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I noticed a small moist patch of the floor near a radon mitigation system. However, I suspect that it has little to do with the radon system but was due to failed caulking at the cove joint, as there seems to be dirt in a small segment of the caulking.

A recent heavy rain likely raised the water table so high that even though our sump pump worked properly, there was still a small amount of water seeping in.

Is my assessment right? If so, what is a good caulk chemistry to use?

There are suggestions such as

Quikrete polyurethane sealant,

silicone caulk,

latex-based Drylok waterproofer.

Updated: A picture of the sump pump. The sump pit is 19 inch in diameter and pretty deep, but the float makes the pump to operate only between a limited depth (as you can see from the water marks in the picture).

Cove joint 1

Cove joint 2

Cove joint 3

Sump Pump

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  • Where is the water coming from? Should you not address that issue first?
    – Alaska Man
    Dec 25 '20 at 19:04
  • Through the cove joint because of high water table? I think our drainage system and sump pump are working for the most part, as this likely happened only after very heavy rains
    – P. B.
    Dec 25 '20 at 19:13
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Water should never come up from below the slab. If water is coming up then your sump pump is undersized and not handling either the influx of ground water or the rain and influx of ground water. The sump pump should have a high level alarm. If the water ever gets anywhere near the bottom of the slab the alarm should be going off.

You aren't going to be able to turn your basement slab into a boat bottom if water is that high in the water table then water will wick up through the porous concrete slab or through any number of penetrations or cracks. You might want to seal the crack to prevent radon / water moisture from entering your basement in that case I'd go for the quikrete product.

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  • I didn't catch it when it was actively leaking, but based on the water stain, I think it was quite limited and recent. I remember that my sump pump was cycling every one minute or so, and it took that long for the sump basin to refill. The sump pump operated like this and gradually slowed down over a day after the rain stopped. This moist spot is almost the opposite corner of the sump pump.
    – P. B.
    Dec 25 '20 at 21:17
  • Your sump cycling every minute isn't great. They have a certain number of activations before they die. You can get an electric float which allows fine grained hi/lo activations which can help but possibly you need to make your sump well larger. Add an image of your sump well. Dec 25 '20 at 21:19
  • A picture and a description of the sump well is added. It's raining heavily again today and I timed it. It's actually 3 minutes between cycles so I may have overestimated the frequency. Still, being able to reduce the frequency further would be nice. I imagine I would have to disable the pump's current float and add the electric float as a piggyback?
    – P. B.
    Dec 25 '20 at 21:45
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    Yes you'd secure the integrate float such that it is on. I like the HydroCheck Hi-Lo Pump Controller with Dual Float Switch (HC6000v2). With your current configuration the pump will go on with a 10"? rise in water. With the hi/lo you can set it below the drain pipe level such that it goes on later and runs longer. You have a 2" pipe and that looks to be a high quality pump so it should drain quick. Probably better to ask as a different question though. Dec 25 '20 at 22:17
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    The latex paint is thin and likely to easily crack. Silicon again coming out of a caulk tube is going to be a bead so harder to spread. The poly urethrane will self level and is meant for concrete to concrete cracks it should to some degree flow into the crack and provide better sealing. Dec 25 '20 at 22:19

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