4b).

The rainfall conditions (RD or DD) did significantly affect NB<sub>std</sub> and M<sub>std</sub> (Mann-Whitney U-tests,
p=1.2 10<sup>-8</sup> and p= 1.9 10<sup>-7</sup>, respectively). Separating the number and mass flows between 6
hydroclimatic categories reduces the observed variability and thus the difference between means and
medians (Fig.4a, b). In contrast, we did not observe any correlation between the wind speed and NB<sub>std</sub> or M<sub>std</sub> (*Spearman's correlations*, rho=-0.02 and rho=-0.09, p=0.8 and p=0.5, respectively).

<figure><img src="image_8.png" /><figcaption>Figure 4: Box plot representation (in log scale) of (a.) macroplastic number flows (NB<sub>std</sub> in items·30min<sup>-1</sup>) and (b.) macroplastic mass flows (M<sub>std</sub> in g·30min<sup>-1</sup>) considering all sessions (n=66), DD only (n=50) and RD only (n=16). Means are symbolized by a dotted line and medians by a solid line. The boxes extend from the 25th to 75th percentile, the black whisker extends from minimum (0) to maximum.</figcaption></figure>

The high variability of the daily number and mass flows, as well as their dependence on complex environmental factors does not allow for direct extrapolation to accurate annual macroplastic flows. Thus, we calculated the number and mass concentrations (in items.m<sup>-3</sup> and g.m<sup>-3</sup> respectively) of macroplastics (Fig. S4) according to rainfall conditions and the water discharge categories. Among all water discharge categories, number and mass concentrations were lower on DD than on RD (Fig.5). On DD, highest number and mass concentrations were observed during low water discharges. In contrast, during RD, number and mass concentrations were higher during moderate water discharges and lower during both low and high water discharges (Fig.5).
<figure><img src="image_9.png" /><figcaption>Figure 5: Box plot representation (in log scale) of (a.) the macroplastic number concentrations and (b.) the macroplastic mass concentrations, according to the rainfall conditions (RD, DD), and the water discharge categories (low, moderate, high). Black dots were plotted for the RD and high water discharge category as n=3. Means are symbolized by a dotted line and medians by a solid line. The boxes extend from the 25th to 75th percentile, the black whisker extends from minimum and maximum non-outliers and crosses show outliers.</figcaption></figure>

The qualitative composition of the samples was also very different between RD and DD and between the 3 water discharge categories (Fig.6). On RD, there was no predominant category of macroplastic in number for low water discharge, whereas during moderate water discharge, cigarette butts were predominant in number (37.2 %). However, the low average mass of cigarette butts (0.17 g) makes their contribution marginal in the total mass (4.1 %) of this hydroclimatic category. PS fragments (20 to 40 % of the total macroplastic items) is also a major category on RD. PET bottles did not appear within the Top 10 of abundance whatever the rainfall and water discharge conditions, but their high average mass (24.5 g) explains the mass proportion up to 26.2% on RD and medium water discharges.
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The monitoring on DD is mainly characterized by light packaging items (24 to 40 %) regardless of the water discharge category. However, up to 62.3 % of the mass is due to 'other plastic items' during the lowest water discharges, which is explained by the presence of uncommon single items (e.g. shoe sole, medicine package or plastic shovel) under those conditions. Heavy macroplastics (> 15 g), such as beach toys on RD and lighters on DD, were mainly transported during high water discharge. Those heavy macroplastics predominated in terms of mass (35-40 %), whereas they were only observed once or twice.

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<figure><img src="image_10.png" /><figcaption>Figure 6: Relative number and mass contributions of the main macroplastics sampled according to rainfall (RD, DD), and water discharge conditions (Low, Moderate, High). The outer circles represent the contribution in number of items and the inner circles the contribution in mass.</figcaption></figure>

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## 3.3. Annual macroplastic flows

We estimated that a mean of 354,000 (±42 %) floating macroplastics were discharged into the Mediterranean Sea by the Têt River each year, over the 4-year-period. This corresponds to an annual mean loading of 0.65 (±51 %) tons.y⁻¹. Table 1 exhibits the annual variability of plastic flows that varied from 202,000 items.y⁻¹ (0.31 tons.y⁻¹) in 2016 (a dry year) to 555,000 items.y⁻¹ (1.10 tons.y⁻¹) in 2018 (a rainy year). Although plastic litter represent 97 % of the total litter items, the mean mass

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flow of total litter reached 1.04 t.y⁻¹, about 60 % more than the mean macroplastic mass flow alone. According to precipitations and water discharge of the Têt River over this 4-year-period, we calculated that 73 % and 76 % of the number and mass flows, respectively, were discharged during RD, which peaked up to 80 % and 85 % in 2018.

