|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 player 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 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 the player 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's inventory regardless of their choice in infecting the water or not.
If the player chooses to infect the water supply with the modified FEV, the player is garnered -1000 Karma points.
- If you choose to infect the water supply, the player 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 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 player's completion of it.
- If you give the modified FEV to Owyn Lyons, you cannot infect the purifier.