I have an existing 42" diameter x 11' deep underground water cistern installed in my backyard that I am starting to use for irrigation. It has HDPE walls and poured concrete bottom. There is no waterproofing applied to it, and so water stored in it slowly seeps into the ground until it reaches 6' full, which appears to be the height of the groundwater.
However, when pumped dry, the tank starts filling up with water immediately, and after about an hour fills to its maximum fill height of 6'. Thus, I can't pump the tank dry to use most waterproofing chemicals, which ask for the concrete to be dry.
What can I do to waterproof the tank? Potability, while not required (this is an irrigation cistern), would be nice in case I needed to ever drink this water in a SHTF situation.