# Highway runoff and wastewater effluents as drivers of platinum dynamics in an urban river: Insights from a comprehensive monitoring study

Maxime Chastanet, Antoine Lerat-Hardy, Jorg Schafer, Melina Abdou, Thomas Gardes, Antoine Marache, Maxime Debret, Alexandra Coynel

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## To cite this version:

Maxime Chastanet, Antoine Lerat-Hardy, Jorg Schafer, Melina Abdou, Thomas Gardes, et al. Highway runoff and wastewater effluents as drivers of platinum dynamics in an urban river: Insights from a comprehensive monitoring study. Applied Geochemistry, 2025, 193, 10.1016/j.apgeochem.2025.106550. hal-05320362

HAL Id: hal-05320362

https://hal.science/hal-05320362v1

Submitted on 31 Oct 2025

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# Highway runoff and wastewater effluents as drivers of platinum dynamics in an urban river: Insights from a comprehensive monitoring study

Maxime Chastanet<sup>a</sup>, Antoine Lerat-Hardy<sup>a</sup>, Jörg Schäfer<sup>a</sup>, Melina Abdou<sup>a</sup>, Thomas Gardes<sup>a</sup>,
Antoine Marache<sup>b</sup>, Maxime Debret<sup>c</sup>, Alexandra Coynel<sup>a, *, e</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

Editorial handling by: G. Bird

Keywords:

Platinum group element (PGE)

Catalytic converters

Cancerostatic platinum compounds (CPC)

Platinum partitioning

Platinum fluxes

First flush

## ABSTRACT

The contribution of a single rain event to daily and annual platinum (Pt) transport in an urban river (Jalle River, South-West of France), assessed from high resolution and annual monitoring, is discussed in terms of dissolved (Pt<sub>d</sub>) and particulate (Pt<sub>p</sub>) partitioning, sources and fluxes. In normal discharge conditions, the Jalle River Pt<sub>d</sub> and Pt<sub>p</sub> concentrations are respectively 1.90 ± 1.62 ng L<sup>-1</sup> and 19.8 ± 6.9 µg kg<sup>-1</sup> and Pt is mostly transported in the dissolved form (~96 %, 110 ± 123 mgPt<sub>d</sub> d<sup>-1</sup>), due to low sediment yield and urban discharges from a wastewater treatment plant (WWTP). The Pt<sub>d</sub> flux in sewage (36–340 mgPt<sub>d</sub> d<sup>-1</sup>) was mainly attributed to excretions from patient undergoing chemotherapy with Pt-drugs (up to 480,000 mgPt<sub>d</sub> yr<sup>-1</sup> at the watershed scale). After a 2-months dry period, an intense rainstorm discharged ≥112 mg of Pt<sub>p</sub> in 4 h from the urban highway, resulting in additional 295 mgPt<sub>p</sub> d<sup>-1</sup> exceptionally transported by the Jalle River. In contrast to normal conditions, during the rainstorm up to ~95 % of Pt was transported in particulate form during the *first flush*, and 80 % of the highway Pt<sub>p</sub> flux during the first 40 min contributing to ~one third of the annual Pt<sub>p</sub> flux. Short, intense discharge events must be monitored at a high resolution to accurately estimate annual Pt fluxes in urban rivers.

## 1. Introduction

The study of emerging contaminants in urban water systems has gained increasing attention in recent years, particularly due to their potential environmental and health impacts. Among these contaminants, platinum group elements (PGE), classified as technology critical elements (TCE), stand out as metals of growing concern (Cobelo-García et al., 2015). Recently, emerging platinum (Pt) contaminations have been reported in surface waters in the North Sea originating from urbanized rivers and estuaries, while specific Pt sources need to be ascertained (Hollister et al., 2024). Platinum is particularly requested for industrial, chemical and medical applications due to its catalytic and cytotoxic properties (Ravindra et al., 2004). Since the 1990s, the widespread use of Pt in catalytic converters to reduce vehicles exhaust emissions has led to the release of Pt particles into the environment, a result of wear and tear on aging converters and harsh operating conditions (e.g. Artelt et al., 1999; Helmers, 1997; Moldovan et al., 2002; Schäfer and Puchelt, 1998). First reported in 1986 in road dust (Hodge

and Stallard, 1986) and subsequently across the world, Pt concentrations have been found to exceed natural and background levels by hundreds of times (e.g. Chellam and Bozlaker, 2015; Kylander et al., 2003; Monteiro et al., 2020; Mukai et al., 1990; Sager et al., 2014; Schäfer et al., 1999). By now, catalytic converters are recognized as the primary anthropogenic source of Pt in the environment, with particles being transported via rainfall runoff and wind erosion (e.g. Mitra and Sen, 2017; Rauch and Peucker-Ehrenbrink, 2015). Accordingly, traffic-related Pt can be washed into drainage systems and ultimately reach urban rivers, either directly through runoff discharge or indirectly via wastewater treatment plant (WWTP; e.g. Abdou et al., 2023; Monteiro et al., 2019; Ruchter and Sures, 2015).

Additionally, domestic and hospital wastewater can drain Pt to WWTP, particularly due to the medical use of cancerostatic platinum compounds (CPC; i.e. cisplatin, carboplatin and oxaliplatin) in chemotherapy (Kümmerer and Helmers, 1997; Vyas et al., 2014). Among these cancer treatment drugs, cisplatin is administered to patients in 50–70 % of chemotherapy (Hannon, 2007) and up to 80 % of administered CPC

<footnote><sup>a</sup> Univ. Bordeaux, CNRS, Bordeaux INP, EPOC, UMR 5805, Pessac, F-33600, France</footnote>

<footnote><sup>b</sup> Univ. Bordeaux, CNRS, Arts et Métiers, Bordeaux INP, INRAE, I2M, UMR 5295, Talence, F-33400, France</footnote>

<footnote><sup>c</sup> Univ Rouen Normandie, Université Caen Normandie, CNRS, Normandie Univ, M2C, UMR 6143, F-76000, Rouen, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: alexandra.coynel@u-bordeaux.fr (A. Coynel).</footnote>
are excreted in dissolved form via urine within the first 24 h, either directly from hospitals or patients' home (Hann et al., 2003; Lenz et al., 2005). Elimination of CPC in WWTP varied between 16 and 63 % (Abdulbur-Alfakhoury et al., 2021; Lenz et al., 2007), with removal efficiencies varying based on treatment methods (e.g. chloride use, residence time; Curtis et al., 2010; Turner and Mascorda, 2015). As a result, WWTP effluents often exhibit elevated Pt concentrations (e.g. Abdou et al., 2023; Goullé et al., 2012; Monteiro et al., 2017) leading to Pt entering aquatic environment, where it may pose risks to aquatic ecosystems.

Historical sediment records and bioindicator species showed a marked rise in Pt concentrations in aquatic system corresponding to its increasing use (Abdou et al., 2016, 2019b; Chastanet et al., 2024; Neira et al., 2015; Rauch and Hemond, 2003). While there is evidence of Pt bioavailability and its potential adverse physiological effects on aquatic organisms (Abdou et al., 2018; Ruchter and Sures, 2015; Zimmermann et al., 2002, 2004), more research is needed to accurately address the extent of Pt contamination, particularly in peri-urban and urban river systems under high anthropogenic pressure and where both the medical and traffic-related sources impact aquatic ecosystems. There is a significant knowledge gap regarding relative contribution from anthropogenic sources, Pt water-particle reactivity and its partitioning between dissolved and particulate phases - factors that are key for determining Pt bioavailability and fate in aquatic systems.

In this context, monitoring dissolved and particulate Pt concentrations in urban rivers is essential for understanding its environmental dynamics and potential impacts. This study focuses on a small urban river in France, the Jalle River, which drains the Bordeaux conurbation, and examines Pt concentrations on both monthly and event-based scales: a year-long monitoring effort at the outlet of the Jalle River and high-frequency measurements during a major rain event, including tracking Pt export from a nearby highway. The rain event examined in this study has already been extensively analyzed in terms of trace metals (TMs) and Rare Earth Elements (REEs) partitioning and fluxes (Lerat-Hardy et al., 2022) while Pt behavior during the rain event is

unknown. Exceptional rain events can cause extensive runoff, flood and riverbed erosion, contributing to significant suspended particulate matter (SPM) fluxes, which can account for 75% of the river annual SPM load (Coynel et al., 2007). Understanding these events is crucial to accurately estimating annual Pt exports in urban rivers. Building on this prior research, this study assesses Pt concentrations and fluxes during both rain events and over an annual cycle to better elucidate Pt behavior in urban water systems, and highlight the need for effective management strategies to mitigate its environmental presence.

