Is the wash-off process of road-deposited sediment source limited or transport limited?

Transport processes of dissolved and particulate nitrogen and phosphorus over urban road surface during rainfall runoff

Nutrient pollutants serve as indicative pollutants in urban stormwater runoff, and usually coexist and transport in particulate and dissolved phase in runoff, which is complex and crucial for effective pollution control. In this study, nitrogen and phosphorus runoff samples were collected during various natural rainfall events to explore its transport process over urban road surface during rainfall runoff. The results showed that nitrogen mainly exists in the dissolved phase (mean proportion: 62.04 %), while phosphorus mainly exists in the particulate phase (mean proportion: 65.58 %). More nitrogen and phosphorus are present and transported in dissolved phase in initial rainfall runoff over urban roads. Nutrient concentration changes during rainfall events were influenced by factors such as rainfall intensity and surface runoff, resulting in multiple peaks. Transport rate peak and concentration peak did not coincide. The proportion of dissolved total nitrogen in the runoff process ranged mainly between 40 % and 80 %, and the proportion of dissolved ammonia was distributed between 60 % and 100 %. The proportion of dissolved phosphorus was more evenly distributed across each proportion interval. Influenced by the differences in phase proportions, first flush processes of nitrogen and phosphorus are not the same. Urban stormwater management measures should prioritize both the initial concentration peaks and the peaks in nutrient transport rates during rainfall. This approach is essential for enhancing the efficiency of stormwater pollutant collection and treatment.

Migration and adsorption of naphthalene in road-deposited sediments from stormwater runoff: Impact of the particle size

In this study, we explored the impact of RDS particle size on the migration dynamics of RDS and naphthalene through rigorous wash-off experiments. The results illuminated that smaller RDS particles showed higher mobility in stormwater runoff. On the other hand, RDS particles larger than 150 μm showed migration ratios below 2 %, suggesting that naphthalene adsorbed on larger RDS primarily migrated in dissolved form. Furthermore, we investigated the migration behaviors of RDS and naphthalene under varied conditions, including rainfall intensity, duration, and naphthalene concentrations. Larger rainfall intensity promoted the naphthalene release from RDS, while long rainfall duration (≥10 min) impeded the migration velocities (≤2.91 %/5 min for RDS, and ≤3.32 %/5 min for corresponding naphthalene) of RDS and naphthalene. Additionally, higher naphthalene concentrations in RDS diminished migration ratios of dissolved naphthalene. Significantly, the maximum uptake of naphthalene on RDS was 6.02 mg/g by the adsorption Langmuir isotherm. Importantly, the adsorption process of naphthalene in RDS is primarily governed by the physical adsorption, as demonstrated by the successive desorption experiments, which showed the desorption rate of up to 87.32 %. Moreover, advanced characterizations such as XPS, FTIR and Raman spectra further confirmed the physical nature of the adsorption process. These findings may help the understanding of the migration behavior of other pollutants in urban surface particulates.

Response of sediment transport capacity to soil properties and hydraulic parameters in the typical agricultural regions of the Loess Plateau

The sediment transport capacity by overland flow (T_c) is a key parameter in process-based soil erosion models and T_c variation is sensitive to changes in soil properties. This study was undertaken to investigate T_c variations with respect to soil properties and establish a universal relationship to predict T_c. The test soils were collected from typical agricultural regions (Guanzhong basin-Yangling (YL), Weibei Dry plateau-Chunhua (CH), Hilly and gully region-Ansai (AS), Ago-pastoral transition zone along the Great Wall-Yuyang (YY), and Weiriver floodplain-Weicheng (WC)) of the Loess Plateau, and subjected to 36 different combinations of slope gradients (S, 5.24-44.52 %) and flow discharge (q, 0.00033-0.00125 m² s^-1) in a hydraulic flume. The results showed that the mean T_c values for WC were 2.15, 1.38, 1.32, and 1.16 times greater than those for YL, CH, AS, and YY, respectively. T_c significantly decreased with clay content (C), mean weight diameter (MWD), and soil organic matter content (SOM). T_c for different soil types increased with S and q as a binary power function, and T_c variation was more sensitive to S than to q. Stream power (w) was the most appropriate hydraulic variable to express T_c for different soils. T_c for different soil types could be satisfactorily simulated using a quaternary power function of S, q, C, and MWD (R² = 0.94; NSE = 0.94) or a ternary power function of w, C, and MWD (R² = 0.94; NSE = 0.94). The new T_c equation can reflect the effect of soil properties on it and facilitate the development of a process-based soil erosion model.

