Urban/guerilla gardens are a possible solution to petroleum dependency and increasing presence of genetically engineered produce on grocery shelves. Unfortunately some urban areas are contaminated with heavy metal (lead, cadmium) and/or organic toxins (pesticide/petroleum residue)..
There are several ways to begin soil remediation of a potentially contaminated area. compost is good for absorbing and breaking down organic toxins via microbes, and can also be used as raised bed 2 raise plant roots above lead..
Phytoremediation uses plants (often grasses/monocots) to take up heavy metals in their roots. These plants would be treated as toxic waste after they have absorbed the metals into their roots. After a few crops are implemented the soil should be free of contaminants, though a test would be a good idea..
http://www.mse-research.mcgill.ca/envr401_2002/brownfields/intro.html