## 2. Material and methods

### 2.1. Study area: Bordeaux urban highway and the Jalle River watershed

The Bordeaux metropolitan area, South-West France, borders the portion of the Garonne River (watershed size = 55,000 km²), forming the main fluvial branch of the Gironde Estuary, one of the main European estuaries (Fig. 1). With a population exceeding 800,000 residents and reaching 1,000,000 during weekdays due to incoming workers, Bordeaux undergoes significant urban sprawl and peri-urban development (INSEE, 2022), implying heavy and growing road traffic, despite a well-developed transport network.

Stretching over 45 km, the Bordeaux peri-urban highway is the longest in France, and is frequently congested, with an average of 250,000 vehicles daily (Lerat-Hardy et al., 2022). The highway corridor serves as key economic zone, hosting numerous commercial areas and providing employment for over 100,000 workers within 1 km radius. To accommodate the high traffic volume, sections of the Western and Northern parts of the highway were expanded from 2x2 to 2x3 lanes between 2009 and 2023. The speed limit is set at 90 km h⁻¹, except in work sections or during periods of atmospheric pollution alerts (70 km h⁻¹). The highway is bordered by green areas or noise barriers, and runoff water is managed through gutters bordering the highway and channeled to rainwater collectors (Lerat-Hardy et al., 2022), buffering the water flow and being directly connected to the Garonne River or its

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Map of the Bordeaux Metropolitan area, the Jalle River watershed including its tributaries (France) and the three sampling sites of the study (outlet of the Jalle River, the Cantinolle WWTP effluent and the NBH collector). Artificial and urbanized areas (data from EEA, 2020) and the main roads are also represented.</figcaption></figure>
urban tributaries after a grit remover step to remove part of coarse traffic particles (Fig. 1).

The Northern Bordeaux Highway (NBH) collector is the largest stormwater runoff collector in the Bordeaux metropolitan area, covering a drainage area of 11.67 km² (Capdeville et al., 2018). It receives runoff from road sections of 105 km in length and 1.04 km² in surface, a third of which comes from the Bordeaux Highway. Industrial sites (~1200 industries), including the Bordeaux-Mérignac international airport, represent >50 % of the NBH drained area (Capdeville et al., 2018). Around 90,000 vehicles, including 6.5 % of trucks, use this section of the highway daily, (Lerat-Hardy et al., 2022). The NBH collector, located in the northwest, discharges its stormwater directly into the Jalle River (Fig. 1).

The Jalle River, with a watershed of 330 km², is a major tributary of the Garonne River in the Bordeaux metropolitan area, extending beyond the metropole and covering 32.4 % of its total area (Fig. 1). The Jalle River has a mean annual discharge of 1.80 m³ s⁻¹ (data from Suez research and expertise center: LyRE; Lerat-Hardy et al., 2022; Pereto et al., 2020). The 32 km long Jalle River drains forested areas in its upper reaches, while its middle and lower watershed sections feature continuous and discontinuous urbanized/industrialized as well as agricultural lands and swamps (EEA, 2020). In the middle section, the Jalle River splits into Northern and Southern branches (Fig. 1). Two mains municipal WWTP are in the Jalle Watershed (Lille-Blanquefort and Cantinolle). While the Lille-Blanquefort WWTP discharge treated water directly into the Garonne River, the Cantinolle WWTP, serving most of the northwestern Bordeaux metropolitan area with treatment capacity of 85,500 people, discharges its effluents into the southern branch of the Jalle River (Fig. 1). This WWTP processes both domestic wastewater and industrial effluents, as well as wastewater from a small clinic's oncologic ward. There is also a small WWTP in the upper reach of the Jalle Watershed which discharge treated water in the Jalle River, with treatment capacity of 25,000 people.

## 2.2. Rain event and sampling strategy

On September 13th, 2016, around 6:30 p.m., a short but intense storm delivered 41 mm of rain over approximately 4 h, accounting for 63 % of the month's total precipitation. Bordeaux, a key route for tourists traveling along the French Atlantic Coast, faces significant traffic congestion during the summer holidays, which leads to the accumulation of traffic-related particles on road surfaces. In Summer (2016), a relatively long dry period without rainfall (~2 months starting from early July until the sampling date) preceded the sampled event. To characterize the urban-related metal fluxes due to the first runoff (flush) after this extended dry period, samples were collected at two sites: the NBH collector and the outlet of the Jalle River on the southern branch, 250 m downstream from the NBH outlet. Although the Jalle River is connected to the Garonne River Branch of the Gironde Estuary, both sampling sites are protected from tidal influences by locks in the downstream Jalle River. Accordingly, the Jalle River site combines runoff from the NBH, effluents from the Cantinolle WWTP and contributions from the Jalle River watershed (Fig. 1).

Sampling at the NBH collector was conducted at high frequency during the rain event: every 5 min from 7:20 p.m. to 8:00 p.m., every 30 min from 8:00 p.m. to 10:30 p.m., and then hourly from 10:30 p.m. to 11:30 p.m., totalizing 15 samples over the course of the 4-h event. At the Jalle River site, water samples were collected using an automatic sampler (SIGMA 900P) over a 24-h period, from 1:30 p.m. on September 13th to 12:30 p.m. on September 14th, covering the duration of the rain event. Each hourly sample was constituted of three sub-samples taken every 20 min. Detailed sampling procedures and storage protocols for both the NBH collector and the Jalle River site are described in Lerat-Hardy et al. (2022).

In addition to the rain event on September 13th, the metal transport at the Jalle River site was monitored monthly from July 2016 to

November 2017. Suspended particles for the analysis of the particulate fraction were retrieved by pumping (peristaltic pump) river water into 40-liter polypropylene (PP) containers. The raw water was then centrifugated (4000 rpm for 10 min) to recover the SPM for particulate Pt (Ptₚ) analysis, except for the Cantinolle WWTP samples, where only the dissolved fraction was studied. Most samples from the Jalle River site had very low SPM concentrations during the 24 h-sampling (except at 8:30 p.m. and 9:30 p.m.). Therefore, these samples were grouped as follows: 1:30 p.m. to 7:30 p.m., 10:30 p.m. to 12:30 a.m., 1:30 a.m. to 6:30 a.m. and 7:30 a.m. to 12:30 p.m. SPM samples were air-dried to constant weight in an oven (50 °C), homogenized and powdered using an agate mortar.

Water samples for the analysis of the dissolved fractions were collected in 1-liter bottles, filtered on-site through 0.2 µm cellulose acetate filters (Sartorius-Minisart®), and stored in pre-cleaned 30 mL PP bottles before being acidified with 14 M HNO₃ (65 % ultrapure, J.T. Baker®).

Effluents from the Cantinolle WWTP were sampled on 6 weekdays between July 2016 and July 2018 to determine to what extent the treated wastewater contributed to Pt discharge into the Jalle River. These samples were filtered on-site using 0.2 µm cellulose acetate filters (Sartorius-Minisart®), placed in HNO₃ pre-cleaned PP bottles, and acidified with 14 M HNO₃. All samples from the NBH collector, the Jalle River and the Cantinolle WWTP were stored in the dark at 4 °C pending analysis.

## 2.3. Sample preparation and analysis

Water discharge (Q) and SPM concentration from the Jalle River site (daily from July 2016 to November 2017 and hourly during the rain event) and the NBH collector (during the rain event) were measured to estimate dissolved and particulate Pt fluxes. The Jalle River discharge was measured daily by the Suez research and expertise center (LyRE) using a Hydreka® Doppler flowmeters associated with piezometric sensors. SPM concentration was determined in-lab using water samples from the monthly field campaign at the Jalle River site. Precise volumes of water were filtered through pre-weighed 0.7 µm glass microfiber filters (Xilab®) and dried in an oven at 50 °C. The SPM concentration was calculated from the mass difference between the final and the initial dry weights. Discharges and SPM concentrations for the 24-h sampling on the Jalle River and the NBH collector were measured similarly and have partly served in previous work (Lerat-Hardy et al., 2022). Physical and chemical parameters (pH, conductivity, water temperature and dissolved oxygen concentration) at the Jalle River site were measured using probes on the monthly field campaigns and every 15 min during the 24-h monitoring.

