Characterisation of road-dust sediment in urban systems: a review of a global challenge

Abstract

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 < 20 μm being most relevant to human health. The use of chemical extraction methods to establish contaminant geochemistry is popular; however, most extraction procedures are not relevant or specific to minerals identified in RDS through microscopic and spectroscopic investigations. 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.

An introduction to advances in sediment science and management

“…This special issue contains papers that review some of these developments (e.g. Collins et al 2020;Haynes et al 2020;Owens 2020), while others provide a perspective on keys aspects and offer recommendations for how the discipline could progress (e.g. Heise et al 2020;Spadaro and Rosenthal 2020;Vercruysse et al 2020).…”

“…Three of these papers (i.e. Collins et al 2020;Haynes et al 2020;Heise et al 2020) provide some developments and perspectives-on sediment source fingerprinting, urban road-deposited sediment, and ecotoxicological testing of sediments and dredged material, respectively-since three of the most widely cited papers on sediment in JSS (i.e. Chapman and Hollert 2006;Taylor and Owens 2009a;Walling 2013).…”

“…PM 2.5 ). Haynes et al (2020) provide a review of RDS that considers the following: (i) global patterns of RDS composition (mainly metals and metalloids); and (ii) the field sampling and analytical approaches used to assemble this information. They show how some contaminants in RDS, such as lead, have tended to decrease while others have either increased or are likely to become a problem in the future (e.g.…”

“…polycyclic aromatic hydrocarbons (PAHs) and microplastics) given technological and societal developments. Haynes et al (2020) also offer advice, via a "workflow", for undertaking future studies on RDS.…”

“…An area touched-on by Haynes et al (2020) is ways to determine if contaminated sediment is detrimental to human health and aquatic ecosystems. In RDS studies, this is often addressed via the use of pollution/contamination indices (such as sediment quality guidelines, enrichment ratio or geoaccumulation index) or chemical sequential extraction procedures.…”