Removal, migration, and distribution of naphthalene in bioretention facilities: the influences of particulate matter

Particulate matter (PM), as an important carrier of carrying and transporting runoff pollutants, can significantly affect the behavior and removal efficiency of pollutants in bioretention facilities. In order to control the pollution caused by naphthalene in bioretention facilities, the removal efficiency and migration characteristics of naphthalene were systematically investigated under the influences of PM. The results showed that the removal efficiency of naphthalene was 74 ~ 97% in bioretention facilities under the influences of PM. With the higher concentration, the lower rainfall return period, and the longer antecedent drying period, the removal efficiency of naphthalene in each medium layer were higher. Furthermore, the PM could increase the naphthalene adsorption capacity onto medium in the first 10 cm depth, which showed more than 80% removal efficiency and lower mobility of naphthalene. The removal efficiency of naphthalene was significantly higher (90 ~ 97%), when the particle size and concentration of PM were 0 ~ 45 μm and 500 mg/L, respectively. This study investigated the important role of PM for naphthalene removal in bioretention facilities, and provided effective guidelines for runoff pollution control, design of stormwater facilities, and assessment risk of naphthalene.

A recent overview of the effect of road surface properties on road safety, environment, and how to monitor them

Road pavements are exposed to traffic loading and external deterioration agents which both can compromise tire-pavement interactions, posing a threat to road safety. The surface wear generates crushed pavement materials or tire rubber wear particles which, combined with other contaminants, negatively impact the environment. In fact, these particles may remain on the road surface; adhere to tires; become airborne; or drain by rainfall to roadsides, waterways, lakes, and even open oceans. Therefore, the presence of road contaminants on road infrastructure pavements is a concern for both road safety and the environment. Although the condition of pavement, traffic intensity, airborne dust emissions, and roadside environments are significantly monitored by road practitioners, especially in urban areas, there is still a need to better evaluate the pollutants remaining on road surfaces. It is known that particles smaller than 40 μm can be trapped within micro-asperities and remain on road surfaces, while particles of larger sizes can be transported by rainfall. However, it is difficult to mobilize particles with sizes larger than 105 μm by storm water runoffs, which tend to remain on road surfaces. Hence, not only rainfall characteristics are responsible of particle's kinetics, but also particle's size and road surface roughness are important. Therefore, this paper presents an overview of road safety and environmental concerns around contaminants, highlighting the importance of the road surface characteristics in their behavior. Finally, the current methods to measure road surface characteristics and their application for environmental and safety issues are discussed.

Leaching risks of antibiotic resistance genes in urban underlying surface sediments during the simulated stormwater runoff and its controls

Recently, increasing attention has been paid to antibiotic resistance genes (ARGs) in urban stormwater runoff. However, no available literature could be found on ARGs leaching from urban underlying surface sediments during stormwater runoff. In this study, surface sediments from commercial public squares around Nanjing (China) was selected for the investigation of target ARGs leaching kinetics, showing that absolute abundances of target ARGs desorption reached at the equilibrium during leaching time of 120-240min with all of the desorption efficiencies about 30%, indicating that there would be low proportion of leaching ARGs in the total ARGs migrating with runoff during rainfall events. Five target ARGs leaching including intI1 (clinic), strA, strB, tetM and tetX can be better described by the pseudo-second-order equation, while qacEdelta1 leaching can be better described by the pseudo-first-order equation, and the leaching for both sul1 and sul2 can be well described by the pseudo-first-order and pseudo-second-order equations. The effects of environmental factors including S/L ratios, pH values and water temperatures indicated that leaching efficiencies of target ARGs enhanced significantly with the increase of S/L ratios and water temperatures, but decreased with the increase of pH values. The transmission experiments after the simulated solar irradiation and heat implied that both large solar UV irradiation (30W/m²) and high temperature (40℃) were conducive to conjugation and transformation frequencies of ARGs. Furthermore, both high Cao and medium PAM levels could be effective for blocking ARGs transmission in the leachate from underlying surface sediments.

