Soil at the community garden at Oak St. and 16th Ave. in Vancouver had concentrations of lead reaching 219 parts per million — nearly four times the background levels in soil in the Lower Mainland.
They found zinc in the soil at 456 parts per million, or more than double background levels. The tests were on native soil, not the raised beds.
The total metal contamination — including air and soil — at the 16 Oaks community garden was even higher than an industrial brownfield they measured, says Gladys Oka, lead author on the study published in the Journal of Soil Science and Plant Nutrition.
"I was surprised...that this community garden which looks beautiful, contains much more metals than the brownfield," says Oka, a UBC masters student in soil science.
The brownfield, near the Georgia Street viaduct, had old rail ties buried in the soil, and other obvious signs of contamination, says Oka.
The 16 Oaks' site had previously been a restaurant and parking lot, but its full history is not clear, says Oka, who notes much of the contamination may come traffic at the busy street corner.
"I think what that tells you is you can't just make assumptions based on how a site looks, that it's going to be contaminated or not contaminated," she says.
Urban agriculture on the rise
Vancouver has been encouraging people to establish new community gardens for several years, and now boasts more than 75 across the city — many along busy streets or on former industrial sites.
The researchers don't want to make gardeners afraid, but do want the city to establish a framework for testing the soil health at urban farming sites.
"Our research has shown that the potential for metal contamination is a concern," says Les Lavkulich, co-author of the study and director of UBC Master of Land and Water Systems program, in a statement.
"Before we start promoting things, we should make sure we have a relatively good idea of what we are promoting."
Oka says gardeners can also choose what to plant, based on the possibility for contamination on their land.
"If you're in a high traffic area, you might also have to think about what's coming in from the air, in which case you might consider planting root vegetables" versus vegetables with a lot of edible foliage, she says.
Oka says she would still eat food grown at the site. It wasn't tested as part of this five-month study.