# Litter in French urban areas - Part 2: transport dynamic and fluxes in stormwater

Lauriane Ledieu, Romain Tramoy, David Mabilais, Sophie Ricordel, Marie-Laure Mosini, Alexandra Mosset, Bernard Flahaut, Laetitia Pineau, Zoé Bridant, Eric Bouchet, et al.

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## To cite this version:

Lauriane Ledieu, Romain Tramoy, David Mabilais, Sophie Ricordel, Marie-Laure Mosini, et al. Litter in French urban areas - Part 2: transport dynamic and fluxes in stormwater. Environmental Science and Pollution Research, 2025, 32 (16), pp.10149-10163. 10.1007/s11356-024-33774-0. hal-04599474

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https://univ-eiffel.hal.science/hal-04599474v1

Submitted on 10 Sep 2025

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Distributed under a Creative Commons Attribution 4.0 International License
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Litter in French urban areas - Part 2: transport dynamic and fluxes in stormwater.

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Lauriane Ledieu¹,²,*, Romain Tramoy³, David Mabilais¹,², Sophie Ricordel¹,², Marie-Laure Mosini¹,², Alexandra

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Mosset¹,², Bernard Flahaut¹,², Laetitia Pineau¹,², Zoé Bridant¹,², Eric Bouchet⁴, Clémence Bruttin⁵, Fabrice

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Rodriguez¹,², Bruno Tassin⁶, Johnny Gasperi¹,²

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¹Univ Gustave Eiffel, GERS-LEE, F-44344 Bouguenais, France

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²Institut de Recherche en Sciences et Techniques de la Ville IRSTV, CNRS, 1 rue de la Noë, 44321 Nantes, France

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³LEESU, Univ Paris Est Créteil, Ecole des Ponts, Creteil, France

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⁴Nantes Métropole, 2 cours du Champ-de-Mars, 44000 Nantes, France

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⁵Citeo, 50 boulevard Haussmann, 75009 Paris, France

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⁶LEESU, Ecole des Ponts, Univ Paris Est Creteil, Marne-la-Vallée, France

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*lauriane.ledieu@univ-eiffel.fr

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**Abstract**

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Urban areas constitute a major hotspot of litter, including plastic litter, that stormwater can wash off
towards waterways. However, few studies quantified and characterized litter densities in urban areas
and fluxes transported by stormwater networks. Moreover, little information is available on litter
transport dynamics in stormwater, and on the factors driving this transport. This study aims at
characterizing and quantifying litter amounts transported by stormwater of two nested French urban
catchments (total surface area of 105 ha). Litter densities relative to land uses and rain events were
investigated in the same catchments as in a compagnon paper (Ledieu et al., 2024). The present study
explores the impact of rain events on the transport of urban litter in stormwater. Litter collection and
characterization combined with a capture/recapture experiment using tagged litter placed in stormwater
inlets show total litter fluxes of 29 kg/yr, in which 14 kg/yr are plastics, exported at the stormwater
outlet. These fluxes confirm that stormwater runoff is a considerable pathway from urban surfaces to
waterways, especially regarding small plastic litter (< 2.5 cm). Item transport dynamic is however not linear and only 0.3% of the urban litter appear remobilized. Litter transport dynamics depends on several factors (urban litter densities, hydrometeorological parameters, item composition and morphologies, and stormwater management systems) that should be considered in global models.

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## Keywords

Plastic litter, litter sources, rain events, stormwater network, mitigation strategies

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# 1. Introduction

By concentrating the human population and activities, high amounts of waste are generated in urban areas. In these areas, waste is managed by collection/cleaning services, or littered (deliberately or accidentally), often depending on the economic development of countries (Hoornweg and Bhada-Tata 2012). Although waste management systems are becoming increasingly efficient in European countries, the population growth and consumer society lead to an explosion in waste production, particularly plastic waste (Heinrich Böll Stiftung et al. 2020; Karimi and Faghri 2021). Littered items can either be reintroduced in the waste management system by municipal services, or stay accumulated on urban surfaces and potentially accumulate in soils, or be transferred towards waterways and subsequently to the sea. Several transfer pathways have been identified in urban areas: domestic wastewater, urban stormwater and combined sewer overflows (Basilico et al. 2020). Urban water networks are thus suspected of being a major source of litter for waterways, particularly plastic litter (Cesarini et al. 2023; Tasseron et al. 2023). However, little is known about plastic transport by stormwater, and no exhaustive and consolidated data are available on the contribution of stormwater discharges to the riverine plastic pollution.

Stormwater can be managed either by combined or by separate sewer systems. These systems, if any, may differ between countries, between cities and even within cities. Combined sewer systems are
traditionally used worldwide, but separate sewer systems are used preferably in developed countries since the second half of the 20th century (De Feo et al. 2014) to achieve the reduction of sanitation impact on waterways. In addition, stormwater volumes tend to increase with more intense rain events and increasing urbanization with impervious surfaces that favors runoff. In France and in several European cities, combined sewage networks are mainly present in city centers and separate sewage networks in peripheral areas, that have been urbanized more recently (Rognard and Finck 2020). This makes it more difficult to distinguish the contributions from different sources and transfer pathways (Basilico et al. 2020). Besides, current regulations do not impose any specific monitoring of litter discharges in combined sewer overflows (Rognard and Finck 2020). Moreover, these inputs occur during extreme weather events, making them even more difficult to monitor.

Some stormwater network infrastructures are designed to limit the entry and the transfer of litter in sewers and to waterways with grates, underflow baffles, hydrodynamic separators (Rognard and Finck 2020; Bruttin et al. 2021) or nets. In recent years, with the growing interest in plastic pollution, such structures are sometimes used to specifically monitor litter inputs from urban runoff (e.g. in Tramoy et al., 2022a, b; Martinez and Griffiths, 2023). Nevertheless, most networks are not equipped with retention devices and litter may be directly exported towards waterways (Basilico et al. 2020), with stormwater outlets becoming a point source of litter. Hydrometeorological parameters (e.g. rainfall intensities and discharge volumes) play a key role in the transfer of litter (Treilles et al. 2021; Malik et al. 2021) but this relationship remains poorly understood (Tramoy et al. 2022b).

