After finishing the Destroy the Bomb Cache job, an alarm will sound and you must talk to Carlson in Wellmasters. Then head on into The Well. For the first part, it's very straight forward, just keep pushing through and killing enemies. Eventually you'll come to a part where you'll have to raise the gates that the enemies keep putting down, but if you have enough ammo this shouldn't be too hard either. Eventually you'll meet up with some rebels for a firefight. After that go up to the big canister and take the ghost toxin to get the Wellness Plan achievement. Then take the elevator back up and go back to Wellspring and take the toxin to Carlson to complete the job. Then he'll tell you to bring the sample to Kvasir for the Deadly Delivery job.