<table><caption>Table 1: Macroplastic number and mass flows estimates over 2016-2019. Annual values are detailed according to each hydro-climatic category: rainy days (RD), dry days (DD) and water discharge categories (low: <5; 5-25 and >25 m³.s⁻¹). Percent of RD = number of RD / total number of day in the considered year * 100; Annual volume = ΣV₃₀.</caption><thead><tr><th></th><th></th><th>2016</th><th>2017</th><th>2018</th><th>2019</th><th>4-year mean</th></tr></thead><tbody><tr><td>Percent of RD (%)</td><td></td><td>4.9 %</td><td>5.5 %</td><td>9.6 %</td><td>5.2 %</td><td>6.3 %</td></tr><tr><td>Annual volume (km³)</td><td></td><td>0.11</td><td>0.17</td><td>0.42</td><td>0.18</td><td>0.22</td></tr><tr><td rowspan="4">Numerical macroplastic flows (items.y⁻¹)</td><td>Low</td><td>16,000</td><td>66,000</td><td>14,000</td><td>60,000</td><td>18,000</td><td>34,000</td><td>12,000</td><td>58,000</td></tr><tr><td>Moderate</td><td>110,000</td><td>10,000</td><td>201,000</td><td>22,000</td><td>192,000</td><td>53,000</td><td>133,000</td><td>23,000</td></tr><tr><td>High</td><td>/</td><td>1,000</td><td>40,000</td><td>2,000</td><td>233,000</td><td>26,000</td><td>91,000</td><td>2,000</td></tr><tr><td>Total</td><td>202,000<br>(62 % in RD)</td><td></td><td>340,000<br>(76 % in RD)</td><td></td><td>555,000<br>(80 % in RD)</td><td></td><td>320,000<br>(74 % in RD)</td><td>354,000<br>(73 % in RD)</td></tr><tr><td rowspan="4">Massic macroplastic flows (t.y⁻¹)</td><td>Low</td><td>0.02</td><td>0.10</td><td>0.02</td><td>0.09</td><td>0.03</td><td>0.05</td><td>0.02</td><td>0.09</td></tr><tr><td>Moderate</td><td>0.16</td><td>0.01</td><td>0.30</td><td>0.02</td><td>0.20</td><td>0.04</td><td>0.20</td><td>0.02</td></tr><tr><td>High</td><td>/</td><td>&lt; 0.01</td><td>0.12</td><td>&lt; 0.01</td><td>0.71</td><td>0.07</td><td>0.28</td><td>&lt; 0.01</td></tr><tr><td>Total</td><td>0.31<br>(61 % in RD)</td><td></td><td>0.56<br>(78 % in RD)</td><td></td><td>1.10<br>(85 % in RD)</td><td></td><td>0.61<br>(80 % in RD)</td><td>0.65<br>(76 % in RD)</td></tr></tbody></table>

# 4. Discussion

## 4.1. Plastic litter composition

97 % of the total floating macrolitter in the Têt River waters were synthetic polymers, confirming the predominance of the plastic pollution. This is, to our knowledge, the highest value ever assessed in rivers, as this rate reached 77 % in the Rhone River (Castro-Jiménez et al. 2019), 82 % in the Tiber River (Crosti et al. 2018) and 82 % in 42 European rivers (Gonzalez-Fernandez, 2021). To the exception of cigarette butts, plastic items observed, including light packaging, PET bottles, PS and plastic fragments were in agreement with the top items listed by the RIMMEL network across Europe (González-Fernández et al. 2021). However, a significant contribution of our study is the the
ability to survey small and light litter items (i.e cigarette butts, light packaging, ...) as our sampling site was only 2 m above the Têt surface waters against > 10 m for most other European rivers. This short distance from the surface allowed us to collect all the visible floating items >2.5 cm (few exceptions during high water discharges) and therefore to assess an accurate averaged mass of 1.8 g per plastic item. This value is therefore significantly lower than the 5.4-6.3 and 10 g.plastic item⁻¹ found in European rivers (Vriend et al. 2020; Schöneich-Argent et al. 2020) and in Saigon rivers (van Emmerik et al. 2019a), respectively. However, van Emmerik et al. (2019b) found a similar value (1.3 g.plastic item⁻¹) after collecting each floating macroplastics from a short distance from the surface in Jakarta's rivers. Obviously, the collection methodology, including the height of the bridges significantly influences the limit visualization size of macroplastic items and so the average mass of floating macroplastics.

