The dump pond is just built as a safe place for LOX to spill until it evaporates into the atmosphere. As opposed to it just spilling on the ground and spreading out and potentially igniting or damaging something with critically cold temperatures. It shouldn't need anything in the pond (it will boil on its own) although I suspect rain water builds up in there on its own (and likely must be pumped out occasionally).
I'm not sure whether that is where it vents to, or if it is a containment pond for contingencies, or both.