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

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/year, in which 14 kg/year 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.

Performance of two contrasting pilot swale designs for treating zinc, polycyclic aromatic hydrocarbons and glyphosate from stormwater runoff.

Swales are a widespread stormwater management solution to reduce pollutant concentrations in runoff. An innovative pilot facility was constructed to evaluate the treatment efficiency of the two main types of water-quality swales, i.e. standard swales and filtering swales. Using stormwater roof runoff, without any additions or spiked with organic micropollutants, 12 runoff simulation runs mimicking frequent storm events were discharged longitudinally or laterally over the pilot swales. The performance of each swale was assessed for 4 micropollutants, i.e. zinc (Zn), glyphosate, pyrene and phenanthrene. These substances were mainly found in the dissolved phase of the stormwater runoff used to supply the pilot swales. The standard swale, constructed from a silt loam soil, partially managed stormwater runoff by infiltration. Micropollutant concentration reductions were higher in the infiltrated water (35-85%) than in the overflow (-13-66%). The filtering swale, made of a sandy central part bordered by silt loam embankments, completely managed stormwater runoff by infiltration, providing high micropollutant concentration reductions (65-100%). Mass load reductions were higher for the filtering swale (67-90% for Zn and ≥89% for organic micropollutants) than for the standard swale (33-73% for Zn, 19-67% for glyphosate and ≥50% for both pyrene and phenanthrene). For both swales, lateral inflow was often associated with significantly higher concentration and mass reductions than longitudinal inflow. Consequently, when designing swales for the treatment of micropollutants, practitioners should preferentially promote filtering swales and installations providing lateral diffuse inflow over the facility.

Analysis of swale factors implicated in pollutant removal efficiency using a swale database.

Swales are traditional basic open-drainage systems which are able to remove stormwater-borne pollutants. In spite of numerous case studies devoted to their performances, parameters influencing the reduction of pollutant concentrations by swales remain elusive. In order to better characterize them, a database was set up by collecting performance results and design characteristics from 59 swales reported in the literature. Investigations on correlations among pollutant efficiency ratios (ERs) indicated that total trace metals (copper (Cu), zinc (Zn), cadmium (Cd), and lead (Pb)), total suspended solids (TSS), total phosphorus (TP), and total Kjeldahl nitrogen (TKN) exhibited many cross-correlated ERs. High ERs were observed for pollutants including a particulate form such as TSS (median ERs = 56%) and total trace metals (median ERs ≥ 62%), suggesting that these pollutants are efficiently trapped by sedimentation in swale bed and/or filtered within swale soil. Medium to high ERs were found for dissolved trace metals (median ERs ≥ 44%), whereas ERs for nutrient species were lower (median ERs ≤ 30%). The inflow concentration was identified as a major factor correlated to ER for most pollutants. For some pollutants, there is also a trend to get higher ER when the geometrical design of the swale increases the hydraulic residence time. Overall, this database may help to better understand swale systems and to optimize their design for improving pollutant removal.

Correction to: Analysis of swale factors implicated in pollutant removal efficiency using a swale database.

The original publication of this paper contains an error. Correct presentation of Equation 1 is presented in this paper.

Linking the management of urban watersheds with the impacts on the receiving water bodies: the use of flow duration curves.

There is growing evidence that changes in the current hydrological behaviour of urbanising catchments are a major source of impacts on the downstream water bodies. However, current flow-rates are rarely considered in studies on urban stormwater management, usually focused on extreme flow-rates. We argue that taking into account receiving water bodies is possible with relatively small modifications in current practices of urban stormwater modelling, through the use of Flow duration curves (FDCs). In this paper, we discuss advantages and requirements of the use of FDCs. Then, we present an example of application comparing source control regulations over an urbanised catchment (178 ha) in Nantes, France.