Many models use levels of mismanaged waste on land to estimate the riverine plastic export into the sea (e.g. Jambeck et al., 2015; Lebreton et al., 2017; Schmidt et al., 2017), but the estimation of urban runoff contribution needs to be completed. Integrating such a parameter should lead to more realistic models but because of low empirical data on litter densities on land, it increases their uncertainty (Roebroek et al. 2022). Previously, we investigated litter composition, origin and accumulation rate on the urban surfaces of two residential and commercial areas located in Nantes (France, see Ledieu et al., 2024). That study has demonstrated that 0.6% of the total amount of waste generated in public places are littered, corresponding to 13 t/yr. Nevertheless, accumulation rates of litter have high spatio-temporal
variations. In addition, some items may have long residence times on urban surfaces especially when they accumulate in green spaces. A better understanding of transfer pathways, transport and potential retention factors for litter in urban systems is therefore needed to better constrain global models and develop new mitigation strategies (Tasseron et al. 2020; Meijer et al. 2021; Tramoy et al. 2022b; Roebroek et al. 2022; Sherow et al. 2023).

The present study aims to (i) characterizing litter composition preferentially transported by urban stormwater, (ii) improving the understanding of litter transport dynamics in stormwater, (iii) quantifying the contribution of stormwater to litter fluxes entering the waterways and (iv) exploring the relationship between litter fluxes and hydrometeorological parameters. Two urban catchments located in Nantes (France) were chosen for this research because their common stormwater outlet is designed as a retention basin allowing the collection of the transported litter. Moreover, litter amount and composition were evaluated on the urban surfaces of these catchments (see Ledieu et al., 2024) and used to compare the typologies and fluxes quantified at the stormwater outlet.

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## 2. Materials and methods

### 2.1. Experimental sites

The survey was carried out at two nested urban catchments with gentle slope, located at the north-east of Nantes Metropole (France): Pin Sec (PS) and Jules Verne (JV, Figure 1a). These catchments have a total surface area of 105 ha and include 5,048 inhabitants. Land use is diverse and representative of the general land use of Nantes Metropole. The Pin Sec catchment is mainly residential with individual and collective housing, and the Jules Verne catchment is mainly commercial. The major part of their surfaces is therefore impervious (76 % of impervious surfaces according to a reconstruction of the land use using the buildings from the BD-Topo®, roadways provided by Nantes Metropole Digital Resources Department and the green spaces map from the campaign Fluxsap 2010 (Mestayer et al. 2011)).
<figure><img src="image_3.png" /><figcaption>**Fig. 1** A) Location of the studied urban catchments at Nantes agglomeration in France. B) Locations of the stormwater inlets of the Pin Sec (outlined in dark green) and Jules Verne (outlined in dark red) catchments represented according to their morphologies (both parameters reported visually using Google Satellite and Google Street View). C) Drainage systems (stormwater and sewage networks provided by Nantes Métropole) and inlets selected for capture/recapture experiments in both catchments. D) Photos of the different types of inlets in the Pin Sec and Jules Verne catchments. Their respective percentages are also indicated</figcaption></figure>

Stormwater is collected by several inlets with various morphologies (Figure 1b). Most of them are equipped with structures to limit the transfer of litter to the sewer network i.e. grates (70%) and underflow baffles (22%; Figure 1b). Photos of each type of inlets are given in the Figure 1d. Stormwater
114 is then transferred through a separate sewer network (total length of 4 km, Rodriguez et al., 2020)
115 towards a common outlet at the south-east of the catchments (Figures 1b and c). This outlet discharges
116 into the perennial Gohards stream. Mainly fed by stormwater runoff, this stream was closed-off in 1945
117 and re-opened in 2012 following a renovation urban planning (Rodriguez et al. 2020). The stormwater
118 outlet was designed in a city park, as a retention basin with an underflow baffle limiting the transfer of
119 litter (Figure S1) to the Gohards stream. Some misconnections between the stormwater and sewage
120 networks exist. The latest conformity survey estimated around 5% of housings misconnected in the Pin
121 Sec catchment, i.e. with wastewater releases in the stormwater network (communication with Nantes
122 Métropole).

123 The investigated area is under an oceanic climate with a mean annual rain amount of 820 mm/yr (1991-
124 2020, Meteo-France station Nantes-Bouguenais; https://www.data.gouv.fr/fr/datasets/r/6734e889-
125 538e-4feb-9e57-d999a1897433, accessed on 17 January 2024). During the five years preceding this
126 study (2016-2020), the mean discharge in the upstream part of the Gohards stream was 27 L/s (0.4-1,691
127 L/s, min-max). As commonly observed for small urban catchments, the hydrological response is fast
128 (Figure S2; Rodriguez et al., 2020). Overflow of the retention basin is rare, but during this study, two
129 were observed after extreme rain events, in April and August 2022 (Table S1).

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131 2.2. Litter collection and characterization

132 After an initial cleaning of the retention basin (C0), 18 campaigns (labelled from C1 to C18) of litter
133 collection were carried out between October 2021 and October 2022. Mainly depending on the rain
134 events, integrated periods varied from 5 to 35 days (Table S1) for a total investigation of 335 days. For
135 each campaign, all items above 1 cm were collected on the banks of the retention basin, and in the basin
136 with dip nets.

137 Items were then dried at ambient air and characterized according to the J-list from the MSFD TG-ML
138 classification (Fleet et al., 2021; Table S2). This list classifies items according to their material category
139 (e.g. plastic, metal, paper, ...), type (e.g. bags, bottles, caps, fragments...) and economic sector (e.g.
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smoking, food-consumption, medical related items, ...). All items were counted and each category of items was weighted. In addition, for smoking and food-consumption related items collected from C1 to C16, brands were recorded when they could be identified.

