I wouldn't worry about it if you are planting flowers. If you are putting in a vegetable garden, I'd choose another place. Who knows what else got dumped there besides rock and tar. Some substances get taken up by the plants that you'd rather not be eating. For instance, I don't grow food plants near my old farmhouse since I know there must be a good amount of lead in the soil from house paint. My veggie garden is a bit away from the house. Ornamentals grow fine there, however.

Contaminated soil can be a serious concern, you may want to have a soil test done by a qualified lab.
The other option is raised beds. The soil should be deep enough so plant roots won't reach into the old soil.

If your in Mass and you buy commercially made compost make sure they are state permitted or licensed by DEP.
The pile has to be tested every other year. I don't know what contaminants there looking for in the tests but why not be safe.
Now start your own compost pile, you'll know for sure.

Ron

The only contaminant that is tested in a basic soil test is lead. Otherwise what you get is percent organic matter, pH, and nutrients (macro and micro). You can get tests for heavy metals and other contaminants, but they are very expensive. As long as you are planting ornamentals you should be fine.

Steve

Thanks everyone. Yes, it is supposed to be vegetable garden and unfortunately the only place i have available to do so. I am planning on putting in raised beds and bringing in new soil to mix in with my huge compost pile. I took out every thing i could find and then covered the spot with fresh soil. the spot is about three feet below level ground. Also I have a friend that's a landscape architect at a local school so I'm going to see if he can do a test for me at the school's lab.

It really makes me sad to see what people used to do to their land and the complete disregard for the environment that they live in.