The Hidden Sources of Pollution in Our Stormwater Systems
When road pollution is discussed, attention usually focuses on vehicle emissions, tyre wear particles, brake dust and fuel leaks. While these are undoubtedly significant sources of contamination, they are not the only pollutants entering our rivers, lakes and oceans through stormwater runoff.
The roads themselves, and even the markings painted onto them, are continuously wearing away. Every vehicle movement, rainfall event and seasonal temperature change contributes to the gradual degradation of road surfaces and road markings. As these materials break down, they release a complex mixture of contaminants including microplastics, heavy metals, hydrocarbons and other toxic compounds.
Road Runoff Pollution Pathway From Road to River
For stormwater engineers, drainage consultants, SuDS designers and environmental professionals, understanding these often-overlooked pollution sources is becoming increasingly important. As water quality regulations tighten and public awareness of microplastic pollution grows, it is essential to consider all contributors to urban runoff contamination.
Roads Are Not Just Stone and Bitumen
Most roads in the UK are constructed using asphalt pavements. Although often perceived as inert infrastructure, asphalt is a carefully engineered material consisting of approximately 90% mineral aggregates and 10% bitumen binder, alongside various additives and modifiers designed to improve durability and performance.
Road surfaces are subjected to constant stress throughout their service life. Vehicle loading, tyre friction, temperature fluctuations, rainfall and ultraviolet radiation all contribute to gradual wear and degradation.
As the road surface abrades, particles are released into the environment and become entrained in stormwater runoff. These particles may contain contaminants originating from the aggregates themselves, the bitumen binder, or chemical additives incorporated during manufacture.
Among the pollutants associated with road pavement degradation are:
- Hydrocarbons
- Polycyclic Aromatic Hydrocarbons (PAHs)
- Heavy metals
- Volatile Organic Compounds (VOCs)
- Microplastics from polymer-modified asphalt
- Rubber crumb particles from recycled tyre additives
- Fine particulate matter and sediment
Many of these contaminants attach themselves to extremely fine particles, particularly those smaller than 64 microns (PM64). These particles remain suspended within stormwater runoff and can travel considerable distances through drainage networks before reaching receiving waters.
For a detailed review of contaminants released from road surfaces and their environmental impacts, see our Technical paper: What Stormwater Runoff Contamination Originates from Road Pavements?
Road Markings: Small Lines, Big Environmental Impact
Road markings play a critical role in road safety and traffic management, yet their environmental impacts are rarely discussed.
Most road markings are composed of resin binders, pigments, additives and reflective glass microbeads. Depending on the application, these materials may include thermoplastics, solvent-based paints, water-based paints or cold plastic systems.
Like road surfaces, road markings are constantly exposed to wear. Vehicle tyres repeatedly pass over them, while rainfall, UV exposure and temperature fluctuations cause cracking, fading and deterioration.
As these materials wear away, they release particles that are readily transported into stormwater drainage systems.
The contaminants associated with road marking degradation include:
- Microplastics from thermoplastic resins
- Heavy metals from pigments and fillers
- VOCs from paint formulations
- PAHs from chemical additives
- Glass microbeads and associated contaminants
Recent research suggests that road markings may represent a significant and under-recognised source of microplastic pollution. However, road marking wear is often grouped into broader categories of road-derived pollution, meaning its contribution remains poorly understood and frequently overlooked during drainage design and environmental assessments.
For a full breakdown of road marking pollution pathways and contaminant sources, see our Technical paper: The Role of Road Markings in Stormwater Runoff Contamination.
What Pollutants Are Entering Stormwater Runoff?
Understanding the contaminants released from roads and road markings is essential because each pollutant behaves differently in the environment and presents unique treatment challenges.
Microplastics
Microplastics are plastic particles smaller than 5 mm and are now found throughout soils, rivers, lakes, oceans and even the atmosphere.
Road-derived microplastics originate from polymer-modified asphalt, thermoplastic road markings and associated plastic additives. Once released, they can persist in the environment for decades, accumulate in sediments and be ingested by aquatic organisms.
Heavy Metals
Heavy metals commonly associated with road surfaces and road markings include lead, zinc, chromium, copper, nickel, vanadium, titanium, barium, arsenic and antimony.
These contaminants often bind to fine sediment particles, making them highly mobile within stormwater runoff. Once deposited in aquatic environments, they can accumulate in sediments and biological tissues.
Polycyclic Aromatic Hydrocarbons (PAHs)
PAHs are toxic organic compounds associated with petroleum-based materials such as bitumen and various road marking additives.
Because PAHs are hydrophobic, they tend to bind strongly to sediment particles. This means they often accumulate within river sediments, where they can persist for long periods and enter aquatic food webs.
