Thoughts on groundwater intrusion vs. abandoned well failure after renovations?

I dealt with a similar mess in my 1940s colonial after we finished the basement. We had an old hand-dug well that was supposedly capped in the 60s, but after a three-day soak, it turned into a fountain right thru the floor. The pressure from a rising water table is incredible; it doesn't care if your yard looks dry. If that old well wasn't decommissioned with bentonite clay or a proper concrete plug all the way down, the storm likely recharged the vein and now it's looking for the path of least resistance.

In my case, trying to seal it from the top with hydraulic cement just made the water migrate to the footer seams. I eventually had to bite the bullet and install an 18-inch basin with a Zoeller M53 pump. Even a 1/3 HP motor was barely keeping up at first. You realy need to give that water a place to go rather than trying to stop it. If you try to plug it, the hydrostatic pressure will just crack your slab somewhere else. Look into a retrofitted interior drainage system—it's messy to install now that you're finished, but it's the only way I've seen work long-term.

I’m going to go against the grain here and suggest you look at your exterior foundation drainage rather than the well itself. When we redid our daylight basement last year, we had a similar 'spring' pop up trough a hairline crack. Everyone blamed the old cistern, but it turned out the renovation contractors had accidentally crushed a section of the 4-inch corrugated footing tile while grading for the new egress window. If your yard looks dry, that water is likely trapped in the backfill zone right against your foundation walls. The storm probaly saturated the deep soil, and now that 'perched' water has nowhere to go but through your slab beacuse of a blockage in the perimeter drain. Before you go through the massive expense of re-sealing a decommissioned well or installing an industrial-grade Zoeller pump, get a camera down your exterior cleanouts. If those pipes are holding water or full of silt, no amount of plugging the floor is going to stop the hydrostatic pressure from finding the next path of least resistance.

If that water is still pushing after five days, you're fighting a losing battle against hydrostatic pressure. I had this happen and trying to patch it is a waste of time. You need to get a Zoeller M98 1/2 HP Sump Pump in a dedicated pit right near that old well head. It’s got a cast iron housing that handles the heat better than the cheap plastic ones when it's running non-stop. Don't bother with the 1/3 HP models if you're dealing with a literal spring; the M98 moves way more volume and will acutally give you some peace of mind while you're sleeping.

Seen this a dozen times back in the day. Folks think 'out of sight, out of mind' with those old well points, but Mother Nature always wins. Back before everyone used those fancy plastic liners, we'd drop a heavy lead weight down to see if the shaft was acutally backfilled or just topped with a thin slab of slate. If your contractors didn't use bentonite chips to swell and seal the vein, you've basially got a straw leading right into your bedroom. Check the temprature of that water; if it's bone-chilling compared to your tap, you're definately fighting an aquifer that's found its way home.