Most of our sampled floating macrolitter belonged to the economic sectors of tobacco (cigarette butts), food (light packaging), construction and transport industry (PS fragments), bringing to light both individual responsibility and inefficient waste management practices. Matching the type of plastic waste to its source of origin would allow industries and stakeholders to take appropriate measures in waste prevention programs (Geyer et al. 2017).

The presence of numerous unidentified plastic pieces as fragments could indicate a significant fragmentation of larger items most commonly found such as PET bottles, light packaging, bags and other plastic materials flowing down the river. This high fragmentation suggests that macroplastics have a long residence time on the watershed soils and along the banks of the Têt River before reaching the mouth. Liro et al. (2020) and Tramoy et al. (2020) have demonstrated that the residence times of macroplastics on land can reach several decades. Macroplastics undergo continuous cycles of transport, deposition and remobilisation, meaning the transfer to the sea is rather chaotic.

The most frequent item was by far light packaging, as they were found in about 70% of the monitoring sessions whatever the weather (DD or RD) and the hydrological (low, medium and high flow rates) conditions. This result, as the rest of the top 10 frequency (Fig.2c) is in phase with the
most frequently found items during marine litter surveys, where light packaging, PET bottles (with tops) and straws are regularly featured (GESAMP 2016). The light packaging items are thin plastic protection from individual food portions that may easily be remobilized by a small wind gust and thus, reach a stream or a river without being drained by the rain. On the other side, we quasi exclusively observed cigarette butts during rainy days. We estimate that ~73,000 cigarette butts (20.5 % × 354,000), reaching 114,000 in 2018, are discharged each year from the Têt River to the Mediterranean Sea. Most of them come from cities where they are consciously littered (Patel et al. 2013). They can easily reach urban waterways during rainy events and, thus, rivers crossing cities and flowing into the sea. According to the OECD (2022), 24% of the French population are daily smokers, with a mean consumption of 12.5 cigarettes a day (World No Tobacco Day, 2020). Moreover, it is assumed that three-quarters of cigarette butts may end up in the environment (Green et al. 2022; World Health Organization, 2022). Thus, with a population of 210,000 inhabitants, we can conjecture that 172 millions of cigarette butts might be discarded in our watershed each year, meaning we only estimated 0.04% of them in the river. First, it is obvious that not all of these butts will reach the sea. This low value may also be explained by the retention time of cigarette butts on soils that can be long enough for causing their fragmentation and degradation into microplastics (Liro et al. 2020; Tramoy et al. 2020). As a result, the majority of cigarette butts that reaches the river are no longer identifiable. The ones collected in our samples were certainly recently discarded in downstream urban areas. Finally, cigarette butts are small litters that can easily be missed during visual counting and be difficult to discriminate among leaves and twigs, despite being our most abundant items.

## 4.2. Hydroclimatic forcing

The hydrological range (< 100 m³.s⁻¹) observed during this survey represented 99.5 % of the daily water discharges recorded over the 2016-2019 period. Exceptionally, the Têt River can reach a daily flow >1,000 m³.s⁻¹ (case of Gloria's storm in January 2020). The counting and sampling of floating
liter becomes impossible in such large flood condition, because our observation site on the fording is
closed over 100 m³.s⁻¹ and also because the turbulence of waters and the darkness associated with
heavy rainfall.