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## 2.3. Capture/recapture experiments

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For a better understanding of litter transport dynamics in the stormwater network and to evaluate the representativeness of litter amounts collected in the basin, two capture/recapture experiments were carried out (a first in April 2022 and a second in November 2022) both with 170 items. Briefly, items collected in the retention basin were tagged with lightweight numbered seals and released in selected stormwater inlets of the catchments. In the Pin Sec and Jules Verne catchments respectively, 8 (labelled from PS1 to PS8) and 9 (labelled from JV1 to JV9) stormwater inlets were selected to represent different morphologies and different distances from the outlet (from 0.1 to 1.3 km of network length, Figure 1c; Table S3). During each experiment, 10 tagged items were placed in each inlet, for a total of 170 items. Then, tagged items have been searched for recovery in the retention basin. If their recovery occurred during a litter collection campaign, these were not included in the litter counts. In case of the retention basin overflows, the downstream part of the Gohards stream was explored. In addition, some parts of the stormwater network (e.g. between JV6 and the retention basin, Figure 1c) were explored by camera to identify litter accumulation zones and/or potential clogging.

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To test the impact of item morphologies in their transport capacity, experiments were carried out with two types of items. The first experiment was led on 4 April 2022 with bottles (55%) and cans (45%) with various capacity (0.15 to 1.5 L). Every item could actually fit through the openings of the different types of inlets except for grated inlets. Bottles were tested with caps (48%), and without (51%, Table S4). The second experiment was led on 2 November 2022 with sweet wrappers of various types, compositions and capacities but with sizes generally lower than 10 cm. These items were placed in the same 17 stormwater inlets than the for the first experiment.

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166 2.4. Litter fluxes in stormwater

167 For each campaign, the mass of items and plastic items collected was extrapolated to kg/yr. This was
168 carried out by dividing the collected mass by the number of days integrated, then extrapolated to 365
169 days. This extrapolation was also carried out for the whole period of investigation using the total mass
170 of items collected and the total number of days integrated.

171 For a greater comparability between studies, litter fluxes are often normalized to the number of
172 inhabitants (expressed in g/inhabitant/yr). This enables the fluxes to be compared to the total waste
173 production per inhabitant in Nantes Metropole (i.e. 397 kg/inhabitant/yr considering glass, household
174 and recyclable waste collected door-to-door by municipal services and at the waste collection sites,
175 Nantes Métropole 2022). Litter fluxes are also normalized to the total surface of the catchment area or
176 to the impervious surfaces (expressed in kg/ha/yr) as Treilles et al. (2021) did, considering only
177 impervious surfaces contribute to stormwater runoff.

178 The calculated fluxes may include inputs of items from other sources than stormwater (direct inputs
179 and/or wastewater releases). Such inputs are suspected using the results of characterization but their
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181 In addition, the calculated annual fluxes were compared to the annual fluxes of litter estimated on the
182 urban surfaces of these catchments (i.e. 13 t/yr of litter; Ledieu et al., 2024) to evaluate the proportion
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185 2.5. Hydrometeorological data collection and treatment

186 Rainfall and stormwater discharges are monitored on these catchments since 2006 as part of the Nantes
187 Urban Environment Observatory (ONEVU) and the National Observation Service (SNO) Observil.
188 Rainfall is recorded at a 1 min frequency at 3 rain gauges (3039, Précis Mécanique®) covering the area.
189 Their average values are used for interpretation. Flow discharges at the stormwater outlets from the Pin
190 Sec and Jules Verne catchments are also recorded separately at a 2 min frequency with a Doppler flow
191 meter (2150 Area Velocity Module, Teledyne Isco®).
A rain event selection program was used to extract several hydrometeorological parameters for each event: duration of rainy and dry periods, maximum rainfall intensity, maximum flow discharge, and runoff volume (without baseflow). Rain events were considered when the total rain amount exceeds 2 mm with a minimum intensity of 1.5 mm/h, and time intervals between two events were 2 h (Berthier 1999). The flood event is considered ended when the runoff volume drained after the rain event is lower than 1/500 of the total runoff volume drained during the rain event, on a 2 min timescale. The baseflow is considered as the flow discharge at the beginning of the rain event and the runoff volume is calculated according to the flow discharge above the baseflow. Total runoff volumes were calculated for each campaign and used to express the litter concentrations in mg/m³.

Correlations between the hydrometeorological parameters and amount of items collected (by count and by mass) or item typologies were evaluated using the Spearman correlation coefficient R (Spearman 1904) calculated with the package "ggplot2" (Wickham 2016) in RStudio version 2022.7.1.554 (RStudio Team, 2022). These correlations were considered as representative and significant if R is equal to or greater than 0.5 and p-values equal to or lower than 0.05.

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## 3. Results

### 3.1. Litter amount, composition and sources

In total, 13,163 items were collected corresponding to 26.74 kg of litter accumulated during a period of 335 days. Per campaign, the number of collected items ranged between 190 (C6) and 2,042 items (C14) and the collected mass between 0.54 (C15) and 3.04 kg (C17, Figure 2a, Table S1).

The collected items belonged to all categories of materials (i.e. plastic, rubber, paper/cardboard, metal, glass, manufactured wood and textile). By count, plastics were always and largely predominant, representing 92% of the total items. Paper/cardboard ranked second (3%) and metal third (2%). By mass, plastics only represented 48% of the total items, glass represented 36% and metal 6% (Figure 2b).

