Common wisdom had it that gardeners growing plants in urban areas are the ones who need to be worrying about high levels of lead and other heavy metals in their soils. But some recent research by Iowa State University agronomists on soils in a suburb of Des Moines suggests that this problem may be more widespread than previously thought.
Researchers took soil samples from suburban residential areas that had been developed at various times from before 1939 up to 2005. They found that the average concentration of lead was even higher than had previously been reported for urban Des Moines, with the older sites having the highest levels, and in some cases higher than the maximum concentration considered "normal." They also found that more recently developed sites had lower levels, which was attributed to the phasing out of leaded gasoline for on-road vehicles in the U.S. by 1996. In the particular area studied, they also found that average concentrations of other heavy metals such as zinc and cadmium varied with the age of the development.
What this suggests is that, no matter where you garden, it's a good idea to start by having a soil test done that includes testing for heavy metals. Then you'll not only know the pH and nutritional status of your soil, you'll also be aware of any possible contamination problems. Your state Cooperative Extension Service or Master Gardeners Program can tell you how to go about submitting a soil sample for testing for minimal fee.