Water and soil pollution implications of road traffic

Abstract

Rising vehicle ownership around the world has resulted in greater traffic volumes on roads and associated traffic-generated pollutants. Vehicular traffic and, by implication, road surfaces are among the most important sources of pollutants to the environment, particularly the urban environment, and can have significant human and ecosystem health impacts. Research studies have confirmed the vulnerability of the population living near high-traffic areas, particularly the elderly. An equal concern is the uptake of trace metals by plants near road environments. Road surfaces also act as repositories for pollutants from other sources that can react with traffic-generated pollutants, leading to changes to pollutant characteristics, influencing their solubility, reactivity, bioavailability, mobility, toxicity and persistence. Pollutants deposited on road surfaces are incorporated in road dust. The primary sources of traffic pollutants are exhaust emissions, fuel and lubrication system leakages, component wear such as tyres, brakes and chassis and road surface wear. Traffic and road surface factors play significant roles in influencing the type, amount and rate of pollutant generation. It has been found that traffic congestion is more significant than traffic volume in terms of pollutant generation. The pollutant emissions from heavy-duty vehicles that use diesel are different compared to light-duty vehicles that use gasoline. Very wide ranges of trace metals and hydrocarbons are emitted into the environment as a result of fuel combustion and vehicle component and road surface wear. The metals most commonly originating from traffic activities include Cd, Cu, Cr, Ni and Zn. Pb is added to road surfaces by the accumulation in soils due to past usage of leaded fuels, as well as ongoing contributions from the wear of vehicle components and road paint. Among the hydrocarbons, polycyclic aromatic hydrocarbons are of most concern, as these compounds represent a significant class of suspected carcinogens. Used lubrication oil and incomplete combustion of fossil fuels, particularly diesel, are significant contributors of polycyclic aromatic hydrocarbons to the environment. It is hypothesised that climate change-driven alterations to climate characteristics will influence changes to traffic pollutant build-up on ground surfaces and wash-off with stormwater runoff.