Head to the area of Utopia marked with an X on the map (near the food stalls) to find a fenced area of porta pottys with a large Caustic-X spill covering the area. There is a number of ways you can clean this up:
 

• Using the Chem Bomb curveball, throw one or two into the area to immediately clean it. This will unlock the achievement, but the area will fill back up with Caustic-X
• Use the nearby jerry can to pour water all over the area, you'll be fighting against the Caustic-X that is still spilling in but can also unlock your achievement this way
• Use a weapon to break the two barrels spilling Caustic-X, then jump onto the dumpster to reach the valve, which you can turn off. This will unlock the achievement and clear the area of Caustic-X permanently