If the mountains are modified to be impassable then you can never access them with a worker to build roads so it would be impossible to hook up any strategic, luxury, or bonus resources that might be found there.
It would also be of little or no value to have a citizen assigned to impassable, unroaded, and unmined mountains with no rails because the net value of the citizen probably be less that the value of a specialist even if it was possible to assign a citizen to the impassable impossible mountains.
Pollution or radiation on a square does not grow like some sort of orange mutant version of the blob. It just sits there and denies you the ability to assign a citizen to work the square and gain benefit. So if the pollution occurs on an impassable square that is impossible to use it will have no effect on your game.
This is sort of like the question of "If a tree falls in the woods and no one is there to hear, then how do we know if it makes a sound?"
You question could be rephrased as "If pollution occurs on a square that no one can get to and no one can possible use because of modifications that I have made to the game, then how do we know if the pollution effects the value of the square that I can never get to?"