The plan was to build a facility that could purify all the water in the Tidal Basin at once. No radiation, no muck, just clear water.”— Doctor Li
Project Purity was created prior to the beginning of the Fallout 3 storyline. It was the ambition and dream of both the Lone Wanderer's parents, Catherine and James, and other scientists, including Madison Li. It was originally supported by the Brotherhood of Steel when they first came to the Capital Wasteland, who would send knights and paladins to defend the project from super mutants and other threats in the ruins. Although the support eroded over time due to the project's lack of results and need for Brotherhood troops elsewhere, e.g. guarding the Citadel or purging the super mutants from the D.C. ruins. Later, more soldiers would be taken from the project in an attempt to make up for soldiers lost to the Brotherhood Outcasts.
This support ended indefinitely after the newborn Lone Wanderer's father James left, thus ending almost all promising research and possibility of future success. After James' abandonment and no support from the Brotherhood, the other scientists abandoned the project, Doctor Li and presumably what was left of the team left to the nearby Rivet City. In Rivet City, Doctor Li will tell the Lone Wanderer that James left Vault 101 to seek her out and restart Project Purity. Doctor Li refused and James moved on. Project Purity is what motivated him to eventually leave the Vault in search of a G.E.C.K. in the hope of completing the project.
After finding James, it is revealed Project Purity was briefly rekindled at the Jefferson Memorial, but the location was captured by Vault 87 super mutants. After enlisting the assistance of Doctor Li and her staff from Rivet City, the location is cleared before the project finally resumes progress. However, the Enclave seizes the Jefferson Memorial and Colonel Autumn attempts to persuade James to cede control to them. James refuses, and kills both himself and the Enclave soldiers with lethal doses of radiation, although Autumn survives. The player is forced to escort the remaining staff to the relative safety of the Citadel, contending with feral ghouls and even more Enclave soldiers.
Though the Enclave managed to capture the Jefferson Memorial, a crucial component, the G.E.C.K., is still required to bring the project to fruition. Once the Lone Wanderer does obtain the G.E.C.K., they are incapacitated by the Enclave and taken to Raven Rock. President John Henry Eden asks that the Lone Wanderer install the modified FEV when they reach the Project Purity control room, which will purge the Capital Wasteland of any beings "tainted" with radiation, regardless of severity.
The Lone Wanderer battles his or her way to the control room and, after finally stopping Colonel Autumn, a final decision is posed. The Lone Wanderer will either choose to insert the modified FEV, fulfill the original intent of Project Purity, or allow Sentinel Lyons to execute the latter. In the original game, as a result of James' actions, the Lone Wanderer ultimately dies if they choose to enter the control room. However, in the downloadable content pack Broken Steel the Lone Wanderer will survive and the decision made at the purifier will have repercussions across the wasteland.
Same old killing, just a different ingredient.”— Talon Company merc
After the decision is made, the water of the Capital Wasteland is purified at last, and life begins anew in the Capital Wasteland. The Lone Wanderer can also, if he/she chooses, simply not do anything, allowing the purifier to self destruct, thus sacrificing the lives of everyone in the facility for no reason. The ending that follows is shorter than the normal ending.
In Broken Steel the purifier is activated, and the Brotherhood of Steel (along with the security team from Rivet City) begins distributing Aqua Pura (purified water) to the various regions of the Capital Wasteland. In addition, the small body of water directly in front of the Jefferson Memorial (the area before the bridge) known as the Tidal Basin is free of radiation.
Any water beyond the bridge is still irradiated as Project Purity will take some time to purify all the water in the Potomac River. If the Modified FEV was inserted into the purifier, then the Aqua Pura will be infected with it.
- It is one of the locations shown in the Bradley-Hercules targeting screen, but cannot be fired upon.
- Inside the water container behind the password entry pad, the Lone Wanderer can see the statue of Thomas Jefferson that the lab was built around and named after.
- Canonically, It appears the Lone Wanderer does indeed purify the water, as characters such as Deacon in Fallout 4, comment on how the water is drinkable in the Capital Wasteland.
Project Purity appears in Fallout 3, Broken Steel, and is briefly mentioned by Deacon in Fallout 4.
- ps3 xbox360 Occasionally, (after activating Project Purity) the tidal basin water will still be irradiated, even though it is supposedly "clean".[verified]
- pc On some occasions a dead Mirelurk may appear to stand on water. If the bugged Mirelurk is shot with any weapon it will fall into the water. Requires the Broken Steel add-on.[verified]
- xbox360 Sarah Lyons will continue to say "Let's get this place locked down before we do anything else." After speaking into the intercom and will continue to do this even though all enemies are dead.[verified]
- ps3 xbox360 Occasionally, the GNR announcement that Project Purity has been taken from the Enclave will go out before the player has started the related quests.[verification overdue]