As gardeners in Portland, OR, struggle to figure out if the city’s manufacturing past is polluting soil now used for growing food, Civil Eats delves into the question of how to detect and mitigate heavy metal contamination in urban gardens. The short answer: it’s not easy.
Testing soil for heavy metals is costly and it’s likely that most urban soil will have some “background” level of these contaminants. There is also no single national standard to define acceptable levels of contaminants in soils. The U.S. Environmental Protection Agency (EPA) has guidelines for heavy metals cleanup at hazardous waste sites, but there is no equivalent for garden soils.
So, as the University of Vermont Extension has written, “It is not clear exactly what levels of heavy metals in soil are safe or unsafe.” As Tulane University School of Medicine research professor Howard Mielke notes, “We have a Clean Air Act and a Clean Water Act, but we don’t have a clean soil act.”
Testing for heavy metals can be expensive, as Rebecca Kessler details for Environmental Health Perspectives. To make matters worse, because contamination may be spread unevenly throughout a site, it can be difficult to get a clear picture of what’s in the ground.
Testing a single sample is rarely sufficient because contaminants occur patchily, says Ganga Hettiarachchi, an environmental chemist at Kansas State University who is studying garden soil contaminants in seven cities and food crops’ absorption of them under various conditions. For instance, lead is often concentrated near foundations of old houses and surface runoff pathways in residential yards, but hot spots can turn up anywhere an old painted board was discarded, say, or a long-gone fruit tree was sprayed with lead-arsenate pesticides.
Furthermore, a recent Brown University study showed that lead contamination can spread farther and penetrate deeper than expected. Soil data from Rhode Island yards showed that lead-based paint spread more than 400 feet from nearby water towers, and often penetrated more than 12 inches below the soil surface. “The heterogeneity of contaminant distribution is one of the biggest challenges,” says Hettiarachchi. “You cannot actually afford to run so many samples.”
For gardeners looking to reclaim land once used for industrial or non-agricultural purposes, scientists at NC Cooperative Extension offer advice on soil testing and methods to minimize risk.
They recommend planting in raised beds where roots can’t touch contaminated soil. Gardeners should wear gloves, wash hands, rinse produce, and limit children’s exposure to potentially contaminated soil.
You can find more on what constitutes a risk and what to do about it from the NC Cooperative Extension here, and from Environmental Health Perspectives here. For more on Portland’s soil crisis, get the full Civil Eats article here.