I am thinking about replacing my sump pit lid with a sealed version made for radon. I have a radon fan system that pulls air from under the slab. If I seal the sump lid airtight (or as close as possible), do I have to add a vent pipe to the outside? Would not having a vent cause any issues with the performance of the pump or the water discharge? The sump is for groundwater collection and discharges into a storm sewer at the street. Thanks for any information.

when water enters the tank it displaces an equal amount of air.
so it is hard to say what happens to that air when the water comes into the tank.
if you did this with a sewer ejector tank it would not work. but a sewer tank must be sealed for sewer gas purposes.
when a toilet is flushed the water rushes into that type tank.
with a ground water tank the water entering comes in much, much slower.
there may be enough space between the gravel/rocks that it will work?
so my answer is ?????