|Main Quest: Project Impurity|
|Travel to the Jefferson Memorial.|
|Infect the purifier with the modified FEV at the purifier's airlock console.|
|Reward: -1000 karma|
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 the one chooses to infect the water supply with the modified FEV, one is garnered -1000 Karma points.
- Upon choosing to infect the water supply, one can still end the game with very good karma by activating Project Purity themselves, losing 1000 karma for assisting in the fulfillment of President John Henry Eden's vision and plan, but gaining +2000 to karma for activating the purifier manually.
- This is one of the very few parts of the main questline of Fallout 3 that the player character can actually fail, accidentally or purposefully. The other failable main quest is Galaxy News Radio.
- This is one of the only parts of the main questline of Fallout 3 that does not garner a trophy or achievement upon the one's completion of it.
- If the player character gives the modified FEV to Owyn Lyons, they cannot infect the purifier.