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<figure><img src="image_4.png" /><figcaption>B) Categories of materials by count and by mass</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>C) Top 10 (by count)</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 2 A) Number (blue triangles) and mass (red triangles) of items collected according to the periods. Seasons are represented.
B) Percentages of the different categories of materials (by count and by mass) sampled during the 18 campaigns (C1 to C18) carried out for litter characterization and during the total period of sampling (Tot.). C) The top 10 type of items by count according to the J-list (Fleet et al., 2021). Percentages of each type of items are indicated</figcaption></figure>

The collected items belonged to 98 categories over the 201 categories listed in the J-list (Table S2). The 10 most abundant type of items represent 80% of the total items by count, and were all plastics. Cigarette butts were the most abundant (J27, 38%), fragments of foamed polystyrene (J81, < 2.5 cm) ranked second (9%) and sweet wrappers ranked third (J30, 8%). The following categories were mostly fragments and containers (Figure 2c). The litter composition was however highly variable according to the campaigns. Only 8 categories were always found in samples: cigarette butts with filters (J27),
fragments of foamed polystyrene (< 2.5 cm, J81), plastic crisps packets/sweets wrappers (J30),
fragments of non-foamed plastic (0 – 50 cm, J79 and J78), plastic tobacco pouches/plastic cigarette
packet packaging (J25), plastic lolly and ice-cream sticks (J31) and metal drink cans (J175, Table S5).

Some categories of items may result from direct inputs or wastewater contributions. According to our
on-site observations, syringes (J99, n = 49, M = 98 g), medical containers (J100, n = 233, M = 314 g),
condoms (J133, n = 21, M = 27 g) and glass bottles (J200, n = 114, M = 9 434 g) may be attributed to
specific activities within the park around the retention basin. Moreover, some typical wastewater items
like sanitary towels (J96, n = 35, M = 219 g), tampons and applicators (J144, n = 3, M = 6 g), toilet
fresheners (J97, n = 2, M = 23 g) and wipes (J237, n = 54, M = 236 g) were observed during some
campaigns (Table S5). These items may be attributed to the disconnected houses in the catchments
considering that these were virtually not observed on the urban surfaces of the catchments (see Ledieu
et al., 2024).

The economic sector was identified for 63% of the items by count, belonging to 7 different economic sectors from the J-list. 'Smoking-related' (26% by count) and 'food-consumption related items' (18% by count) were predominant. 'Medical' and 'personal, hygiene and care related items' were less present, representing median percentages of 3% and 1% by count, respectively. Items from 'recreation', 'building and construction activities' as well as 'clothing' were also collected but in low proportions (< 1% by count, Figure 3a).

Among the smoking related items, the brand of 312 items (7% of the total) could be identified and 35 different tobacco brands were recorded. The top 10 brands however represents 87%. Marlboro was predominantly identified, followed by Winston and Camel. The Philip Morris International group therefore lead the top 3 industrial groups in this category (Figure 3b and c).

Among food-consumption related items, the brand of 364 items (22% of the total) could be identified
and 161 different brands were recorded. The top 10 brands represents 47% with Kinder as leader,
followed by Cristaline (water bottle) and Mars. The top 3 industrial groups followed this podium (Figure
3b and c).
A Economic sectors (n = 13,163)

<figure><img src="image_7.png" /></figure>

B Top 3 groups

<figure><img src="image_8.png" /></figure>

Smoking related items
(n = 312)

Food-consumption related items
(n = 364)

<figure><img src="image_9.png" /></figure>

C Top 10 brands

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 3** A) Boxplots representing the variability (first quartile, third quartile and median; vertical bars: 1.5 times the interquartile range, red crosses: mean percentages) of item percentages by count according to their economic sectors. B) Top 3 industrial groups and C) top 10 brands recorded for smoking related and food-consumption related items from C1 to C16. The number of items for which the brands were identified is indicated. Brands were colored according to the podium of the Top 3 groups.</figcaption></figure>

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## 3.2. Transport dynamic of the tagged items

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Among the 340 items used for the capture/recapture experiments, 157 (46%) were recaptured in the basin, 33% of the released bottles/cans and 58% of the released sweet wrappers (Figure 4a and b, Table S3). For bottles/cans, 2 items (1%) were recaptured at the outlet after the first rain event, illustrating a rapid, but low transport by stormwater. The highest number (32 items, 19%) was recaptured 21 days following their release, mostly resulting from an intense rain event (Figure 4a). For sweet wrappers, the highest number (59 items, 35%) was recaptured at the outlet after the first rain event following their release (Figure 4b), illustrating an easier transport of these small items than bottles/cans.

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Two months after their release, the transport dynamic of bottles/cans became random with 1 item recaptured from time to time regardless of the rain amount (Figure 4a). Three months after their release, a similar dynamic was observed for sweet wrappers (Figure 4b). The last bottle/can and sweet wrapper were recaptured more than seven (231 days) and nine months after their release (279 days), respectively (Figure 4a and b).

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In addition, more bottles were recaptured (36%) than cans (28%), and most of the recaptured bottles were those with caps (71% of the recaptured bottles, Table S4).
<figure><img src="image_12.png" /><figcaption>(A) Bottles/cans</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>(B) Sweet wrappers</figcaption></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

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<caption>Fig. 4 Number of tagged items recaptured at each campaign (black diamonds) and cumulative percentages (black line) according to the daily rain amount for A) bottles/cans (n=170) and B) sweet wrappers (n=170). Maps indicate the number of items recaptured per stormwater inlet according to its morphology (see the corresponding colors in Figure 1b)</caption>

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The number of recaptured items highly varied according to stormwater inlets. For 3 inlets (JV3, JV5 and JV7, Figure 1c), neither bottles/cans, nor sweet wrappers were recaptured. Two of them were underflow baffle inlets. During the first experiment, no inlet exhibited a 100% recovery rate of the bottles/cans.

For 4 inlets (PS1, PS3, PS5 and JV4, Figure 1c), the number of recaptured items was similar during both experiments. For all other inlets, the number of recaptured items was highest for sweet wrappers.

Besides, the recovery rate of sweet wrappers reached 100% for 2 opening curb inlets (JV2 and PS4, Figure 1c). No trend according to the distance between the stormwater inlets and the outlet was observed (Table S3).

### 3.3. Litter fluxes in stormwater
Given the litter amount collected during the investigated period, 29.1 kg/yr of litter are in total released in the Gohards stream, representing around 6 g/inhabitant/yr and 0.4 kg/imp.ha/yr. Plastic litter fluxes represent 14.1 kg/yr, or around 3 g/inhabitant/yr and 0.2 kg/imp.ha/yr (Table S6).

