I don't know much about cisterns, but I know if you fill it with water, and it empties out by itself, it is leaking ;)

But seriously, can you get down in it to look at it? Check it out, and if you find any cracks, I am sure if you ask someone at the hardware store they can point you to some sealants, then you just have to make sure they won't poison your vegetables.

I would fill it with diverted rainwater, and give it a try. "A liter saved is a liter earned" and all that. As long as you aren't drinking it, I don't see why it wouldn't be useable. And if it is well covered, that should help keep animals from falling in and drowning.

Has anyone tried gravity pumps?

While remodeling we found a ferrocement cistern, buried under the bathroom floor. Had been used to collect rainwater and volume appears to be aprox 1200 gallons. Want to be able to collect rain water once again and use it for most everything but comsumption.

Searching for info about how to pump the water out of the tank, whether filters are needed and any other pertinent info. Have requested a book by SB Watt on ferrocement water tanks but thought I'd post this in the meantime. Thanks

Ames Blue Max is a drinking water approved coating that goes on nicely. Takes 2 - 3 weeks to cure and must be applied to dry clean surface. I have built a cistern from cement blocks and used Blue Max. It works.

Here is a link that might be useful: cistern pictures and biological water filters