From parking lots to logistics platforms, every rainfall event carries a continuous but often invisible load of organic pollutants into soils and groundwater systems. Hydrocarbons and polycyclic aromatic hydrocarbons (PAHs), generated largely by transportation-related activities, are now a central challenge in urban stormwater management.

Understanding their nature, environmental behavior, impacts, and the limitations of existing treatment systems has become a technical necessity for infrastructure and urban development projects facing increasingly stringent Water Framework Directive requirements.

Part 1 | The nature, behavior, and impacts of hydrocarbons and PAHs

Hydrocarbons are a broad family of organic compounds composed exclusively of carbon (C) and hydrogen (H) atoms. Naturally present in fossil fuels such as oil, coal, and natural gas, they exist in a wide range of chemical structures with very different properties. They generally fall into several major categories:

• Aliphatic hydrocarbons, structured as linear or branched chains and commonly used in fuels and mineral oils

• Aromatic hydrocarbons, characterized by one or more benzene rings

• Light aromatic hydrocarbons such as BTEX compounds (benzene, toluene, ethylbenzene, and xylenes), known for their volatility and ability to contaminate both ambient air and groundwater

Within this broader family, polycyclic aromatic hydrocarbons (PAHs) represent a particularly critical category due to their environmental persistence and associated health risks, especially in aquatic environments and stormwater runoff systems.

PAHs are organic compounds composed of multiple fused benzene rings, ranging from two rings in molecules such as naphthalene to seven rings in more complex compounds like coronene. Their high molecular weight, elevated melting points, and very high boiling points reflect strong chemical stability combined with low water solubility and a strong affinity for both organic matter and mineral particles.

PAHs originate either from the natural formation of fossil fuels, explaining their presence in coal, crude oil, tar products, and creosotes, or from the incomplete combustion of organic matter. Road traffic, domestic heating, industrial processes, coal coking, oil refining, and waste incineration are all major environmental sources of PAHs. They are also present in many commonly used materials and products, including asphalt pavements, industrial oils, tires, and creosote-treated wood.

The unique challenge of PAHs: toxic yet conditionally biodegradable

Unlike heavy metals, which cannot be chemically destroyed and must instead be immobilized or extracted, PAHs are organic compounds that can potentially be mineralized into water, CO₂, and mineral salts through microbial activity. This distinction creates treatment opportunities that do not exist for heavy metals.

Scientific research has documented bacterial biodegradation pathways for PAHs for decades. Aerobic degradation processes involve enzymes such as dioxygenases and dehydrogenases, while anaerobic pathways involving sulfate-reducing and nitrate-reducing bacteria have also been identified for more persistent compounds.

However, the primary limitation remains bioavailability. PAHs adsorbed onto suspended particles and sediments become significantly less accessible to microorganisms, limiting natural biodegradation in real-world environments.

A recognized environmental and health concern

Not all PAHs present the same level of risk, but many are recognized as persistent, genotoxic, and carcinogenic pollutants for which no safe exposure threshold can be clearly established. Even low-level chronic exposure may generate adverse impacts on human health and ecosystems due to bioaccumulation and long-term sublethal effects.

In 1984, the US EPA classified 16 PAHs as priority pollutants, a reference list now widely adopted internationally and reflected in standards such as ISO 13859:2014. This list continues to underpin environmental monitoring programs and water quality regulations worldwide.

Several figures illustrate the scale of the issue:

• 16 PAHs classified as priority pollutants by the US EPA

• 10 priority PAHs commonly identified in stormwater runoff

• 6 of these 10 PAHs classified as confirmed or probable carcinogens

• 0.1 µg/L: European limit value for PAHs in groundwater intended for drinking water production

• 10-500 ng/L: median concentrations measured in urban stormwater runoff

Part 2 | PAH biodegradation and the importance of source treatment

In urban and peri-urban environments, sources of hydrocarbons and PAHs are multiple, continuous, and closely linked to everyday infrastructure use. Combustion engines generate PAH-laden particles that settle onto roads and parking areas. Tire wear, brake pad abrasion, asphalt degradation, fuel leaks, and oil drips gradually accumulate on impermeable surfaces over time.

During rainfall events, these pollutants are washed away by runoff water and transported toward stormwater systems, receiving environments, and infiltration structures. Studies conducted downstream of highway runoff discharges have shown elevated concentrations of PAHs in sediments, particularly phenanthrene, pyrene, and fluoranthene. This is a form of chronic pollution associated not with accidental spills, but with the normal operation of urban and transport infrastructure.

Dissolved phase vs particulate phase: a critical distinction

The distribution of PAHs between particulate and dissolved phases plays a major role in treatment performance. Heavy PAHs strongly adsorb onto suspended solids and fine particles, representing more than 50% of the pollutant load in roadway runoff. Lighter PAHs remain partially dissolved in water and are therefore more mobile, representing a greater risk of migration toward groundwater systems.

Measured concentrations in stormwater runoff

• 10-500 ng/L: median PAH concentrations measured in urban stormwater runoff

• < 5 mg/L: typical chronic hydrocarbon concentrations in runoff water, often below the operating thresholds of conventional oil separators

• ×10 increase in PAH concentrations measured in temperate soils since industrialization

Environmental and health consequences

PAHs belong to a category of pollutants capable of generating harmful biological effects at concentrations in the microgram-per-liter or even nanogram-per-liter range.

