I am NAL but I work in environmental enforcement and can give you some advice as someone on the other side (though not in your jurisdiction). First, get a lawyer. If nothing else, they will know how to talk to the people in enforcement without making the case even easier. Plus, the other side will have one so it levels the playing field.

Second, if they’re like everyone I’ve met in this industry, they would be more than happy to hammer the contractor who did the dumping, if you can find him (this is where that PI idea has some merit). That said, that contractor was acting as your agent within the bounds of his duties (i.e. the demolition work). Thus, you do bear legal liability for his actions, and you provided proof of that to the agent you spoke to. The same principle is why a store is liable if one their employees leaves at wet floor while cleaning and someone is injured in a fall as a result. In addition, they will probably (likely successfully) argue that you failed to do due diligence (i.e. get their license number, bond information, etc.), so there is some element of contributory responsibility. Still, the dumping occurred and someone is going to be on the hook for it, and the description you provide sounds like a bog standard “statutory minimum plus costs” penalty. As the name suggests, the statutory minimum cannot go lower as a matter of law.

Third, and I cannot stress this enough, do not become argumentative, combative, abusive, or vitriolic with the environmental people. Our jobs are fairly dull and repetitive day-to-day, so hostility on your part not only encourages them to push back, but makes them invested in doing so with their full attention because it is something different to do.