Credit to Ultamisia.
Continue to progress through the DLC, you'll eventually come across the top right area of the map, called Utopia . It'll have a huge pool as a reference point.
https://imgur.com/V3mWyrASimply:
If you look at the map, you'll see words marked: Wake up people" and a "X" right under it, this is where you'll get your cheevo.
Take out a caustic-X green barrel in front of a white fence and open up the gate.
Several anti-caustic zombies will spawn in, dispatch them easily
Find water containers and get rid of the green goo on the floor AND turn off the valve to the caustic container
https://imgur.com/Bq2MkOACheevo should pop right after