Benzo[a]pyrene, a five-ring PAH commonly used as an indicator compound, is classified as carcinogenic to humans by the International Agency for Research on Cancer (IARC). Other PAHs are classified as probable carcinogens or remain insufficiently studied despite growing evidence of toxicity.

Insert | Cocktail effect

In stormwater runoff, PAHs coexist with heavy metals, alkylphenols, and microplastics. Their interactions can significantly amplify environmental and toxicological impacts through what is commonly referred to as the “cocktail effect.”

Microplastics in particular can act as transport vectors for lipophilic PAHs, increasing their bioavailability to aquatic organisms beyond what concentration data alone would suggest.

Regulatory pressure continues to increase

Under the European Water Framework Directive (2000/60/EC), eight PAHs are classified among the 41 priority substances subject to strict Environmental Quality Standards. The French Biodiversity Office identifies PAHs, particularly benzo[g,h,i]perylene and indeno[1,2,3-cd]pyrene, among the substances most frequently responsible for downgrading water body chemical status in France.

The limit value established for PAHs in groundwater intended for drinking water production is just 0.1 µg/L, a particularly demanding threshold that highlights the importance of controlling dissolved pollutant fractions.

At the national level, successive French micropollutant action plans have increasingly emphasized source control stormwater management strategies focused on infiltration and pollutant retention before discharge into natural environments occurs.

Part 3 | Existing stormwater treatment solutions and their limitations

Oil separators

Oil separators remain the most widely used conventional stormwater treatment systems. Based on gravity separation and flotation principles, they rely on density differences between water and free-floating hydrocarbons and are governed by NF EN 858 standards.

However, their effectiveness remains limited. Chronic hydrocarbon concentrations in stormwater runoff are often below their operating thresholds, and these systems do not treat dissolved pollutant fractions. In addition, poor maintenance can transform these devices into secondary pollution sources during intense rainfall events.

Insert | PAHs: a regulatory and technical blind spot

The NF EN 858 standard governing oil separators does not specifically address PAHs. Performance testing focuses only on free-floating hydrocarbons. Yet dissolved and particulate PAHs represent some of the most significant environmental and health risks associated with stormwater pollution and are among the primary causes of Water Framework Directive non-compliance.

This creates a structural gap between oil separator regulations and broader water quality objectives.

Vegetated systems and sustainable urban drainage systems (SUDS)

Vegetated systems such as swales and constructed wetlands form part of a more ecological stormwater management approach. Through filtration, adsorption, and biological activity within the rhizosphere, they can effectively retain suspended solids and certain metals.

However, their performance against dissolved hydrocarbons and PAHs remains variable and highly dependent on soil properties, hydraulic design, vegetation health, and long-term maintenance practices. Management of contaminated plant waste also remains an often-overlooked operational challenge.

Sustainable Urban Drainage Systems (SUDS), including permeable pavements, infiltration basins, drainage trenches, rain gardens, and green roofs, primarily aim to manage runoff hydraulically and promote infiltration at source. While effective from a flow management perspective, they do not always incorporate dedicated pollutant treatment mechanisms.

Without appropriate treatment systems, these infrastructures may unintentionally facilitate the transfer of hydrocarbons and PAHs toward soils and groundwater, creating an environmental paradox in areas exposed to traffic and industrial pollution.

Depolluting aquatextiles and geotextiles

A depolluting aquatextile is a technical textile specifically designed for direct integration into stormwater infiltration systems. Unlike conventional geotextiles, which mainly provide separation, filtration, or protection functions, depolluting aquatextiles are designed specifically to treat hydrocarbons and PAHs in stormwater runoff.

Their operating principle is inspired by the natural purification capacity of organic-rich soils. As runoff water passes through soil, hydrocarbons are adsorbed and gradually biodegraded by microorganisms. Depolluting aquatextiles reproduce, optimize, and control this natural process within a dedicated engineered textile structure integrated directly into infiltration systems.

This represents a major shift from conventional separator-based approaches. Treatment is no longer handled through an external device requiring emptying and maintenance, but through a continuous biological treatment process integrated directly within the stormwater infrastructure itself.

Research on infiltration systems such as swales, infiltration trenches, and vegetated basins shows that PAHs and trace metals are primarily retained within the first 50 cm of soil or filtration substrate, corresponding to the sediment deposition zone and the organic-rich surface layer.

This is precisely where depolluting textiles become particularly relevant. OSMORIA aquatextiles were developed to optimize this critical interface zone by maximizing the capture of fine particles carrying particulate-phase PAHs while simultaneously supporting adsorption and biodegradation of dissolved fractions. Integrated directly into stormwater management systems, they provide a robust, scalable, and controllable first barrier against pollution before contaminants reach receiving environments.

Conclusion

Hydrocarbons and PAHs represent a chronic, diffuse, and persistent form of stormwater pollution. The limitations of conventional treatment systems highlight the need for a different approach: leveraging the biodegradation potential of PAHs directly at the source.

By integrating active depolluting solutions directly within infiltration systems, technologies such as OSMORIA aquatextiles provide a practical response to Water Framework Directive requirements, chemical status objectives, and the operational realities of modern infrastructure projects.