Clean Water Act (Virgil)
Get this story mission by talking to Virgil after liberating the Hope County Jail in Faith's region.
He'll give you a task at the Deep North Water Treatment Plant, southwest of the Marina.
The plant is as heavily guarded as an outpost, but there's no bonus for stealth, so sniping from on high is a quick way to clear the grounds. Prioritize the flamethrower heavy, and the VIP mixed in with the crowd.
There aren't a lot of exits, so even if the enemies manage to charge en masse, they're pretty easy to mow down. You yourself can enter the grounds via a cut hole in the southeast of the complex.
The air here is Blissy, but not cripplingly so. Before tackling the main quest, you might snag the local map off a blue-covered crate here.
Both pump rooms are accessed via the water. The key is to look for the sky blue pipe exiting each room, and following them inside. They even have helpful white arrows pointing the way.
Once inside, each pipe room acts the same way- there's a safe to crack, and machines to explode. Both rooms have large red gas canisters that can be shot for the necessary oomph.
After completing one pump room and exiting, a horde of cultists will descend from the hills. Don't play this one too breezily- there are a ton of these guys coming from all directions, so you will want to fire from cover, and exercise some crowd control.
Note that one of the pump room tunnels is blocked by debris, including an oil drum. To clear the block, go back above water, and shoot at that spot until you hear an explosion.
This will clear the block, though you might give it a few moments for the pieces to settle.
Destroying the machines in both pump rooms completes the mission.