leaky cistern

The cistern is at least 75 years old. I'm sure it's just plastered to the dirt excavation since that seems to be the most common way they were built 'way out in the country' at that time.

It holds about 2500 gallons; went unused for a couple of years; leaked down slowly for several months after being put back in use but leakage has accelerated suddenly.

Thanks for any help.

You need to be very careful here as any tank can cause death in a heart beat from the lack of oxygen. Make sure you have a good air supply with a helper on stand by.

What I would do is drain the tank then pressure wash the walls and floor then wash with muratic acid ( a fresh air supply mask is needed from here on).

Then I would mix 3 parts fine sand with 1 part portland cement, add 1/4tsp dish soap, mix well, then plaster the walls and floor and finish smooth.

Adding the dish soap breaks the surface tension on the sand so the portland sets water tight.

T_Bone

Haven't looked for either in many years, don't know if they are even still on the market.

Soundguy

T-Bone, your mix ratio is what I needed. An ' old-timer' said sand and portland with 1/2 cup salt for half a wheelbarrow; mix the powder first, then add water; should be able to 'make a snowball,' but he didn't remember the ratio. I guess the salt is to make it sticky. The mix needs to be both stiff and sticky. I'll use dish soap--I've seen plasterers add dish soap to their mud.

I'll keep air supply uppermost in mind. The cistern is 17 feet deep, round and shaped like a little brown jug. The vertical sides taper at the shoulder to a 2-foot diameter neck made of pottery.

Red Dave, it's good to know that UGL is available bagged as a powder. I've used it in pre-mixed gallon cans on foundation walls. Powder would get the cost down. UGL might be part of the solution to seal up high on the walls where there's less water pressure. I'm convinced that cracks down low need to be plastered thick and wide, like at least 1/4-inch thick by about 2 feet wide.

Soundguy, I've used hydraulic cement to patch cracks in foundation walls after v-grooving. In this case, large areas need to be plastered so a regular portland cement mix seems to be what's required.

There are modern rubber/plastic coatings that remain stretchy and claim to be suitable for potable water, but I'm leery of anything that might leave a chemical taste or odor to the water (T-Bone, I think I remember you writing that your taster didn't get along with plastic drinking glasses--I'm kinda the same way).

I'm letting it leak out now for the last time, and running a tape down to water level each day to get an idea of where the worst leakage is.

Thanks again.