The dynamic of plastic transport is obviously influenced by the hydroclimatic forcing. Al-Zawaidah et al. (2021) noted that no studies have been conducted to support the assumption that more litter are supplied within rivers during windy events. Here, we assumed that the effect of wind might enhance the movement of light plastics further away from the original pollution location, as light packaging were the predominant items during dry days and our region is well-known for its continental intense wind. Nevertheless, we did not find any link between wind speed and macroplastic litter loading, probably because the leakage from the urban center to the river is not linear, unlike rainfall which leads to a rapid hydric response of the river. Macroplastic flows were significantly higher during RD than DD, meaning that precipitation events facilitate new litter inputs to the watercourse and remobilize items already deposited upstream on the river banks. In some river systems, such as coastal Mediterranean rivers, the majority of plastic transport can occur in a very limited time (van Emmerik et al. 2019b). Indeed, 73 % and 76% (number and mass respectively) of macroplastics are discharged by the Têt River during rain events (~6 % of the full year). The great contribution of meteorological forcing in the macroplastic flows dynamic was already highlighted in the Huveaune watershed (Gulf of Lion, France) by Tramoy et al. (2022), who evidenced “rainfall events” (wet periods $2 \le P< 30$ mm and heavy rainfall episodes $\ge 30$ mm) contribute to much more mass inputs, in comparison to their timespan of the year. However, these authors estimated the mass contribution of macroplastic inputs during “rainfall events” to 36.7 %, far from our 76 % value.

We assume that the floating macroplastics may come from two distinct sources (Fig.7). The first
source is the watershed and all abandoned litter that may be easily remobilized because of their
buoyancy during rainfall (the "runoff" source). We presume that this source is enhanced within an
urban area such as Perpignan, which explains the high amount of cigarette butts during rain events.
These floating macroplastics reach the Têt River through its tributaries and artificial streams. The
second source (called "remobilization") is the stock of litter stranded along the banks and the riparian vegetation. Within the framework of a participatory research with a local non-profit association (Citeco66), Kerhervé et al. (2021) estimated an averaged stranded mass of litter of 4.1 t per km of river banks between Perpignan and the Têt mouth (distance: 11 km). This stranded litter is not only composed of floating macroplastics but we can easily assume that this stock may be remobilized by rising and turbulent waters during rain events (Fig. 7). We also have to consider that the Têt River, as most rivers, is dammed in order to regulate the water flow throughout the year (Fig. 1b). This may partly explain water runoff variation during dry days in its downstream part. However, information on dam functioning, reservoir levels and release mechanisms are sensitive and could not be obtained for this study.

<figure><img src="image_11.png" /><figcaption>Figure 7: Scheme of the two main sources of macroplastics within the Têt River watershed. 1) the water runoff within the watershed during rainfall and 2) the remobilization of stranded macroplastics along the river banks during water rise. Moderate water rise may also occur during DD due to some dam release.</figcaption></figure>

We reported that the highest concentrations of macroplastics occurred during moderate water discharge in RD. These unprecedented results show that plastic waste can be largely remobilized at
the beginning of the rise of water after a rainy event (max NBstd=1,752 items.30min⁻¹), especially after long dry periods, as in October 2018 (the fourth RD after a 3-months-long dry period, with a moderate daily water discharge of 21.3 m³.s⁻¹). This assumption is consistent with van Emmerik et al. (2022), who evidenced plastic transport was up to six times higher during rising discharge events than under normal conditions.

We did not observe whether macroplastic flows exhibited seasonal dynamics, as it was shown in Cesarini at al. (2023). Previously, Bourrin et al. (2008) explained that flash flood events are more common in small coastal Mediterranean rivers and probably more influential than seasonal floods, rather favorable on larger river systems.

### 4.3. Plastic export from the Têt River to the Mediterranean Sea

The Mediterranean Sea has been identified as one of the most plastic-polluted seas on its surface (Suaria & Aliani 2014). This observation is linked to its semi-enclosed structure, the high population density along its coastline and the cumulative influence of small watersheds around the Mediterranean Sea (Sadaoui et al. 2018). Floating plastics may persist in surface seawater for years, but either a significant part would sink after being biofouled or would strand on beaches (Constant et al. 2019; Lots et al. 2017). Thus, establishing reliably and accurately the plastic loading discharged into the sea by these coastal rivers represents a major challenge for the understanding of the extent of this pollution and its impacts on the environment. We chose not to extrapolate our concentrations beyond the sampling period (2016-2019) for several reasons. First, the year 2020 witnessed two exceptional consecutive massive floods (up to 1,000 m³.s⁻¹ in January and April), and we can not extrapolate the floating macroplastic concentrations for water discharge over 100 m³.s⁻¹. Second, the composition of floating litter could have changed during the last few years, since some of the major plastic items have been banned from commercialization in France (cotton buds and disposable
dishware since January 2020; plastic straws, since January 2021) and others have appeared since the
COVID-19 worldwide pandemic (masks, gloves, self-tests...) (Savuca et al. 2022).

