So cool to find the obvious leak! I would be tempted to use a PVC pipe to direct appropriate cement mixture directly into the hole. Follow TJs advice though.
If cement, use the material that contains glass fibers. Mixed just a little thin so it flows into the initial water path, and not using coarse aggregate. At least at first if it just keeps getting sucked in, then I’m not sure.
I am not a mason, so I don’t know all of the products available. If these is such a thing as cement that swells when it cures, that would likely help, combined with fibers.
I suppose bentonite may work directed into the hole, but I have heard very few success stories.
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