### 2.3.1. Dissolved platinum analysis

Dissolved Pt (Pt₉) concentrations were analyzed by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS, Thermo® X Series II) using the standard addition method with a daily prepared Pt stock solution from a mono-elementary Pt standard (1000 µg mL⁻¹ PLASMACAL, SCP Science®). The ¹⁹⁵Pt isotope was used for quantification. Hafnium interference (¹⁷⁹Hf¹⁶O) was monitored simultaneously with Pt and found negligible, eliminating the need for mathematical correction (Soyol-Erdene et al., 2011). The detection limit for Pt₉ by ICP-MS was estimated to 0.02 ng L⁻¹ (3 times the blank standard deviation, n = 10). As there is no available Certified Reference Material (CRM) for Pt₉ in natural water, the analytical quality was assessed by determining Pt in spiked river water from a downstream Garonne River site (La Réole), achieving ~ 95 % recovery and 8 % RSD.

### 2.3.2. Particulate platinum analysis

Suspended particulate matter was acid-digested in aqua regia (2 mL HCl and 1 mL HNO₃; 30 % HCl and 65 % HNO₃ Merck® Suprapur) at 110 °C in closed PP tubes (DigiTUBEs®, SCP SCIENCE), placed in a
Teflon-coated heating block for 3 h. The cooled contents were diluted with 10 mL Milli-Q® water and centrifuged at 4000 rpm for 10 min at 20 °C prior to analysis. Platinum concentrations in SPM were quantified by ICP-MS using standard addition method as previously described.

The detection limit for Ptₚ measurements (3 times the blank standard deviation, n = 10) was 0.7 µg kg⁻¹ for a typical sample mass of 30 mg. Quality control used CRM for Pt concentrations in solid matrices: JSd-2 sedimentary rocks (indicative value from GSJ), BCR-723 road dust (IRMM). Results showed ~100% and 95% recovery for JSd-2 and BCR-723, with RSD of ~10% and 5%, respectively (n = 3).

## 2.4. Estimation of dissolved and particulate platinum fluxes

During the rain event, both dissolved and particulate Pt fluxes at the NBH collector and the Jalle River site were estimated using the following equations:

$$Pt_dFlux_i = (Q_i \times t_i) \times [Pt_d]_i$$

$$Pt_pFlux_i = ((Q_i \times t_i) \times [SPM]_i) \times [Pt_p]_i$$

where Qᵢ is the water discharge at the sampling time (m³ s⁻¹), tᵢ is the time period represented by the sample (s, integrate 5, 30 or 60 min), [SPM]ᵢ the sample SPM concentration (mg L⁻¹) and [Pt₉]ᵢ and [Pt₁₀]ᵢ the dissolved and particulate concentrations (respectively ng L⁻¹ and µg kg⁻¹).

Total Pt flux was deduced from the sum of the dissolved and the particulate Pt fluxes, allowing the estimation of the relative Ptₚ contribution to the total Pt flux. Daily Pt fluxes were calculated on the monthly samples over the annual monitoring of the Jalle River in the same way as for the rain event fluxes. Estimate of annual Pt₉ and Pt₁₀ fluxes in the Jalle River were calculated as follows (Meybeck et al., 1993):

$$Annual Pt_d Flux = \left( \frac{\sum (Q_i \times [Pt_d]_i)}{\sum Q_i} \right) \times Q_{yr}$$

$$Annual Pt_p Flux = \left( \frac{\sum (Q_i \times [SPM]_i \times [Pt_p]_i)}{\sum Q_i} \right) \times Q_{yr}$$

where Qᵢ is the average daily water discharge (L s⁻¹), [SPM]ᵢ the sample SPM concentration (mg L⁻¹), [Pt₉]ᵢ and [Pt₁₀]ᵢ the dissolved and particulate concentrations (respectively ng L⁻¹ and µg kg⁻¹) and Qₐᵣ the Jalle River average annual water discharge (L yr⁻¹, based on an average instantaneous water discharge of 1.80 m³ s⁻¹).

## 2.5. Estimation of Pt flux from medical use in cancerostatic drugs

We aimed to quantify the total annual Pt load from CPC excreted into the environment via municipal and hospital WWTP in a year, first at the national level for France and then specifically for the Cantinolle WWTP. This analysis compares the theoretical Pt flux from CPC (PtCPC) to the Pt₉ in the Jalle River. The calculation was based on the following formula:

$$Pt_{CPC}Flux = (Adm(Pt)_{Ci-Pt} \times N_{Ci-Pt}) + (Adm(Pt)_{Ca-Pt} \times N_{Ca-Pt}) + (Adm(Pt)_{Ox-Pt} \times N_{Ox-Pt})$$

where Ci-Pt = cisplatin, Ca-Pt = carboplatin, Ox-Pt = oxaliplatin, Adm(Pt)_{Ci-Pt}, Ca-Pt, Ox-Pt is the administered dose of Pt per dose of the respective CPC (mg) and N_{Ci-Pt}, Ca-Pt, Ox-Pt is the number of doses of respective CPC administered per year. The administrated Pt dose is further defined by:

$$Adm(Pt)_{Ci-Pt,Ca-Pt,Ox-Pt} = q(Pt)_{Ci-Pt,Ca-Pt,Ox-Pt} \times d_{Ci-Pt,Ca-Pt,Ox-Pt} \times BSA$$

where q(Pt)_{Ci-Pt, Ca-Pt, Ox-Pt} is the Pt mass per unit CPC (mgPt mgCPC⁻¹); 0.65 mg mgCi-Pt⁻¹, 0.52 mg mgCa-Pt⁻¹ and 0.49 mg mgOxPt⁻¹ (calculated from Vyas et al., 2014); d_{Ci-Pt, Ca-Pt, Ox-Pt} is the

administered CPC dose (mgCPC m⁻²) from the ANSM (French national agency on medication security): 50–120 mgCi-Pt m⁻², 400 mgCa-Pt m⁻² and 85 mgOx-Pt m⁻² (ANSM, 2023a, 2023b, 2024) and BSA is the average body surface area (m²): 1.6 m² for women and 1.8 m² for men (Bender, 1995).

$$N_{Ci-Pt,Ca-Pt,Ox-Pt} = \%_{0}^{0}_{0} \times N_{patients} \times N_{session}$$

where %Ci-Pt, Ca-Pt, Ox-Pt is the distribution of CPC in chemotherapy, Npatients the number of patients in chemotherapy per year and Nsession the number of treatment session per patient.

# 3. Results

## 3.1. Monitoring of the rain event at the NBH collector

### 3.1.1. Water discharge and SPM concentrations

At the start of the high-resolution monitoring at the NBH collector, water discharge showed a sharp increase from 1.8 m³ s⁻¹ at 7:20 p.m. to 4.4 m³ s⁻¹ at 7:40 p.m., followed by rapid decline, dropping to 2.0 m³ s⁻¹ at 8:00 p.m. (Fig. 2a). After this, the discharge steadily decreased, reaching 0.8 m³ s⁻¹ by the end of the monitoring at 11:30 p.m.

The SPM concentration profile exhibited a continuous decrease throughout the event, initially from 697 mg L⁻¹ at 7:20 p.m. to 221 mg L⁻¹ at 8:00 p.m., followed by a slower decrease to 14 mg L⁻¹ at 11:30 p.m. (Fig. 2b).

### 3.1.2. Dissolved and particulate Pt concentrations and fluxes

During the high-resolution monitoring period, Pt₉ concentrations ranged from 0.16 to 1.07 ng L⁻¹, with the maximum occurring at the beginning of the event at 7:20 p.m. Then, Pt₉ dropped to 0.23 ng L⁻¹ by 8:00 p.m., with a noticeable intermediate peak at 7:40 p.m. (0.43 ng L⁻¹; Fig. 2c). From 8:00 p.m. to 11:30 p.m., Pt₉ remained relatively stable and low, averaging 0.24 ± 0.04 ng L⁻¹, yet peaking at 0.47 ng L⁻¹ at 10:30 p.m.

In contrast, Ptₚ concentrations varied throughout the monitoring without a clear trend (22.8 ± 6.34 µg kg⁻¹; Fig. 2d). The highest Ptₚ concentration was 38.6 µg kg⁻¹ at 8:30 p.m., while the lowest was 15.0 µg kg⁻¹, measured by the end of the rain event at 10:30 p.m.

