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Purpose The proportion of people living in urbanised areas is predicted to rise to > 65% by 2050, and therefore, more humans than ever will be exposed to urban environmental pollution. Accumulation of organic and inorganic substances on street and road surfaces is a major global challenge requiring scientifically robust methods of establishing risk that inform management strategies. This aim of this contribution is to critically review the global literature on urban road–deposited sediment contamination with a specific focus on variability in sampling and analytical methods. Materials and methods In order to assess the concentration of contaminants in global road-deposited sediment (RDS), a comprehensive search of published RDS studies was completed. We review methodological approaches used in RDS studies to highlight the variability in datasets as a result of sampling technique, grain size fractionation, geochemical and mineralogical characterisation methods and establishing the influence of local geology on contaminant concentrations. We also consider emerging contaminants in RDS, and we provide a workflow diagram which promotes a standardised sampling and analysis regime that we believe can reduce data variability and promote collaboration when it comes to tackling the important issue of RDS contamination. Results and discussion Across the literature, Asia (except China) and Africa are underrepresented in RDS studies despite these continents having the largest and fastest growing populations, respectively. The removal of tetraethyl lead from gasoline produced a noticeable decrease in lead concentrations in global RDS, and platinum group element (PGE) concentrations in RDS were consistent with catalytic converter usage. Research into the impact of electric vehicles on non-exhaust emissions suggests other contaminants such as zinc may become more prominent in the future. Most RDS studies consider grain size fractions larger than > 20 μm due to sampling constraints despite RDS Conclusions This review highlights considerable variability in sampling and analytical approach which makes it difficult to identify broad global patterns in RDS contamination. To remove this variability from future RDS research, this review suggests a workflow plan which attempts to improve the comparability between RDS studies. Such comparability is crucial in identifying more discrete RDS trends and informing future emission policy.

The urban sedimentary system is attracting increasing interest because of its role in influencing air and water quality. A large amount of road-deposited sediment (RDS) lies on the road network of Prince George, a city of about 80,000 people, in British Columbia, Canada. The objectives of this study were: (1) to determine the total mass of RDS within the city, and how this varied over time and space; and (2) to determine the temporal and spatial variations in the particle-size fractions of the RDS. Samples of RDS were collected using a grid network during two different time periods in the snow-free season in 2009. Composite samples for each grid (n = 46) were fractionated into five grain-size classes (>500, 250–500, 125–250, 63–125, 500-μm fraction, and there was a trend of decreasing amounts (by mass) for the finer particle-size fractions. In terms of the most important particle-size fraction from an air and water quality perspective, the

Over 50% of the global population live in urban centres and, therefore, an understanding of the processes acting upon urban systems is a global issue. The nature of human-made, often impervious, land surfaces and heavily engineered waterways results in hydrological and sedimentological systems in urbanised basins which contrast significantly to those within more natural (i.e. pristine, forested, agricultural) aquatic systems. In addition, the abundance of contamination sources in urban systems results in chemical pressures often manifested as high pollution concentrations or loadings, which in turn have detrimental impacts on human and ecosystem health. These lead to management and sustainability issues not generally encountered in more natural environments. The purpose of this review is to provide a state-of-the-art assessment of sediment sources, pathways and storage within urban river systems, to consider sediment management within urban systems and river basins, and examine the role of local and global environmental changes on sediment processes and management. Inevitably, much of the sediment that is transported within urbanised basins is contaminated, so this review also considers sediment–contaminant sources and interactions. We reach a number of conclusions and recommendations for future research. There is a need for better sampling and monitoring of sediment and sediment-associated contaminant fluxes and cycling in urban river channels and basins. This should include better techniques and studies to identify sources and transfers of road-deposited sediment (RDS), airborne particulate matter and sediments in the river system. Greater interdisciplinary research, combining sedimentologists, hydrologists, urban planners, urban archaeologists, chemists and biologists, is needed. More attention needs to focus on upscaling and connecting urban areas to the rest of the river basin, both upstream and downstream. Finally, there is a need to balance multiple needs (urban population, water resources) with likely trends in both urban development and global environmental change.

An important component of monitoring pollution of urban road-deposited sediment (RDS) is an understanding of the temporal variability in its composition and physical characteristics. This study set out to determine what the monthly variability in metal concentrations, organic matter content, grain-size and grain-size fraction metal-loadings are in inner city sites in Manchester, northwest England. The results show that there is significant temporal variability in metal (Pb, Zn, Fe, Mn) concentration in RDS from inner city Manchester. There was no significant temporal variability in grain-size characteristics or organic matter content, indicting that these metal variations were the result of variation in sources and accumulation processes. Pb and Zn displayed local variability, suggesting local controls on variability, whereas Fe and Mn displayed consistent variability across all sites, suggesting a common, larger-scale control on variability. The finest grain-size fraction ( 300 μm) commonly contained the highest concentrations, again suggesting differing controls. For all metals, due to the weight percent dominance of the coarser fractions, the dominant loading of metals is in the coarser fractions. This has implications on management strategies, via street sweeping and the subsequent waste disposal, and on the modelling of the input of RDS and associated metals into surface waterways. The recognition of significant temporal variability of metal concentrations in RDS, independent of grain-size changes, implies that the monitoring of urban road sediment pollution will require not just consideration of spatial variability, but the design of schemes that will capture temporal variability also.

Remobilisation of contaminant metals from sediments can occur by chemical, biological or physical changes. This in turn can lead to contaminant fluxes to the porewaters and ultimately the water column. The aim of the research presented here is to document post-depositional controls on metal mobility and fluxes in a heavily metal-contaminated estuary. This will allow for an improved understanding of the impact of contaminated sediments on water quality from both a short-term and long-term perspective. Dulas Bay is situated on the east coast of Anglesey, North Wales, and receives polluted waters from Parys Mountain. Metals within Dulas Bay sediments show surface enrichment and variations in mineralogical form. Diagenesis clearly plays a role in post-depositional behaviour of the metals, forming sulphides and potentially carbonates. The presence of a dominant exchangeable/carbonate fraction, and elevated porewater metals, in this sulphidic system is significant and could indicate the presence of freshwater diagenesis, or, reflect the high levels of metals in the sediment.