Check out this post I found at TheDailyGreen.com. This is pretty interesting stuff:
"You may have purchased organic seeds, carefully worked your own compost into the ground, and pulled weeds by hand. But all your hard work could still be undermined, and not just by gophers, mildew or beetles. As the Boston Globe points out, decades of use of lead in gasoline and paint have left a toxic legacy in the ground in many areas, and the heavy metal may be entering your hard-earned produce..."