The Cliffspring River

The Cliffspring River has begun turning foul and corrupted. It empties into the Mist's Edge, and I fear the wash will affect Auberdine soon. I suspect the Blackwood furbolgs up-river are the cause of the taint, but I also suspect that they aren't the true root of it. Take this sampling tube and go to the mouth of the river to our north. Proceed inland to the first waterfall and draw a sample from the pool there. You'll see a bridge overhead. Once you have a sample, return to me in Auberdine.