drainaquifer has already been run.

look on levels 107,108,109, and 110. You will see blobs of damp clay/sand. These tiles will continue to generate water if breached.

On a later save, I've managed to turn the water flow off using 'liquids' but it still randomly starts flowing again after some amount of time.

The specific tile is right behind the Mason's workshop on level 106 (not sure if I have dug it out or not yet on this save)