Among these litter fluxes, considering the masses of items that can be attributed to specific activities within the park around the retention basin (section 3.1.), 10.7 kg/yr may be attributed to direct inputs (37% of total litter fluxes) in the basin. Considering the masses of the typical wastewater items (section 3.1.), 0.6 kg/yr may be attributed to wastewater releases from the misconnected houses (2% of total litter fluxes). However, such contributions include few plastic items, i.e. 0.4 kg/yr from direct inputs and 0.5 kg/yr from wastewater releases (3% and 4% of total plastic litter fluxes, respectively, Table S6). At least 61% of the litter fluxes and 93% of the plastic litter fluxes may therefore be attributed to stormwater runoff.

### 3.4. Relationship with hydrometeorological data

Campaigns integrated various hydrometeorological conditions. Excluding C9 for which the rain amount was not sufficient to consider a rain event generating runoff according to the selection program, campaigns integrate between 1 and 7 rain events and dry periods ranging from 3 to 36 days. The total rain amounts ranged from 3 to 108.6 mm with rainfall intensities varying between 2.4 to 48.6 mm/h. For the campaigns during which flow discharges were accurately recorded (C4, C6, C8, C10 to C13, C15 and C16), the maximum flow discharges generated at the outlet ranged from 188 to 3,008 L/s and runoff volumes ranged from 1,178 to 11,416 m³ per event. Reported to these volumes, litter and plastic concentrations ranged from 168 to 1,181 mg/m³ and from 47 to 681 mg/m³, respectively (median values equal to 250 mg/m³ and 182 mg/m³, respectively; Tables 1 and S7).

Excluding C9, the collected masses of items were significantly correlated with the antecedent dry period and with rain amount (Figure 5a and b). The number of transported items was positively correlated to the maximum rainfall intensity (Figure 5c). A seasonality appears with increasing amount of litter (by
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count and by mass) collected at the stormwater outlet in summer (Figure 2a) as a result of increasing
dry periods and more intense rain events due to thunderstorms.

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<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

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<caption>Fig. 5 Mass of items collected during each campaign (C1 to 18) according to A) the antecedent dry period and B) the rain amount. C) Number of items collected during each campaign (C1 to 18) according to the rain event maximum intensity (median value in case of several rain events). Campaigns for which an overflow of the retention basin was observed are represented with red circles. For each correlation, Spearman correlation coefficients and associated p-values are indicated</caption>

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# 4. Discussion

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## 4.1. Stormwater as a transfer pathway for urban litter

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Item typologies and brands identified at the stormwater outlet were consistent with the activities of the Pin Sec and Jules Verne catchments, i.e. transport facilities, public places, bar/tobacco, fast-food restaurants, stores and a hospital. The predominance of smoking and food-consumption related items, the top 3 industrial groups of smoking related items as well as the most frequent brand of the food-consumption related items were similar on the urban surfaces of these catchments (Ledieu et al. 2024). Urban litter is therefore a major source of litter in waterways. In fact, the most littered items are broadly
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similar on urban surfaces (Martinez and Griffiths 2023; Ledieu et al. 2024) leading to a general predominance of smoking and food-consumption related items in stormwater and receiving urban streams in France (Tramoy et al., 2022a, b), in the Netherlands (Tasseron et al. 2020), in New Zealand (Martinez and Griffiths 2023), in Australia (Sherow et al. 2023), or in South Africa (Weideman et al. 2020).

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<table><caption>Table 1: Annual total litter and plastic litter fluxes normalized per inhabitants (in g/inhabitant/yr), per drained surfaces (in kg/ha/yr) or per impervious surfaces (in kg/imp.ha/yr), and concentrations (in mg/m³), in catchments from different countries according to their sewer systems (SW = stormwater, C = combined) and land uses (Ind. = industrial, Com. = commercial, Res. = residential).</caption><thead><tr><th></th><th></th><th></th><th>Per inhabitant<br>(g/<br>inhabitant<br>/yr)</th><th>Per<br>surface<br>(kg/ha<br>/yr)</th><th>Per<br>impervious<br>surface<br>(kg/imp.ha/yr)</th><th>Per<br>runoff<br>volumes<br>(mg/m³)</th></tr></thead><tbody><tr><td colspan="7" style="text-align: center; font-weight: bold;">Total litter fluxes</td></tr><tr><td rowspan="3">Cape Town<br>(South Africa)<sup>1</sup></td><td>Ind.</td><td></td><td>137.6</td><td></td><td></td><td></td></tr><tr><td>Com. &<br>Res.</td><td></td><td>42.0</td><td></td><td></td><td></td></tr><tr><td>Res.</td><td></td><td>0.9</td><td></td><td></td><td></td></tr><tr><td>Northland (New<br>Zealand)<sup>2</sup></td><td>Mixed</td><td></td><td>1.8</td><td></td><td></td><td></td></tr><tr><td>Sucy-en-Brie<br>(France)<sup>3</sup></td><td>SW<br>Res.</td><td></td><td></td><td></td><td>28-182</td><td></td></tr><tr><td>Goussainville<br>(France)<sup>4</sup></td><td>Mixed</td><td>10-20</td><td>1</td><td></td><td></td><td></td></tr><tr><td>Malaunay<br>(France)<sup>4</sup></td><td>Com.</td><td>2</td><td>0.1</td><td></td><td></td><td></td></tr><tr><td style="background-color: #d3d3d3;">This study</td><td>Com. & Res.</td><td style="background-color: #d3d3d3;">6</td><td style="background-color: #d3d3d3;">0.3</td><td style="background-color: #d3d3d3;">0.4</td><td style="background-color: #d3d3d3;">158-<br>1,181</td><td></td></tr><tr><td>Grenoble (France)<sup>4</sup></td><td>C<br>Res.</td><td>0.4</td><td>0.03</td><td></td><td>25</td><td></td></tr><tr><td colspan="7" style="text-align: center; font-weight: bold;">Plastic litter fluxes</td></tr><tr><td rowspan="3">Cape Town<br>(South Africa)<sup>1</sup></td><td>Ind.</td><td></td><td>71.1</td><td></td><td></td><td></td></tr><tr><td>Com. &<br>Res.</td><td></td><td>21.6</td><td></td><td></td><td></td></tr><tr><td>Res.</td><td></td><td>0.5</td><td></td><td></td><td></td></tr><tr><td>Northland (New<br>Zealand)<sup>2</sup></td><td>Mixed</td><td></td><td>1.1</td><td></td><td></td><td></td></tr><tr><td>Sucy-en-Brie<br>(France)<sup>3</sup></td><td>SW<br>Res.</td><td>19</td><td></td><td>1.7</td><td>7-134</td><td></td></tr><tr><td>Goussainville<br>(France)<sup>4</sup></td><td>Mixed</td><td>5-10</td><td>0.5</td><td></td><td></td><td></td></tr><tr><td>Malaunay<br>(France)<sup>4</sup></td><td>Com.</td><td>2</td><td>0.1</td><td></td><td></td><td></td></tr><tr><td style="background-color: #d3d3d3;">This study</td><td>Com. & Res.</td><td style="background-color: #d3d3d3;">3</td><td style="background-color: #d3d3d3;">0.1</td><td style="background-color: #d3d3d3;">0.2</td><td style="background-color: #d3d3d3;">47-681</td><td></td></tr><tr><td>Grenoble (France)<sup>4</sup></td><td>C<br>Res.</td><td>0.1</td><td>0.006</td><td></td><td>5</td><td></td></tr></tbody></table>

