 Stormwater runoff is rain that lands on a hard surface and travels overland because it can’t soak into the ground. When rain falls onto lawns and gardens and forests (called pervious areas), much of it soaks into the soil and groundwater and will then slowly seep into streams, flowing into the sea. As we build houses, roads and other buildings we increase the percentage of land sealed by concrete, tarseal and roofs (called impervious areas). When rain falls onto impervious surfaces it cannot soak into the ground and so runs off the surface. When it rains, water rushes off these surfaces into stormwater drains carrying dirt, oil and grime to the nearest watercourse, often flooding our small streams. You don’t see this same water flow in forested or vegetated areas because there the rain soaks into groundwater and slowly seeps into streams, maintaing a baseflow between rain events.
Stormwater drains were built to take rainwater to the nearest natural waterway to protect urban areas against flooding. They don’t carry stormwater to a treatment plant. And so, they carry everything washed from the roads, paths and driveways of our city untreated and unfiltered into the streams, lakes and seas that surround us. In the past, the main problem with stormwater was flooding. Now stormwater is known to be one of the main causes of water pollution in the world.
How does stormwater get polluted?
Water in rivers, creeks and wetlands becomes polluted when run-off (stormwater) from factories, gardens, building sites, roads and other hard surfaces enters these water bodies. When this stormwater passes over urban or rural areas, it picks up pollutants with the flow of the water. The faster the flow of water, the more pollutants it is likely to pick up! Even simple activities like washing your car on the street can add pollutants such as phosphorous (from detergents), which increase the chance of nuisance algal blooms. Frequently pollution occurs when materials are spilled or discharged onto the ground and flow into stormwater drains. These drains are designed to carry rain water directly to the nearest natural water body. Unfortunately they will also transport untreated pollutants as well. Many discharges occur daily throughout the Auckland Region. Whether in tiny or large amounts and regardless of the type of pollutants, they can compromise our waterways.
Our community can improve the quality of our waterways by practising good stormwater management (preventing anything other than water entering stormwater drains). But people need to know how and what to change. That’s where we come into the equation. By monitoring water quality, and learning more about our valuable waterways, we can tell our family, friends and neighbours what they can do to help improve our rivers and wetlands. Stormwater doesn’t just stop at the gutters, it travels through our rivers, creeks and wetlands, eventually reaching the sea!
There are two main ways ways in which contaminants enter our stormwater drains. These are:
- ‘Non point source’ Pollution. Rain generally flows over roads, carparks, yards and other surfaces before it gets to the stormwater drains – on the way it picks up contaminants such as oil, diesel, petrol, sediment, litter and heavy metals such as copper and zinc from car tyres and copper from brake linings. These pollutants then end up untreated in our streams, harbours and at our beaches. For more information about non point sources of pollution, see Wai Care Manual Book 2. Types and sources of stormwater pollutants.
- ‘Point source’ Pollution. The second way our stormwater system gets polluted is from direct discharges – such as oil or chemical spills, paint brushes being washed over stormwater drains and vehicles being washed on driveways – where soapy water washes straight into a stormwater drain. To find out which of your own daily activities affect stormwater quality, see Wai Care Manual Book 7 Clean Up Your Act.
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