Factorio Forums

Attaching an underflow Valve to a Tailings Pond, should, according to its tooltip at least, prevent overflowing the pond. The tooltip says that it allows flow only if the *output* (Pond) is under 80%.
Once this ghost is replaced with the actual valve though, it will let the liquid through and lead to ground pollution conversion.

I assume this a bug, certainly I did not find a description or tooltip that makes note of this problem.

Thats not a bug it works as intended. The tooltip is telling you that it is voiding machine and you need pump to get fluid out that means tailing pond is lower than normal pipe, so valve like that wont work there.