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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.

  • Is this a 42" diameter HDPE tube resting on a concrete pad or was the concrete poured inside the tube? Is there any sealing or bonding of the tube to the concrete? – Jim Stewart Aug 10 '17 at 9:08
  • The concrete was poured in the tube. I don't know if there's concrete under the tube as a footer as well, but I think that the pour inside the tube is all that you were really asking. There's no sealing or bonding. – David Pfeffer Aug 10 '17 at 13:40
  • So what you have is as much a well as it is a cistern. – Jim Stewart Aug 10 '17 at 15:51
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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.

  • Is there nothing that can be applied to do the waterproofing "wet"? – David Pfeffer Aug 10 '17 at 13:41
  • I know of no sealing agent or bonding material that would work in this situation. – Michael Karas Aug 10 '17 at 14:40
  • You might be able to get a bladder to fit in the cistern. One problem with that would be that ground water would move in between the bladder and the concrete bottom and the HDPE tube and if you ever drew down the level in the bladder below the 6' level then a mass of water would move in. – Jim Stewart Aug 10 '17 at 16:18
  • I would likely have more success drilling a hole for a pipe through the bottom of the cistern (which I can pump dry-ish and climb into), and then sealing around the pipe and capping the pipe top, rather than digging an 11+ foot hole next to the tank in clay soil. But the general idea here seems sound. – David Pfeffer Aug 10 '17 at 16:51
  • I'm holding out hope someone will come along with a brilliant suggestion for "nondestructive" waterproofing, otherwise I'll mark this as the answer. – David Pfeffer Aug 10 '17 at 16:52
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There are epoxies which cure underwater and are commonly available as pool repair kits. One thing you might try is lowering the water level and caulking around the base of the tube with one. With any luck that will slow the seepage enough that you can seal over the entire thing with something that will work in damp conditions.

Alternately you could try pumping it down and going over the entire bottom with an underwater grout, which again may slow the seepage enough to let you apply something waterproof.

If the tube is actually round you could stuff an insert down it; either one from a fabricator or a tube you capped yourself. Between finding a fabricator and possibly paying freight if they aren't nearby that option is going to come with an appreciable price tag though.

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I think you might have to put a bladder or liner in it. A liner similar to one used in an in-ground vinyl pool might work, but I bet there are liners specifically for this purpose. If you can get it dry, there is a product called Thoroseal that can be used to waterproof the concrete. It even says its good for cisterns in their FAQ: http://www.thoroproducts.com/faq.htm. However, I think the issue would be the seal between the walls and the concrete.

  • A custom made "pool liner" is a good idea if it can't be sealed. water above the water line would slowly push the water back out, I have seen that happen on cement bottomed swimming pools with liners that were emptied for two long. – Ed Beal Aug 10 '17 at 19:11

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