## 4.2. Transport dynamic of litter through stormwater networks

Even if urban areas concentrate plastic litter, this category of materials is even more predominant by count in urban stormwater (Martinez and Griffiths 2023; Sherow et al. 2023), like in receiving streams

<footnote n="1">¹Weideman et al., 2020; ²Martinez and Griffiths, 2023; ³Treilles et al., 2021; ⁴Tramoy et al., 2022b</footnote>
369 (Weideman et al. 2020; Tasseron et al. 2020; Treilles et al. 2021; Tramoy et al. 2022a) and in rivers
370 (Crosti et al. 2018; González-Fernández et al. 2021; Cesarini et al. 2023). Most authors suggest a better
371 transport of these non-degradable, lightweight and buoyant items compared to paper, glass or metal
372 (Pasternak et al. 2021; Hadiuzzaman et al. 2022; Tramoy et al. 2022b; Cesarini et al. 2023). In this study,
373 the proportions of plastic by count increased from 62% on urban surfaces (Ledieu et al. 2024) to 92% at
374 the stormwater outlet, where the top 10 items only includes plastics (Figure 2c). The 5 types of paper
375 items frequently observed on the urban surfaces of the Pin Sec and Jules Verne catchments (see Ledieu
376 et al., 2024) are not transported, probably because of their degradation (Crosti et al. 2018; Pasternak et
377 al. 2021), except in case of intense rain events. The recurrence of foamed polystyrene fragments and
378 plastic tobacco pouches at the stormwater outlet (Figure 2c) while these were not commonly observed
379 on urban surfaces (Ledieu et al. 2024) also highlight the promoted transport of lightweight and buoyant
380 items. Moreover, capture/recapture experiments and the predominance of items < 2.5 cm in the top 10
381 (Figure 2c) confirmed an easier transport of small items (Pasternak et al., 2021). Cesarini et al. (2023)
382 suggested that the break-up processes of litter start before items reach the sea. In this study, the
383 increasing proportions of unidentified plastic fragments from the urban surfaces (9%, Ledieu et al.,
384 2024) to the stormwater outlet (24%, Table S5) even demonstrates that these processes start on urban
385 surfaces, probably due to the road traffic or other factors e.g. the cutting of grass (Ledieu et al., 2024).
386 As a result, their transport by stormwater is enhanced. Item composition and sizes play a key role in
387 their transport dynamic and litter transfer through stormwater networks is thus selective.

388 Litter fluxes transported by stormwater exhibit a seasonality in several studies (Treilles et al. 2021;
389 Tramoy et al. 2022a; Martinez and Griffiths 2023). In the case of the Pin Sec and Jules Verne
390 catchments, litter fluxes tend to increase in summer (Figures 2 and 6) when rain events are more intense
391 considering that high intensity events encourage the transport of a greater number of items (Figure 5c,
392 Tramoy et al., 2022a; Treilles et al., 2021). Hydrological parameters are however not sufficient to
393 explain this variability (Tasseron et al. 2020; Roebroek et al. 2022). As cleaning services do not recover
394 all the items, litter accumulate on urban surfaces, particularly in summer (Figure 6) when outdoor
395 activities are more intense and promote litter generation (Ledieu et al. 2024). Thus, the greater the time
interval between two rain events, the greater the quantity of litter exported (Kim et al. 2006) as demonstrated by the increasing mass of items collected at the stormwater outlet with increasing antecedent dry periods (Figure 5a). In contrast, studies monitoring stormwater inlets observe the highest litter fluxes during wet periods with more rain events (Malik et al. 2021; Martinez and Griffiths 2023). In fact, the 46% recovery rate of the capture/recapture experiments suggests that even if stormwater removes items from urban surfaces, these were not necessarily transferred towards the outlet. The not recaptured items could (i) have been recovered during a cleaning of the stormwater inlets, (ii) be stuck in the sewer network, (iii) be transferred downstream the retention basin, or (iv) be accumulated at the bottom of the basin. After overflows of the retention basin, a small percentage (3%) of tagged items was recovered in the Gohards stream but no item was found far downstream. During dry weather, 5% of tagged items (mostly cans) were identified at the bottom of the pond and recaptured. No item was observed during the video exploration of some parts of the stormwater network (Figure S3). In fact, some bottles/cans placed during the first experiment were still in some underflow baffle inlets when the sweet wrappers were placed inside 7 months after. Litter may thus be blocked by structures (e.g. selective inlets, Treilles et al., 2021) and/or create clogging (Basilico et al. 2020) leading to an intermittent transport dynamic in urban stormwater networks (Figure 4a and b). Like observed in rivers (Tramoy et al. 2020; Ledieu et al. 2022), items can have longer residence times in stormwater networks than those considered in global models (Treilles et al. 2021).