Road-deposited sediments mediating the transfer of anthropogenic organic matter to stormwater runoff

It has been regarded that road-deposited sediment (RDS) is one of the important sinks of anthropogenic pollutants as well as the major source of pollutants in stormwater runoff. However, the role of RDS, as a mediator of pollutants to the stormwater runoff, has not yet been investigated so far. Therefore, in this study, the leaching of dissolved pollutants, especially dissolved organic matter (DOM) from RDS, in synthetic precipitation was investigated. A significant amount of metals, nutrients, dissolved compounds, and DOM was leached. The leaching of DOM during 10 sequential leachings was 1811.3 and 2301.7 mg C/kg for larger (63 μm-2 mm) and smaller (< 63 μm) RDS, respectively. The results of UV/Vis spectroscopy, fluorescence spectroscopy, and size exclusion chromatography showed that the leached DOM was of anthropogenic/abiotic origins with lower molecular weight and humification degree. It is ubiquitous in stormwater runoff and industrial discharges and differs from natural organic matter. The results strongly suggest that RDS is an important mediator transferring anthropogenic pollutants to stormwater runoff. In addition, the removal of RDS, such as sweeping, would significantly reduce the pollutants input to the runoff.

Potentially toxic elements pollution in road deposited sediments around the active smelting industry of Korea

Potentially toxic elements (PTEs) were investigated in the different sizes of road deposited sediments (RDS) around the active smelting industry to understand their sources and to assess the pollution and ecological risk levels. The highest PTEs concentrations was shown near the raw materials import port and the smelting facilities. The fine particles of RDS showed extremely high PTEs concentrations. Zn has the highest mean concentration in the < 63 μm particle size of RDS, followed by Pb > Cu > As > Cr > Ni > Cd > Hg. The PTEs concentrations of this study were the highest values compared to the soils around the smelter and the RDS in urban and industrial areas in the world. This indicates that these PTEs pollution in RDS were mainly attributed to the transportation of raw materials for the smelting industry. According to nemerow pollution index calculation, RDS at all sampling sites with particles of less than 250 mm was seriously polluted with PTEs. The ecological risk was also found to be very high in all RDS fractions and highly toxic elements such as Cd, Pb and Hg pose extremely risk. Given the total amounts PTEs in the road surface, it is necessary to apply RDS removal management plan to reduce the PTEs pollution.

Heavy metal pollution by road-deposited sediments and its contribution to total suspended solids in rainfall runoff from intensive industrial areas

Understanding the relationship between road-deposited sediments (RDS) and total suspended solids (TSS) is essential for managing non-point pollution. Studying the heavy metal concentrations of RDS and TSS in rainfall is important to the development of RDS management strategies and to the design of effective stormwater management practices. We investigated the heavy metal (V, Cr, Mn, Co, Ni, Cu, Zn, As, Mo, Cd, Sn, Pb) in RDS and TSS in rainfall runoff to assess the metal pollution level and to evaluate the contribution of RDS heavy metal pollution in the TSS. The heavy metal pollution in RDS and TSS in industrial areas was relatively higher in small particles (<125 μm), while TSS had a higher heavy metal concentration than RDS. In addition, the concentration of heavy metals in TSS increased rapidly during the initial rainfall. The amount of particles larger than 125 μm also increased, suggesting that large metal particles accumulated in industrial areas were also discharged in the form of non-point pollution. The amount of RDS per unit of industrial area (g/m²) and the accumulation of heavy metals (Pb, Cu, and Zn) (mg/m²) were 15- and 8-54-fold higher than those of urban areas, respectively. Considering particles <125 μm, which can be easily transported or discharged during rainfall, the contribution rate of RDS to TSS was 41.3%, but the average contribution rate to heavy metals in TSS was 22.1%. The average load of heavy metals from industrial areas in TSS was 77.9%. The load of Cu, Ni, As, Cd, and Sn exceeded 90%, indicating that most of these metals were attributed to industrial activities related to metal processing. Our results suggest the importance of efficient road cleaning and rainfall runoff management strategies to solve the heavy metal pollution problem caused by non-point sources in industrial areas.

