Troubled Waters

After retaking the Castle and with Radio Freedom on, you’ll eventually get this quest. Radio Freedom will report about message from a robot from Graygarden. This is directly south of Jalbert Brothers Disposal. Head to the greenhouse and talk to the marked robot, Supervisor White. Next, you have to go to the Weston Water Treatment Plant as marked in the map.

The exterior of the plant is filled with super mutant shacks. At least one of them is a suicider. If you're powerful enough to take it out before it self-detonates, you can obtain the mini-nuke it's carrying. Check the wooden shacks to find supermutant gauntlets, bladed helmet, ammo boxes, and a duffel bag.

Inside the plant, there's a duffel bag inside the locker room. Beside the stairs is an advanced door. This door only opens the storage room with some adhesives. The stairs Lead to the plant's rooftop. The rooftop office has an expert safe and a Novice terminal. The terminal has controls to the spotlights and the safe. Return back to the plant.

Take the elevator this time then destroy the ceiling turret that will shoot at you immediately. Next, you can check the broken wall of the restroom or pick the lock of the Novice door to find two ammo boxes. Next, push the button of the security door to the south then enter the office to the left. There should be an Expert safe there and an issue of Picket Fences.

The security door below is not functioning so backtrack to the previous room and go downstairs this time. Activate the plant's pump to lower the Water level. Open the security door and head out. There will be multiple mirelurks hiding under the water so use your VATS as necessary.

Head to the next security door to the NE and open it. There's an Advanced door at the end of the hallway with no notable loot or container. Head to the next room and go through the next door to reach the second pump control switch. There will be a mirelurk that will ambush you as well so be ready.

After flipping the switch and lowering the Water level, backtrack to the walkways then head to the third switch. More mirelurks will appear as well. Make your way to the fourth switch and activate it as well. This will fully drain the water. Push the security door nearby and you'll have access to the elevator leading outside and a green steamer trunk. Now return to Supervisor White to complete the quest and get Graygarden as your additional settlement.