Water quality performance

Surface water runoff is a major cause of pollution in watercourses. Pervious surfaces, together with their associated substructures, limit the direct discharge of pollutants to receiving waters and reduce the impacts of pollution.

The pollutants of most concern in highway or car park runoff are:

Sediments.
Metals (zinc, copper, cadmium).
Hydrocarbons (oil and fuel) including poly-cyclic aromatic hydrocarbons (PAH).
Pesticides and herbicides (from landscaping maintenance).
Chlorides (from de-icing).

The majority of pollution is washed off urban surfaces at the beginning of a rainfall event and this is known as the first flush. In this regard, sloping roof surfaces make the most significant contribution to the first discharges within the drainage system, but the later contributions from the paved surfaces deliver the more significant pollution contribution.

All evidence to date has demonstrated an improvement in water quality by the use of pervious surfaces. Pervious surfaces can be designed to provide a number of interception mechanisms that mitigate against the risks posed to controlled waters. Available methods include:

Filtration.
Biodegradation of organic pollutants, such as petrol and diesel.
Adsorption of pollutants (pollutants attach or bind to surfaces within the construction) which depends on factors such as texture, aggregate structure, moisture content.
Settlement and retention of solids.
Use of sealed bases to prevent infiltration to groundwater.
Use of additional treatment methods at the outflow from the pervious surface, such as wetlands or lagoons.
Use of enhanced soils to improve treatment within the pervious pavement system.

The use of pervious surfaces should also give a benefit to water quality as a result of reduction in peak flows to receiving waters, which enhances the settlement and biodegradation of pollutants. Where the outflow is released to surface waters the reduced peak flow causes less of a short term shock pollutant load to the receiving waters and allows increased dilution. This is generally an improvement over traditional systems where the first flush effect can be pronounced. The delay and treatment provided by pervious systems can attenuate this effect.

A fully documented qualitative risk assessment of the impacts of a pervious surface on the receiving waters should always be undertaken and the results fully communicated to the developers/owners. This should take account of the sensitivity of the receiving environment, and the likely maintenance regime.