The total Pt flux during the 4-h rain event at the NBH collector amounted to 7.2 mg of Pt₉ and 112 mg of Ptₚ (Fig. 5a and b). The majority of Pt transport occurred within the first 40 min of monitoring (i.e. before 8:00 p.m.), with 89 mg of Ptₚ and 3.7 mg of Pt₉ being discharged.

## 3.2. Monitoring on the Jalle River site during the rain event and the monthly survey

### 3.2.1. 24-Hour sampling (September 13th – 14th, 2016)

#### 3.2.1.1. Physical and chemical parameters during the rain event.
The hourly water discharges at the Jalle River site remained stable at 0.26 m³ s⁻¹ from 1:30 p.m. until the onset of the rain event at approximately 6:30 p.m. on September 13th, 2016 (Fig. 3a). Following the rain, the water discharge surged, peaking at 4.05 m³ s⁻¹ at 8:30 p.m. The flow remained high throughout the event, and gradually decreased overnight; falling from 3.78 m³ s⁻¹ at 1:30 a.m. to 0.62 m³ s⁻¹ at 12:30 p.m. the next day.

The pH remained relatively stable, varying from 7.4 to 7.9, with a slight increase during the afternoon of September 13th, peaking at 7.9 at 8:30 p.m. A subsequent drop occurred, followed by a stabilization in the morning of September 14th (Fig. 3c). Conductivity varied from 190 to 550 µS cm⁻¹ and was initially high and steady (525–550 µS cm⁻¹) before plummeting to 190 µS cm⁻¹ at 8:15 p.m. It then fluctuated until 3:30 a.m. before gradually decreasing to 290 µS cm⁻¹ by the morning of September 14th (Fig. 3c).

Water temperature varied between 19.5 °C (9:00 a.m. to 11:30 a.m.
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Temporal variations of (a) water discharge (Q; m³ s⁻¹), (b) SPM concentration (mg L⁻¹) and (c) dissolved (ng L⁻¹) and (d) particulate (µg kg⁻¹) Pt concentrations in the NBH collector from 7:20 p.m. to 11:30 p.m. during the storm event of September 13th, 2016. Discharge and SPM concentrations are from Lerat-Hardy et al. (2022).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Temporal variations of (a) water discharge (Q; m³ s⁻¹), (b) SPM concentration (mg L⁻¹), (c) pH and conductivity (µS cm⁻¹), (d) water temperature (°C) and dissolved O₂ (mg L⁻¹) and (e) dissolved (ng L⁻¹) and (f) particulate (µg kg⁻¹) Pt concentrations in the Jalle River on the 13th and September 14, 2016 covering the storm event that occurred in the evening of September 13th. Physical and chemical parameters are from Lerat-Hardy et al. (2022). *: Corresponding sampling period at the NBH collector.</figcaption></figure>

on September 14th) and 22.1 °C (7:00 p.m. on September 13th). It decreased at 8:30 p.m. from 21.7 °C to 20.6 °C and remained stable through the night before decreasing again in the morning (Fig. 3d). Dissolved oxygen (O₂) concentrations fluctuated between 3.5 and 8.1

mg L⁻¹ with increasing trend during the afternoon of September 13th, reaching its peak before the rain event, followed by a decrease to the lowest concentration during the night and an increase during the following morning (Fig. 3d).
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Daily variations of (a) average water discharge (Q; m³ s⁻¹) and rainfall (mm d⁻¹) and (b) SPM concentration (mg L⁻¹) in the Jalle River from July 2016 to November 2017. Circles represent Q and SPM concentration of the Jalle River on the monthly samples. (c) pH and conductivity (µS cm⁻¹), (d) water temperature (°C) and dissolved O₂ (mg L⁻¹) and (e) Pt₉ (ng L⁻¹) and (f) Ptₚ (µg kg⁻¹) Pt concentrations of the Jalle River on the monthly samples from July 2016 to November 2017. *: 24 h monitoring of the Jalle River on September 13th – 14th, 2016.</figcaption></figure>

Before the rain event, SPM concentrations averaged 5.7 ± 1.8 mg L⁻¹ from 1:30 p.m. to 7:30 p.m. During the rain, SPM concentrations increased to 635 mg L⁻¹ at 8:30 p.m., then quickly dropped to 135 mg L⁻¹ at 9:30 p.m., and steadily decreased towards initial levels throughout the night, reaching 6.5 mg L⁻¹ at 12:30 p.m. on September 14th (Fig. 3b).

### 3.2.1.2. Dissolved and particulate Pt concentrations and fluxes during the rain event.
The Pt₉ concentrations ranged from 0.95 to 3.15 ng L⁻¹ and reached their maximum at 1:30 p.m. on September 13th. As the rain event began, Pt₉ concentrations decreased from 3.15 ng L⁻¹ to 1.37 ng L⁻¹ at 8:30 p.m. (Fig. 3e). Post-rain, Pt₉ concentrations fluctuated overnight, reaching a secondary peak of 2.28 ng L⁻¹ at 4:30 a.m. before stabilizing at 1.00 ± 0.04 ng L⁻¹ on the morning of September 14th.

The Ptₚ concentration ranged from 17.7 µg kg⁻¹ to 54.7 µg kg⁻¹ and were initially around 33.7 µg kg⁻¹ in the afternoon of September 13th. During the rain event at 8:30 p.m., Ptₚ levels decreased to 17.7 µg kg⁻¹ but rose towards the end of the event, peaking at 54.7 µg kg⁻¹ early on September 14th (Fig. 3f). By the end of the 24 h-campaign on the Jalle River, the Ptₚ concentrations returned to 31.6 µg kg⁻¹, which is close to pre-rain concentrations.

The total Pt₉ and Ptₚ fluxes during the 24-h monitoring at the outlet of the Jalle River was estimated at respectively 267 mg d⁻¹ and 295 mg d⁻¹ with hourly Pt₉ fluxes ranging from 1.5 mg h⁻¹ to 26 mg h⁻¹ and hourly Ptₚ fluxes from 0.1 to 164 mg h⁻¹ (Fig. 5c and d). The majority of Pt transport occurred in the middle of the 24-h monitoring.

### 3.2.2. Annual survey (July 2016–November 2017)

#### 3.2.2.1. Physical and chemical parameters during the annual survey.
From July 2016 to May 2017, daily water discharge remained relatively low

and stable, averaging 0.59 ± 0.24 m³ s⁻¹, with occasional small increase due to rainfall (Fig. 4a). A notable increase occurred from May to July 2017 related to intense rainfall, reaching 8.45 m³ s⁻¹, followed by low water conditions in August (0.43 ± 0.05 m³ s⁻¹). The rainy Autumn 2017 caused the water discharge to rise by the end of the monitoring. On the sampled days, water discharge varied from 0.37 to 3.40 m³ s⁻¹.

The pH varied from 7.1 to 7.9 throughout the monitored period with seemingly more acid waters during Autumn 2016 and winter 2017 (Fig. 4c). Conductivity ranged from ~330 µS cm⁻¹ to ~680 µS cm⁻¹ peaking on October 9th, 2017 and reaching a minimum on March 1st, 2017 (Fig. 4c). Water temperatures varied significantly over the period, with the lowest temperature recorded at 6.5 °C during the Winter 2017 and exceeding 20 °C during both summers 2016 and 2017 (Fig. 4d). Dissolved O₂ concentrations (1.7–8.6 mg L⁻¹) were inconsistent, fluctuating around ~7 mg L⁻¹ from July 2016 to March 2017 (maximum concentration = 8.6 mg L⁻¹) and seemed to decrease in the late Summer 2017 until the lowest measured dissolved O₂ concentration recorded at 1.7 mg L⁻¹ (Fig. 4d).

The daily SPM concentrations in the Jalle River ranged from 0.2 to 91 mg L⁻¹ and averaged 5 ± 7 mg L⁻¹ over the survey period. The SPM concentrations were highest from May to July 2017 when water discharge was high and remained higher than average during this period (11.9 ± 14.4 mg L⁻¹; Fig. 4b). Outside of this period, daily SPM concentrations were low, about 4.1 ± 3.7 mg L⁻¹. On the sampled days, SPM concentrations (0.4–7.6 mg L⁻¹) were similar in magnitude.

