Since 1978, lead has been banned by the federal government. It is a known toxic agent to humans and our pets, and ingesting lead can cause a wide variety of issues and even death.
The most common source of lead contamination in the garden is from lead paint, and in particular in the soils close to structures that may have had lead paint used in the past. Before its toxicity was known, lead was used in paints since it helped paint to dry quickly and to produce vibrant colours. Typically, houses build before 1960 will most likely contain lead based paint. If the paint chips and flakes off into the garden, this can be a source of contamination.
One interesting piece of trivia we learned recently is that sometimes when people got sick and needed to move to the country or seaside for fresh air, their health improved because they were no longer poisoning themselves from the lead paint inside their homes.
The best way to test for lead is to take a soil sample and send it off to a lab for analysis. When combined with testing for other heavy metals, you’re most likely looking at around a $100-$130 test in Ontario.
If your test comes back with a high proportion of lead (the City of Toronto considers a value of 340mg/KG and the EPA considers 500 parts per million hazardous), we suggest you take a few precautions to avoid further exposure. Wear a dust mask, good gloves, and leave your shoes outside whenever you work in the garden.
If you wish to continue using the garden, there are several options we can suggest.
Phytoremediation is the process of using plants to mitigate environmental problems without the need to excavate the containment material and dispose of it in another place. When it comes to lead, there are a few plants that can be used including Indian mustard (Brassica juncea), Ragweed (Ambrosia artemisiifolia), Hemp Dogbane (Apocynum cannabinum) and Poplar trees.
2. Construct and Grow Using Raised Beds
Instead of worrying about using contaminated soil, start off with clean, fresh uncontaminated soil. Build a raised bed with a bottom consisting of a thick material to keep native soil out. This can be something simple like several layers of cardboard or something more robust like plywood.
Raised beds offer peace of mind that the produce you grow will be clean and free of contaminants (assuming you start off with clean soil and don’t add anything nasty)
3. Minimize Potential Exposure
If you decide to continue using the existing soil, there are a few things The City of Toronto recommends. The first is to dilute the concentration by adding in new clean soil and/or compost. If you add enough volume, you can essentially dilute and lower the concentration to a safer level.
Next, you can raise the pH to a more neutral level, usually over 6.5. By increasing the pH, lead and other contaminants become much harder for plants to access.
Finally, you can focus on growing fruiting plants like tomatoes, beans, and fruit trees as there is less chance of lead and other contaminants being passed into the fruit.
For more information, the City of Toronto has a good Soil Assessment Guide (pdf) to help you learn more and make an informed choice.