Quite obviously, the Duchy can only be responsible for the activities they control. So far as they are, I don’t think it’s inconsistent with their stated aims when this issue was landed on them. I mean, you can say you are a careful and responsible driver, and that can be perfectly true, and someone can drive into you.
Personally, I think central government should be responsible for clearing this up - serious environmental hazards should be centrally funded. If they disclaim responsibility then yes I think the Duchy should contribute to some of the costs. But I don’t think it’s Charles’ job to step in and find what should be Central Government costs - that is unconstitutional.