#### 3.2.2.2. Dissolved and particulate Pt concentrations and fluxes along the annual monitoring.
Both Pt₉ (0.59–5.83 ng L⁻¹; Fig. 4e) and Ptₚ (11.3–31.9; Fig. 4f) concentrations followed similar trends in the Jalle River throughout the monitored period. From July to September 2016, concentrations were low and variable (Pt₉ = 1.52 ± 0.36 ng L⁻¹; Ptₚ =
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** (a) Dissolved and (b) particulate cumulative Pt fluxes (mg) at the NBH collector during the rain event. (c) Dissolved and (d) particulate Pt fluxes (mg h⁻¹) in the Jalle River during the 24-h monitoring, the four periods are represented with different background colors and the dash line is to separate the sub-periods in period 3. (e) Dissolved and (f) particulate Pt fluxes (mg d⁻¹) in the Jalle River during the annual monitoring. The vertical dash lines indicate the rain event. In (e), the Cantinolle WWTP average Pt₉ discharge with uncertainty is represented as a band in the background. (g) Particulate Pt distribution (%) over the total Pt flux during the three monitoring. For the 24-h monitoring, colored circles represent the four periods from (c) and (d). (h) Particulate Pt distribution (%) as function of SPM concentration (mg L⁻¹) for Log (K₉) varying between 2 and 6. Full blue triangles are for the NBH collector samples, full red circles for the 24-h monitoring and empty green square for the annual monitoring on the Jalle River. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

16.3 ± 3.66 µg kg⁻¹). In autumn 2016, concentrations increased, with Pt₉ reaching 30.7 µg kg⁻¹ on October 6th and Pt₉ peaking at 5.83 ng L⁻¹ on November 17. During the Winter 2017, Pt concentrations decreased to their lowest levels, with Pt₉ at 0.59 ng L⁻¹ and Pt₉ at 11.3 µg kg⁻¹ on February 13th, 2017. However, concentrations increased again in late spring and early summer 2017, with Pt₉ rising to 3.80 ng L⁻¹ and Pt₉ peaking at 31.9 µg kg⁻¹ (Fig. 4e and f). After a decline in late Summer

2017 to levels similar to those observed in the Summer 2016. Both Pt₉ and Pt₉ peaked again on October 9th, 2017, reaching concentrations of 1.98 ng L⁻¹ and 28.9 µg kg⁻¹, respectively.

The daily Pt₉ fluxes ranged from 25 to 580 mg d⁻¹ and exhibited important temporal variability (110 ± 123 mg d⁻¹; Fig. 5e). The daily Pt₉ fluxes ranged from 0.4 to 9 mg d⁻¹ and averaged 2.9 ± 2.2 mg d⁻¹ (Fig. 5f).
### 3.3. Monitoring at the Cantinolle WWTP

The data on daily discharged water flux from the Cantinolle WWTP was provided by the LyRE – Suez and ranged between 10,800 and 17,100 $m^3 d^{-1}$ (Table 1), i.e. between 0.12 and 0.20 $m^3 s^{-1}$. The six water samples collected from the Cantinolle WWTP effluent showed variable $Pt_d$ concentrations averaging $12.0 \pm 6.24 \text{ ng L}^{-1}$, with a minimum of $3.02 \text{ ng L}^{-1}$ and a maximum of $19.9 \text{ ng L}^{-1}$ (Table 1). On September 28th, 2017 an unfiltered sample ($Pt_{tot}$) was analyzed in addition to the dissolved sample. The result showed that at this day, $Pt_{tot}$ concentration was similar to $Pt_d$ ($Pt_d = 19.7 \text{ ng L}^{-1}$; $Pt_{tot} = 17.9 \text{ ng L}^{-1}$), suggesting that $Pt_d$ may be dominant in the WWTP effluent. Considering the discharged water and the $Pt_d$ concentration, daily $Pt_d$ fluxes ranged between 36 and 340 mg $d^{-1}$, i.e. between 1.5 and 14 mg $h^{-1}$. It appears that the maximum discharge and $Pt_d$ concentration both occurred on June 14th, 2018, however the correlation between water discharge and $Pt_d$ concentration cannot be verified on the other sampling days.

## 4. Discussion

### 4.1. Platinum dynamics and distribution in road runoff during the rain event

Since the introduction of catalytic converters in the 1970s, Pt levels in road dust have significantly increased, systematically exceeding the natural geochemical background (UCC) of $0.5 \text{ µg kg}^{-1}$ (e.g. Peuck-er-Ehrenbrink and Jahn, 2001; Rudnick and Gao, 2003). Platinum concentrations in road dust vary widely, depending on factors like traffic conditions and volume (urban or highway) and particles settling (e.g. under a tunnel or bridge), ranging from tens to hundreds of $\mu g kg^{-1}$ (e.g. Das and Chellam, 2020; Prichard et al., 2008; Sager et al., 2015; Wise-man et al., 2016) to even few thousands of $\mu g kg^{-1}$ (Gómez et al., 2001; Nischkauer et al., 2015; Schäfer and Puchelt, 1998). However, during the 4-h rainfall event, $Pt_p$ concentrations at the NBH runoff collector were relatively moderate ($22.8 \pm 6.4 \text{ µg kg}^{-1}$; Fig. 2d) compared to these data. The $Pt_p$ concentration observed did not correlate with peak water discharge (Fig. 2a) or with the SPM concentrations, which decreased steadily after 7:20 p.m. (Fig. 2b). Several factors might explain these moderate concentrations, including the presence of green borders along the highway that may introduce non-Pt-bearing particles into the runoff (Ely et al., 2001; Schäfer and Puchelt, 1998), dilution by particles derived from industrial and residential runoff (as previously observed for traffic-related Pt in sewage sludge; Jackson et al., 2010), and potential disruption of road dust due to ongoing highway expansion works.

Both $Pt_d$ and SPM concentrations exhibited a similar pattern during the monitored rain event (Fig. 2b and c). This pattern likely reflects the “first flush” phenomenon where initial intense rainfall washes accumulated road dust after a long drought period (Bertrand-Krajewski et al., 1998; Dang et al., 2023; Deletic, 1998; Lerat-Hardy et al., 2022). In fact, on an urban road section with approximately 16,000 vehicles per day, $Pt_d$ concentration reached $1000 \text{ ng L}^{-1}$ in runoffs at the very beginning of a rain event associated with very high SPM concentrations (>1000 mg $L^{-1}$; Laschka et al., 1996). Unfortunately, the earliest minutes of the rain event, when the highest concentrations are to be expected, were not

<table><caption>**Table 1**
Water discharge ($m^3 d^{-1}$), $Pt_d$ concentrations ($ng L^{-1}$) and $Pt_d$ flux ($mg d^{-1}$) at the Cantinolle WWTP on the six days sampled between 2016 and 2018.</caption><thead><tr><th>Date</th><th>Q (m<sup>3</sup> d<sup>-1</sup>)</th><th>Pt<sub>d</sub> (ng L<sup>-1</sup>)</th><th>Pt<sub>d</sub> flux (mg d<sup>-1</sup>)</th></tr></thead><tbody><tr><td>Tuesday 2016.07.19</td><td>10,800</td><td>5.78</td><td>62</td></tr><tr><td>Wednesday 2016.11.16</td><td>11,318</td><td>11.4</td><td>129</td></tr><tr><td>Thursday 2017.09.28</td><td>11,491</td><td>19.2</td><td>221</td></tr><tr><td>Tuesday 2017.11.28</td><td>11,923</td><td>3.02</td><td>36</td></tr><tr><td>Thursday 2018.06.14</td><td>17,107</td><td>19.9</td><td>340</td></tr><tr><td>Thursday 2018.07.19</td><td>13,478</td><td>12.4</td><td>167</td></tr></tbody></table>

covered in this work. The correlation between $Pt_d$ and SPM concentrations ($r = 0.70$, $p(uncorr) < 0.05$) suggests that either $Pt_d$ desorption from particles occurred during road dust transport to the NBH collector or that parts of the Pt-bearing road dust particles were smaller in size than $0.2 \mu m$ due to the particle concentration effect, i.e. in the form of Pt nanoparticles and/or colloids (O'Connor and Connolly, 1980). Therefore, the noted increase in $Pt_d$ concentration between 10:00 p.m. and 11:30 p.m. could be related to (i) Pt-bearing nanoparticles and/or colloids, synchronous to an increase in SPM concentration (Fig. 2b and c), needing further studies on the particle concentration effect, and/or (ii) the delayed inputs from upstream retention structures along the NBH collector. These potential inputs may be investigated by monitoring specifically the NBH collector watershed and all its retention basin. By the end of the event, $Pt_d$ concentrations ($\sim 0.2 \text{ ng L}^{-1}$) were still high compared to natural river systems ($<0.05 \text{ ng L}^{-1}$; Cobelo-García et al., 2013; Padan et al., 2020) and rainwater ($0.03-0.12 \text{ ng L}^{-1}$; Mashio et al., 2016).

