Project Impurity

Project Impurity is a main quest in Fallout 3.

Detailed walkthrough
Once the Lone Wanderer approaches the entrance of the Jefferson Memorial as required by the related main quest Take it Back!, this quest will activate soon after. The exact objective outlined is: "Infect the Project Purity water supply... or not".

The player character can choose to infect the water supply with the modified FEV given to them by President Eden once inside of the Rotunda which will complete this quest immediately after. This is done via the purifier's console just to the right of the entrance to the purifier's primary chamber.

Should one choose to not infect the water, the quest will fail. It grants no XP upon its completion and the modified FEV will remove itself from the player character's inventory regardless of their choice in infecting the water or not.

If one chooses to infect the water supply with the modified FEV, one is garnered -1000 Karma points.