By Marcus Wilson 21/11/2019 4


Apparently, some people enjoy washing their cars. Each to his or her own, I suppose. I mean, some people like duck shooting, some people follow Coronation Street, and some people’s idea of a good day out is to sit on a grass bank at Seddon Park and watch cricket all day. (Guess which one I’ll be doing a week Saturday…)

Now, in the news last week was the new “ban” by Napier City Council on washing your car on the drive. I used this article in our Science Communication class on Tuesday as an example of how to dig a bit deeper and find the science behind a story. I told the students to imagine they’d been given half an hour to prepare for a local radio interview about the proposal, and to get themselves ready. It was a bit unfair given that only a handful are studying environmental science – the rest are doing subjects from across the full range of science disciplines offered here at Waikato – but they did well.

So, in 30 minutes, what did we find out?

Well, the original source is a good place to go to. Getting hold of the council’s plan and the proposed bylaw wasn’t too challenging. Interestingly, the plan says nothing about car washing as such. What it does say is that pollutants should not be allowed to enter waterways. A separate fact sheet then advises that this means that car washing on the drive would likely breach this requirement, since the run-off, which contains detergent, would most probably find its way into the storm water system and out into the Ahuriri estuary.

Are car washing detergents harmful? After a bit of searching in the science literature on aquatic effects of sulphates (which are named explicitly in the online article) the class quickly found out that they can have considerable effect on aquatic life – basically they remove dissolved oxygen from the water and asphyxiate the fish and other marine life. We did, however, uncover some discussion that suggested that salt water environments (such as the Napier estuary) were more susceptible than freshwater and so one might need to be careful assuming that the ban should be countrywide.

Also, some students looked at alternative, ‘eco-friendly’ car detergents and concluded that these, in terms of their likely effect on life in the estuary, are not much better than traditional car washing mixes.

Car washing on the lawn would likely be permitted, because run-off would be filtered through the ground before any found its way to the stormwater system. Likewise, hosing down your car with tap water would likely be permissible because no detergent chemicals are added. However, there may be problems there – further reading uncovered the possibility of contaminants from brake pads and car fluids being flushed from the car and down the drain. This would happen too in the rain, so the implication is that you could be breaching the bylaw by simply parking a poorly-serviced car on your drive.

While a “ban on car washing on the drive” might sound a bit extreme, no-one in class yesterday, after 30 minutes of science research, thought it unreasonable.

But personally, this will change my behaviour very little. Why waste precious cricket-time by washing a car? That’s what rain is for.

The post Is car washing so bad we need to ban it? appeared first on Physics Stop.


4 Responses to “Is car washing so bad we need to ban it?”

  • Well, I’ve got to say I do wash my car regularly and I’m not sure I understand people who don’t. It can be painful seeing vehicles on the road on Monday that have dirt or even things growing in them. And the insides also need regular car to remove the build up of pollutants and bacteria that are introduced to the interior.

    I don’t think there’s any environmentally-friendly way to own a car, which is why I prefer to commute by bicycle 🙂
    But there are some options to reduce detergent rain off into drains and the like. One is to just wash it where the water flows through soil or similar, and not direct to stormwater. As you note. Stopping it entering the storm water is the first best step.
    Another trick is to go waterless. So between washes I use a detailer spray and a spray-wax to maintain the car’s cleanliness and shine.

  • Our cars do get the occasional trip to the car-wash at the garage, which I believe (not that I’ve independently checked) manages the wastewater in a better way. And, sometimes, I do wash them myself, and often on the lawn. Certainly the left-over water in the bucket goes on the lawn, not down the drain.

  • Do you know if the biodegradable stuff breaks down into harmless stuff in soils, before it has a chance to leach into the stormwater?
    I mean, I purposely choose the stuff that breaks down. It’d be good to have a clearer idea on what biodegradable means.
    I always wash my own car because, well, I don’t trust anywhere else to do a proper job 🙂 Plus, it’s vital for the 3 monthly-wax treatment (wash car, apply clay bar to paint, then a paint cleaner, then a polish, then wax and buff). I don’t think any service station’s car wash can match that level of detail.