Size distributions and heavy metal pollution of urban road-deposited sediments (RDS) related to traffic types

Little is known about the particles attached on the surfaces of coarse (> 150 μm) road-deposited sediments (RDS), which are potential contributors of stormwater pollution. Therefore, the size distributions and heavy metal (including Cu, Pb, Zn, Cr, and Ni) pollution of RDS were evaluated taking these attached particles into consideration. Moreover, the relationships of RDS contamination with traffic types were further discussed. The results showed that < 22, 22-38.5, 38.5-150, > 150 μm RDS and attached particles accounted for 2.1 ± 1.8%, 8.5 ± 5.7%, 50.7 ± 6.8%, 38.6 ± 8.3%, and 2.0 ± 1.2% of total RDS mass, respectively. The size distributions of attached particles were comparable to that of representative stormwater particles, suggesting their great potential to contribute stormwater particles by desorption. The Zn pollution of attached particles was remarkable, which was significantly higher than that of both fine (< 150 μm) and coarse (> 150 μm) RDS. The Cu and Pb pollution were comparable to that of fine RDS, which were significantly higher than that of coarse RDS. Conversely, the Cr and Ni pollution were comparable to that of coarse RDS, which were significantly lower than that of fine RDS. Traffic types were found to be closely related with RDS contamination, and thus are indicative of specific RDS pollution. For example, electrombile in old residential area is indicative of abnormal Pb pollution due to past emissions of leaded gasoline; lorry is indicative of remarkable Zn pollution of > 22 μm RDS due to abrasion of tires and metallic substances. Accordingly, traffic type is accessible to identify the typical pollutants in RDS to promote effective RDS management.

Variation in the Concentration of Metals in Road Dust Size Fractions Between 2 µm and 2 mm: Results from Three Metallurgical Centres in Poland

The composition of road dust is influenced by emissions from local industry as well as by traffic emissions. Thus, the composition of urban road dust can be used as an indicator for environmental pollution. Pollutants contained in road dust also are transferred into the atmosphere by resuspension and into the aquatic system by wash-off. In this transfer, the particle size of the road dust particles is of extreme importance. Therefore, information about the composition of road dust in dependence of the particle size is crucial. In this study, road dust samples were separated by air classification into size fractions down to 2 µm. The chemical analysis of the size fractions also revealed a significant size dependence of the metal concentrations in the finest size fractions. The least polluted size fraction was generally the fraction 200-500 µm, whereas the highest concentrations were measured in the finest size fraction < 2 µm. These results are important for the assessment of the mass fraction of the various pollutants in the mobile size fractions in re-entrainment as well as in run-off during rainfall.

Influence of rainfall intensity and pollution build-up levels on water quality and quantity response of permeable pavements

