Aquatic ecosystems are very sensitive to water quality changes as a result of stormwater runoff.
This has been observed in the direct effects of toxic pollutants, the effects of combinations of different contaminants, and the accumulation of persistent chemicals within animal food webs (Hauer & Hill, 1996). Stormwater runoff can contain elevated levels of nutrients, metals, pesticides, temperature and organic contaminants (Paul & Meyer, 2001). The PAUP identifies sediment, metals and temperature as key contaminants to be managed in the urban environment. Potential contaminant sources are discussed in further detail in Section E in relation to 'treatment train' responses.
The previous approach to water quality treatment in Auckland has been to require the capture and treatment of stormwater runoff from 80% of all storms. A treatment pond is a common stormwater device which generally utilises the process of sedimentation to capture contaminants. Sedimentation assumes granular material in suspension will 'drop out' to the bottom of the pond and embed the contaminants that are bound to them. This process can be enhanced through physical filtration and biological uptake (wetland planting) or flocculation (through chemical additives).
High flows can re-suspend sediments in ponds, and runoff temperatures can increase dramatically during periods of low flow. Ponds are also limited in their ability to settle silts and clay particles and remove dissolved contaminants from solution.
A WSD approach avoids potential sources of contaminants (impervious surfaces and exposed hazardous materials) and additionally promotes the treatment of stormwater runoff close to source. Examples of WSD approaches to water quality treatment include:
- Minimising the use of materials that leach contaminants such as copper, galvanised metal and treated timber
- Applying appropriate land management practices for fertiliser and pesticide application to minimise effects of harmful substances on stormwatersystems
- Maximising landscape elements such as trees to capture and metabolise airborne and waterborne pollutants and to provide shade to reduce thermal effects
- Reducing the extent of impervious surfaces to limit areas where contaminants can accumulate
- Where impervious surfaces do occur, directing surface runoff to landscape areas to allow the 'first flush' to achieve some preliminary level of treatment
- Creating permeable surfaces (pervious paving, living roofs, etc.) to infiltrate stormwaterand capture dissolved contaminants
- Integrating treatment devices such as raingardens, swales and wetlands, which utilise natural microbial activity at the plant-water-soil interface to transform and take up contaminants.