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Rainwater will flow over surfaces such as roads, roofs, and footpaths to become stormwater. Heavy rain events can cause flooding. Image: Reuters

When rainwater falls onto hard, sealed surfaces such as roofs, roads, footpaths, and driveways it cannot soak into the ground. Instead, it runs over and off these surfaces. This runoff is called stormwater.

Permeable and impermeable surfaces

When rain falls onto surfaces like lawns and gardens, some of it soaks into the soil to become groundwater. This is because theses surfaces are permeable, meaning water can soak through them.

Surfaces such as roads, roofs, and footpaths are impermeable, meaning water cannot soak through them. Rainwater will flow over these surfaces to become stormwater.

Where does stormwater go?

In towns and cities there are a lot of solid surfaces and therefore a lot of stormwater. This stormwater needs to be collected so it doesn’t cause flooding and erosion. Most stormwater is directed into an underground stormwater network of drains and pipes. Manhole covers and grates you might have seen on roads lead into this system. Streams and open stormwater channels are also important pathways for stormwater.

This stormwater network empties into our rivers, streams, lakes, and harbours at selected points. Unlike wastewater, stormwater does not get treated.

The stormwater cycle. Image: Environment Canterbury.

Why is knowing about stormwater important?

Knowing about stormwater is important because it is a lot of water that can quickly become polluted. As stormwater runs over rooftops, roads, car parks and industrial sites, it picks up pollutants such as animal waste, dust, rubber, petrol, oil, lead, and other metals or materials left behind on city roads. People can also directly cause pollution by pouring waste such as paint and oil down stormwater drains, littering, or by washing their cars on the road or driveway.

Remember, stormwater is not treated or ‘cleaned’ in any way, so all these things can pollute stormwater as it travels though the stormwater network and into streams, rivers, and estuaries. This can harm plant and animal life and affect the health of people using these waterways for recreation and food gathering.

Polluted stormwater enters an open drain. Image: Pete Pattinson, NIWA.

Reducing stormwater pollution

Only water should go down the drain.

Here are some tips to reduce stormwater pollution and overflows.

  1. Wash your car on the lawn.
  2. Use cleaning products carefully and follow the instructions on the label.
  3. Pour any left-over cleaning water down the laundry sink, or on the garden, not down the gutter.
  4. If using water-based paints, clean brushes in the laundry - don’t wash paint down a stormwater drain. Save any leftover paint.
  5. Take toxic chemicals to a hazardous waste facility. Do not put these chemicals down the stormwater drain or into the wastewater system.
  6. Clear leaves and other rubbish away from stormwater drains.

Only water should go down the drain. Image: LEARNZ.

Try the stormwater quiz.

Wet weather overflows – read about how stormwater can overload public wastewater pipes.

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