We estimated an average annual macroplastic number and mass flows of 354,000 items.y⁻¹ and 0.65 t.y⁻¹, respectively, during the considered period. Our estimates are based on macroplastic items only, microplastic items not being considered. Shirinzi et al. (2020) found annual floating plastic mass flow within the same range (0.1 and 0.4 t.y⁻¹) for two small Catalan rivers (Llobregat and Besos Rivers, respectively) but they used plastic category/size/weight ratios and did not collect and weigh macro-litter. Furthermore, the 4.1 t of stranded litter on the downstream part of the river bed, as well as the dense litter flowing through the sub-surface and bottom layers are not taken into account.

Also, we extrapolated the calculated macroplastic flows over the entire width of the river, whereas the water regime is not constant over the entire width (peak velocity in the center, smoother on the edges). Thereby, we consider our assessment of plastic inputs from the Têt River to the Mediterranean Sea as a low estimate. Castro-Jiménez et al. (2019) estimated the floating macroplastic inputs of the Rhone River to the NW Mediterranean Sea to ~223,000 items.y⁻¹ for a total mass of ~0.7 t.y⁻¹. These results are on the same order of magnitude than our estimates, whereas the mean water flow of the Rhône River is more than 100 times higher than the Têt River's one. Our higher floating plastic load can be linked to an easier transport of plastic litter within small coastal basins than in large basins (González-Fernández et al. 2021), to a greater population density in the Têt River (1.7 times higher) (Constant et al. 2020) and to a methodological advantage on the Têt River as we were able to consider smaller items (>80 % of sampled macroplastics were <10 cm, Fig. S5). González-Fernández et al. (2021) estimated plastic flows to be >3,000 t.y⁻¹ for the entire European coastline, considering a mean mass of 5.4 g per plastic items, much higher than our 1.8 g.plastic item⁻¹. This highlights the importance of data collection methods in the resulting estimates and the complexity of cross-referencing available data.

It is noteworthy that our estimate is ~7 times higher than the microplastic estimates derived from samples collected at the same site and simultaneously to our macrolitter sampling in 2016 (Constant
et al. 2020). Annual microplastic flows were evaluated to be 0.09 t yr⁻¹ for the Têt River, bringing the total flow of plastics (macro- and microplastic combined) to 0.74 t.y⁻¹ for this river.

As Castro-Jiménez et al. (2019), we noticed a significant discordance between the results derived from MPW modeling and our estimates derived from direct observation/weighing of floating items. To our knowledge, no other study (except for the Têt and Rhône Rivers) have assessed the number and mass flows of floating macroplastics discharged by rivers into the Mediterranean Sea. Boucher & Billard (2020) estimated from the MPW index a macroplastic flow of 710 t.y⁻¹ for the French Mediterranean inland sources and 140.000 t.y⁻¹ for the whole Mediterranean inland sources.

Assuming that the French Mediterranean inland sources are mainly due to 8 main small coastal rivers, like the Têt River (0.65 t.y⁻¹ × 8), plus the Rhône River (0.7 t.y⁻¹), we can roughly estimate to 6 t.y⁻¹ the mass of floating macro-plastics discharged into the sea. This rough estimate highlights the high discrepancy (2 orders of magnitude) between in-situ measurements and MPW-related modeling.

