It isn’t uncommon for home gardeners in older cities to find a significant amount of lead in their backyard soil, so many build raised beds and bring in freshly composted soil to avoid the problem.

That was exactly what The Food Project, an organization that promotes sustainable urban agriculture, did in the Boston communities of Roxbury and Dorchester, where yard soils were found to have lead levels about the U.S. EPA limits of 400 micrograms of lead per gram (µg/g) of soil.

But then researchers and students from Wellesley College who were studying the community gardens project found that the imported soil in the raised beds, which started 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.

According to an article in the Geological Society of America, solving this mystery was the subject of a presentation to the group at a recent meeting in Denver.

Lead contamination in the soils of most cities, the researchers noted, is primarily attributed to the residual effects of leaded gasoline and lead paint, but their use has been outlawed for years, so other factors had to be at work.

They concluded that lead was likely being transported into the raised beds by the wind and rain — for example, rain splattering on the ground was carrying contaminated material into the raised beds.

Fortunately, the article also stated, the lead found in the Boston-area raised beds was not particularly good at being absorbed by the human body.  Still, the researchers recommend that home gardeners scoop away the top inch or two of the soil every year from raised beds  in order to keep the lead out of garden produce.