Elemental BracersCollect 5 Intact Elemental Bracers and use the Divining Scroll on them. Afterwards, bring the Divined Scroll to Sentinel Velene Starstrike at the Silverwind Refuge.
Since you must destroy the bracers to kill the elementals, it will probably be difficult to find the specific ones that contain the information the divining scroll needs. You can trade bracers with others in order to obtain the ones you need, or perhaps you have the divined paper already?
Well done, <name>, this should give us the information we need to have the culprit discovered. How would you feel about taking out the mage who fills our pools with corrupt elementals? The corruption of Ashenvale and beyond makes my stomach turn on its own--to have a wizard actively attempting to destroy it further sickens me to no end. We shall make him pay.