I guess that's the risk you take. I remember reading in my local paper a couple years ago that some guy in my town poured some gasoline from a lawnmower can into a storm drain. Some neighbor saw this and called the police. Apparently that resulted in a visit from the police, fire department, and regional hazmat team, who went downstream to plug the storm line and then collect everything upstream. IIRC, criminal charges were filed as well. Considering how hard it is to dispose of gasoline around here (once or twice a year county hazardous waste day, drive it way over there, wait in line, etc. and they only started those collections 2 or 3 years ago, there used to be NO way to do it, that I'm aware of) I'm not surprised that the guy did what he did, and I don't know what happened to him, but I would bet he had to pay for the hazmat team response.
My point is it only takes one phone call to put you in a world of hurt, for what might be federal charges. And you can't plead ignorance because you already talked to the EPA (you didn't give them your name, did you? lol). Of course, I don't know how they would prove you did over 8 cars a week, anyway. That's actually almost unbelievable that they let you do 8 cars instead of having a zero-tolerance policy, unless the 8 cars applies to homeowners as well. Hmm...if you talked to the EPA lady that would be a federal law?