WATER QUALITY DESIGN OBJECTIVE

Manage the quality of runoff to prevent pollution

Diffuse urban pollution (ie pollution from widespread multiple sources – see Chapter 1) is a significant factor in compromising groundwater and receiving water standards that are required under the EU Water Framework Directive. The UK government recognises that tackling diffuse pollution originating from urban runoff is a high priority and the increased use of SuDS is an important means of reducing urban runoff and improving the water quality of that runoff (Defra, 2012).

Agricultural land can also be a significant source of pollution, but this is not covered by this manual. Guidance on SuDS for agricultural sites is provided by Avery (2012).

Pipes are usually designed to convey water at velocities that keep sediment in suspension, preventing build-up within the pipe but transferring the runoff and any associated pollution directly to the receiving surface waters. Although some gullypots and catchpits can trap sediment, their efficacy is strongly linked to the frequency of maintenance, and there are significant risks associated with poor-quality water that is stored in them being remobilised and washed downstream. SuDS can treat and clean surface water runoff from urban areas so that the receiving environment is protected, while at the same time conveying, storing and infiltrating surface water to protect flood risk, river morphology and water resources, and delivering amenity and biodiversity value for the development.

There is large variability in the level of pollutants in urban runoff. Sources of pollution from impermeable surfaces are summarised in Table 4.1. Evidence relating to urban runoff pollution is presented in Section 26.4. Untrafficked areas are usually the least contaminated, with levels of contamination tending to rise with traffic intensities (particularly manoeuvring frequencies and lorry movements) and with higher risks of spillages and process contaminants from commercial and/or industrial activities.

04 ^Chapter This chapter explains the objective of designing for water quality and the design criteria that should be followed to deliver this objective. Good practice design standards are also presented.

This chapter should be read alongside Chapters 3, 5 and 6 to understand how the different SuDS design criteria relate to each other and Chapter 7 to understand when and how to apply these criteria.

Guidance on designing individual SuDS components for treatment can be found in Chapters 11–23.

Information regarding urban runoff contaminants, together with methods for assessing level of hazard, is presented in Chapter 26.

Factors affecting pollution levels in urban runoff are set out in Box 4.1, with antecedent weather conditions (which affects the build-up of contaminants on the surface) and rainfall characteristics influencing the amount of pollution washed off the site in any individual rainfall event.

Information regarding urban runoff contaminants is presented in Chapter 26 (Annex 1) together with methods for assessing the level of hazard posed (Section 26.7).

The pollution risk posed by the site will depend on the sensitivity of the receiving environment, the pathway between the source of the runoff and the receiving waters, and the level of dilution available. The variety, scale and complexity of diffuse urban pollution can potentially lead to a range of intermittent acute (short-term) impacts and chronic (longer-term) impacts.

The overall impact of the site on water quality in the receiving waters is dependent on the following:

▪▪the types of pollutants on the site, as these have different effects on the receiving water body (Table 4.1)

▪▪the peak pollutant concentrations in the runoff from the site, as these can cause acute (short- term) toxicity in the receiving waters

▪▪the total pollutant load likely to be conveyed in the runoff from the site to the receiving environment, as this can cause chronic (long-term) pollution and gradual deterioration, owing to cumulative build-up of pollutants.

The relationship between peak pollutant concentrations and total pollutant load is discussed in Box 4.2.

Potential impacts on receiving surface waters include the blanketing of river beds with sediment and the reduction of light penetration from suspended solids causing negative impacts on ecosystems. In some cases, this can result in the slow decline in biodiversity and ultimately the “death” of the river.

Dissolved pollutants and hydrocarbons can lead to reductions in natural oxygen levels in surface waters, toxic conditions, metals bioaccumulation, contamination of benthic organisms, and the death of fish and other animals. In extreme cases (often because of misconnections with the foul sewerage system), significant levels of pathogens may also be present in the runoff, and these can be hazardous to human health in the event of exposure.

Pollution of groundwater, although less obvious than pollution of surface waters, tends to be

irreversible and permanent. Groundwater quality is at risk from both point source pollution (eg a leak from BOX4.1 Factors influencing pollution levels in urban runoff

The amount and type of pollution washed off a surface will depend on many things including:

▪ planned activities on, above and adjacent to the surface that affect the deposition of pollutants, their retention on the surface and the extent to which they are mixed with runoff (including pollution prevention strategies – see Chapter 27)

▪ unplanned activities (accidents and spillages) that can cause temporary unexpected high pollutant concentrations – such as from a road accident or poor pollution prevention practices on construction sites, housing estates, commercial and industrial zones or waste management areas

▪ the surface location and type, affecting wash-off rates and contaminant movement mechanisms

▪ the drainage path

▪ the length of the dry weather period before the rainfall event

▪ the intensity and duration of the rainfall, and the associated flow velocities

▪ any further pollutant transformations occurring during residence and conveyance within gullies, chambers, pipe or channel networks, gravels, soils and vegetation and quiescent bodies of water.

an oil storage tank) and diffuse pollution (eg leaking sewers or infiltration of contaminated runoff). Good quality groundwater is crucial for water-dependent plants and animals, and as a source of drinking water.

Nitrates, pesticides, solvents, metals, hydrocarbons and other pollutants can potentially find their way into groundwater with the level of risk posed depending on the following:

▪▪The type of pollutant. Trace metal contaminants are conservative and will ultimately migrate through the unsaturated zone – the soil layer between the land surface and the groundwater level.

Organic and some inorganic compounds, however, have the potential to undergo degradation as they pass through the soil. Usually biodegradation is the most important process affecting organic compounds, but other processes such as hydrolysis, reduction and substitution may be relevant to specific compounds and subsurface environments.

