"It's not a static situation," said one of the researchers, Daniel Brabander, an associate professor at Wellesley College. "It's very prudent to characterize it at the start, but depending on neighborhood where you're doing this, it is evolving."
The Food Project has recommended growers also take simple yet potentially effective steps to reduce exposure to contaminated soil by washing their hands after gardening, washing vegetables thoroughly and trying not to track soil indoors.
Murray McBride, director of Cornell Waste Management Institute, said its analysis of garden beds in New York City generally has been encouraging, with one pilot study of 44 gardens finding less than 10 percent had high lead levels in the soil. He said efforts there to bring in clean soil and compost for raised beds may be why lead was less of a problem.
A lack of standard practices as urban agriculture expands has made the problem difficult to assess. Dave Weatherspoon, an associate professor at Michigan State University who studies food issues in Detroit, where urban farming is taking off, said more research is needed to provide a better understanding of what soil contamination could mean for crops and what should be done about it.
"We don't want people to feel that their food isn't safe," Weatherspoon said. "That is the worst thing that can happen to the U.S. food system."