414
<figure><img src="image_19.png" /><figcaption>Fig. 6 Number of items collected (blue triangles) during each campaign according to the periods. Litter densities quantified on the urban surfaces of the catchments (grey cubes) for the similar period (from Ledieu et al., 2024), rain amounts and periods of school holidays (grey stripes) are represented</figcaption></figure>

4.3. Litter fluxes through urban stormwater networks

The compagnon paper consisting in the assessment of litter on the urban surfaces of the present study area demonstrated that the total proportion of littered items only represented 0.6% of the total waste production of the Pin Sec and Jules Verne catchments (Ledieu et al., 2024, Figure 7). As highlighted by the impact of public place cleaning frequencies in Chicago (USA, Vincent and Hoellein, 2017) and in several French cities (Goussainville, Malaunay and Grenoble, Tramoy et al., 2022b), a major part of the litter stock on urban surfaces is removed by the cleaning services. Remaining items are mostly accumulated in green spaces (Ledieu et al. 2024) where their removal is limited (Gholami et al. 2020; Mellink et al. 2022). As a result, the estimated litter fluxes entering the stormwater inlets of the Pin Sec and Jules Verne catchments only represent 0.3% of the mass estimated littered on the urban surfaces. In comparison to the total waste production per inhabitant at Nantes Métropole, litter fluxes measured at the stormwater outlet represent 0.001%. This leak is higher for plastic litter considering their promoted transport by stormwater. The 0.7% of plastic leakages rates quantified in this study (Figure 7) are however far lower than the 15-40% considered by Jambeck et al. (2015) to estimate the riverine plastic
export into the sea. In Sucy-en-Brie (France), Treilles et al. (2021) actually quantified a plastic litter fluxes in stormwater corresponding to less than 0.03% of the amount consumed per inhabitant. The buffering effect of the stormwater network and the presence of selective inlets further reduce the litter fluxes that manage to reach the stormwater outlet. In this study, litter fluxes quantified at the stormwater outlet represent only half (46%) of the fluxes reaching the stormwater inlets as ¾ of them are equipped with grates and underflow baffles (Figure 1b). Nevertheless, more regular cleanings of stormwater inlets could further reduce litter fluxes as suggested by the long residence time (up to 7 months) exhibited by some bottles/cans. Moreover, it clearly appears through the results of this study that the most upstream actions (cleaning services) reduce more efficiently litter emission towards receiving waterways than when they reach the stormwater network. This low leakage rate also suggests that further reducing the plastic litter in waterways would mainly involve a reduction in their consumption.

445

<figure><img src="image_20.png" /></figure>

446

Fig. 7 Conceptual scheme of total litter and plastic litter fluxes (in blue) on the urban surfaces, at the stormwater inlet and the stormwater outlet of the Pin Sec and Jules Verne catchments. *Fluxes expressed at the stormwater inlet are calculated assuming that stormwater releases identified at the outlet represent 46% of the fluxes reaching the inlets. **Fluxes expressed at the stormwater outlet include the fluxes suspected from direct inputs and wastewater releases. The percentages represented by these leaks are expressed on the left in relation to the waste production per inhabitants (Nantes Métropole, 2022), and on the right in relation to the masses of littered items in public places (taken from Ledieu et al., 2024 – Litter in French urban areas - Part 1).

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455

456

## 4.4. Recommendations for the assessment of stormwater contribution to the litter pollution in hydrosystems

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462

463

Even if the litter fluxes transported by stormwater are relatively low compared to the waste production and littering, these represent high litter loads transferred towards receiving waterways when considering population of the catchments. In the urban litter studies, fluxes are expressed as a function of the number of inhabitants or the impervious surface area of the catchment (in ha, Table 1). Using the number of inhabitants takes greater account of the generation of litter, compared with the overall production of waste per inhabitant. In contrast, using the impervious surface area takes a hydrological approach, taking greater account of the transfer pathway, runoff.

464

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480

At a national scale, extrapolated to the national number of inhabitants in France (i.e. 65.8 million in 2023; https://www.insee.fr/fr/statistiques/5225246), litter fluxes calculated at the stormwater outlet in this study (Table 1) would represent an export of around 395 t/yr of litter, whose 197 t/yr of plastics. It corresponds to the French annual input of plastics released from land estimated by González-Fernández et al. (2021) but the extrapolation using the number of inhabitants is subject to certain biases. In fact, litter is mainly the result of on-the-go consumptions and passersby contribute greater to the litter amount than inhabitants (Kawecki and Nowack 2019; Ledieu et al. 2024). Moreover, littering trends cannot be easily generalized. For example, Weideman et al. (2020) observed higher litter fluxes at the stormwater outlets draining industrial and commercial areas than the one draining residential areas (Table 1), consistently with the observations from urban litter studies (Keep Britain Tidy 2020; Gholami et al. 2020; Pietz et al. 2021; Youngblood et al. 2022; Xiong et al. 2022) including in the Pin Sec and Jules Verne catchments (Ledieu et al. 2024). Nevertheless, in Australia, Sherow et al. (2023) observed lower litter fluxes at stormwater outlets from industrial areas than from city centres, shopping areas and transport hubs, and in New Zealand, Martinez and Griffiths (2023) observed higher litter fluxes from hospital grounds, fast foods and retail than from hotel/motel, residential and commercial areas. To date, there are few studies on the variation in litter fluxes between sites, and discrepancies in land use definitions make interpretation of the results difficult. In addition, including information on litter
481 generation and waste management systems (waste collection and cleaning activities) in the litter studies
482 is essential to include such variables in the extrapolations.

