One way to poison underground drinking water is pump CO2 into the ground and maybe with the CO2 a virus or bacteria or some other toxin or foreign substance. Today we already pump CO2 into the ground into oil wells to extract more oil from older wells. But if you start randomly pumping CO2 into the ground, you will only do harm.
CO2 Will turn water into acid. The best way to deal with CO2 is let the plants and trees breathe it and turn it back into fresh oxygen so all of us who exhale co2 can breathe.
CO2 is poisoning underground drinking water today and used antifreeze and nuclear waste as well as other toxins from the industrial age dumped into the ground and rivers and streams and into the ocean and then left to evaporate into the air.