Introduction

Every rainfall event generates runoff water across impermeable surfaces such as roads, parking lots, and logistics areas. As this water flows over urban and industrial surfaces, it collects pollutants including engine oil, fuel residues, tire particles, and other contaminants that accumulate between rainfall events. These pollutants are then transported into stormwater networks, infiltrated into the ground, or ultimately discharged into natural environments.

Among these pollutants, hydrocarbons and polycyclic aromatic hydrocarbons (PAHs) are particularly concerning. Persistent, bioaccumulative, and toxic, they threaten aquatic biodiversity, groundwater quality, soil health, and ultimately human health.

As the European Water Framework Directive (WFD) sets the objective of achieving good ecological and chemical status for all European water bodies by 2027, stormwater management has become a critical environmental issue. Understanding runoff pollution is the first step toward controlling it.

1 | Runoff water: an underestimated source of pollution

Runoff water is rainwater that cannot infiltrate impermeable surfaces and therefore flows across the ground surface. The problem intensifies when runoff passes over heavily urbanized or industrialized areas such as roads, parking lots, and industrial sites. Pollutants accumulate during dry periods and are then mobilized during the first rainfall events.

Hydrocarbons are carbon-based organic compounds that form the characteristic iridescent sheen often visible on runoff water. PAHs are primarily generated by incomplete combustion processes, including vehicle engines, asphalt-based materials, industrial activity, and tire wear. Although biodegradable under certain conditions, PAHs are highly persistent in the environment and strongly bind to sediments and suspended particles.

According to data from the Seine-Normandy Water Agency, concentrations of PAHs in combined sewer overflows during rainfall events can reach between 1 and 4.8 µg/L, significantly higher than during dry weather conditions. The same studies indicate that roadway runoff contributes up to 54% of aliphatic hydrocarbon loads measured within urban watersheds. According to the French Biodiversity Office (OFB), two-thirds of pollutants identified in French waterways are PAHs.

2 | Why is this pollution becoming such a critical issue?

Urbanization intensifies the problem

Urbanization and soil sealing significantly increase both the volume and velocity of runoff water while reducing the natural filtration capacity of soils. Hydrocarbons, heavy metals, pesticides, and other pollutants accumulated on impermeable surfaces are transported directly into aquatic environments and coastal waters.

At the same time, aging or combined sewer systems can become saturated during heavy rainfall events, leading to untreated discharges directly into natural environments.

A direct threat to groundwater and aquatic ecosystems

Due to their hydrophobic properties and low water solubility, PAHs strongly adsorb to sediments and suspended solids in aquatic environments. This allows them to persist in the environment long after their initial release.

Recent groundwater monitoring data in France identified PAHs among the micropollutants detected in groundwater systems. In parallel, a significant number of abandoned drinking water abstraction sites between 1980 and 2025 were linked to hydrocarbon and organic pollutant contamination.

Current chemical status data for surface water bodies remains concerning: only 44% of French surface waters currently achieve good chemical status. Hazardous substances, including PAHs, remain among the primary causes of non-compliance.

Documented impacts on soil biodiversity

When polluted runoff infiltrates untreated into surface soils, hydrocarbons and PAHs accumulate within the soil structure. As persistent organic pollutants, PAHs degrade slowly and can disrupt soil ecological functions over long periods.

The impacts on biodiversity are measurable. Soil microorganisms, fungi, bacteria, and earthworms are all affected by hydrocarbon contamination. Research conducted on PAH-contaminated industrial soils demonstrated clear toxicity impacts on earthworms (Eisenia fetida), plants, and other terrestrial organisms.

Although soil microorganisms can naturally biodegrade certain hydrocarbons and PAHs, this self-purification capacity depends heavily on soil characteristics including granulometry, organic matter content, pH, and moisture levels. In practice, natural remediation mechanisms are often too slow and limited to fully manage long-term contamination.

A human health concern

Many PAHs are recognized as mutagenic and carcinogenic. Several are classified as probable or possible human carcinogens by organizations including the International Agency for Research on Cancer (IARC), the US EPA, and the European Union. Benzo[a]pyrene, one of the most toxic PAHs, is specifically recognized for its ability to damage cellular DNA.

When polluted runoff reaches drinking water catchment areas, the integrity of the human water supply chain can be compromised. Clean infiltration is therefore essential for protecting the aquifers that supply drinking water.

Increasingly demanding regulatory requirements

The European Water Framework Directive (2000/60/EC) establishes a clear objective: achieving good ecological and chemical status for all European water bodies by 2027. This deadline now represents a major compliance challenge for project owners, engineers, and infrastructure developers.

River basin management plans and associated environmental regulations are becoming increasingly restrictive. Projects located in watersheds failing to achieve regulatory targets may face significant permitting constraints or refusals. Environmental liability obligations for project owners are also becoming stronger across Europe.

3 | Solutions and best practices: managing runoff pollution at the source

Alternative stormwater systems and source infiltration

Today, infrastructure and environmental professionals have access to a wide range of complementary stormwater management solutions. The most effective approach is to treat pollution where it forms, before discharge into drainage systems or natural environments occurs.

Alternative systems such as vegetated swales, drainage trenches, retention basins, permeable pavements, and rain gardens provide an initial level of hydraulic control by slowing flows and promoting controlled infiltration. However, infiltration alone is not enough. The quality of infiltrated water must also be addressed to avoid transferring pollution into groundwater systems.

Aquatextiles: an innovative approach to stormwater treatment

This is where innovative treatment technologies become essential. Solmax developed OSMORIA® aquatextiles specifically to address hydrocarbon and PAH pollution in stormwater systems.

These depolluting aquatextiles integrate directly into source control stormwater infrastructure, helping treat hydrocarbons and other pollutants before infiltration or discharge into the environment occurs. They are designed to support engineers, consultants, contractors, and municipalities in meeting increasingly demanding environmental and regulatory requirements.

Conclusion

Runoff water is a major source of diffuse pollution whose impacts accumulate over time, threatening groundwater quality, aquatic ecosystems, soil biodiversity, and public health. The hydrocarbons and PAHs carried by runoff do not simply disappear. They persist, migrate, and accumulate within sediments and living systems.

Addressing this challenge requires more than hydraulic management alone. It demands a broader understanding of water quality and source treatment from the earliest stages of project design.

The solutions already exist. They rely on integrating source treatment strategies, sustainable stormwater management practices, and proven treatment technologies into infrastructure projects. With the 2027 Water Framework Directive deadline rapidly approaching, action can no longer be postponed.

Designing or managing a stormwater infrastructure project? Our teams are available to help evaluate OSMORIA solutions suited to your technical and regulatory requirements.