483 Extrapolations using urban surface areas are also biased since only impervious surfaces contribute to
484 stormwater runoff. By only considering impervious surfaces, Treilles et al. (2021) actually quantified
485 higher fluxes in Sucy-en-Brie than in other French cities (Table 1). Considering the impervious surfaces
486 in mainland France (i.e. 3,300,000 ha, Loisier and Petel, 2019), litter fluxes calculated at the stormwater
487 outlet in this study (Table 1) would represent 1,320 t/yr, of which 660 t/yr are plastics, released in French
488 hydrosystems. This is three times higher than the extrapolations using the number of inhabitants. An
489 approach using the impervious surface area appears more consistent with the concept of litter production
490 and transport. Nevertheless, the question of whether the total impervious surfaces should be considered
491 or only roadways and sidewalks (where there is potentially litter) remains open. In this study, buildings
492 were taken into account, resulting in impervious surfaces representing 76% of the catchment area,
493 compared with 27% in the study of Treilles et al. (2021). In addition, not all impervious surfaces in
494 urban areas are connected to separate sewers and little information is available on the potential additional
495 contribution of combined sewer overflows. On the scale of Greater Paris (183,000 ha, France), Treilles
496 et al. (2021) demonstrated that the impervious surfaces connected to separate sewers represented only
497 10% of the surface area (i.e. 19,000 ha), which considerably reduces the potential inputs of litter by
498 stormwater. Moreover, stormwater networks may be buried or open, which modifies the potential of
499 direct inputs of items. Finally, such extrapolations do not take spatial variations of hydrometeorological
500 parameters into account. Expressing litter concentrations as a function of runoff volumes (in mg/m³)
501 may be a good way to include these variables.

502 It is thus difficult to extrapolate the contribution of stormwater to plastic litter pollution in waterways.
503 Litter fluxes transported by stormwater depend as much on litter generation on urban surfaces as on
504 runoff and potential obstructions. Detailed information on these parameters in further studies would
505 provide a better understanding of the impact of these factors in order to identify ways of combining them
506 to refine extrapolations. Finally, considering that the selective and intermittent transport dynamic of
507 litter in urban areas appears regulated by the same rules as in natural environments (e.g. the presence of
508 vegetation, an enhanced transport of lightweight plastic items and a transport promoted during intense
509 rain events), modeling litter transport through stormwater networks may be addressed by hydrological
510 or erosion models.

511

512 **Conclusions**

513 By concentrating small and lightweight plastic items, urban areas create a considerable hotspot of items
514 that can be transported by stormwater. Litter at stormwater outlets therefore reflect the global urban
515 plastic litter composition, i.e. mainly smoking and food consumption related items. In this study, the
516 plastic litter fluxes represent around 6 g/inhabitant/yr and 0.4 kg/imp.ha/yr. Even if these fluxes
517 normally remain in the retention basin, results of this study confirm that stormwater runoff is a
518 significant transfer pathway for plastic litter making their outlets considerable point sources of litter
519 towards receiving streams.

520 Litter fluxes can be highly variable depending on: (i) the stock of litter lying on urban surfaces, (ii) the
521 hydrometeorological variables, (iii) the item transport capacity, and (iv) the presence of stormwater
522 management structures limiting their transfer. These several influencing factors raise the question about
523 which parameter to use for extrapolating the results as considering the litter generation (normalizing per
524 inhabitants) or the transfer pathway (normalizing by the drained area) can lead to high variations in the
525 global estimations of litter fluxes. Litter transport dynamic is therefore not linear and difficult to interpret
526 but a way to integrate all these parameters in global models needs to be found.

527 Finally, this study illustrates the valuable impact of upstream actions to reduce the litter fluxes
528 transported by stormwater. Thanks to cleaning services, stormwater actually remove few proportions of
529 items compared to the amount littered on urban surfaces. Moreover, high litter loads remain accumulated
530 in green spaces. Consequently, in the case of the Pin Sec and Jules Verne urban catchments, only 0.3%
531 of the littered items and 0.7% of the littered plastics appear to reach the stormwater inlet and their
532 structures to limit the transfer of litter towards the sewer network (i.e. grates and underflow baffle)
533 further reduce these fluxes. Such conclusions imply that further fluxes reduction would require a
534 diminution in plastic consumption, in littering as well as a more regular maintenance of stormwater
535 inlets.

536

537 Acknowledgments

538 Authors gratefully thanked the Nantes Urban Environment Observatory (ONEVU-IRSTV) for
539 providing the hydrometeorological data, the Digital Resources department and the Diagnosis of
540 Domestic Installations and Sewerage Networks (DIDRA) department of Nantes Métropole for providing
541 the sewer networks and their video exploration. In addition, authors are grateful to the televised
542 inspection teams, as well as to the road, the cleaning and sewerage teams of the Erdre et Loire division
543 for their collaboration.
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659 areas based on machine learning – A case study of Beijing. Waste Management 142:88-100.
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663

664

665 **Statements and Declarations**

666 **Funding**

667 This work was co-funded by the Région des Pays de la Loire (France) and Nantes Métropole, as part of
668 the Plasti-nium Project, and by Citeo, as part of the Stritter project.

669

670 **Competing Interests**

671 The authors have no relevant financial or non-financial interests to disclose.

672

673 **Author Contributions**

674 Johnny Gasperi, Bruno Tassin, Romain Tramoy, Clémence Bruttin, Eric Bouchet and Lauriane Ledieu
675 contributed to the study conception and design, and to the data analysis. Litter collection and
676 characterization were performed by Lauriane Ledieu, David Mabilais, Sophie Ricordel and Zoé Bridant.
677 Hydrometeorological data collection and curation were performed by Marie-Laure Mosini, Alexandra
678 Mosset, Bernard Flahaut et Laëtitia Pineau. Marie-Laure Mosini, Fabrice Rodriguez and Alexandra
679 Mosset carried out the hydrometeorological data analysis. The first draft of the manuscript was written
680 by Lauriane Ledieu and all authors commented on previous versions of the manuscript. All authors read
681 and approved the final manuscript.

682

683 **Ethical Approval**

684 Not applicable.

685
686 Consent to Participate

687 Not applicable.

688

689 Consent to Publish

690 Not applicable.

691