How to unlock the quest: Talk to Supervisor White in Graygarden (M4,1)
- Investigate the Water Treatment Plant
- Drain the flooded Water Treatment Plant
- Restart the main pump
- Return to Supervisor White
Investigate the Water Treatment Plant
You'll get this task from Supervisor White in Graygarden. The greenhouse has a problem with water. You need to check what's happening. Go to Weston Water Treatment Plant, south of your current location. The place is heavily guarded by super mutants, so be careful. Enter the main building, look for an elevator and use it. Now you have to find the pumping station. Go downstairs and turn left. The objective will be updated as soon as you reach the room.
Pump the water out of the plant
You need to activate four pumps that will get water out of the building. Each of them will lower the water level and uncover new ramps and corridors. A group of mirelurks will appear each time. The first switch is located in the room you're currently in . The second pump can be found on the other side of the building . Follow the ramps. When you activate it, the water level will decrease enough for you to see the third switch . Go there and activate it. You'll see the fourth switch  right in front of you. To reach it, jump into the water and get through the partially flooded door. The console is on the first floor.
Restart the main pump
You'll gain access to the console that controls the flow by pumping all of the water out of the room. Your only task is to activate it. Go to the switch  and activate it.
Return to Supervisor White
Your last task is to return to your current employer. To do it, use the elevator in the room next to the switch. Go to Graygarden and talk to Supervisor White. You will receive vegetables as a reward and you'll be able to turn the greenhouse into a settlement.
Rewards for completing the quest: bottlecaps, vegetables, access to a workshop.