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.
Best Answer
From the potability standpoint I think you will want to definitely plan to have high performance hand operated water filter unit available to filter any water you would try to drink from this tank. Check at a place like REI to find a filter like the type used by canoeists when they make trips on rivers and lakes and filter lake water for drinking.
It seems to me that it is going to be very difficult to get this tank dry enough to seal it. One way that may work is to dig down five or six holes that are about a foot in diameter to a depth of maybe 13 to 14 feet deep (i.e. a good amount below the existing concrete tank floor). Then insert a sump type pump into the bottom of each hole that pumps out accumulated water as fast as it can accumulate. You would want to spill the pumped water out in the street or rain sewer so as to carry the water away from your cistern area. It is possible that this could lower the water table near the tank bottom enough to keep it dry for a period of time.
Of course you would want to do this at a time of the year when there has been little or no rain for a goodly length of time.