Why can't you go through it?
[SPOILER]
You can find people who know about the spring in Ebron, Witton, Parverhill, Thremten, Red keep and Aldlyn. In Ebron talk with child, in Witton talk with person near snowman, in Parverhill talk to lonely woman in empty house, in Thremten talk with green-haired man, in Red keep talk with sleepy guard near tent, in Aldlyn talk the visitors of the Inn
Then walk to the gambling den (South fort) and interact with the correct tile to reveal the spring location
You need Mira and Mirel in your party, and the relationship with both high enough for the events to trigger.
