It's been a while since I played this game, but did you do this first:
See the cage pulled up when the chain is wound up by the water pump.
Use the rope on the well. Adrian ties it on the column.
Go back to the library and down the spiral staircase of the column to the sewer.
See the cage suspended on the chain. The rope tied to the well is hanging inside the cage.
Talk to Valentina about the plan.