Project Purity

 is a water purification project that started before the beginning of Fallout 3. In Rivet City, Doctor Li will tell the PC that his father, James, left Vault 101 to travel to Rivet City to start the project up again. Dr. Li refused and James moved on.

Quotes

 * The plan was to build a facility that could purify all the water in the Tidal Basin at once. No radiation, no much, just clear water. - Doctor Li

Related quests
The project is tied to Following in His Footsteps, and possibly tied to The Waters of Life, as well as the Jefferson Memorial location.