Project Impurity

Project Impurity is a main quest in Fallout 3.

Detailed walkthrough
Once the player approaches the entrance of the Jefferson Memorial as required by the related main quest Take it Back!, this quest will activate. The exact objective is to "Infect the Project Purity water supply... or not". The player can choose to infect the water supply with the modified FEV given to him/her by President Eden once inside of the Rotunda which will complete the quest. This is done via the console at the entrance of the purifier structure before turning left to activate the purifier itself.

Should the player elect to not infect the water, the quest will fail. It grants no Experience Points upon 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 loses 1000 Karma points.