This “first flush” within the first 40 min accounted for over 80 % of the $Pt_p$ flux and 52 % of the $Pt_d$ flux of the monitored rain event (Fig. 5a and b). Approximately 95 % of the Pt discharged at the NBH collector was in particulate form (Fig. 5g), likely from abrasion of catalytic converters. This abrasion is caused by harsh physical (e.g. velocity, high temperature) and chemical conditions (e.g. sulfur poisoning, oxidation) inside converters (Artelt et al., 1999; Ravindra et al., 2004), resulting in emitted particles ranging from nanometers to over 63 $\mu m$ in size (Rauch and Morrison, 2008), with overall increasing size for the older catalysts (Artelt et al., 2000). The relative distribution of the dissolved Pt fraction observed here (5 % of $Pt_d$; Fig. 5g) is consistent with previous studies of road dust (4–10 %; Ermolin et al., 2022). The dissolved fraction (defined as < 0.2 $\mu m$) may also contain Pt nanoparticles, which have been reported to constitute 1–3 % of the total Pt in road dusts (Ermolin et al., 2022; Folens et al., 2018).

By the end of the rain event, the $Pt_d$ reached nearly 40 % of the instantaneous total Pt flux at the NBH collector (Fig. 5g). A similar late increase of the dissolved fraction has been observed for other automobile-related metals such as Cu, Zn, Cd, Ni, Sb, in a previous study at the NBH collector (Lerat-Hardy et al., 2022). This trend may indicate the near-complete rinsing of the road dust in the NBH watershed or the introduction of runoff from further less traffic-impacted (i.e. with less Pt-bearing particles) areas.

### 4.2. Deciphering platinum sources and contribution during a storm event in an urban river

A previous study on the September storm event in the Jalle River identified four distinct periods based on variations in physical and chemical parameters, as well as TMs and REEs concentrations (Lerat-Hardy et al., 2022). Dissolved and particulate Pt concentrations were also categorized into these 4 periods defined as follow.

#### 4.2.1. Period 1: pre-rain event Pt baseline in an urban river

During the afternoon of September 13th (1:30 p.m. to 7:30 p.m.), the Jalle River exhibited typical low-water conditions with low SPM concentration (Fig. 3a and b). This period was characterized by high conductivity, rising water temperature and increased dissolved $O_2$ related to photosynthesis (Fig. 3c and d). Initial $Pt_d$ concentration were relatively high ($3.15 \text{ ng L}^{-1}$ at 1:30 p.m., Fig. 3e) before decreasing by half during the afternoon, similar to trends observed for most of REEs, Zn, Pb and Sn (Lerat-Hardy et al., 2022). The initial variations in dissolved TMs were attributed to the geochemical influence of the forested upper reaches of the watershed and the podzolic soils, despite the river being mainly sustained by the downstream calcareous aquifer during this period (Lerat-Hardy et al., 2022). Furthermore, the observed variations in $Pt_d$ concentrations may be linked to upstream WWTP discharges into the Jalle River (Table 1). In total, 19.9 mg of $Pt_d$ and 2.5 mg of $Pt_p$ were discharged at the Jalle River site during the period 1 (Fig. 5c and d).
The Ptₚ concentration was relatively high for a river environment (33.7 µg kg⁻¹; Fig. 3f), for instance 0.7 µg kg⁻¹ in SPM from the Garonne River (Cobelo-García et al., 2014) and 1.4–1.6 µg kg⁻¹ in other major French river sediments (Chastanet et al., 2024), but comparable to those in urban rivers facing substantial anthropogenic pressure (e.g. road runoff) with reported Ptₚ between ~10 and 45 µg kg⁻¹ (Rauch and Morrison, 1999; Ruchter and Sures, 2015). Despite high Ptₚ concentration, Pt was mainly transported in the dissolved form (90.2 ± 3.8 % of Pt₉; Fig. 5g) because of very weak mechanical erosion in the floodplain system (~1 t km⁻² yr⁻¹).

### 4.2.2. Period 2: Pt inputs from road runoffs during the rain event

The sample collected at 8:30 p.m. is the single sample of the second period and represents the rain event starting at the Jalle River site ~2h after the rainfall began (~6:30 p.m.; Lerat-Hardy et al., 2022). This period coincides with the peak water discharge (4.05 m³ s⁻¹; Fig. 3a) and the maximum SPM concentration (635 mg L⁻¹; Fig. 3b), resulting in a hourly SPM flux of 9250 kg h⁻¹. Additionally, this period is characterized by drops in conductivity and water temperature, along with a notable rise in pH (Fig. 3c and d). These changes likely reflect the arrival of a new water body at the Jalle River site, i.e. runoff from the NBH collector and upstream runoff enhanced by the rainfall (Lerat-Hardy et al., 2022).

Interestingly, the Pt₉ concentration remained higher at the Jalle River site than at the NBH collector and similar to pre-rain event levels (~1.4 ng L⁻¹; Fig. 3e), suggesting that the Pt₉ originated from sources other than road runoffs. Meanwhile, Ptₚ concentration halved at this period (17.7 µg kg⁻¹; Fig. 3f) and was then similar to Ptₚ concentrations in the NBH collector, supposed to provide the Pt signal derived from road runoff (Fig. 2d) and potentially modified by dilution involving particles from additional geogenic and non-road particle inputs caused by the intense rainfall. However, it is worth noting that the pre-rain event Ptₚ concentration (1:30 p.m. to 7:30 p.m.) was based on a single composite sample, making it difficult to detect any potential concentration fluctuations during the afternoon.

In terms of fluxes, the Jalle River discharged 20 mg h⁻¹ of Pt₉ and 164 mg h⁻¹ of Ptₚ between 7:30 p.m. and 8:30 p.m. (Fig. 5c and d), increasing the hourly Ptₚ flux by more than 400 times compared to the first period. This demonstrates the importance of considering flux variations at high temporal resolution, rather than just concentrations, to assess the role of urban rivers during storm events. In contrast to the pre-rain Pt baseline, Pt was dominantly transported in the particulate form (89 % of Ptₚ; Fig. 5g), suggesting that Pt-bearing particles probably derived from catalytic converters were the main Pt source in the Jalle River at this time. This observation is supported by the transport of other traffic-related metals (e.g. Cu, Zn, Sb, Pb and Ce) identified as inputs from the NBH collector to the Jalle River during the same period (Lerat-Hardy et al., 2022).

Estimates of Pt inputs from the Bordeaux highways through the NBH collector before 8:00 p.m. suggest that at least 54 % of Ptₚ and 18 % of Pt₉ transported in the Jalle River between 7:30 p.m. and 8:30 p.m. originated from the highway. These contributions are likely underestimated, as the first minutes of the rain event could not be sampled at the NBH collector. Such mass balances clearly suggest that urban highways are major, if not primary, sources of Ptₚ to urban rivers during rain events. However, the role of unmonitored runoffs from urban roads (i.e. with driving speed between 30 and 50 km h⁻¹), where catalytic converter Pt instantaneous emissions might be higher due to frequent speed changes (Artelt et al., 1999; Moldovan et al., 2002), should not be overlooked.

During the entire rain event (24-h monitoring), the NBH collector contributed 38 % of Ptₚ and 3 % of Pt₉ to the Jalle River Pt fluxes. This result supports the idea that although road runoffs play a key role in Ptₚ transport, they cannot explain the relatively high Pt₉ flux in the Jalle River over a longer timescale. In fact, the Ptₚ contribution from the NBH collector was event-related and very limited in time, similar to that of Ce

(another element emitted from catalytic converter washcoats; Rauch et al., 2000; Wiseman et al., 2016), along with other traffic-related metals (e.g. Ni, Zn, Pb; Lerat-Hardy et al., 2022).

### 4.2.3. Period 3: mixed Pt urban sources post-rain

The third period spans from 9:30 p.m. on September 13th to 5:30 a. m. on September 14th (Lerat-Hardy et al., 2022). During this time, water discharge remained high but began to decrease slowly after 1:30 a.m. and the SPM concentration (initially at 135 mg L⁻¹) decreased throughout the period (Fig. 3a and b). This period can be divided into two sub-periods based on Pt₉ and Ptₚ dynamics.

