Rhubarb is very efficient at bioaccumulating toxic heavy metals. Food garden, Moree, NSW.
Recycling is good, except when it is car tyres being up-cycled in food gardens. Permaculturists often misguidedly include tyres in ‘earth ship’ designs. What could possibly be wrong?
In June 2017, I posted about a ‘Dystopian treasure island’ on Facebook. It related to a news story about a playground created in Victoria, Australia, for Fitzroy public housing estate kids – dubbed a ‘sustainable sculpture’ – using old car tyres and recycled steel and rubber conveyor belts. The belts had been sourced from the mining industry and they contain a similar range of materials as car tyre rubber.
Car tyre rubber leaches toxins over many years. The rate of leaching is faster in areas of higher rainfall, so in dry regions leaching occurs over longer periods. Leaching is also affected by soil acidity and other weathering effects, but many of the compounds released are not good for soil, crop, or human health.
Different impacts for animal and plant health complicate matters. As a horticulturist with a grounding in science, I know that humans can effortlessly excrete excess zinc from our diet with no negative health impacts. However, excess zinc in soil generally harms plant growth. I’ve had quite a bit of experience with damage caused by elevated zinc levels in products like artificial planting mixes containing ‘natural’ manures sourced cheaply from industrially farmed livestock, such as cow manure.
It is even more important to note that in terms of exposure to toxins, young children are not merely smaller versions of adult humans. The response of a child – which does not yet have a fully developed immune system to toxins – will not be the same as for an adult.
The best source of information I can find on the risks associated with up-cycling old tyres comes from the Brighton Permaculture Trust, England (BPT). The accuracy of the associated risks is complicated, BPT say, because “we don’t know the exact amount (of compounds) used because of commercial secrecy.” This secrecy means that even advocates of tyre up-cycling cannot predict the environmental impacts.
We have also known since the 1970’s that heavy metals contaminating soils are soaked up by plants – they can bioaccumulate in crops. Some very efficient bioaccumulators of heavy metals include silverbeet and rhubarb and root crops like turnip, potato, radish and beetroot. Toxins also accumulate in sediment in waterways, lakes and mangroves.
The practise of adding lead to petrol in the UK stopped many years before Australia phased out leaded petrol, however to this day lead contamination of soil close to busy roads will still exclude them from cropping certain food plants. Copper, another heavy metal released on roads by traffic, also accumulates in roadside soil. Have your nature strip soil laboratory tested before you start cropping. I contacted the National Measurement Institute for advice and had a test crop of silverbeet analysed before I started cropping my footpath garden. If you treat footpath food gardens as being constructed on semi-industrial land, you start appreciating the associated risks.
Millions of discarded car tyres pose significant environmental risks.Above ground stores can catch fire. They contain lots of empty air space, so they use up large amounts of space in landfill. Sometimes, in wet landfill locations, these pockets of air are sufficient to allow masses of tyres to ‘bubble up’ to the surface again. Old tyres are burned to generate electricity, releasing dioxins and furans amongs...
A particularly disastrous reuse of tyres is in the creation of artificial reefs as a method of disposal. Old tyre reefs have been constructed in the USA, the Gulf of Mexico, Indonesia, Malaysia, Australia, and Africa. None have ever succeeded in enhancing game fishing or conservation. Typically, when these ‘reef dumps’ break up during cyclones they pummel surrounding coral before being washed ashore, up river systems and into mangroves.
Waste tyres are found in many everyday items, including the soles of shoes, for shock absorbing play surfaces and as a flexible surface around tree bases (as an alternative to rigid structures made from bricks or concrete), in backfill for pipelines and roadworks and in making asphalt.
What do we know a typical tyre will contain? We know the following elements are found in an ‘average’ tyre:
Natural rubber, which is OK;
Synthetic rubber compounds, including butadiene, a known carcinogen;
Benzene, a solvent, and a known carcinogen;
Toluene, a solvent with negative health effects;
Xylene, an irritant;
Petroleum naphtha, a toxin;
Polycyclic aromatic hydrocarbons: These include phenols – some are endocrine-disruptors – and benzo(a)pyrene – compounds linked to cancer;
Heavy metals: zinc, chromium, nickel, lead, copper and cadmium. These are the most common contaminants found in foot path garden soil;
Carbon black, which is possibly carcinogenic;
Vulcanising agents: sulphur and zinc oxide;
Polychlorinated biphenyls, which are known carcinogens;
Recycling is, generally speaking, good. But some products are not simply or quickly made safe for everyday use. When you see recycled tyres and their products being used in food gardens or children’s play spaces, it is time to have an adult discussion about risk reduction and healthy childhood development.
A great source book for de-toxifying schools and playgrounds is ‘The Toxic Playground’ by Jo Immig, National Co-ordinator of the National Toxics Network Inc.
Patron, National Toxics Network
24th April 2019