Stormwater runoff from urban areas has been shown to contain a wide variety of pollutants from both natural and man-made sources, with key contaminants including:
- sediment / suspended solids
- nutrients (nitrogen and phosphorous)
- heavy metals
- hydrocarbons (oil and grease)
- micro-biological organisms.
To minimise the impact of pollutants on receiving waterways many authorities have now set specific Water Quality Objectives (WQO) for the treatment of stormwater runoff from new developments.
The figure below, adapted from Ecological Engineering 2003, demonstrates how specific pollutants must be targeted by higher level treatment measures and how it is helpful to separate them into primary, secondary, and tertiary categories.