Permeable pavements are part of stormwater management practices known as sustainable urban drainage systems (SUDS). This study describes the influence of several environmental variables, such as the rainfall regime or the pollution build-up level, on the hydraulic and water quality performance of permeable pavements. Four infiltrometers with different configurations of pavement layers were used to study the influence of two rainfall regimes (Atlantic and Mediterranean) and two rainfall intensities (0.5 and 2.2mm/min). The influence of the progressive pollution build-up level was studied by dry sprinkling of road deposited sediments collected with a mechanical street sweeper with a dose of 5g/m²/d. The results show that permeable pavements retained a significant rainwater volume and improved the infiltrated water quality in terms of suspended solids, organic matter and nutrients when compared to the corresponding surface runoff potentially generated from an impervious pavement. The volume of rainwater retained inside them varied between 16 and 66% depending on the variables studied. The water infiltrated from permeable pavements subjected to a Mediterranean rainfall regime contained, in general, higher concentrations of organic matter (22 to 89mg Chemical Oxygen Demand/l) and nutrients (0.6 to 2.1mg Total Nitrogen/l and 0.05 to 0.45mg Total Phosphorus/l) than those under Atlantic regime. However, the latter infiltrated higher loadings in terms of mass. Nitrogen was the substance that infiltrated the most, reaching a 25% of the total mass of nitrogen deposited on the pavements surface. The concentration and mass loading in infiltrated water increased as the pollution build-up level did. The leachability of nutrients and organic matter was greater for high rainfall intensities. The results suggest that it is essential to carry out an adequate cleaning in dry conditions, especially when high intensity rainfall events are foreseen, because of its greater capacity to mobilize pollutants.

The upper size limit of the dust samples in road dust heavy metal studies: Benefits of a combined sieving and air classification sample preparation procedure

The concentrations of heavy metals and other components in road dust from a Polish coal mining district were investigated. The sample was split into eight size fractions ranging from <5 μm to 1-2 mm applying the combined classification procedure of dry sieving and air classification. For most elements, significantly higher concentrations were found in the finest size fractions. In combination with the mass fractions of the various size fractions the concentrations of the components were calculated for this road dust assuming different upper size limits. The results demonstrate the important influence of the upper size limit applied on the composition of road dust. An analysis of the composition of road dust by particle size, instead of analysing whole road dust samples, proves to have several advantages. On the one hand, a reliable comparison of the results with the concentration data available in the literature is possible and on the other hand, the results can be used in assessments, for example resuspension or wash-off by rainfall, which require different upper size limits. Upper size limit applied in road dust sampling significantly influences the results. Combined sieving and air classification sample preparation procedure allows flexible use of the results.

Reduction of non-point source contaminants associated with road-deposited sediments by sweeping

Road-deposited sediments (RDS) on an expressway, residual RDS collected after sweeping, and RDS removed by means of sweeping were analyzed to evaluate the degree to which sweeping removed various non-point source contaminants. The total RDS load was 393.1 ± 80.3 kg/km and the RDS, residual RDS, and swept RDS were all highly polluted with organics, nutrients, and metals. Among the metals studied, Cu, Zn, Pb, Ni, Ca, and Fe were significantly enriched, and most of the contaminants were associated with particles within the size range from 63 μm to 2 mm. Sweeping reduced RDS and its associated contaminants by 33.3-49.1% on average. We also measured the biological oxygen demand (BOD) of RDS in the present work, representing to our knowledge the first time that this has been done; we found that RDS contains a significant amount of biodegradable organics and that the reduction of BOD by sweeping was higher than that of other contaminants. Significant correlations were found between the contaminants measured, indicating that the organics and the metals originated from both exhaust and non-exhaust particles. Meanwhile, the concentrations of Cu and Ni were higher in 63 μm-2 mm particles than in smaller particles, suggesting that some metals in RDS likely exist intrinsically in particles, rather than only as adsorbates on particle surfaces. Overall, the results in this study showed that sweeping to collect RDS can be a good alternative for reduction of contaminants in runoff.

Influence of urban surface roughness on build-up and wash-off dynamics of road-deposited sediment

An in-depth understanding of the impacts of surface roughness on road-deposited sediment (RDS) build-up and wash-off is essential for the estimation of surface runoff loads and design of RDS control measures. In this study, RDS build-up and wash-off dynamic processes were investigated on paired asphalt and concrete road surfaces with 35 days of continuous sampling during different natural rainfall events. Our results showed that RDS build-up loads and grain size composition were affected by surface roughness, while the impact of surface roughness on the length of the dynamic equilibrium period was not notable. Selective wash-off of RDS with different effects according to grain size are more likely to occur on asphalt road surfaces during rainfall-runoff, but the RDS wash-off percentage is not affected by surface roughness during snowmelt-runoff. Both total apparent depression depth and micro-depression structures influence RDS build-up and wash-off dynamics. These results imply that surface roughness has combined effects on RDS build-up and wash-off dynamics during the generation and control of urban diffuse pollution.

