A water table is a geological layer on a planet. Comprised of water, this layer was a key precondition to supporting life on Class M world. The ability to access and exploit the water table was a key part of the Federation's terraforming process.
In 2366, Beta Agni II's water table was decontaminated of tricyanate poisoning unnaturally quickly by the introduction of hytritium. This pointed the finger of suspicion at Kivas Fajo, who had sabotaged the water table. (TNG: "The Most Toys")