Ablutions

Ablutions is a quest in the Fallout 4 add-on Far Harbor.

Detailed walkthrough
For this quest, Mai at the Nucleus asks the Sole Survivor to acquire a pump regulator to repair the decontamination arches near the entrance. Her directions lead to the Vim! Pop factory which is inhabited by a large number of super mutants and their hounds. The regulator can be found in the Vim Factory Floor area with the two large mixing vats.

Upon returning to Mai, questioning her about her "repairs" reveals that she is actually attempting to circumvent the decon arches' filter systems in order to have them irradiate instead of decontaminate everyone passing through. She then leaves the honor of installing the pump regulator to the Sole Survivor. Reaching the mechanism at the bottom of the submarine dock involves trudging through the dock's heavily irradiated water, so suitable preparation is recommended. With an Intelligence of 7 or higher, the Survivor can opt to reverse Mai's alterations to the system before installing the new regulator, thus making the decon arches actually decontaminate as they were meant to do. Return to Mai to claim the unique Radical Conversion radium rifle as just reward regardless of how the quest was resolved.