I can't remember the quest all that clearly but i think after you meet up with Morrok in that building, he tells you to scout around & look for the source of the pollution.
If i remember correctly, you need to go outside the building & walk up & down the river bank, quite close to the building you met Morrok in. I think the quest says something about the waterfall but i got the quest complete message just near the building, on the river bank where a ramp goes down to the water.
"I can stand brute force but brute reason is quite unbearable. There is something unfair about its use. It is hitting below the intellect."
Oscar Wilde