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What happened at Rietvlei?
It is believed that years of mainly organic pollutants draining into the wetland from primarily stormwater and river catchment runoff caused the oxygen content of the water to drop. When too much organic matter enters a water body, bacteria start to proliferate, and use up oxygen.
The warm, calm weather conditions last week contributed to oxygen levels going below the threshold necessary to support many aquatic species, with large-scale organism death the result.
What is stormwater and how does it get polluted?
‘Stormwater’ is the water you see running down the street gutter after heavy rain; essentially, it’s any water that doesn’t go directly down a plughole or a pipe. It includes rainwater that runs off from roofs and paved areas, as well as waste water that doesn’t go down indoor pipes e.g. the water you’ve used to wash your car, or runoff from your garden.
The stormwater drainage system is separate from the sewage system. Waste water from pipes and drains inside buildings leads to the sewage plant where it is treated before re-entering the environment.
The trouble with stormwater runoff is that it eventually enters natural waterways untreated i.e. pollutants that get into stormwater also get into our rivers, wetlands and seas. There’s nothing to stop that oil you’ve just washed off your car from ending up on the beach.
As stormwater travels, it can pick up all kinds of undesirable substances that have no business in water. These include litter (anything that floats) like plastic bags and cigarette butts, chemicals like detergents, oil, fertiliser and paint, and organic material such as grass clippings, animal faeces and soil. Some of these are toxic. Others, such as many organic substances, aren’t poisonous but may accumulate in such great quantities that natural systems can’t cope – as occurred dramatically at Rietvlei.
In addition to causing chemical changes, sediments can also physically clog waterways.
Preventing stormwater pollution
The following are simple measures we can all take to keep our stormwater as clean as possible.
- Don’t litter, and spread the message by example - pick up litter in your neighbourhood.
- Don’t pour oil, paint or other chemicals, or the waste water from washing dirty equipment, down the stormwater drain or gutter.
- If you have a pile or area of loose sand or cement – as is often the case during building projects – keep it covered when possible to prevent it from being washed into the stormwater system.
- Preferably wash your car on a grassed surface instead of in the street or driveway where detergents and dirt can go down stormwater drains.
- Keep your car well maintained, especially as regards fixing leaks.
- Don’t hose down driveways and other paved surfaces: sweep or rake up garden refuse, and re-use as much as possible in your garden as compost.
- Use fertilisers and pesticides sparingly and away from drains.
- Plant water-wise plants: they use less water, resulting in less runoff, and they drop fewer leaves.
- Make sure swimming pools are backwashed into the sewer system, not the stormwater drain.
- Don’t let dog or other animal faeces get into the gutter – bury them in the garden or put them in a sealed bag in the bin. When walking your dog, take a plastic bag or a poop-scoop - and use it.
- Olivia Rose-Innes, EnviroHealth, Health24, January 2007
