Use the Winterhoof Cleansing Totem at the Winterhoof Water Well, then return to Mull Thunderhorn.
I have crafted the totem to cleanse the Winterhoof Water Well. Now, you must take it to the well and perform a cleansing ritual. You will find it to the southeast, guarded by foul goblins!
This will not be an easy task, but you must do it if we are ever to use its waters again.
Fortune to you, [player].