Is my garden poisonous?

My family moved into a new house 3 years ago primarily for the possibility of hobby farming the large acreage. I immediately set about clearing and turning about 1/3 of an acre of it into an organic garden. I even harbor dreams of opening a small farmer's market stand in a few years.

Problem is, whenever it rains there are rainbow slicks across every puddle down there (and it is only *just* above the water table, so there are a LOT of puddles) and I'm growing less and less certain of whether the "organic" veggies I grow might actually be infused with toxic chemicals and hazardous to our health.

I finally got around to looking up soil tests, but it seems that most soil testing places won't test for chemicals, pesticides, gasoline, oil, etc. (I don't even know if those are what the problem is!) I looked up "environmental testing" on Angie's List and it seems most places only test for mold and asbestos.

I really want to find out WHAT is causing the rainbow slicks, and whether it is hazardous to eat vegetables grown in that soil.

Where do I go and who do I call? Extra points for concrete recommendations in the Baltimore/DC metro area or someplace I could ship a sample to.