Preparedness Pro says that it is okay to store water on cement as long as it is not hot. Here's her full quote:
Water Storage Myth: Don’t store your water barrels on cement.I wish I knew her sources on this, but I don't. But to me, it makes sense. I would appreciate my readers' comments on this. The rest of the article is excellent.
Water Storage Fact: Actually, there’s always a missing component to this myth. The key is not to store your water barrels on HEATED cement, and even that’s questionable advice. To store your water in your basement on the cement floor is just fine. There’s no need to make your barrels less stable by putting them on 2 x 4s. Cement only leaches chemicals when it gets hot. If you’re going to store your water in your garage, where the sun heats up the connecting driveway cement, then yes, I’d consider raising your barrels up on floor boards or such.
Here is another article on water storage.