published on 22 March 2017 in water

Road runoff and environmental pollution

What is road runoff?
Rain, in falling to the ground, can be contaminated in two ways: as a result of the dissolution of compounds present in the atmosphere due to anthropogenic and/or natural events (mainly metals, chlorides and sodium), or as a result of the road runoff phenomenon. The latter is the one that can mostly pollute the environment, especially in the first few minutes of intense rainfall, the so-called “storm water”.
It may surprise you to know that among the sources of water pollution there is the road, travelled on every day by motor vehicles of all kinds. When rainwater washes the roads, it brings with it pollutants deposited on the tarmac by vehicles.

Oils and hydrocarbons deposited on the tarmac due to vehicular traffic and dissolved following precipitation (http://www.bayareaperviousconcrete.com/).

The environmental hazard of this water depends on the nature of the ground (structure, inclination, permeability, surface type, etc.), on the type of use of the ground itself (agriculture, civil, manufacturing, services, transport, etc.) and finally on the substances dispersed on it or that fall from the atmosphere due to human activities.
This is a significant source of pollution because it includes a large quantity of different substances, all potentially dangerous, contained in the rain water that washes the tarmac and in the solid particles resting on it (road sediment). For example, exhaust gases and lubricants release lead, hydrocarbons, nickel and bromine. Iron and chromium detach from corroded bodywork, while sulphur, chlorine and cyanide are dispersed via cooling liquids. Finally, tyres deposit rubber particles containing lead, cadmium and zinc on the tarmac.

Waste on the ground washed away by rain (http://www.texastrees.org/).

Why is storm water important?
The reference national law, the Consolidated Environment Act (TUA) of Legislative Decree 152/2006, defines that in any atmospheric event, storm water corresponds to a rainfall lasting approx. 15 minutes, which corresponds to 5 mm evenly distributed on the surface.
Storm water has the ability to wash road surfaces, both by washing away the largest fraction of particulate matter, as well as by dissolving the pollutants that have been deposited between one atmospheric event and another (or between road cleaning operations).

The pollutants present on road surfaces come from atmospheric deposits during dry weather (of natural or anthropogenic origin), from vehicular traffic (combustion of fuels, tyre wear, vehicle mechanical parts and braking system, vehicle bodywork corrosion, etc.), from mainly organic waste, from vegetation, from erosion of the road surface caused by vehicular traffic and from barrier corrosion.

Diagram of road runoff and main pollution. Sources https://www.nsa.gov/resources/everyone/commitment-to-environment/

What comes out of vehicle exhaust gases?
The gaseous emissions from motor vehicle exhausts can fall back on the ground in greater or lesser proximity to the point of emission, resulting in accumulation of pollutants on the ground or in surface water. Exhaust gases, in addition to water vapour, carbon dioxide and nitrogen gas, contain numerous other pollutants such as carbon monoxide, unburnt hydrocarbons, nitrogen oxides, ammonia, hydrogen sulphide, carbonyl sulphide, sulphur dioxide, sulphates, organic sulphur compounds, aldehydes, ketones, phenols, organic amines, nitramines, alcohol, polycyclic aromatic hydrocarbons, particulate matter, organic compounds related to particulate matter, metals and metal compounds.

What does road runoff contain?
Numerous international studies have shown that, very often, the concentrations of pollutants measured exceed the limits and specifications of Italian legislation, especially as regards environmental quality standards.
Of particular interest is the concentration of hydrocarbons (polycyclic aromatic hydrocarbons) that abundantly exceed the specifications of the legislation.
In a study conducted on the deep sediments of Lake Ontario (Canada), a significant presence of hydrocarbons was found, whose accumulation increased as a result of the increased urbanization of the drainage basin, as well as of commercial vessels and recreational activities on the lake and its tributary rivers. For example, in Lake Washington, the major source of hydrocarbons, according to some experts, is probably due to runoff from paved roads in the drainage basin, estimating that approx. 85% of petroleum hydrocarbons found in the lake’s sediments come from rainfall runoff. The similarities between increases in oil and lead have also suggested a connection with residues from car exhausts. 

Can road runoff pollute the environment?
At the international level, numerous studies have been conducted showing that the water and sediments from the runoff from road paving and related appurtenances (car parks, service areas, parking areas, etc.) can be very contaminated and cause a significant negative impact on the quality of the environment (surface water, groundwater, soil, river and lake sediments).
The table shows the main categories of pollutants contained in surface runoff by category, parameters and sources that determine its presence and its effect on the environment.

Main categories of pollutants contained in surface runoff, sources and effects.

It is important to note that it is generally not possible to find road runoff devoid of pollutants. Consequently, it is very important that road runoff water and sediment, especially along busy roads or adjacent to environmentally sensitive areas, is properly treated before being released into the environment.
The reference legislation also specifies that storm water must be sent, in compliance with emission limits, in preferential order to the sewer system, to surface water bodies, to the ground or to the surface layers of the subsoil and possibly be first subjected to treatment to reduce the pollutant content.

By Dario Colucci

To find out more:

  • Ball J.E., Jenks R. and Ausborg D. (1998). An assessment of availability of pollutant constituents on road surfaces. The Science of the Total Environment, 209, 243-254.
  • Da Deppo L. (2011). Raccolta e trattamento delle acque di prima pioggia. IX giornata sul drenaggio urbano sostenibile. University of Brescia.
  • Fabiani C., Donati A., De Gironimo G., Bernabei S., Dell’osso D., Munafò M., Cecchi G. (2006). Strumenti per la valutazione degli impatti provocati dalle acque di prima pioggia nelle aree urbane. APAT (in http://www.isprambiente.gov.it/files/pubblicazioni/statoambiente/ambiente-urbano-3-2006/appendice.pdf).
  • Papiri S. and Todeschini S. (2004). Qualità e controllo delle acque di dilavamento di infrastrutture viarie. University of Pavia
  • Consolidated Environmental Act, Legislative Decree 152/2006, as amended and supplemented.
  • S. EPA (1993). Handbook of Urban Runoff Pollution, Prevention and Control Planning. EPAI625/R-93-004 September 1993.
  • Wakeham SG (1977a). A characterization of the sources of petroleum hydrocarbons in Lake Washington. J. Water Pollut Control Fed 49: 1680-1687
  • Wakeham SG (1977b) Hydrocarbon budget for Lake Washington. Limmol Oceanogr 22:952957.
  • Bourbonniere and Meyers. Sedimentary geolipid records of historical changes in the watersheds and productivities of Lakes Ontario and Erie. Limnol. Oceanogr., 41(2), 1996, 352-359 0 1996, by the American Society of Limnology and Oceanography, Inc.
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