Volunteers participate in a soil remediation project at Hayes Valley Farm in San FranciscoPhoto: Zoe KrollAs someone who works on urban agricultural policy, I'm often asked, "Is city-grown food safe?" The question comes from aspiring urban gardeners and concerned eaters alike. And it seems to stem from both a fear of the known and a fear of the unknown.  First, the fear of the known: Common urban contaminants include lead, arsenic, and other heavy metals leaked into soil from old paint, leaded gasoline, modern car exhaust, and industrial land-use. These metals are responsible for a whole host of maladies. Heavy …