**Sub-period 1:** 9:30 p.m. to 12:30 a.m. During this sub-period, both Pt₉ and Ptₚ concentrations increased following the peak outflow from the rain at 8:30 p.m. (Fig. 3e and f). The mean hourly Pt₉ flux is about 18.9 ± 2.7 mg h⁻¹, showing an increasing trend (Fig. 5c) while the Ptₚ fluxes decreased from 36 to 11 mg h⁻¹ (Fig. 5d) associated with decreasing SPM flux. As a result, the proportion of Pt transported in particulate form dropped from 70 % to 33 % (Fig. 5g). The Pt in this period was likely sourced from residual runoffs from additional roads in the watershed, given that most road runoff had already been discharged into the Jalle River before 8:00 p.m. as observed at the NBH collector.

**Sub period 2:** 12:30 a.m. to 5:30 a.m. In this sub-period, both Pt₉ (1.99 ± 0.23 ng L⁻¹; Fig. 3e) and Ptₚ (54.7 µg kg⁻¹; Fig. 3f) reached their highest concentrations during the monitoring period. The Jalle River discharged the most Pt₉ on the night of September 13th to 14th, with a total of 113 mg over 5 h (22.6 ± 2.2 mg h⁻¹; Fig. 5c). However, the daily Ptₚ flux continued to decrease averaging 6.3 ± 1.7 mg h⁻¹ (Fig. 5d). The dissolved fluxes of some other TMs and REEs (i.e. Gd, Ag and Cd) also increased during this period (Lerat-Hardy et al., 2022) which had been attributed to inputs from the Cantinolle WWTP. This WWTP experienced an exceptional by-pass due to excessive inflow, releasing ~6000 m³ of wastewater into the Jalle River around 10:30 p.m., with ~60 % of it being untreated. This by-pass contributed to 46 % of the water discharge in the Jalle River during this period (Lerat-Hardy et al., 2022).

Platinum can be present in wastewater due to its use in CPC for oncological treatments and excretion through urines (Hann et al., 2003; Kümmerer and Helmers, 1997; Lenz et al., 2005). Due to incomplete elimination of CPC in WWTP (e.g. Abdulbur-Alfakhoury et al., 2021; Lenz et al., 2005, 2007; Monteiro et al., 2017; Trommetter et al., 2022), Pt can be discharged into aquatic environments with reported Pt₉ concentrations in WWTP effluents ranging from 2 to 25 ng L⁻¹ (e.g. Abdou et al., 2023; Abdulbur-Alfakhoury et al., 2021; Gouillé et al., 2012; Monteiro et al., 2017; Trommetter et al., 2022). The Cantinolle WWTP showed similar Pt₉ concentrations (3–20 ng L⁻¹; Table 1), with daily dissolved flux exported to the Jalle River ranging from 36 to 340 mg d⁻¹ (Table 1), corresponding to a Pt₉ contribution per inhabitant of 0.4–4.0 µg inhab⁻¹ d⁻¹.

Although Pt is supposedly discharged from the Cantinolle WWTP in the dissolved phase, the increase in Ptₚ concentration during the night might be due to the by-pass. Previous studies have shown that CPC tend to adsorb onto river SPM over time (Curtis et al., 2010; Turner and Mascorda, 2015). Despite the short time between the by-pass discharge (~10:30 p.m.) and sampling downstream at the Jalle River site (12:30 a. m. to 5:30 a.m.), the adsorption of Pt might have affected Ptₚ contents due to the low SPM concentration (12 ± 4 mg L⁻¹; Fig. 3b). Thus, the by-pass probably has contributed to the increased Pt flux into the Jalle River during this period (75 ± 32 mg by-passed), with Pt predominantly in the dissolved form (78 ± 4 % of Pt₉; Fig. 5g).

### 4.2.4. Period 4: post-rain event and upstream influence

The last period (6:30 a.m. to 12:30 p.m.) reflects the situation after the by-pass event. Water discharge continued to decrease and SPM concentration was low, approaching pre-rain levels (Fig. 3a and b).

Elevated concentrations of dissolved TMs and REEs (e.g. Al, Fe, Sb, Pb, La, Ce) were thought to reflect the geochemical influence from the podzol-dominated forest areas in the upper reaches of the watershed
(Lerat-Hardy et al., 2022). Although Pt<sub>d</sub> concentration had clearly decreased after the rain event (Fig. 3e), it still remained relatively high compared to natural rivers, likely due to residual impact from the by-pass or ongoing discharge from the WWTP. The Pt<sub>p</sub> concentration returned to levels similar to the first period (31.6 µg kg<sup>-1</sup>; Fig. 3f), indicating that the Jalle River system had not fully returned to pre-rain conditions in terms of Pt discharge by the end of the monitoring period.

### 4.3. Understanding platinum dynamics in an urban river at the annual scale

The Pt<sub>d</sub> and Pt<sub>p</sub> concentrations (1.90 ± 1.62 ng L<sup>-1</sup> and 19.8 ± 6.8 µg kg<sup>-1</sup>) in the Jalle River during the annual survey were within the same range as those measured during the 24-h monitoring (Fig. 4e and f). They were also similar in magnitude to those reported in other urban rivers, with Pt<sub>d</sub> ranging from 0.56 to 2.6 ng L<sup>-1</sup> after a rain event (Monticelli et al., 2010) and from <0.01 to 6.9 ng L<sup>-1</sup> in the densely populated area of Tokyo (Mashio et al., 2016; Obata et al., 2006) and with Pt<sub>p</sub> ranging from ~10 and up to 45 µg kg<sup>-1</sup> at the outlet of highway runoffs (Rauch and Morrison, 1999; Ruchter and Sures, 2015). This observation suggests notable and permanent Pt enrichment in the Jalle River water and SPM. Higher Pt<sub>p</sub> concentrations occurring during Autumn 2016; Spring 2017 (Fig. 4f) were unrelated to water discharge or SPM concentration, and might be explained by adsorption of Pt<sub>d</sub> from the Cantinolle WWTP onto biogenic particles such as phytoplankton. The latter may transfer Pt from the dissolved to the particulate phase either by direct adsorption onto the phytoplankton surface or by bio-uptake, changing Pt partitioning in rivers and coastal marine systems (Abdou et al., 2020, 2023).

The Pt distribution coefficient (K<sub>d</sub>), calculated as the ratio of Pt<sub>p</sub> to Pt<sub>d</sub> concentrations and expressed in Log<sub>10</sub>, varied significantly with environmental conditions (Fig. 5h). In normal conditions (i.e. 2016 – 2017 monitoring), the Log(K<sub>d</sub>) for the urban Jalle River, heavily impacted by urban discharges, ranges from 3.64 to 4.55 (4.12 ± 0.25). During the studied intense rain event (2016.09.13th-14th), average Log (K<sub>d</sub>) values tended to increase to 4.44 ± 0.12 (4.11–4.59), attributed to runoff from highways and roads (NBH collector) in which Pt-rich particles emitted from catalytic converters caused even greater Log(K<sub>d</sub>) values, ranging from 4.47 to 5.17 (4.84 ± 0.19). The lower Log(K<sub>d</sub>) values in the Jalle River under normal conditions could be attributed to dissolved CPC discharged by the WWTP, assuming their Log(K<sub>d</sub>) values ranging from 1.82 for oxaliplatin to 2.88 for cisplatin in river sediments (Turner and Mascorda, 2015; Vyas et al., 2014). Such contrasting K<sub>d</sub> values may reflect two distinct Pt sources in urban rivers and associated sorption/desorption processes: the highly dissolved CPC (Log(K<sub>d</sub>) < 3) and the road-derived Pt<sub>p</sub> (Log(K<sub>d</sub>) ~5). The rain event-driven increase in particulate Pt flux and K<sub>d</sub> (Fig. 5g and h) clearly illustrates the phase shift induced by surface runoff inputs, confirming that hydrological events can transiently inverse the partitioning of Pt in urban rivers. Among the very few data on Pt K<sub>d</sub> in the environment, our calculated Log(K<sub>d</sub>) for the urban Jalle River in normal conditions align with those reported for the urbanized Genoa Harbor (3.30–4.36 for 0.2 µm filtered water samples; Abdou et al., 2019a) and the Tagus Estuary (4.0 ± 0.4 for 0.45 µm filtered water samples; Monteiro et al., 2021) where both WWTP discharges and road runoffs were reported as Pt sources.

