Does the Enviromental Protection Agency have an enforcement and investigative unit?

Yes and they will fine the Cr*p out of you!

Likelihood of getting caught is far higher than likelihood of getting sued. Their enforcement is primarily legal, so they tend to only go for the biggest fish. My friend works for them, her continuing frustration is the lack of actual enforcement power the agency has. As with many regulatory agencies, they're totally understaffed in both these departments.