Volatile Organic Compounds (VOCs)
VOCs are released from road marking materials and asphalt throughout their service life.
Some VOCs dissolve directly into water, while others attach to suspended particles. Many are resistant to degradation and can be transported over large distances within drainage systems.
Glass Microbeads
The reflective glass microbeads used in road markings are often overlooked as a pollutant source.
Although relatively inert compared with other contaminants, they are persistent, non-biodegradable particles that can act as carriers for attached heavy metals and chemical residues.
Why Does This Matter?
The contaminants released from roads and road markings do not simply disappear once they enter drainage systems.
Traditional drainage networks are designed primarily to move water away from urban areas as quickly as possible. Consequently, many contaminants are transported directly into rivers, lakes and coastal waters with minimal treatment.
Once released into the environment, these pollutants can affect ecosystems in several ways.
Harm to Aquatic Ecosystems
Microplastics, heavy metals and organic contaminants can negatively affect aquatic organisms at every level of the food web.
Exposure may impair growth, reproduction and survival rates among fish, invertebrates, algae and microorganisms that form the foundation of aquatic ecosystems.
Bioaccumulation and Biomagnification
Many contaminants accumulate within living organisms over time.
As smaller organisms are consumed by larger predators, contaminant concentrations can increase at each stage of the food chain. This process, known as biomagnification, can ultimately affect top predators, including humans.
Human Health Concerns
Research has linked exposure to heavy metals, PAHs and VOCs with a wide range of health effects, including:
- Respiratory illness
- Cardiovascular disease
- Neurological damage
- Reproductive impacts
- Organ damage
- Increased cancer risk
While exposure pathways vary, protecting water quality remains a key component of reducing overall environmental risk.
Increasing Regulatory Pressure
Across the UK and Europe, greater emphasis is being placed on water quality outcomes and environmental protection.
Regulators and planning authorities are increasingly expecting developments to address not only flood risk but also the treatment of pollutants carried within stormwater runoff.
Why Traditional Drainage is No Longer Enough
Conventional drainage systems perform a vital function in managing runoff volumes, but they are generally not designed to remove contaminants effectively.
As urbanisation increases and climate change leads to more frequent and intense rainfall events, larger volumes of polluted runoff are being generated.
This means that both the quantity and quality of runoff must be considered during drainage design.
Simply conveying water away from a site is no longer sufficient.
How SuDS Can Reduce Road Runoff Pollution
Sustainable Drainage Systems (SuDS) are specifically designed to manage both runoff quantity and water quality.
Rather than rapidly transporting runoff away from a site, SuDS seek to slow flows, encourage natural processes and remove pollutants before discharge.
One of the most effective approaches is the use of a treatment train, where multiple treatment stages work together to target different contaminant types and particle sizes.
Common treatment mechanisms include:
- Sedimentation
- Filtration
- Adsorption
- Biological treatment
- Ion exchange
The effective treatment of road runoff requires more than simply capturing gross pollutants. Research shows that many contaminants associated with road pavements and road markings (including heavy metals, PAHs, hydrocarbons, VOCs and microplastics) are preferentially associated with fine suspended solids, particularly particles smaller than 64 microns (PM64).
A treatment train comprising the 3P HydroShark and 3P HydroSystem is designed to target these road runoff pollution pathways. The HydroShark utilises hydrodynamic vortex separation to remove settleable solids and reduce sediment loading, while the HydroSystem provides secondary treatment through sedimentation, filtration, adsorption and ion exchange mechanisms that capture finer suspended solids and dissolved contaminants. Together, these systems provide a robust water quality treatment solution for highway runoff, urban drainage networks and SuDS schemes where protection of receiving waters is a key design objective.
Looking Beyond Vehicle Pollution
Road runoff pollution is often viewed through the lens of vehicle emissions, tyre wear and brake dust. However, growing evidence shows that road surfaces and road markings are also important contributors to stormwater contamination.
As these materials wear over time, they release microplastics, heavy metals, hydrocarbons, PAHs and other contaminants that are transported through drainage networks and into natural water bodies.
While research into these pollution sources continues to develop, the message is already clear: protecting water quality requires us to consider the entire road environment, not just the vehicles using it.
For drainage engineers, SuDS designers and environmental professionals, road runoff pollution presents both a challenge and an opportunity. By designing systems that target these emerging pollutants, we can help create more resilient drainage infrastructure and healthier aquatic ecosystems for future generations.
Further Information
3P Technik’s technical team can provide project-specific advice and support. Please contact us to discuss your requirements.
3p Technical Papers
- What Stormwater Runoff Contamination Originates from Road Pavements? (pdf opens in new window),
- The Role of Road Markings in Stormwater Runoff Contamination (pdf opens in new window).