Jumbo will toss objects at you which can be easily dodged with the Dash, and if he runs at you simply sprint to the other side of the room. I recommend unloading into him with the assault rifle during every opening you get, and activating Overdrive for bonus damage if you have it. Once Jumbo is dead grab the supplies from the room then head to the quest marked door. Go inside where you will find even more supply boxes, then head up the stairs where you will find a Data Pad on the table, and another valve door. Open the door and you will run into a short encounter with some guards (and guess what: MORE supply boxes).
Proceed forward into the sewers, continuing to grab all the random loot along the way. You will see a Storage Container behind some metal bars. Enter the valve door to the right, grab the Data Pad off the shelf on your left, then blast the supply boxes inside the next room to reveal a small opening in the wall. Crawl through to reach the other side of the bars and loot the Container. Head back through the pipe and then up the ladder.
Continue down the path and you will see some goons at the end of the hall. Destroy them with a cooked grenade, then head down the hall where you encounter another small horde of goons. Dispatch them with your weapon of choice and press on (and be sure to break open the supply boxes floating in the pool of water). There are also a few side rooms with various supplies, so be sure to stop in and loot whatever you can. Continue forward and you will eventually enter the tank room. Use your rifle to pick off the goons defending the tank, then head up to the tank platform and grab the Data Pad from the table behind it. There is also a Storage Container hidden in a drain directly under the entrance.
Once you grab all the loot head out the valve door near the tank. This door will lead you to what I would call the Hanging Body room, which is of course filled with more goons. This room is quite large, so I would recommend getting up on the catwalk and picking off enemies with your assault rifle from above. As usual where there are a lot of enemies there is a lot of loot, so take some time while killing goons to bust open the supply boxes and pocket all the loot. Once you are done killing/looting use the elevator to head back into Wellspring.
Once you exit the elevator head into the alleyway on your left to find a Data Pad on a trash can. Then head down the path in front of the elevator to find two more Data Pads (one on a box on the left, another in a short alley on the right). Head back to the mayor’s office, take the elevator up, then speak with Hagar to complete the mission.
From here you will need to raise your reputation with Hagar to level 5, at which point you will unlock the next Hagar story mission, Beneath the Surface.