Story related, cannot be missed.
You will gain a quest called "Hijacked Well" from the Mayor of Wellspring once you return from having the Feltrite sample examined. He will have you drop in on the local water treatment area (very close to his office). Inside you'll meet Carlson who will give you this quest and send you into the well. Kill or be killed as always, and once you retrieve the toxin, you retrieve 10 gamerscore as well from this achievement.
A quick helpful piece of information as well. This is an area that you cannot come back to once it's complete. So if you are going for the Gotta Collect Em All achievement, note that there are 2 collector cards in this area. One is up a ladder in a long corridor right before you meet up with some town soldiers. The other is right after you meet them, head towards a dead end past the gate they are trying to open. You'll see it sitting on some crates there.