The findings are significant for urban and suburban gardeners, who according to a 2012 estimate from the U.S. Department of Agriculture, provide about 15 percent of the world's food, much of that on vacant lands. The American Community Gardening Association also reported in 2013 that the number of urban residents in food gardening increased 29 percent between 2008 and 2013, from 7 million to 9 million.
Ganga Hettiarachchi, an associate professor of soil and environmental chemistry at Kansas State University, noted that many urban gardens are on land affected by previous use, such as brownfield sites, or abandoned properties with real or perceived contamination issues. In the United States, there are approximately 425,000 brownfield sites covering an estimated 5 million acres, according to the U.S. Government Accountability Office.
"We tested many urban gardens in seven cities across the country for the presence of contaminants in foods grown on contaminated soils," Hettiarachchi said. "We were interested in understanding the bioavailability of the contaminants — how much of these might be absorbed by the food being grown."
In areas where garden plots were established, Hettiarachchi said that lead was the most common contaminant, but several sites also contained elevated levels of arsenic, zinc or polycyclic aromatic hyrdocarbons, or PAHs.
Those contaminants, however, were scarcely present in the food harvested from these gardens. Hettiarachchi said the research team found elevated levels of lead in root crops — such as carrots, beet root and radishes — but data indicate that consumers would have to be exposed to these vegetables every day for their entire life to experience adverse effects.
"If you consider all that and realistically how much of those root crops people would grow in gardens, the risk is very low," Hettiarachchi said. "People still are going to be concerned, but it's important to understand that the science indicates there is very little risk to humans who consume these foods."
Data on leafy vegetables and fruit grown on contaminated soil indicate even less concern. The research found that contaminated dust would sometimes show up on fruit or the leafy parts of vegetables, which is a risk that can be reduced by simply cleaning those foods before eating them, according to Hettiarachchi.
The research also established some best management practices to further decrease the risk of exposure to contaminants:• Get the soil tested. Then, if needed, apply lime to neutralize the soil pH, which prevents soil from becoming acidic.• Apply adequate nutrients. "When plants are not deprived of nutrients, they will not try to uptake other nutrients," said Hettiarachchi, who suggests maintaining a high level of phosphorus if the soil contains elevated levels of lead.• Apply organic matter, such as compost, manure or peat moss. Hettiarachchi said her team's recommendation is adding compost at one-third or one-fourth by volume of soil so that it has the immediate effect of diluting contaminants that may be in the soil.• Dilute the contaminated soils with clean soil, if available.
"Total concentration of a contaminant is not equal to bioavailable concentration," Hettiarachchi said. "Our research suggests that the bioavailability of the contaminants we tested is very low. In most cases, it was less than 15 percent of the total for both lead and arsenic."
Hettiarachchi said that while the health risks from eating foods grown on contaminated soils is very low, gardeners are still at risk from direct contact with the soil and appropriate precautions should be used.
Funding for this study was provided in 2009 by the U.S. Environmental Protection Agency and ends in September 2015. Hettiarachchi and colleagues currently are seeking additional funding for follow-up studies, such as ways in which the contaminants can be immobilized in the soil, and studying forms of contaminants in produce to better understand bioavailability in food crops.
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