▪▪The depth of the unsaturated zone. Greater depths will tend to increase the time taken for

contaminants to migrate down to groundwater and potentially reduce the contaminant concentrations at this point, where degradation processes occur in the soil profile.

▪▪The characteristics of the unsaturated zone. Some soils will provide better contaminant retention and storage, increasing the length of time before contaminants migrate down through the soil to groundwater, and better facilitating contaminant degradation. For example, fine grained materials will provide a better barrier to pollutant migration than materials with fissure or fracture flow paths.

▪▪ The level of build-up of contaminants within the soil profile. This will be a function of the contaminant loading rate and the length of time over which contaminants have accumulated. Higher loading rates are likely to reduce the period over which contaminants are retained within the soils and prevented from downward migration.

It is therefore important to design drainage systems to protect both surface waters and groundwaters, by assessing the potential risk posed by the site and putting in place adequate measures to reduce the risk to acceptable levels (Section 4.2.2). This helps ensure that all discharges meet the requirements of relevant legislation, and that discharges from SuDS are sufficiently low risk that they will not require

“permitting” or “licensing” by the environmental regulator.

Designing for water quality using a risk-based approach is discussed in Section 4.2.

BOX4.2 Pollutant concentrations and loads

A peak “flush” of pollutants often occurs during the early stages of a storm event, before the flow rate in the system reaches its peak. It is possible to get a high initial pollution concentration for relatively small rainfall events, as it does not take a great deal of rain to wash off the pollutants. This is why it is important to manage the frequent small events effectively.

Figure 4.1 shows how pollution concentration (red) and cumulative pollution load (green) change over time for sediments transported during a rainfall event, compared to the flow rate (blue). This shows the initial “flush” of pollutants shortly after the start of the event, and then there is a second peak later in the event, which coincides with a further increase in flow rate, as runoff from more distant parts of the site reaches the downstream system.

These peaks in sediment concentration are typical of sediments transported from urban surfaces and also for the pollutants that are predominantly attached to the sediments during an event, such as hydrocarbons, organic compounds and heavy metals. Other pollutants (including dissolved pollutants) can show different runoff patterns, but all show high initial concentrations due to initial wash-off of pollutants from the catchment surface.

Figure 4.1 Example of flow, pollutant concentration and pollutant load build-up during a rainfall event

Note

1 Heavy metals include: lead, cadmium, copper, chromium, nickel, zinc, mercury. Not all heavy metals are present in all cases.

2 Methyl tert-butyl ether.

TABLE

4.1 Sources of pollution from impermeable surfaces (after Wilson et al, 2004) Source Typical pollutants Source details

Atmospheric deposition

Phosphorous, nitrogen, sulphur, heavy metals¹, hydrocarbons, particulates

Industrial activities, traffic air pollution and agricultural activities all contribute to atmospheric pollution. Rain also absorbs atmospheric pollutants, which are then present in runoff. Atmospheric pollutants can be deposited on, or absorbed by roofing materials and discharged into roof runoff – flat urban roofs are particularly vulnerable.

Traffic – exhausts

Hydrocarbons, MTBE², cadmium, platinum, palladium, rhodium

Vehicle emissions include polycyclic aromatic hydrocarbons (PAH) and unburnt fuel and particles from catalytic converters.

Traffic – wear and

corrosion Particulates, heavy metals¹ Abrasion of tyres and corrosion of vehicles deposit pollutants onto the road or car parking surfaces.

Leaks and spillages (eg from road vehicles)

Hydrocarbons,

phosphates, heavy metals¹, glycols, alcohols

Engines leak oil, hydraulic and de-icing fluids and spillages occur when refuelling. Lubricating oil can contain phosphates and metals. Accidental spillages also occur.

Litter/animal faeces Bacteria, viruses, phosphorous, nitrogen

Litter typically includes items such as drinks cans, paper, food, cigarettes, animal excreta, plastic and glass. Some of this will break down and cause pollutants to be washed off urban surfaces. Dead animals on roads decompose and release pollutants including bacteria. Pets and other animals leave faeces that wash into the drainage system.

Vegetation/ landscape maintenance

Phosphorous, nitrogen, herbicides, insecticides and fungicides, organic matter

Leaves and grass cuttings are an organic source.

Herbicides and pesticides used for weed and pest control in landscaped areas such as gardens, parks, recreation areas and golf courses can be a major source of pollution.

Soil erosion

Sediment, phosphorous, nitrogen, herbicides, insecticides and fungicides

Runoff from poorly detailed landscaped or other areas can wash onto impervious surfaces and cause pollution of runoff.

De-icing activities

Grit, chloride, sulphate, heavy metals¹, glycol, cyanide, phosphate

Salt is commonly used for de-icing roads and car parks.

Rock salt used for this purpose comprises sodium chloride and grit. It can also include cyanide and phosphates for anti- caking and as corrosion inhibitors, heavy metals, urea and ethylene glycol.

Cleaning activities

Sediment, phosphorous, nitrogen, detergents, hydrocarbons

Washing vehicles, windows, bins or pressure washing hardstandings leads to silt, organic matter, detergents and hydrocarbons (mobilised by the detergents) entering the surface water drainage.

Sewer misconnections

Bacteria (including pathogens), detergents, organic matter and textiles

Accidental (but illegal) connections of foul sewers to surface water sewers – where separate sewers exist.

Illegal disposal of chemicals and oil

Hydrocarbons, various chemicals

Illegal disposal of used engine oils or other chemicals can occur at small (domestic) or large (industrial) scales.