Estimates of annual Pt fluxes suggest Pt<sub>d</sub> flux of ~97 g yr<sup>-1</sup> and Pt<sub>p</sub> flux of ~3 g yr<sup>-1</sup>, with highly variable monitored daily Pt<sub>d</sub> fluxes (110 ± 123 mg d<sup>-1</sup>; Fig. 5e). Excluding occasional high discharge events during the monitoring period, the daily Pt baseline flux in the Jalle River is approximately 55 ± 19 mg d<sup>-1</sup> for Pt<sub>d</sub> and 2.9 ± 2.2 mg d<sup>-1</sup> for Pt<sub>p</sub>, representing an average inhabitant-specific Pt<sub>d</sub> flux of 0.6 ± 0.2 µg d<sup>-1</sup> inhab-1 and an average surface specific Pt<sub>p</sub> flux of 8.7 ± 6.8 µg d<sup>-1</sup> km<sup>-2</sup> (n = 15). Estimates of Pt<sub>d</sub> inhabitant specific flux are lower than those from the two Brussels WWTP (2.9 µg d<sup>-1</sup> inhab<sup>-1</sup>, herein calculated based on Pt<sub>d</sub> = 4.2 g d<sup>-1</sup> in the WWTP outflow and a treatment capacity of 1,460,000 people; Abdulbur-Alfakhoury et al., 2021),

probably because the two Brussels WWTP receive sewage directly from the UZ Brussels hospital. At the annual scale Pt is typically transported in the Jalle River in the dissolved form (96.2 ± 3.2 % of Pt<sub>d</sub>; Fig. 5g), consistent with observations before and after the rain event on September 13th, 2016.

To further understand Pt<sub>d</sub> concentration in the Jalle River and the WWTP contribution to urban rivers, we aimed to quantify the Pt fluxes excreted by patients undergoing chemotherapy at the scale of France and of the Cantinolle WWTP as described in Section 2.5. Based on available statistics, cisplatin accounts for ~60 % of chemotherapy (50–70 %; Hannon, 2007), with carboplatin and oxaliplatin each assuming for accounting for 20 % of the remaining treatments. In France, an estimate of 363,000 patients (0.6 % of the French population) underwent chemotherapy in 2021, with a gender distribution of 57 % men and 43 % women, and each patient typically pertaining in 4–6 treatment sessions (INCa, 2023; e-cancer.fr). Our calculations suggest that between 174 kg (assuming 4 treatment sessions and the minimum dose of cisplatin) to 363 kg (assuming 6 treatment sessions and the maximum dose of cisplatin) of medical Pt are released into the environment annually via French WWTP. At the Cantinolle WWTP (85,500 inhabitants), this corresponds to an estimated annual flux of 229 g yr<sup>-1</sup> to 477 g yr<sup>-1</sup> into the Jalle River. However, this estimate is subject to uncertainties due to spatial and gender variations in cancer incidence, low Pt removal efficiency in WWTP, and the fact that no major oncological ward discharge into the Cantinolle WWTP. It is also noteworthy that up to 30 % of CPC is directly excreted in hospitals (Lenz et al., 2005; Vyas et al., 2014). Despite these uncertainties, our estimates exceed the measured Pt<sub>d</sub> flux in the Jalle River (97 g yr<sup>-1</sup>), confirming that most of the Pt<sub>d</sub> in the Jalle River (and more generally in urban rivers) may derive from CPC excretion, highlighting the need for further investigations on their ecological impact on aquatic ecosystems.

Furthermore, industrial wastewater discharges could also contribute to Pt levels, adding to the complexity of urban Pt sources (Chastanet et al., 2024; Helmers and Kümmerer, 1999; Lottermoser, 1994; Zereini et al., 1998). While industrial contributions cannot be entirely ruled out, the absence of regulatory monitoring for Pt in industrial effluents clearly limits our ability to assess their potential role. The close agreement between observed flux and model predictions based on medical Pt use provides strong evidence that medical effluents are a dominant source of dissolved Pt in the Jalle River. It is still crucial to consider both WWTP effluents and potential industrial inputs when assessing Pt pollution in urban waterways.

Rainfall events drastically influence the distribution of Pt between dissolved and particulate forms. The September 13th-14th, 2016 rain event alone contributed significantly to the annual Pt budget, with a Pt<sub>p</sub> budget equivalent to 120 days of moderate fluxes in less than 4 h. The high daily Pt<sub>d</sub> flux of 267 mg d<sup>-1</sup> during the event represents five times the baseline flux, emphasizing the substantial impact of exceptional rainfall events. These findings underscore the importance of monitoring these episodic inputs when evaluating Pt fluxes in urban rivers.

### 5. Conclusion

The comparison of Pt concentrations, partitioning and fluxes in an urban river (Jalle River, South-West of France) obtained from an annual monitoring and during an exceptional rain event has provided original information on Pt sources and transport dynamics. The concentrations of dissolved Pt (Pt<sub>d</sub>) and particulate Pt (Pt<sub>p</sub>) reported here are consistent in magnitude with previous studies on urban rivers. Under typical discharge conditions, the Jalle River exports Pt predominantly in the dissolved form, owing to low sediment yield and the presence of infrastructures (e.g. locks, mills) which are efficient traps for particles. This observation and estimates of the Pt fluxes originating from cancerstatic platinum compounds (CPC) uses in France suggest that effluents from wastewater treatment plants (WWTP) are the primary source of Pt<sub>d</sub> in urban rivers throughout the year. While WWTP
appeared as the main source, potential contributions from industrial wastewater should not be overlooked. In contrast, during intense rain events, Pt partitioning between the dissolved and particulate phases tends to be inversed due to major Ptₚ inputs and transport. The latter has been attributed to intense road runoff and increased river discharges limiting sedimentation and/or enhancing erosion, especially during the "first flush". The first few minutes of rainstorms (>80 % of the Ptₚ flux) may increase Pt fluxes by two orders of magnitude, equivalent to 120 days of normal Ptₚ discharge. Consequently, high resolution monitoring during rain events is essential for accurately estimating annual Pt fluxes in urban rivers. Given the lack of information on the environmental distribution, reactivity and potential impact of Pt from different sources (traffic, medical, industry) on ecosystem health, further research on speciation, bioavailability and toxicity are needed, covering a wider scale of aquatic environments including fresh, brackish and marine systems.

## CRediT authorship contribution statement

**Maxime Chastanet:** Writing – original draft, Visualization, Investigation, Data curation, Conceptualization. **Antoine Lerat-Hardy:** Writing – review & editing, Validation, Investigation. **Jörg Schäfer:** Writing – review & editing, Validation, Supervision. **Melina Abdou:** Writing – review & editing, Validation, Investigation, Data curation. **Thomas Gardes:** Writing – review & editing, Validation, Supervision. **Antoine Marache:** Writing – review & editing. **Maxime Debret:** Writing – review & editing, Supervision. **Alexandra Coynel:** Writing – review & editing, Visualization, Validation, Supervision, Resources, Project administration, Investigation, Funding acquisition, Conceptualization.

## Funding sources

This work has been supported by the Regional Water Agency (Agence de l'Éau Adour-Garonne) and the "Agence Française pour la Bio-diversité" in the frame of the REGARD Project: "Comprehensive and integrated program against micropollutants (Bordeaux Métropole)." The author Maxime Chastanet has benefited from a PhD grant by the French Ministry of "Enseignement Supérieur, de la Recherche et de l'Innovation". The funding sources had no involvements in the conduct of the research and the preparation of the article.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgement

This work has been supported by the Regional Water Agency (Agence de l'Éau Adour-Garonne) and the "Agence Française pour la Bio-diversité" in the frame of the REGARD Project: "Comprehensive and integrated program against micropollutants (Bordeaux Métropole)" for the sampling campaigns. We are also grateful to Marion-Justine Cap-deville and Damien Granger from the Suez research and expertise center (LyRE) for their assistance and implication during sampling campaigns. The analyses were performed by the instrumental platform "Majeurs et Métaux traces" (M&Ms, UMR 5805 EPOC, University of Bordeaux). The author Maxime Chastanet has benefited from a PhD grant by the French Ministry of "Enseignement Supérieur et de la Recherche".

## Data availability

Data will be made available on request.

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