Can Cob be used for drinking water catchment?

I'm new to cob, and looking for fun new applications. I'm toying with an idea of catching rainwater off a greenhouse roof and then directing the runoff into a catchment system inside the greenhouse to use later for drinking water. I'd like to build a big basin out of cob that could hold 200 to 300 gallons of water. I realize that such a thing would require waterproofing the cob. So, here are my questions:

1) What do you permies use to waterproof your cob?
2) Do you know of any methods of waterproofing that will be safe for use with drinking water?
3) Am I barking up the wrong tree? Should I be considering other methods of storing drinking water?

Any advice is greatly appreciated.

I've never done any waterproofing of cob other than using oil to resist some moisture, but I'm thinking you might want to go a different route. At 300 gallons you are looking at almost 2500lbs of weight, so the thickness of cob would have to be large enough to withstand the force of the water pushing against the side of whatever catchment you make. I think you are better off getting a poly water collector, they cost about $1 a gallon, but you might find one cheaper on craigslist or somewhere local.

Anyone else done anything like this?

That makes sense. I'll keep looking for other options. Thanks for the input.