Indeed, when we extrapolate the two main indices - MPW and human development index (HDI) - used in modeling to our Têt River watershed (Table 2), our annual mass flow of floating macroplastics (0.65 t.y⁻¹) only represents 0.06-0.28 % of the available plastic waste in the Têt River watershed (APTRW). This means that ‘probability of emission’ into the sea is very low (<1 %) for this river, and is consistent with results from Meijer et al. (2021) who also found <1% of probability of emission on larger French rivers (Seine: 0.06 % ; Rhône: 0.12 %). However, as this has been recently demonstrated for riverine microplastic flows (Weiss et al. 2021), MPW and HDI are probably not the best predictors in order to extrapolate the budgets to larger regional and global scales.
<table><caption>Table 2: Comparative table of the use of two modeling methods (MPW and HDI) to calculate the available plastic litter in the Têt River watershed in tons per year (AP<sub>TRW</sub>) and the relative importance (% of annual AP<sub>TRW</sub>) of our estimated mean annual macroplastic flow (M<sub>out</sub>) compared to this AP<sub>TRW</sub>. MSW: municipal solid waste in kilograms per capita and per year; P: percentage of plastic in MSW; Pop: population of the Têt River watershed in capita; %MPW: percentage of mismanaged waste in France; M<sub>out</sub>= 0.65 t.y<sup>-1</sup>).</caption><thead><tr><th>Author</th><th>Method</th><th>Parameters</th><th>Available Plastic in TRW (AP<sub>TRW</sub>)</th><th>% of annual AP<sub>TRW</sub></th></tr></thead><tbody><tr><td>Jambeck et al. (2015) &amp; Lebreton and Andrady (2019)</td><td>MPW</td><td>MSW = 540 kg.pers<sup>-1</sup>.y<sup>-1</sup><br>P = 10 %<br>Pop = 210 000 pers<br>%MPW = 2 %<br>AP<sub>TRW</sub> = Pop x MSW x P x %MPW</td><td>227 t.y<sup>-1</sup></td><td>(M<sub>out</sub>/AP<sub>TRW</sub>) x 100 = 0.28 %</td></tr><tr><td>Mai et al. (2020)</td><td>HDI</td><td>MSW = 540 kg.pers<sup>-1</sup>.y<sup>-1</sup><br>P = 10 %<br>Pop = 210 000 pers<br>%MPW = 2 %<br>AP<sub>TRW</sub> = Pop x MSW x P x (1 - HDI)</td><td>1,134 t.y<sup>-1</sup></td><td>(M<sub>out</sub>/AP<sub>TRW</sub>) x 100 = 0.06 %</td></tr></tbody></table>

## 5. Conclusion

This study on a typical small coastal Mediterranean river (Têt River) provides the first field-based, long-term and accurate monitoring of floating macroplastics assumed to leak into the Gulf of Lion (NW Mediterranean Sea). Regional environmental factors (heavy rainfall and sudden water discharge rise) have a substantial influence on the river-specific levels of macroplastic inputs, and affect the actual amount of riverine litter discharged into the sea. The Têt River exhibited significantly higher plastic emission levels during rainy days, especially at the beginning of the flood events, meaning we could avoid the great majority of macroplastics inputs (73 %) by targeting actions on a few days of the year (~6 %), and before the floods take place.

Taking into account hydroclimatic local features, we managed to minimize the large discrepancies observed between mean and median annual plastic emission rates calculated and thus provide more precise annual estimates (354,000 items.y<sup>-1</sup> ; 0.65 t.y<sup>-1</sup>). We have demonstrated that long-term visual counting in proximity to the water surface and weighing of floating macrolitter is a suitable
monitoring strategy to achieve estimation accuracy that may contribute to further enhancement of waste prevention programs.

It seems relevant to associate the litter with the economic sector of concern because matching the type of plastic waste to its source of origin would allow stakeholders to take appropriate measures of prevention and pollution mitigation. The large share of cigarette butts (20.5 %) and other single-use items (light packaging, 16.3 %) in number demonstrates that beyond the improvement of waste management strategies, behavioral changes are also needed to stem waste production at the source.

The methodological limitation of this study concerns the lack of knowledge on plastic flows during major flood events (> 100 m³.s⁻¹). We do not know what happens during such extreme events: this belongs to the speculative and requires in-depth study. The use of a hyper-spectral camera in order to overcome visual issues related to rainfall periods and turbulent flood waters may fill this knowledge gap.

## Acknowledgment

Authors would thank Christophe Menniti (CEFREM) and Master students (Flore Oudin, Heloïse Gendre, Baptiste Grassi) for their help on the field. We would like to also thank Mahrez Sadaoui for his valuable knowledge on the hydrodynamic of the Têt River. Margaux Laverre's research internship (Master BDD-UPVD) was fund by the REDPLAST66 project (2019-2022; Littoral et mer, Fondation de France, project number: 1910). D. González-Fernández was supported by the European Union (H2020-MSCA-IF-2018 846843 - LitRivus) and by the 2014-2020 ERDF Operational Programme and the Department of Economy, Knowledge, Business and University of the Regional Government of Andalusia: Project reference FEDER-UCA18-107247.
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