A treatment train approach enables the capture of a range of predicted contaminants by directing stormwater runoff through a complementary sequence of stormwater management responses (refer Figure 37). For example, a roadside swale might capture gross contaminants and fine gravels in stormwater runoff, which could then be directed to a raingarden to capture fine sediments and hydrocarbons.
Many Auckland catchments generally contribute fine sediments of Waitemata group silts and clays into stormwater runoff. Additional contaminants of concern are heavy metals entering estuarine and harbour environments such as copper, zinc and cadmium. For more information, please refer to Auckland Council Technical Report TR2013/035 Auckland Unitary Plan Stormwater Management Provisions: Technical Basis of Contaminant and Volume Management Requirements.
WSD promotes the direction of stormwater runoff to enhanced natural systems or practices that incorporate natural processes, such as raingardens and swales. The capture of fine particles and contaminants occurs at the plant-soil-water interface where they are captured, metabolised and transformed through physical, chemical and biological processes. The treatment processes inherent to natural systems are described in more detail in Auckland Regional Council Technical Report TR2009/083 Landscape and Ecology Values within Stormwater Management (Lewis et al., 2010).
The treatment potential of natural systems and processes can be optimised by incorporating the following approaches (discussed in the following relevant descriptions of treatment types):
- Increase hydraulic residence time and provide a low-turbulence environment in practices incorporating natural systems
- Use medium to coarse textured soils for enhanced permeability
- Provide dense planting of fine leaved plant species to maximise filtering potential and prevent preferential flowpaths
- Provide a neutral pH to create the aerobic and anaerobic conditions for cycling nitrogen, sulphur, etc.
- Raise organic content, soil exchange capacity, and provide for a neutral pH in order to precipitate metals
- Ensure a high soil-water-plant root interface for phosphorus uptake.
Potential contaminant sources
A treatment train should be selected from a suite of potential stormwater management responses in order to target specific land use contaminants (refer to Figure 38). Some of the more common land uses and their associated contaminants are discussed below.
Residential developments commonly contribute fine sediments, nutrients, pathogens and general organics to stormwater runoff. Landscape areas also contribute pesticides, herbicides and organic debris such as displaced mulch and grass clippings. Zinc and copper can commonly occur in stormwater runoff from building roofs and spouting, and fertilisers and fungicides (Kennedy & Sutherland, 2008).
The contaminant load from streets and highways is impacted by automobile behaviour (stopping and turning), and this increases with vehicle counts (Shaver, 2010a). Sediments and associated contaminants stem from pavement abrasion, car wheels and atmospheric deposition. Engine wear and emissions contribute to a lesser degree. Contaminants specific to transport infrastructure include hydrocarbons and heavy metals from wearing of parts, fuel and lubricants.
Industry contributes common urban contaminants as well as higher concentrations of less common and potentially toxic contaminants depending on the activity. Sources of contaminants include structures, roofing and spouting, vehicle and waste storage, and stockpiles. Improperly connected sewer pipes and drains may lead to contamination of groundwater or increased combined sewer overflow events. Large areas of impervious surfaces can also lead to spikes in water temperature in stormwater runoff, which can elevate the toxicity of other contaminants.