Influence of urban surface roughness on build-up and wash-off dynamics of road-deposited sediment

An in-depth understanding of the impacts of surface roughness on road-deposited sediment (RDS) build-up and wash-off is essential for the estimation of surface runoff loads and design of RDS control measures. In this study, RDS build-up and wash-off dynamic processes were investigated on paired asphalt and concrete road surfaces with 35 days of continuous sampling during different natural rainfall events. Our results showed that RDS build-up loads and grain size composition were affected by surface roughness, while the impact of surface roughness on the length of the dynamic equilibrium period was not notable. Selective wash-off of RDS with different effects according to grain size are more likely to occur on asphalt road surfaces during rainfall-runoff, but the RDS wash-off percentage is not affected by surface roughness during snowmelt-runoff. Both total apparent depression depth and micro-depression structures influence RDS build-up and wash-off dynamics. These results imply that surface roughness has combined effects on RDS build-up and wash-off dynamics during the generation and control of urban diffuse pollution.

A contribution of nanoscale particles of road-deposited sediments to the pollution of urban runoff by heavy metals

Road-deposited sediments (RDS) present a sink for traffic-related pollutants including heavy metals (HMs). HMs associated with RDS particles enter the urban aquatic environment during rainfall events and have adverse effects for biota. RDS nanoscale particles (NSPs) require special consideration due to their specific properties, extremely high mobility in the environment, and ability to penetrate into living organisms. In the present work, the contribution of NSPs of RDS to the pollution of urban runoff by HMs has been evaluated for the first time. It has been shown that bulk RDS samples are polluted by HMs as compared to background urban soils (geo-accumulation indexes of Cu and Zn may attain 2-3). Meanwhile, NSPs of RDS are enriched by HMs as compared to bulk samples; concentration factor for Ni, Cu, Zn, Cd, Sn, and Pb in NSPs being varied from 2 to 10. The water-soluble fractions of RDS samples were also analyzed. Results have shown that the content of water-soluble HMs in RDS is insignificant and rarely exceeds 0.5% of the total contents of HMs in the bulk samples; the highest contents are identified for Cu and Pb. It should be noted that the water-soluble fraction is nearly free from Zn and this element is almost entirely present as particulate matter (NSPs). In general, the overall contribution of NSPs and water-soluble fraction of HMs to the pollution of urban runoff is comparable.

Evaluation of the methods for quantifying particle wash-off loadings in urban impervious surfaces at small scales

Quantifying road-deposited sediment (RDS) wash-off is critical in estimating pollutant load and for supporting the designs of urban storm water low-impact development (LID) best management practices (BMPs). The objective of this study was to identify a simple and effective small-scale wash-off load estimation and prediction method suitable for BMP engineering. The reliabilities, complexities, and applicabilities of four prediction methods (i.e., the event mean concentration (EMC) equation method, the exponential equation method, and the RDS mass difference before and after rainfall event method, and the RDS index model) were tested for three sites (i.e., a newly built area, and built-up area, and conservation area) during five natural rainfall events. Taking the calculations by the EMC method as standard results revealed the following. (1) The degree of agreement of the methods with the standard results in descending order was exponential equation method > RDS index model > RDS mass difference. (2) The degree of complexity of the studied methods in descending order was exponential equation method > the EMC equation method > the RDS index model > the RDS mass difference. (3) When considering the applicability of the models in terms of LID-BMP engineering, the RDS index model could be considered optimal for supporting the design of urban storm water BMPs.

