After watching an item on Campbell Live about contaminated soil in a vege garden in Christchurch, I thought I'd follow it up with info on soil testing for those who would like to know more. Soil testing in gardens should go beyond testing the soil pH, particularly if you are planning on growing food crops.
Become a detective
A good way to start is to find out about the history of your site. It is fascinating looking up Auckland Council GIS viewer to check out historic aerial photos of your location. There is a scroll bar which will show images as far back as 1940. Look for past uses of the property, such as orchards, glasshouses, farm and implement sheds and rubbish dumps. These are likely to be a source of soil pollutants. Dioxins are a problem in New Zealand after past use of DDT.
Copper hotspots are also an issue and can be found where old orchards have been, as is arsenic leaching from treated timber.
If you are planning to buy a block of land it is prudent to invest in soil tests to avoid problems in the long run. You can also get plant material tested to ensure crops are safe to eat and that they have not taken up chemicals or heavy metals in the soil.
Look out for lead
If you have a pre-1945 house it is almost certain that the paint will have had lead in it -- under newer layers of modern paints. Even paint jobs pre-1980 may contain lead. If the paint has been sanded or is deteriorating (particularly in high-use areas such as windows) and flakes are landing in the soil immediately around your house then you are highly likely to have some degree of lead contamination.
At pharmacies you can buy a lead paint test of 5 per cent sodium sulphide solution. Expose the deepest layers of paint by cutting into it at an angle with a sharp blade. If the paint goes black after testing it contains lead. Prevention is better than cure.
When prepping for a paint job, reduce the contamination of soil around your house by laying out disposable polythene drop sheets. Never "dry-sand" lead paint. Use a wet-stripping or sanding technique on flaking paint -- not waterblasting -- or use a sander with a filter vacuum attachment. Alternatives include low toxicity gel paint strippers. Lead and other soil contaminants can become a problem if you have children or pets, and both often eat soil. Pets also lick their paws and fur which is how they can ingest contaminants. Always ensure veges are washed well before eating.
Which spot do I test?
To avoid the chance of condemning your entire section, it is often best to test several zones. If you have an area that is cause for concern based on past land use, take a grid series of soil plug samples from that area, mix together and label. If there is a spot where you are keen to grow veges, or are already growing them there, do another grid of soil samples.
There are a range of soil testing options at Hills Labs to choose from depending on what you would like to find out. These include tests for general vege garden and residential soil contamination, plus specific tests for organic certification, lifestyle blocks, horticulture and agriculture.
It can be tricky to assess whether the level of pollution in your soil is likely to be an issue. Just changing the land use on your site can considerably reduce your family's exposure. Have a look at the organic guideline for soil contaminants on the vege garden testing page on the Hills Lab's site.
As discussed on Campbell Live, building a raised bed with untreated timber, and importing new garden mix is a good solution for safe gardening.
Relocate kids' play areas and avoid growing veges in areas that are testing high for lead.
By using soft cover such as bark mulch or laying new turf you can reduce your lead exposure by up to 80 per cent, even more if you pave over it.
To assess the level of hazard of your soil, and to find out all there is to know about managing lead, visit health.govt.nz for more information.
Although removing soil isn't necessarily the best solution, levels of soil pollution that are above public health guidelines sometimes require drastic action.
Notify your regional council if you are removing contaminated fill so that it can be dealt with properly. The council will also give you an idea of whether the contamination in your soil is a public health issue and can advise on whether you will need an environmental consultant.