What is urban stormwater?
Stormwater, or rainwater runoff, is rainwater that runs off land and moves away from the area where it originally falls. In urban or built up areas, it is best defined as rain that runs off surfaces where water cannot penetrate such as roofs, driveways and roads. It is carried away by a series of pipes known as the stormwater drainage network to our natural water bodies—creeks, rivers and the sea. In some areas stormwater is discharged directly to underground water. Urban areas have many more impermeable surfaces or surfaces where water cannot penetrate, which generate more runoff and lead to higher stormwater volumes and discharges.
In natural environments, before an area is urbanised, the ground is permeable to water. Rain infiltrates the soil and recharges groundwater or slowly runs off into creeks, rivers and the coast as the soil becomes saturated with water.
This diagram illustrates some of the processes and movement of water in a more natural or non-urbanised environment where most of the soil is permeable to water.
Urban areas have large areas of hard surfaces, impermeable to water such as roofs, roads, paved and concrete areas. These do not allow rain to infiltrate the soil and groundwater recharge is greatly reduced. Instead larger volumes of water run quickly off these surfaces becoming what we commonly call urban stormwater. Many natural watercourses have been concreted to assist in draining this water away as quickly as possible.
Depending on the soil type, rainfall pattern and amount of impervious area, the urban area will greatly increase runoff compared to an undisturbed area where runoff is minimal. These higher runoff rates alter the flow regime in waterways and increase scouring (erosion), affecting aquatic life and increase flows discharging to the coast. As it passes over impermeable surfaces it also picks up a range of pollutants.
This diagram illustrates some of the processes and movement of water in an urbanised environment where there are many hard surfaces.
The stormwater drainage network is separate from the sewage system. All the pipes and drains inside buildings, such as the kitchen, laundry, toilet and bathroom are connected to the sewer or to a septic tank or treatment system. Outside the building there may be a sewer connection under a tap over a gully trap. The sewer takes wastewater to treatment works, where it is treated, before being piped to the sea or irrigated over land. All other outside drains such as the roof downpipe are connected to the stormwater drainage network. Stormwater drainage flows through outdoor drains into a pipe network and into our natural water bodies (creeks, rivers, groundwater, wetlands and the sea). It generally flows from the streets and drains untreated into natural water bodies, taking with it a range of pollutants collected on its journey.
As stormwater travels over surfaces it picks up pollutants that are not normally found in receiving waters. Pollutants include litter, nutrients such as phosphate and nitrogen, metals such as zinc, copper and lead, oil and grease from roads, garden waste, bacteria and sediments. Some of these are toxic even in small amounts. Some pollutants may not be toxic on their own, but in large quantities they can overwhelm natural systems.
Stormwater pollution is usually referred to as diffuse pollution because it does not come from one readily identifiable source, rather from a range of sources over a large area that cumulatively have significant environmental impacts.
Increased runoff or stormwater has also changed the water flow regime in our creeks, rivers and the coast. With less infiltration into the soil and groundwater, and higher flow in urban waterways, there is greater erosion in streams further adding to the pollutant load. Changing the flow regime in this way has implications for the aquatic species that live in the urban waterways.
Sediment (soil) in a stormwater drain
Impacts from stormwater pollution
Why is stormwater pollution an issue?
Polluted stormwater may cause significant ecological changes to receiving waters—groundwater, urban streams and coastal waters, including
- Algae blooms from increased nutrients
- Reduced oxygen availability for aquatic organisms
- Pollutants such as lead, copper, zinc, hydrocarbons may be toxic threatening viability of aquatic communities or entering the food chain
- As sediment is deposited it may smother habitats
- Reduced water clarity limiting sunlight penetration and photosynthesis of aquatic plants.
In some areas such as Mount Gambier, stormwater discharges directly to the aquifer (underground water). This ultimately finds its way to the Blue Lake, the main water supply for Mount Gambier. The city's had adopted best practice stormwater management practices.
In coastal waters not only has the pollution affected aquatic life, reducing biodiversity, it may also cause the water to be unsafe for swimming and other recreational uses. In time, this may potentially impact the fishing and tourism industries.
The Adelaide Coastal Waters Study found that the loss of more than 5,000 hectares of seagrass along the Adelaide metropolitan coast could be attributed to pollution some of which is due to stormwater pollution. Key recommendations:
- reduce the volumes of stormwater, wastewater and industrial input
- 50% reduction in suspended solids, the majority of the suspended solids come from stormwater pollution that enters Gulf St Vincent
- 75 % reduction in nitrogen from all sources including stormwater
- reduce the amount of coloured dissolved organic matter discharged by rivers and stormwater drains.
The EPA continues to monitor marine and other waters in the state to assess their condition and to provide information that can be used to guide management decisions. This monitoring is used to produce the aquatic ecosystem conditions reports.
Impacts from stormwater have also been observed from coastal towns in some regional coastal areas such as Port Lincoln, Whyalla and coastal towns on Yorke Peninsula. Further information can be found in the marine aquatic ecosystem reports of the Tiparra Nearshore Marine Biounit, Orontes Nearshore Marine Biounit, Yonga Nearshore Marine Biounit and the Jussieu Nearshore Marine Biounit.
Polluted stormwater discharging at the coast
Stormwater as a resource
Stormwater is also a valuable resource and a potential alternative water supply. With predicted changes to our climate including longer dry periods and less rainfall, it is important to make the most of all alternative water supplies such as stormwater.
The quality of stormwater is critical in determining its suitability as an alternative water supply and if we are to use this resource we need to prevent stormwater pollution. The water must be fit for the intended purpose and pose no risks to the environment, or to public health and safety. Excessive stormwater pollution may be too costly to treat and the stormwater pollution becomes a lost opportunity for stormwater reuse schemes.