Development of an indicator for characterizing particle size distribution and quality of stormwater runoff

Stormwater particles washed from road-deposited sediments (RDS) are traditionally characterized as either turbidity or total suspended solids (TSS). Although these parameters are influenced by particle sizes, neither of them characterizes the particle size distribution (PSD), which is of great importance in pollutant entrainment and treatment performance. Therefore, the ratio of turbidity to TSS (Tur/TSS) is proposed and validated as a potential surrogate for the bulk PSD and quality of stormwater runoff. The results show an increasing trend of Tur/TSS with finer sizes of both RDS and stormwater runoff. Taking heavy metals (HMs, including Cu, Pb, Zn, Cr, and Ni) as typical pollutants in stormwater runoff, the concentrations (mg/kg) were found to vary significantly during rainfall events and tended to increase significantly with Tur/TSS. Therefore, Tur/TSS is a valid parameter to characterize the PSD and quality of stormwater. The high negative correlations between Tur/TSS and rainfall intensity demonstrate that stormwater with higher Tur/TSS generates under low intensity and, thus, characterizes small volume, finer sizes, weak settleability, greater mobility, and bioavailability. Conversely, stormwater with lower Tur/TSS generates under high intensity and, thus, characterizes large volume, coarser sizes, good settleability, low mobility, and bioavailability. These results highlight the need to control stormwater with high Tur/TSS. Moreover, Tur/TSS can aid the selection of stormwater control measures with appropriate detention storage, pollution loading, and removal effectiveness of particles.

Characterization of wash-off from urban impervious surfaces and SuDS design criteria for source control under semi-arid conditions

Knowledge about pollutant wash-off from urban impervious surfaces is a key feature for developing effective management strategies. Accordingly, further information is required about urban areas under semi-arid climate conditions at the sub-catchment scale. This is important for designing source control systems for pollution. In this study, a characterization of pollutant wash-off has been performed over sixteen months, at the sub-catchment scale for urban roads as impervious surfaces. The study was conducted in Valencia, Spain, a city with a Mediterranean climate. The results show high event mean concentrations for suspended solids (98mg/l), organic matter (142mgCOD/l, 25mgBOD₅/l), nutrients (3.7mgTN/l, 0.4mgTP/l), and metals (0.23, 0.32, 0.62 and 0.17mg/l for Cu, Ni, Pb, and Zn, respectively). The results of the runoff characterization highlight the need to control this pollution at its source, separately from wastewater because of their different characteristics. The wash-off, defined in terms of mobilized mass (g/m²) fits well with both process-based and statistical models, with the runoff volume and rainfall depth being the main explanatory variables. Based on these results and using information collected from hydrographs and pollutographs, an approach for sizing sustainable urban drainage systems (SuDS), focusing on water quality and quantity variables, has been proposed. By setting a concentration-based target (TSS discharged to receiving waters <35mg/l), the results indicate that for a SuDS type detention basin (DB), an off-line configuration performs better than an on-line configuration. The resulting design criterion, expressed as SuDS volume per unit catchment area, assuming a DB type SuDS, varies between 7 and 10l/m².

Investigation of the wash-off process using an innovative portable rainfall simulator allowing continuous monitoring of flow and turbidity at the urban surface outlet

Development of appropriate models, based on an in-depth understanding of the wash-off process, is essential to accurately estimating pollutant loads transported by stormwater, thereby minimizing environmental contamination. To this end, we developed an innovative rainfall simulator, which simulated an intense rainfall (120mm/h) and permitted the acquisition of runoff samples as well as the in situ monitoring of continuous flow and turbidity dynamics. Relationships between deposited sediments and total suspended solids in simulated runoff were thus investigated on two different types of surfaces within the Paris region in terms of loads and particle size distribution. Results demonstrate the occurrence of first flush phenomenon on the sidewalks even under constant flow. Results also show that the highest fraction conveyed by runoff consisted of fine (<16μm) and medium-sized (<100μm) particles, whose detachment was more favorable from smooth surfaces than from rougher ones. In terms of stormwater quality modelling, results suggest that the integration of a wash-off fraction based on both particle size and rainfall intensity could be an entrance for a better prediction of stormwater pollution.

