If you’re a vegetable gardener in a lot of older cities, there’s a fair chance you have a significant amount of lead in your soil. One common mitigation approach is to build a raised bed and fill it with freshly composted, low-lead soil from elsewhere, right? Maybe not, according to researchers studying the mysterious case of the lead contamination found within raised beds in community gardens in the Boston communities of Roxbury and Dorchester.

“Raised beds are surrounded by a sea of contaminated soil,” said Daniel Brabander of Wellesley College. Brabander, his students and colleagues have been studying the lead in 144 backyard gardens in coordination with The Food Project, an organization committed to food security, nutrition and sustainable urban agriculture. Eighty-one percent of the gardens they studied were found to have lead levels above the U.S. EPA limits of 400 micrograms of lead per gram (µg/g) of soil.

To solve that problem, raised wooden beds with freshly composted soil were installed in backyard and community gardens by the Food Project. But the researchers have found that the soil in raised beds that starts with as little as 110 micrograms of lead per gram of soil rose to an average of 336 µg/g of lead in just four years.

Just how this is happening is the focus of a Nov. 1 presentation by Emily Estes at the meeting of the Geological Society of America in Denver.

“We’re trying to get a better handle on the mode of transport and the source,” said Estes. That means some pretty detailed monitoring and chemical analyses of the minerals in the soils as well as the kind of lead that’s in the soil.