we need a Norman Torpedo :devil:

MARK

... an inverted inflatable packer -- this is basically an elongated balloon made of thick robust rubber with an rough outer surface (this is sometimes impregnated with something cheap like iron pyrites). Push it up the borehole clear of the bottom section -- because it would probably be fractured rock - and inflate (cylinder of compressed air) -- this would only be a temporary measure.

To make a more efficient seal I would have a tube going up the centre of the packer with it protruding some way beyond the packer. It would have a non-return valve on its end. Then I would pump up a slurry of fast drying underwater cement. Could be expensive. ...

Obviously the seal at the bottom needs to be strong enough to withstand the pressure of water in the borehole above it. How deep is the borehole? Is it open at surface?

Is the water going into the workings from the borehole going to cause a potential risk to the public? If that is the case then I am sure that whoever drilled the borehole, if they are still around, could be made to seal it!

In the coal mining industry every borehole had to be sealed on abandonment for reasons just like this -- even considerable thought had to be given when drilling through old workings to prove reserves below them. It was always a risk and aquifers had to be sealed off. I once had an aquifer seal fail on me and had to use an inflatable packer to stop a colliery being flooded!