Contribution of atmospheric dry deposition to stormwater loads for PAHs and trace metals in a small and highly trafficked urban road catchment

A deep understanding of pollutant buildup and wash-off is essential for accurate urban stormwater quality modeling and for the development of stormwater management practices, knowing the potential adverse impacts of runoff pollution on receiving waters. In the context of quantifying the contribution of airborne pollutants to the contamination of stormwater runoff and assessing the need of developing an integrated AIR-WATER modeling chain, loads of polycyclic aromatic hydrocarbons (PAHs) and metal trace elements (MTEs) are calculated in atmospheric dry deposits, stormwater runoff, and surface dust stock within a small yet highly trafficked urban road catchment (~ 30,000 vehicles per day) near Paris. Despite the important traffic load and according to the current definition of "atmospheric" source, atmospheric deposition did not account for more than 10% of the PAHs and trace metal loads in stormwater samples for the majority of the events, based on the ratio of deposition to stormwater. This result shows that atmospheric deposition is not a major source of pollutants in stormwater, and thus, linking the air and water compartment in a modeling chain to have more accurate estimates of pollutant loads in stormwater runoff might not be relevant. Comparison of road dust with water samples demonstrates that only the fine fraction of the available stock is eroded during a rainfall event. Even if the atmosphere mostly generates fine particles, the existence of other sources of fine particles to stormwater runoff is highlighted.

Effect of VOC Emissions from Vegetation on Air Quality in Berlin during a Heatwave

The potential of emissions from urban vegetation combined with anthropogenic emissions to produce ozone and particulate matter has long been recognized. This potential increases with rising temperatures and may lead to severe problems with air quality in densely populated areas during heat waves. Here, we investigate how heat waves affect emissions of volatile organic compounds from urban/suburban vegetation and corresponding ground-level ozone and particulate matter. We use the Weather Research and Forecasting Model with atmospheric chemistry (WRF-Chem) with emissions of volatile organic compounds (VOCs) from vegetation simulated with MEGAN to quantify some of these feedbacks in Berlin, Germany, during the heat wave in 2006. The highest ozone concentration observed during that period was ∼200 μg/m³ (∼101 ppb_V). The model simulations indicate that the contribution of biogenic VOC emissions to ozone formation is lower in June (9-11%) and August (6-9%) than in July (17-20%). On particular days within the analyzed heat wave period, this contribution increases up to 60%. The actual contribution is expected to be even higher as the model underestimates isoprene concentrations over urban forests and parks by 0.6-1.4 ppb_v. Our study demonstrates that biogenic VOCs can considerably enhance air pollution during heat waves. We emphasize the dual role of vegetation for air quality and human health in cities during warm seasons, which is removal and lessening versus enhancement of air pollution. The results of our study suggest that reduction of anthropogenic sources of NOx, VOCs, and PM, for example, reduction of the motorized vehicle fleet, would have to accompany urban tree planting campaigns to make them really beneficial for urban dwellers.

Physicochemical conditions and properties of particles in urban runoff and rivers: Implications for runoff pollution

In this study, to gain an improved understanding of the fate and fractionation of particle-bound pollutants, we evaluated the physicochemical conditions and the properties of particles in rainwater, urban runoff, and rivers of Yixing, a city with a large drainage density in the Taihu Lake Basin, China. Road runoff and river samples were collected during the wet and dry seasons in 2015 and 2016. There were significant differences between the physicochemical conditions (pH, oxidation-reduction potential (ORP), and electroconductivity (EC)) of rainwater, runoff, and rivers. The lowest pH and highest ORP values of rainwater provide the optimal conditions for leaching of particle-bound pollutants such as heavy metals. The differences in the physicochemical conditions of the runoff and rivers may contribute to the redistribution of pollutants between particulate and dissolved phases after runoff is discharged into waterways. Runoff and river particles were mainly composed of silt and clay (<63 μm, 88.3%-90.7%), and runoff particles contained a higher proportion of nano-scale particles (<1 μm) but a lower proportion of submicron-scale particles (1-16 μm) than rivers. The ratio of turbidity to TSS increased with the proportion of fine particles and was associated with the accumulation of pollutants and settling ability of particles, which shows that it can be used as an index when monitoring runoff pollution.