The drillshaft is encased by one -- often two courses -- of concrete casing well past the water table. That's in order to block the very circumstance you've posed.
Makes sense to me — plug the hole up so that the fracking liquid actually WILL frack and not just, under pressure, come back up the hole that was drilled.
Maybe short lived radioactive tracers could/should be added to frack liquid just to prove to doubting Thomasinas, once and for all, that no it’s not getting into the water table.