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I have a pretty high levels of radon in the basement (~400 Bq/m3 when windows are not open), so I want to seal obvious entry points, one of which is what looks like a 10 inch weeping tile cleanout access point (see photo). At the moment it is covered by a plywood circle cutout. I dropped radon detector in there and it shows 900 Bq/m3 after a day.

I expect a lot of radon is seeping from there, from the sewer drain or from another 10 inch hole I haven't opened yet.

The question is, how do I air-seal this access point but still keep cleanout accessible and level with the rest of the floor? (it is kinda in the middle of the room).

I don't expect cleanout to be needed often (?), so at worst a permanent but removable cap siliconed in would do. Ideally though, there is a sealed access hatch made for the purpose that I can install into recessed concrete or something, but I'm having trouble googling for that.

Update: I have 3 of those cleanouts actually, after digging into the gravel for a bit it seems like it is gravel all the way down (and around). I think that's what slab on grade with wiping tile looks like?

This is 1995 house in Canada for reference. No sump pump or hole for it.

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    I'd start with cleaning those stones out of there so that you (and we) can see what you're dealing with. There should be a cap under there and, ideally no gaps around it. Might want to remove the debris and post another picture.
    – gnicko
    Sep 13, 2021 at 19:01
  • Looks like this gravel is what basement slab is sitting on, if I dig under the concrete, I feel underneath of the slab. I expect if I dig all the way down I'll hit the dirt after a while.
    – Eugene
    Sep 13, 2021 at 23:32
  • So my goal is to seal off gravel air from the basement air. Something like @r13 proposed, but maybe less drastic than a concrete plug.
    – Eugene
    Sep 13, 2021 at 23:34

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The concrete plug can be easily chipped out when the need for the cleanout arises.

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