What is a Washoff (of pollutants)

In the field of urban drainage and environmental management, the term washoff refers to the process by which pollutants accumulated on surfaces such as roads, rooftops, car parks, pavements, and other impervious areas are mobilised and transported by rainfall runoff into the drainage system. This process is a primary contributor to the degradation of surface water quality in urban environments and is a key consideration in the design and implementation of sustainable drainage systems (SuDS), combined sewer overflow (CSO) mitigation, and urban pollution control strategies.

Washoff is a naturally occurring but highly dynamic phenomenon, influenced by rainfall intensity, catchment characteristics, surface type, antecedent dry periods, and the types of pollutants present. Understanding the mechanics and implications of pollutant washoff is essential for professionals involved in urban drainage planning, water quality modelling, and regulatory compliance.

What is Pollutant Washoff?

Pollutant washoff occurs during rainfall events when precipitation dislodges and entrains pollutants that have built up on urban surfaces during dry weather periods. These pollutants, once mobilised by surface runoff, are carried into the stormwater drainage network, where they may either be treated, stored, or directly discharged into receiving water bodies such as rivers, canals, or estuaries.

This process is often overlooked compared to point-source pollution (e.g. sewage discharges), but it represents a significant non-point source of contamination, particularly in highly urbanised areas where the proportion of impervious surfaces is high.

Sources of Pollutants Subject to Washoff

Pollutants that can be mobilised through washoff are typically deposited on surfaces through a variety of human and natural activities. These include:

  • Atmospheric deposition of dust and particulate matter

  • Vehicle emissions and tyre wear residues

  • Oil, grease, and hydrocarbons from traffic

  • Animal faeces and decaying organic material

  • Sediments from construction activities

  • Fertilisers, pesticides, and herbicides from gardens or landscaped areas

  • Litter and other anthropogenic debris

These materials accumulate over time and, when rainfall begins, are rapidly flushed into nearby surface drains or combined sewers.

Key Factors Affecting Washoff Dynamics

The rate, volume, and concentration of pollutants washed off a surface are affected by a number of interrelated variables:

1. Rainfall Characteristics

  • Intensity: High-intensity rainfall generates greater flow energy, increasing the capacity of runoff to mobilise and transport heavier or more adhesive particles.

  • Duration: Longer rainfall events result in more complete washoff of surface pollutants.

  • Frequency: Infrequent storms lead to greater pollutant accumulation between events, resulting in more concentrated washoff during the next storm.

2. Surface Type and Condition

  • Impervious surfaces (e.g. asphalt, concrete, rooftops) produce the most rapid and pollutant-laden runoff, as infiltration is minimal.

  • Pervious or vegetated areas offer some filtration and retention, reducing washoff.

  • Surface roughness and slope influence how readily water flows across a surface and entrains particles.

3. Antecedent Dry Period (ADP)

The length of time since the last rainfall event is a critical factor. Longer ADPs typically result in higher pollutant accumulation, especially in urban catchments with heavy traffic or industrial activity.

4. Catchment Size and Connectivity

Larger catchments with well-connected drainage networks may experience more efficient washoff, as water quickly moves towards outfalls without retention or filtration.

Types of Pollutants Washed Off

The pollutants mobilised during washoff can be broadly categorised into the following:

Sediment-Bound Pollutants

These include heavy metals, hydrocarbons, and nutrients that are adsorbed onto fine particles. They tend to settle in slow-flowing areas or sedimentation tanks but may be re-suspended during subsequent storm events.

Dissolved Pollutants

Soluble substances such as nitrates, phosphates, and certain hydrocarbons are transported in solution. These are more difficult to remove using physical separation techniques and require chemical or biological treatment.

Gross Pollutants

Items like plastics, packaging, cigarette butts, and larger debris fall into this category. They are often targeted through litter traps and gross pollutant screens.

Pathogens

Bacteria and viruses from animal waste, decaying matter, and sewage misconnections can also be mobilised during rainfall events, posing health risks to both human and aquatic life.

Modelling and Predicting Washoff

Washoff processes are highly variable, but they can be modelled using hydrological and hydraulic simulation tools. The most commonly used models include:

  • SWMM (Storm Water Management Model): Developed by the United States EPA, this model simulates runoff quantity and quality over urban catchments.

  • InfoWorks ICM: Widely used in the UK, integrates washoff with sewer and river flow simulations.

  • MIKE URBAN: Used for integrated urban water modelling including water quality.

These tools account for rainfall characteristics, surface types, and land use to estimate pollutant loads and concentrations during runoff events. They can be used to design mitigation strategies and evaluate compliance with discharge regulations.

Environmental and Regulatory Implications

In the United Kingdom, pollutant washoff is a major concern in meeting the objectives of the Water Framework Directive (WFD), which requires all water bodies to achieve good ecological and chemical status. Urban surface runoff is a significant pressure on water bodies, especially in dense metropolitan areas.

Water companies, local authorities, and developers must account for washoff in:

  • Environmental permits for surface water discharges

  • Surface water management plans

  • Sustainable drainage strategy documents

  • Impact assessments for new developments

Uncontrolled washoff from development sites, roads, and industrial estates can lead to:

  • Eutrophication of water bodies due to nutrient loading

  • Fish kills from oxygen depletion

  • Accumulation of toxic metals in sediments

  • Visual and aesthetic degradation of urban rivers

Control and Mitigation Measures

The negative impacts of pollutant washoff can be reduced through a range of source control, conveyance, and end-of-pipe measures.

Source Control Measures

  • Street sweeping: Regular removal of dust and debris reduces pollutant build-up.

  • Public education: Encouraging responsible disposal of waste and limiting fertiliser use.

  • Surface sealing controls: Limiting new impervious areas in development planning.

  • Use of permeable paving: Allows infiltration and reduces surface flow velocity.

Conveyance and Treatment Measures

  • Swales and filter strips: Slow runoff and provide initial filtration.

  • Sedimentation basins and retention ponds: Settle out heavier particles.

  • Constructed wetlands: Provide biological treatment for nutrients and pathogens.

  • Hydrodynamic separators and vortex devices: Separate gross pollutants and sediments.

  • Oil and grease interceptors: Remove hydrocarbons from industrial sites and car parks.

The design of SuDS must consider the nature and quantity of pollutants likely to be mobilised in each catchment and should integrate multiple treatment stages for effective pollutant removal.

Washoff in the Context of Climate Change

Climate change is expected to increase the frequency of extreme rainfall events, especially intense short-duration storms that generate high runoff volumes and velocities. These events are likely to:

  • Increase the magnitude of washoff events

  • Mobilise larger quantities of sediment and pollutants

  • Overwhelm existing drainage systems and treatment infrastructure

Adaptation strategies must therefore incorporate enhanced washoff control, resilience in SuDS design, and investment in stormwater treatment solutions capable of handling future rainfall regimes.

Conclusion

Washoff of pollutants is a key process in the urban water cycle, with profound implications for drainage design, environmental compliance, and water quality protection. It represents the interface between surface activity and aquatic impact, bridging disciplines from road maintenance to hydraulic engineering and environmental science.

As urbanisation continues and climate variability increases, the importance of understanding and managing pollutant washoff will only grow. Integrating smart drainage design, proactive surface management, and public awareness will be essential in addressing the challenges posed by this persistent and complex phenomenon. For engineers, planners, and environmental managers alike, pollutant washoff remains both a challenge and an opportunity for creating cleaner, more sustainable cities.