Aqua Pura

 is literally "Pure Water" that is available once the add-on Broken Steel is installed, and the quest Take it Back! is completed. This water is from the Potomac River Basin and has been completely cleansed of its radioactivity by the Purifier at the Jefferson Memorial.

Characteristics
If the water is not infected with the Modified FEV, then Aqua Pura is just normal, filtered water that restores 20 HP; identical to Purified water.

Drinking the water after choosing to infect it with the modified FEV will result in a player debuff (-2 STR and -1 INT) and, if you drink too many, you will die (this is usually any more than two). The reason for this danger is the fact that the Lone Wanderer was actually born outside of a Vault and thus he or she is not immune against the modified FEV, which was designed to kill anyone who had prolonged exposure to the radiation and other mutational effects of the Wasteland.

Locations
The yellow barrels of Aqua Pura can be found across the Capital Wasteland (once Broken Steel is installed). Scavengers with pack brahmin carrying barrels can be seen making deliveries on Merchant routes, protected by Brotherhood of Steel Knights and Paladins. Stolen barrels can be found in the possession of Raiders, Talon Company mercs, Enclave troops, Super mutants, and Slavers, (since the Brotherhood refuses to give them water) and are often used as bait for ambushes. Bottles of Aqua Pura can be found in abundance within the Citadel, with at least 5 bottles per room, or in crates.

Bugs

 * Giving any amount of Aqua Pura to Water Beggars will result in their death at the next map change, regardless of whether or not the Aqua Pura contains Modified FEV.
 * Sometimes after drinking Aqua Pura a message will appear saying that "Aqua Pura has worn off". This is like when a chem wears off but there were no effects to begin with. This message may appear at random times, days after drinking Aqua Pura. This is not really a bug but a scripting quirk. Because tainted Aqua Pura has a debuff effect it's treated like a chem. The taint effect is still applied to the Player, but since the effect fails a condition check, the actual debuff and player death chance aren't applied. The pure Aqua Pura is thus also treated as a chem though it has no ill effects; the end result being the "worn off" messages.
 * NPCs use this as an aid item during firefights. It has the same effects on them if there is Modified FEV added to the water. However, NPCs drinking it will trigger the script that attributes the negative effect to you. In essence, if NPCs drink many infected Aqua Puras, the script will kill you instead of them.
 * Aqua Pura crates can appear before Take It Back! has been completed if Broken Steel is installed.