When they no longer have potable water. When their crops wither and die due to subterranean poisoning of the land (there are lots of wheat farmers up there). When they begin to suffer from debilitating illnesses, cancers, etc.
Then they will come whining to the federal government. The gubermint should have protected them against such things!!! The gubermint now needs to pay for all of this.
The sad part is that all these folks have been so poor for so long that they have sold their souls for some quick cash. And, as you said, they will expect the gubbamint to fix it all for them when they find that they are dying from all this. It's much the same story with Montana, the governor is all for it since it's giving his state tax base some rapid infusion, and he's also pushing for more coal mining not far from the tar sands mess.
This is what the Kochs and their ilk were planning on, make us all so poor that we will clamor for these shit jobs atht will kill us off earlier...