Road run-off

London’s rivers are more polluted than they should be, and one of the causes is that sediment,  heavy metals, petrol and other pollutants from cars are washing into them from roads when it rains.

Check out our story map here: Spatial Quantification of Road Runoff in Greater London 20-08-19

The long term impacts of road run-off pollutants are not fully understood, but the short term effects can be acute and contribute to fish kills in so-called ‘first flush’ events. This happens when a long dry period is followed by rain, as seen in the 2018 drought in London. Organic material, hydro-carbons (major components of petroleum) and other chemicals build up on our roads, becoming more concentrated. When it does eventually rain, this sediment washes into London’s rivers. Oxygen levels crash as bacteria use oxygen to break down the organic material entering the river, leaving fish struggling to breathe. In the worst instances, they die.

Dead fish float on the surface after pollutants are washed in from roads

Thames21 is leading a partnership project with Middlesex University and ZSL (funded by the Environment Agency, Transport for London and the Greater London Authority) that aims to quantify the amount of pollution from roads entering our rivers. This project will use ‘big data’ from transport models to calculate the quantity of  pollutants deposited on roads from traffic throughout the capital. The report was completed in 2019 and is available below:

Spatial Quantification of Road Runoff in Greater London 20-08-19



Identifying the pollution hotspots to help target solutions

Inspired by the need to improve water quality in London, Thames21 and partners have identified the road sources which are likely to contribute the most amount of pollution to our rivers. Now areas at risk are identified and quantified, we have provided authorities  (the Greater London Authority,  the Environment Agency, Transport for London and local councils) with a methodology to calculate the risks associated with pollutants. This data is presented in a heat map to help local authorities and communities locate areas in which water quality should be targeted, potentially through  new green infrastructure such as SuDS (Sustainable Urban Drainage Systems).  Appropriately sited green infrastructure has been shown to cut the volume of pollutants entering rivers, by soaking them up like a sponge and filtering impurities.

In areas with separate sewers, road runoff goes straight into the river


Future investigations, based on the evidence gained from this work, will be needed to develop catchment scale mitigation and treatment plans. This project has the potential to improve the health of London’s rivers by pinpointing the worst road run off pollution hotspots and paving the way for new green spaces to be located where they will have maximum pollution—busting impact.

Read the full report:Spatial Quantification of Road Runoff in Greater London 20-08-19